[image: 4825.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: Wolf-Parkinson-White syndrome (WPW), abnormal QRS (ABQRS), sinus rhythm (SR) Explanation for Wolf-Parkinson-White syndrome (WPW) Wolf-Parkinson-White (WPW) syndrome is a condition where an extra electrical pathway (accessory pathway) connects the atria and ventricles, bypassing the AV node. This allows electrical signals to arrive at the ventricles earlier than normal, leading to "pre-excitation." The three hallmark signs of WPW on a standard 12-lead ECG are often referred to as the WPW Triad: 1. Short PR Interval Definition: The time from the start of the P wave to the start of the QRS complex is usually less than 120 ms ( 0.12 seconds), or less than 3 small squares. Why it happens: Normally, the AV node holds the electrical signal briefly to allow the ventricles to fill with blood. In WPW, the signal travels down the accessory pathway (which has no delay mechanism) and activates the ventricles early, shortening this interval. 2. Delta Wave Definition: A slurred or slow-rising upstroke at the very beginning of the QRS complex. Why it happens: This represents the early, slow depolarization of the ventricular muscle tissue via the accessory pathway (pre-excitation) before the normal signal from the AV node catches up and rapidly depolarizes the rest of the ventricles. 3. Wide QRS Complex Definition: The total duration of the QRS complex is usually greater than 110–120 ms (typically > 0.11 or 0.12 seconds). Why it happens: The QRS is widened because it is a "fusion beat"—a combination of the early activation from the accessory pathway (the Delta wave) and the normal activation from the AV node. Secondary Signs & Classification Beyond the classic triad, you may see other specific changes depending on the location of the accessory pathway. Secondary ST-T Wave Changes Because the depolarization (activation) of the heart is abnormal, the repolarization (recovery) is also abnormal. You will often see ST-segment depression or T-wave inversion that is discordant (opposite in direction) to the QRS complex and Delta wave. "Pseudo-Infarction" Patterns The abnormal Delta waves can sometimes be negative (pointing down) in certain leads (often inferior leads II, III, aVF). This can mimic pathological Q waves, leading to a misdiagnosis of a past heart attack (myocardial infarction). Type A vs. Type B Pattern Historically, WPW was classified by the appearance of the QRS in the precordial leads (V1–V6), which helps localize where the extra pathway is located. Type A (Left-sided pathway): Positive Delta wave and QRS complex in lead V1. Looks similar to a Right Bundle Branch Block (RBBB) pattern. Indicates the pathway is likely on the left side of the heart. Type B (Right-sided pathway): Negative Delta wave and QRS complex in leads V1 and V2. Looks similar to a Left Bundle Branch Block (LBBB) pattern. Indicates the pathway is likely on the right side of the heart. Note: It is clinically important to distinguish "WPW Pattern" from "WPW Syndrome." A patient has the Pattern if they simply have these ECG findings. They are diagnosed with the Syndrome only if they have the ECG findings plus symptoms of arrhythmias (such as palpitations or SVT). Read more on LITFL: https://litfl.com/pre-excitation-syndromes-ecg-library/ Explanation for abnormal QRS (ABQRS) An abnormal QRS complex on a 12-lead ECG is typically identified by deviations in three main categories: Duration ( width), Amplitude (height/voltage), and Morphology (shape/contour). 1. Abnormal Duration (Wide QRS) A normal QRS complex lasts between 0.08 and 0.10 seconds (80–100 ms). A duration of > 0.12 seconds (>120 ms or 3 small squares) is considered abnormally wide. Bundle Branch Blocks (BBB): A blockage in the electrical conduction system causes one ventricle to depolarize later than the other, widening the QRS. Right Bundle Branch Block (RBBB): Characterized by an rSR' ("bunny ears") pattern in leads V1–V2 and a wide, slurred S wave in leads I and V6. Left Bundle Branch Block (LBBB): Characterized by a deep, broad S wave in V1 and a broad, notched, or " M-shaped" R wave in leads I, aVL, V5, and V6. Ventricular Rhythms: Rhythms originating from the ventricles (rather than the atria) do not use the fast conduction system, resulting in a wide QRS. Examples include Premature Ventricular Complexes (PVCs), Ventricular Tachycardia (VT), and Idioventricular rhythms. Hyperkalemia: High potassium levels can slow conduction, leading to a bizarrely wide QRS that may merge with the T wave (sine-wave pattern). Wolff-Parkinson-White (WPW) Syndrome: An accessory pathway allows early activation of the ventricles ( pre-excitation), causing a Delta wave (slurring of the initial upstroke) and a widened QRS. 2. Abnormal Amplitude (Voltage) The height of the QRS complex represents the electrical force generated by the ventricular muscle mass. High Voltage (Hypertrophy): Left Ventricular Hypertrophy (LVH): The muscle wall is thickened, generating stronger electrical forces. A common sign is the Sokolow-Lyon criteria: Depth of S wave in V1 + Height of R wave in V5 or V6 > 35 mm. Right Ventricular Hypertrophy (RVH): Often causes a dominant R wave in lead V1 (height > 7 mm) and a deep S wave in V5 or V6. Low Voltage: Defined as QRS amplitude < 5 mm in all limb leads and < 10 mm in all precordial (chest) leads. Causes: Anything that insulates the heart or dampens the signal, such as pericardial effusion (fluid around the heart), COPD (air trapping), obesity, or hypothyroidism (myxedema). 3. Abnormal Morphology (Shape) Even if the width and height are normal, the shape of the wave can indicate pathology. Pathological Q Waves: Q waves are the first downward deflection of the QRS. While small "septal" Q waves are normal in some leads, * pathological* Q waves indicate dead myocardial tissue (previous Myocardial Infarction). Signs: Duration > 0.04 s (40 ms) or depth > 25% of the following R wave height. Poor R Wave Progression: Normally, the R wave grows larger as you move from lead V1 to V6. If the R wave remains small or absent in leads V1–V3, it is termed "poor progression." Causes: Anterior Myocardial Infarction (old or new), LBBB, or lead misplacement. Fragmented QRS: Presence of additional spikes, notches, or slurs within the QRS complex (not fitting a typical BBB pattern). This often represents myocardial scarring or fibrosis. Electrical Alternans: The height of the QRS complex alternates between beats (large, small, large, small). This is a specific sign of a large pericardial effusion (cardiac tamponade) as the heart swings back and forth in the fluid. Read more: LITFL (interventricular conduction delay): https://litfl.com/intraventricular-conduction-delay-qrs-widening/ LITFL (low QRS voltage): https://litfl.com/low-qrs-voltage-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 8626.png] Diagnostic Code Explanations (2) Diagnoses for this EKG: incomplete right bundle branch block (IRBBB), sinus rhythm (SR) Explanation for incomplete right bundle branch block (IRBBB) An Incomplete Right Bundle Branch Block (IRBBB) is a conduction delay where the electrical impulse travels to the right ventricle more slowly than normal, but not slowly enough to be classified as a "complete" block. On a standard 12-lead ECG, the diagnosis is made by looking for specific changes in the QRS duration and the wave shapes in leads V1 and V6. 1. QRS Duration (The Main Criteria) The primary difference between an incomplete and a complete block is the duration of the QRS complex. Incomplete RBBB: The QRS duration is prolonged but remains between 100 and 119 milliseconds (0.10 to < 0.12 seconds). Complete RBBB: The QRS duration is ≥ 120 milliseconds (≥ 0.12 seconds). 2. Signs in the Right Precordial Leads (V1 and V2) The most distinct sign of IRBBB appears in leads V1 and V2 (the leads placed over the right ventricle). RSR' Pattern ("M-Shaped" Complex): You will typically see a three-phase wave described as rSr', rsR', or * *rSR'**. The first wave (r) represents normal activation of the septum. The second wave (s) represents the normal activation of the left ventricle. The third wave (R') is a second upward spike that represents the delayed activation of the right ventricle. R' is often taller than r: The second upward spike (R') is usually larger than the initial one. 3. Signs in the Lateral Leads (V6 and Lead I) While V1 shows the delayed "push" of electricity toward the right, the leads on the left side of the chest (V6 and Lead I) show that same electrical force moving away from them. Slurred S Wave: You will see a wide, "slurred" S wave at the end of the QRS complex. This S wave is wider than the preceding R wave (or > 40ms) and looks "messy" or slow compared to the sharp upstroke of the R wave. Summary of Diagnostic Criteria | Feature | Incomplete RBBB Signs | QRS Duration: 100 – 119 ms (Narrower than complete block) Lead V1 / V2: rSR' pattern (Bunny ears / M-shape) Lead I / V6: Wide, slurred S wave Clinical Significance It is important to note that an incomplete RBBB is frequently a normal variant (benign finding), particularly in children, young adults, and well-conditioned athletes. However, it can occasionally be associated with conditions that strain the right side of the heart, such as an atrial septal defect (ASD) or right ventricular hypertrophy. Read more on LITFL: https://litfl.com/right-bundle-branch-block-rbbb-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 3636.png] Diagnostic Code Explanations (4) Diagnoses for this EKG: non-specific intraventricular conduction disturbance (block) (IVCD), first degree AV block (1AVB), ventricular premature complex (PVC), sinus rhythm (SR) Explanation for non-specific intraventricular conduction disturbance (block) (IVCD) A nonspecific intraventricular conduction disturbance (often abbreviated as IVCD or NICD) is an electrocardiographic finding that indicates a delay in the electrical activation of the ventricles, but the pattern does not fit the specific criteria for the well-defined "named" blocks (Right or Left Bundle Branch Block). This diagnosis is essentially a diagnosis of exclusion. 1. The Core Criteria To identify a nonspecific intraventricular conduction disturbance, the ECG must meet the following two conditions: Widened QRS Complex: The QRS duration is prolonged, typically defined as > 110 ms (some guidelines use ≥ 100 ms or > 120 ms). Absence of Specific Bundle Branch Block Morphology: The QRS shape does not meet the classic criteria for Left Bundle Branch Block (LBBB) or Right Bundle Branch Block (RBBB). 2. What You Will See (Signs on the 12-Lead) Because this is a "nonspecific" finding, there is no single characteristic shape (morphology) like "rabbit ears" or "slurred R waves" that defines it. Instead, you look for the absence of specific patterns. A. Check V1 and V6 Not RBBB: Look at lead V1. In a nonspecific IVCD, you will not see the classic rSR' (rabbit ears) pattern. Look at lead V6. You will not see the wide, slurred S-wave typical of RBBB. Not LBBB: Look at lead V1. You will not necessarily see the deep, broad QS complex typical of LBBB. Look at leads I, aVL, V5, and V6. You will not see the classic broad, notched, or slurred R-waves (monophasic R waves) that define LBBB. B. General Appearance Bizarre Morphology: The QRS complexes often look "bizarre" or generally widened without a recognizable pattern. Axis Deviation: Left or Right axis deviation may be present but is not required. Notching or Slurring: You may see slurring of the QRS upstroke or downstroke, but it does not fit the specific locations required for RBBB or LBBB. 3. Common Causes (Why this happens) Since the conduction system (the "highway") isn't blocked in a specific branch, the delay often occurs in the heart muscle (myocardium) itself or generally throughout the Purkinje system. Common causes include: Electrolyte Abnormalities: Hyperkalemia (high potassium) is a critical life-threatening cause that widens the QRS universally. Medication Toxicity: Sodium channel blockers (e.g., Tricyclic Antidepressants like Amitriptyline, or antiarrhythmics like Flecainide) can cause a "bizarre" wide QRS. Myocardial Scarring: Previous heart attack (MI) or fibrosis (from cardiomyopathy) can slow conduction through the scarred tissue. Ventricular Hypertrophy: Severe thickening of the heart muscle can simply take longer to depolarize, widening the QRS. Summary for Diagnosis If you see a wide QRS (> 110 ms): Is it LBBB? (Deep S in V1, Broad/Notched R in V6?) -> No. Is it RBBB? (Rabbit ears in V1, Slurred S in V6?) -> No. Is it a Paced Rhythm? (Pacer spikes?) -> No. Is it Pre-excitation (Wolff-Parkinson-White)? (Delta wave?) -> No. Then it is a Nonspecific Intraventricular Conduction Disturbance. Read more on LITFL: https://litfl.com/intraventricular-conduction-delay-qrs-widening/ Explanation for first degree AV block (1AVB) First-degree atrioventricular (AV) block is characterized by a delay in the electrical conduction from the atria to the ventricles. Despite the name "block," the electrical impulse is not actually blocked; it is simply slowed down as it passes through the AV node. The hallmark sign on a standard 12-lead ECG is a prolonged PR interval that remains constant. Key Diagnostic Criteria To diagnose first-degree AV block, the ECG must show the following features: Prolonged PR Interval: Adults: The PR interval is consistently greater than 200 milliseconds (ms). On standard ECG paper (where 1 small square = 40 ms), this is wider than 5 small squares (or 1 large square). Pediatrics: The threshold is lower and age-dependent. For example, a PR interval >160 ms in an infant or >180 ms in a young child may qualify as first-degree block. 1:1 Relationship (One P for every QRS): Every P wave is followed by a QRS complex. Every QRS complex is preceded by a P wave. There are no dropped beats (unlike second-degree or third-degree heart blocks). Constant PR Interval: The duration of the PR interval is fixed and does not change from beat to beat. (If the PR interval progressively lengthens before a dropped beat, that is Second-Degree Type I / Wenckebach). How to Measure Properly Best Lead: Look at Lead II (or sometimes V1), as P waves are typically most visible there. Measurement: Measure from the beginning of the P wave (where it first leaves the isoelectric line) to the beginning of the QRS complex (the very first deflection, whether it is a Q wave or an R wave). Consistency: Verify the measurement across multiple beats to ensure it is fixed. Clinical Context & Nuances "Marked" First-Degree Block: If the PR interval exceeds 300 ms, it is often termed "marked" first-degree AV block. While usually asymptomatic, intervals this long can cause "pacemaker syndrome-like" symptoms (fatigue, dizziness) because the atria contract while the mitral/tricuspid valves are still closed from the previous beat. P Wave Location: In cases of marked block, the PR interval may be so long that the P wave is buried in the preceding T wave. You may notice a "camel hump" appearance on the T wave or a T wave that looks different than usual. Causes: Common causes include high vagal tone (common in athletes), medications (beta-blockers, calcium channel blockers, digoxin), electrolyte imbalances, or age-related degeneration of the conduction system. Summary PR Interval: > 0.20 seconds (more than 5 small boxes) Rhythm: Regular (assuming the underlying sinus rhythm is regular) P Waves: Normal upright P waves in Lead II; always present Conduction Ratio: 1:1 (Atria : Ventricles) Dropped Beats? None Read more on LITFL: https://litfl.com/first-degree-heart-block-ecg-library/ Explanation for ventricular premature complex (PVC) On a standard 12-lead ECG, a Premature Ventricular Complex (PVC) is identified by a specific set of electrical features that distinguish it from normal sinus beats. These signs relate to timing, morphology (shape), and the pause that follows the beat. 1. Key Diagnostic Criteria The three most reliable signs of a PVC are: Prematurity: The complex occurs earlier than the next expected sinus beat. Wide QRS Complex: The QRS duration is prolonged, typically >120 ms (0.12 seconds). Because the impulse originates in the ventricle, it travels through the heart muscle (cell-to-cell) rather than the fast-conducting His-Purkinje system, resulting in a slower, wider depolarization. Absence of a P Wave: There is usually no P wave preceding the QRS complex because the impulse did not originate in the atria. (Note: A retrograde P wave may sometimes be seen after the QRS or buried within it). 2. Morphological Signs (Shape) Bizarre Appearance: The QRS complex often looks large, jagged, or distinctively different from the patient's normal sinus beats. Discordance: The ST segment and T wave are typically directed opposite to the main vector of the QRS complex. If the QRS is largely upright (positive), the T wave will be inverted (negative). If the QRS is largely downward (negative), the T wave will be upright. 3. Compensatory Pause Most PVCs are followed by a complete compensatory pause. This means the interval between the normal beat before the PVC and the normal beat after the PVC is exactly equal to two normal R-R intervals. Why? The PVC renders the ventricle refractory, blocking the next normal sinus impulse. However, the sinus node itself is usually not reset, so it fires the subsequent beat on schedule. 4. Localization Signs (Where the PVC comes from) The shape of the PVC on the 12-lead ECG can help identify its origin within the ventricles: Left Bundle Branch Block (LBBB) Pattern: If the PVC looks like an LBBB (dominant S wave in V1, wide upright R wave in V6), it likely originates from the Right Ventricle. Right Bundle Branch Block (RBBB) Pattern: If the PVC looks like an RBBB (dominant R wave in V1, wide S wave in V6), it likely originates from the Left Ventricle. 5. Patterns and Variants PVCs are often described by their frequency and pattern: Bigeminy: Every other beat is a PVC. Trigeminy: Every third beat is a PVC. Couplet: Two consecutive PVCs. Multifocal: PVCs with different shapes (morphologies), suggesting they originate from different irritable foci in the heart. Interpolated PVC: A rare variant where a PVC occurs specifically between two sinus beats without a compensatory pause, effectively "squeezing" in without disrupting the rhythm. R-on-T Phenomenon: A dangerous sign where the PVC occurs very early, landing on the T wave of the preceding beat. This can precipitate ventricular fibrillation. Read more on LITFL: https://litfl.com/premature-ventricular-complex-pvc-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 13705.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 2508.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 20652.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus arrhythmia (SARRH) Explanation for sinus arrhythmia (SARRH) On a standard 12-lead ECG, sinus arrhythmia is characterized by a normal sinus rhythm that has an irregular rate. The "irregularity" is usually patterned and predictable, corresponding to the patient's breathing cycle. Here are the specific signs and criteria used to identify it: 1. The Core Criterion: R-R Interval Variation The hallmark sign of sinus arrhythmia is a variation in the timing between heartbeats (the R-R interval) that originates from the sinus node. Variation Threshold: The difference between the longest and shortest R-R interval (or P-P interval) is greater than 0.12 seconds (which equals 3 small squares on standard ECG paper). Pattern: The rhythm typically speeds up and slows down in a cyclical pattern. 2. Normal P-Wave Morphology Because the electrical impulse still originates from the sinoatrial (SA) node, the atrial depolarization looks normal. Appearance: P-waves are upright in leads I and II. Consistency: The P-waves have a constant shape and structure beat-to-beat. There is one P-wave for every QRS complex. 3. Constant PR Interval The time it takes for the impulse to travel from the atria to the ventricles remains stable. The PR interval is constant and typically within the normal range of 0.12 to 0.20 seconds. 4. Normal QRS Complex The ventricular contraction is unaffected, so the QRS complex is narrow (usually < 0.12 seconds) and follows every P-wave. 5. Correlation with Respiration (Respiratory Sinus Arrhythmia) This is the most common form, especially in children and young, healthy adults. You can often see this "phasic" change on the rhythm strip at the bottom of the 12-lead page: Inspiration (Breathing in): The heart rate increases (R-R intervals get shorter). Expiration (Breathing out): The heart rate decreases (R-R intervals get longer). Note on Clinical Context: While usually benign and respiratory in nature, non-respiratory sinus arrhythmia (where the irregularity does not align with the breathing cycle) can occur. This is more commonly seen in older adults and can be associated with underlying heart disease or digitalis toxicity. Read more on LITFL: https://litfl.com/sinus-arrhythmia-ecg-library/

[image: 19222.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 1839.png] Diagnostic Code Explanations (4) Diagnoses for this EKG: anteroseptal myocardial infarction (ASMI), left anterior fascicular block (LAFB), Q waves present (QWAVE), sinus rhythm (SR) Explanation for anteroseptal myocardial infarction (ASMI) An anteroseptal myocardial infarction (MI) is characterized by ischemia and necrosis affecting the anterior wall of the left ventricle and the interventricular septum. It typically results from an occlusion of the Left Anterior Descending (LAD) coronary artery. On a standard 12-lead ECG, the signs of an anteroseptal MI are primarily observed in the precordial leads V1 through V4. Key ECG Findings ST-Segment Elevation Leads: The hallmark sign is ST-segment elevation in leads V1, V2, V3, and V4. Morphology: The elevation is often significant and may evolve into a "tombstone" pattern in severe cases. Specifics: Septal leads: V1 and V2. Anterior leads: V3 and V4. The involvement of both groups defines the "anteroseptal" pattern. Q Waves (Pathological) As the infarction evolves (or if it is age-indeterminate), pathological Q waves will develop in leads V1–V4. A QS complex (a deep negative deflection with no preceding positive R wave) is commonly seen in V1 and V2. Poor R-Wave Progression (PRWP) In a healthy heart, the R wave (the first positive spike of the QRS complex) should get progressively taller as you move from lead V1 to V6. In an anteroseptal MI, the destruction of heart muscle in the anterior wall leads to a loss of these positive forces. Consequently, you will see small or absent R waves in leads V1–V3 (or even up to V4). T-Wave Changes Hyperacute Phase: Immediately after occlusion, you may see tall, peaked "hyperacute" T waves in V1–V4 before ST elevation becomes obvious. Evolving/Subacute Phase: As the MI progresses, the T waves typically become inverted (negative) in the same leads (V1–V4). Reciprocal Changes Unlike inferior or lateral MIs, pure anteroseptal MIs often lack distinct reciprocal changes (ST depression) in the standard limb leads. Why? The electrical "opposite" of the anterior wall is the posterior wall. Standard 12-lead ECGs do not have leads on the patient's back to record this directly. Exception: If the infarction extends to the high lateral wall (leads I and aVL), you may see reciprocal ST depression in the inferior leads (II, III, aVF). Summary Checklist for Anteroseptal MI ST Elevation: Leads V1, V2, V3, V4 Q Waves: Developing in V1–V4 R Waves: Loss of height in V1–V3 (Poor R-wave progression) Reciprocal Changes: usually absent (unless lateral extension is present) Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (anterior STEMI): https://litfl.com/anterior-myocardial-infarction-ecg-library/ Explanation for left anterior fascicular block (LAFB) Left anterior fascicular block (LAFB), also known as left anterior hemiblock, is a conduction abnormality where the anterior fascicle of the left bundle branch is blocked. This forces electrical impulses to depolarize the left ventricle via the posterior fascicle, altering the direction of the electrical vector. Diagnosing LAFB relies on specific changes in the QRS axis and morphology. Below are the primary signs seen on a standard 12-lead ECG. 1. Pathological Left Axis Deviation (LAD) This is the hallmark sign of LAFB. Because the anterior fascicle is blocked, the electrical wavefront travels from the posterior/inferior wall towards the anterior/superior wall. This creates a net electrical vector that points upward and to the left. Criterion: The axis is typically between -45° and -90°. Note: While an axis between -30° and -45° suggests possible LAFB, it is less specific and can be a normal variant or due to left ventricular hypertrophy. A shift beyond -45° is generally required for a definitive diagnosis. 2. Lead-Specific QRS Morphology The redirected electrical vector creates a "divergent" pattern between the lateral and inferior leads. Lateral Leads (I and aVL): Pattern: qR complex (small Q wave, tall R wave). Explanation: The initial forces go downwards/rightwards (creating the small 'q'), followed by the major forces going upwards/leftwards (creating the tall 'R'). Inferior Leads (II, III, and aVF): Pattern: rS complex (small R wave, deep S wave). Explanation: The initial forces move briefly towards these leads (small 'r'), but the bulk of the depolarization moves away from them (deep 'S'). Key Check: Lead aVF should be negative. Lead II should also be negative (which confirms the axis is more negative than -30°). 3. Normal or Minimally Prolonged QRS Duration Unlike a complete Left Bundle Branch Block (LBBB), the main conduction trunk is intact, so ventricular depolarization is not significantly delayed. Criterion: QRS duration is usually < 120 ms (typically 80–110 ms). Context: If the QRS is ≥ 120 ms, a complete bundle branch block is likely present (e.g., RBBB + LAFB). 4. Prolonged R-Wave Peak Time in Lead aVL This measures the time from the onset of the QRS to the peak of the R wave (intrinsicoid deflection), representing the time it takes for depolarization to reach the lateral wall. Criterion: R-wave peak time ≥ 45 ms in lead aVL. Comparison: This is often delayed compared to lead V6. Summary of Diagnostic Criteria (Rule of Thumb) To quickly identify LAFB, look for: Left Axis Deviation (Positive Lead I, Negative Lead aVF). Negative Lead II (Deep S wave). Small Q waves in I and aVL. Normal QRS width (< 120 ms). Important Clinical Note: LAFB is a diagnosis of exclusion. You must ensure that the Left Axis Deviation is not caused by other conditions, such as a preexisting inferior myocardial infarction (which causes deep Q waves in II, III, and aVF) or severe Left Ventricular Hypertrophy (LVH). Read more on LITFL: https://litfl.com/left-anterior-fascicular-block-lafb-ecg-library/ Explanation for Q waves present (QWAVE) In a standard 12-lead ECG, a Q wave is defined as the first negative deflection of the QRS complex that is not preceded by any positive deflection (R wave). If there is any positive wave before the dip, that dip is an S wave, not a Q wave. While small Q waves can be normal in certain leads, "pathological" Q waves are a key sign of myocardial infarction ( heart attack) or other structural heart issues. The following breakdown details how to identify Q waves and distinguish normal from abnormal. 1. Identifying a Q Wave Visual Check: Look at the QRS complex. Does it start immediately with a downward line? That is a Q wave. QS Complex: If the entire QRS complex is just one deep downward stroke with no upward R wave at all, this is called a QS complex and is considered a Q wave equivalent. 2. Normal (Physiological) Q Waves Not all Q waves are bad. Normal "septal" Q waves represent the normal depolarization of the interventricular septum ( from left to right). Appearance: Narrow (short duration) and shallow. Duration: Less than 0.03 seconds (less than 1 small box). Amplitude: Less than 25% of the height of the following R wave. Location: Commonly seen in the lateral leads (I, aVL, V5, V6). Note: Deep Q waves can be normal in lead aVR (because it looks at the heart from the opposite angle) and sometimes in lead III (especially if they disappear when the patient takes a deep breath). 3. Pathological (Abnormal) Q Waves Pathological Q waves usually indicate that there is an electrical "hole" in the heart tissue, often scar tissue from a previous myocardial infarction (MI). Since dead tissue cannot conduct electricity, the electrode "looks through" the dead patch and records the electrical forces moving away from it on the opposite side of the heart, resulting in a deep negative deflection. Criteria for Pathological Q Waves: Duration (Width): ≥ 0.04 seconds (1 small box or wider). This is the most specific sign of pathology. Amplitude (Depth): ≥ 2 mm deep OR > 25% of the height of the subsequent R wave. Location: Leads V1–V3: Any Q wave in these leads (except a QS complex in V1) is generally considered abnormal. Contiguity: To be significant, Q waves must appear in two or more contiguous leads (leads that look at the same area of the heart). 4. Locating the Damage (Infarct Localization) The leads where pathological Q waves appear tell you which part of the heart has scarring: Inferior: II, III, aVF (Right Coronary Artery - RCA) Lateral: I, aVL, V5, V6 (Left Circumflex - LCx) Anterior: V3, V4 (Left Anterior Descending - LAD) Septal: V1, V2 (Left Anterior Descending - LAD) Posterior: Tall R wave in V1, V2* (RCA or LCx) (Note: Posterior MI is often indicated by a "reciprocal" change: a tall, wide R wave in V1 or V2, which is essentially an upside-down Q wave.) 5. Other Causes of Q Waves (Mimics) While a prior heart attack is the most common cause, other conditions can create Q waves or Q-wave-like patterns: Lead Placement Errors: Switching limb leads can create fake Q waves. Left Bundle Branch Block (LBBB): Can cause QS complexes in leads V1–V3. Ventricular Hypertrophy (LVH/RVH): Enlarged heart muscle can distort electrical forces, creating deep Q waves in certain leads. Wolff-Parkinson-White (WPW) Syndrome: The delta wave (pre-excitation) can sometimes be negative, mimicking a Q wave (pseudo-Q wave). Pulmonary Embolism: Classic "S1Q3T3" pattern includes a Q wave in lead III. Read more on LITFL: https://litfl.com/q-wave-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 2154.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus bradycardia (SBRAD) Explanation for sinus bradycardia (SBRAD) On a standard 12-lead ECG, sinus bradycardia is identified by a heart rate of less than 60 beats per minute (bpm) while maintaining all the characteristics of a normal sinus rhythm. To diagnose it, you must confirm that the rhythm originates from the sinus node (the heart's natural pacemaker) and is conducting normally to the ventricles. Here are the specific signs and criteria to look for: 1. Heart Rate Criterion: Ventricular rate is < 60 bpm. Calculation: On standard ECG paper (25 mm/s), the distance between consecutive R-waves will be greater than 5 large squares (since $300 \div 5 = 60$). 2. Rhythm Criterion: The rhythm is regular. Check: The R-R intervals (distance between heartbeats) are consistent. Slight variation (respiratory sinus arrhythmia) is common and acceptable, but the overall pattern should be steady. 3. P-Wave Morphology (Crucial for "Sinus" Origin) To confirm the rhythm is sinus (coming from the sinoatrial node) rather than an atrial or junctional escape rhythm, the P-waves must have a specific vector: Leads I, II, and aVF: P-waves must be upright (positive). Lead aVR: P-waves must be inverted (negative). Consistency: All P-waves should look identical, indicating they originate from the same focus. 4. P-to-QRS Relationship Criterion: There is a 1:1 relationship. Check: Every P-wave is followed by a QRS complex, and every QRS complex is preceded by a P-wave. This rules out Complete Heart Block (3rd-degree AV block), where P-waves and QRS complexes beat independently. 5. Intervals PR Interval: Normal and constant (between 0.12 and 0.20 seconds; 3–5 small squares). This rules out 1st-degree AV block (where the PR is > 0.20s). QRS Duration: Usually normal (< 0.12 seconds; less than 3 small squares), unless there is a pre-existing bundle branch block. Note on Differential Diagnosis: If the rate is slow but the P-waves are inverted in lead II or absent entirely, the rhythm is likely a Junctional Escape Rhythm, not sinus bradycardia. If there are multiple P-waves for a single QRS, you are looking at an **AV Block **. Read more on LITFL: https://litfl.com/sinus-bradycardia-ecg-library/

[image: 21399.png] Diagnostic Code Explanations (4) Diagnoses for this EKG: anterolateral myocardial infarction (ALMI), Wolf-Parkinson-White syndrome (WPW), abnormal QRS (ABQRS), sinus rhythm (SR) Explanation for anterolateral myocardial infarction (ALMI) An anterolateral myocardial infarction (MI) is characterized by specific changes on a standard 12-lead ECG that reflect damage to both the anterior (front) and lateral (side) walls of the left ventricle. Here are the key signs to look for: 1. Affected Leads The hallmark of an anterolateral MI is ST-segment elevation in a combination of precordial (chest) and limb leads: Anterior Leads: V3 and V4 (representing the anterior wall). Lateral Leads: V5, V6, I, and aVL (representing the lateral wall). Note: You may often see involvement of leads V1 and V2 as well if the infarction extends to the septum ( anteroseptal-lateral), often referred to simply as an extensive anterior MI. 2. Characteristic Waveform Changes ST-Segment Elevation: Look for J-point elevation in the leads mentioned above (V3-V6, I, aVL). This is the primary indicator of acute injury. Hyperacute T Waves: In the very early stages (minutes after onset), you may see tall, broad, and peaked T waves in the anterolateral leads before distinct ST elevation develops. Pathological Q Waves: As the infarction evolves (hours to days), deep and wide Q waves may develop in leads V3-V6, I, and aVL, indicating necrosis (tissue death). Poor R-Wave Progression: Normally, the R wave grows larger as you move from V1 to V6. In an anterolateral MI, the R waves in V3 and V4 may remain very small or disappear entirely. 3. Reciprocal Changes ECG leads that look at the heart from the opposite angle often show "mirror image" changes. For an anterolateral MI, you will typically see: ST-Segment Depression: Most prominent in the inferior leads (II, III, and aVF). Summary of Diagnostic Criteria ST Elevation: V3, V4, V5, V6, I, aVL Reciprocal ST Depression: II, III, aVF Culprit Artery: Usually the Left Anterior Descending (LAD) artery or a large Diagonal branch (D1). Less commonly, it can involve the Left Circumflex (LCx) if it supplies the lateral wall. Visual Tip: If you see ST elevation stretching across the chest leads (V3-V6) and "high lateral" leads (I, aVL), combined with ST depression in the bottom leads (II, III, aVF), the diagnosis is highly likely an anterolateral STEMI. Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (anterior STEMI): https://litfl.com/anterior-myocardial-infarction-ecg-library/ LITFL (high lateral STEMI): https://litfl.com/high-lateral-stemi-ecg-library/ LITFL (lateral STEMI): https://litfl.com/lateral-stemi-ecg-library/ Explanation for Wolf-Parkinson-White syndrome (WPW) Wolf-Parkinson-White (WPW) syndrome is a condition where an extra electrical pathway (accessory pathway) connects the atria and ventricles, bypassing the AV node. This allows electrical signals to arrive at the ventricles earlier than normal, leading to "pre-excitation." The three hallmark signs of WPW on a standard 12-lead ECG are often referred to as the WPW Triad: 1. Short PR Interval Definition: The time from the start of the P wave to the start of the QRS complex is usually less than 120 ms ( 0.12 seconds), or less than 3 small squares. Why it happens: Normally, the AV node holds the electrical signal briefly to allow the ventricles to fill with blood. In WPW, the signal travels down the accessory pathway (which has no delay mechanism) and activates the ventricles early, shortening this interval. 2. Delta Wave Definition: A slurred or slow-rising upstroke at the very beginning of the QRS complex. Why it happens: This represents the early, slow depolarization of the ventricular muscle tissue via the accessory pathway (pre-excitation) before the normal signal from the AV node catches up and rapidly depolarizes the rest of the ventricles. 3. Wide QRS Complex Definition: The total duration of the QRS complex is usually greater than 110–120 ms (typically > 0.11 or 0.12 seconds). Why it happens: The QRS is widened because it is a "fusion beat"—a combination of the early activation from the accessory pathway (the Delta wave) and the normal activation from the AV node. Secondary Signs & Classification Beyond the classic triad, you may see other specific changes depending on the location of the accessory pathway. Secondary ST-T Wave Changes Because the depolarization (activation) of the heart is abnormal, the repolarization (recovery) is also abnormal. You will often see ST-segment depression or T-wave inversion that is discordant (opposite in direction) to the QRS complex and Delta wave. "Pseudo-Infarction" Patterns The abnormal Delta waves can sometimes be negative (pointing down) in certain leads (often inferior leads II, III, aVF). This can mimic pathological Q waves, leading to a misdiagnosis of a past heart attack (myocardial infarction). Type A vs. Type B Pattern Historically, WPW was classified by the appearance of the QRS in the precordial leads (V1–V6), which helps localize where the extra pathway is located. Type A (Left-sided pathway): Positive Delta wave and QRS complex in lead V1. Looks similar to a Right Bundle Branch Block (RBBB) pattern. Indicates the pathway is likely on the left side of the heart. Type B (Right-sided pathway): Negative Delta wave and QRS complex in leads V1 and V2. Looks similar to a Left Bundle Branch Block (LBBB) pattern. Indicates the pathway is likely on the right side of the heart. Note: It is clinically important to distinguish "WPW Pattern" from "WPW Syndrome." A patient has the Pattern if they simply have these ECG findings. They are diagnosed with the Syndrome only if they have the ECG findings plus symptoms of arrhythmias (such as palpitations or SVT). Read more on LITFL: https://litfl.com/pre-excitation-syndromes-ecg-library/ Explanation for abnormal QRS (ABQRS) An abnormal QRS complex on a 12-lead ECG is typically identified by deviations in three main categories: Duration ( width), Amplitude (height/voltage), and Morphology (shape/contour). 1. Abnormal Duration (Wide QRS) A normal QRS complex lasts between 0.08 and 0.10 seconds (80–100 ms). A duration of > 0.12 seconds (>120 ms or 3 small squares) is considered abnormally wide. Bundle Branch Blocks (BBB): A blockage in the electrical conduction system causes one ventricle to depolarize later than the other, widening the QRS. Right Bundle Branch Block (RBBB): Characterized by an rSR' ("bunny ears") pattern in leads V1–V2 and a wide, slurred S wave in leads I and V6. Left Bundle Branch Block (LBBB): Characterized by a deep, broad S wave in V1 and a broad, notched, or " M-shaped" R wave in leads I, aVL, V5, and V6. Ventricular Rhythms: Rhythms originating from the ventricles (rather than the atria) do not use the fast conduction system, resulting in a wide QRS. Examples include Premature Ventricular Complexes (PVCs), Ventricular Tachycardia (VT), and Idioventricular rhythms. Hyperkalemia: High potassium levels can slow conduction, leading to a bizarrely wide QRS that may merge with the T wave (sine-wave pattern). Wolff-Parkinson-White (WPW) Syndrome: An accessory pathway allows early activation of the ventricles ( pre-excitation), causing a Delta wave (slurring of the initial upstroke) and a widened QRS. 2. Abnormal Amplitude (Voltage) The height of the QRS complex represents the electrical force generated by the ventricular muscle mass. High Voltage (Hypertrophy): Left Ventricular Hypertrophy (LVH): The muscle wall is thickened, generating stronger electrical forces. A common sign is the Sokolow-Lyon criteria: Depth of S wave in V1 + Height of R wave in V5 or V6 > 35 mm. Right Ventricular Hypertrophy (RVH): Often causes a dominant R wave in lead V1 (height > 7 mm) and a deep S wave in V5 or V6. Low Voltage: Defined as QRS amplitude < 5 mm in all limb leads and < 10 mm in all precordial (chest) leads. Causes: Anything that insulates the heart or dampens the signal, such as pericardial effusion (fluid around the heart), COPD (air trapping), obesity, or hypothyroidism (myxedema). 3. Abnormal Morphology (Shape) Even if the width and height are normal, the shape of the wave can indicate pathology. Pathological Q Waves: Q waves are the first downward deflection of the QRS. While small "septal" Q waves are normal in some leads, * pathological* Q waves indicate dead myocardial tissue (previous Myocardial Infarction). Signs: Duration > 0.04 s (40 ms) or depth > 25% of the following R wave height. Poor R Wave Progression: Normally, the R wave grows larger as you move from lead V1 to V6. If the R wave remains small or absent in leads V1–V3, it is termed "poor progression." Causes: Anterior Myocardial Infarction (old or new), LBBB, or lead misplacement. Fragmented QRS: Presence of additional spikes, notches, or slurs within the QRS complex (not fitting a typical BBB pattern). This often represents myocardial scarring or fibrosis. Electrical Alternans: The height of the QRS complex alternates between beats (large, small, large, small). This is a specific sign of a large pericardial effusion (cardiac tamponade) as the heart swings back and forth in the fluid. Read more: LITFL (interventricular conduction delay): https://litfl.com/intraventricular-conduction-delay-qrs-widening/ LITFL (low QRS voltage): https://litfl.com/low-qrs-voltage-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 10337.png] Diagnostic Code Explanations (2) Diagnoses for this EKG: ventricular premature complex (PVC), sinus rhythm (SR) Explanation for ventricular premature complex (PVC) On a standard 12-lead ECG, a Premature Ventricular Complex (PVC) is identified by a specific set of electrical features that distinguish it from normal sinus beats. These signs relate to timing, morphology (shape), and the pause that follows the beat. 1. Key Diagnostic Criteria The three most reliable signs of a PVC are: Prematurity: The complex occurs earlier than the next expected sinus beat. Wide QRS Complex: The QRS duration is prolonged, typically >120 ms (0.12 seconds). Because the impulse originates in the ventricle, it travels through the heart muscle (cell-to-cell) rather than the fast-conducting His-Purkinje system, resulting in a slower, wider depolarization. Absence of a P Wave: There is usually no P wave preceding the QRS complex because the impulse did not originate in the atria. (Note: A retrograde P wave may sometimes be seen after the QRS or buried within it). 2. Morphological Signs (Shape) Bizarre Appearance: The QRS complex often looks large, jagged, or distinctively different from the patient's normal sinus beats. Discordance: The ST segment and T wave are typically directed opposite to the main vector of the QRS complex. If the QRS is largely upright (positive), the T wave will be inverted (negative). If the QRS is largely downward (negative), the T wave will be upright. 3. Compensatory Pause Most PVCs are followed by a complete compensatory pause. This means the interval between the normal beat before the PVC and the normal beat after the PVC is exactly equal to two normal R-R intervals. Why? The PVC renders the ventricle refractory, blocking the next normal sinus impulse. However, the sinus node itself is usually not reset, so it fires the subsequent beat on schedule. 4. Localization Signs (Where the PVC comes from) The shape of the PVC on the 12-lead ECG can help identify its origin within the ventricles: Left Bundle Branch Block (LBBB) Pattern: If the PVC looks like an LBBB (dominant S wave in V1, wide upright R wave in V6), it likely originates from the Right Ventricle. Right Bundle Branch Block (RBBB) Pattern: If the PVC looks like an RBBB (dominant R wave in V1, wide S wave in V6), it likely originates from the Left Ventricle. 5. Patterns and Variants PVCs are often described by their frequency and pattern: Bigeminy: Every other beat is a PVC. Trigeminy: Every third beat is a PVC. Couplet: Two consecutive PVCs. Multifocal: PVCs with different shapes (morphologies), suggesting they originate from different irritable foci in the heart. Interpolated PVC: A rare variant where a PVC occurs specifically between two sinus beats without a compensatory pause, effectively "squeezing" in without disrupting the rhythm. R-on-T Phenomenon: A dangerous sign where the PVC occurs very early, landing on the T wave of the preceding beat. This can precipitate ventricular fibrillation. Read more on LITFL: https://litfl.com/premature-ventricular-complex-pvc-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 21828.png] Diagnostic Code Explanations (4) Diagnoses for this EKG: inferior myocardial infarction (IMI), ventricular premature complex (PVC), abnormal QRS (ABQRS), sinus rhythm (SR) Explanation for inferior myocardial infarction (IMI) An inferior myocardial infarction (MI) is a heart attack affecting the inferior wall of the heart, which rests on the diaphragm. It is most commonly caused by an occlusion of the Right Coronary Artery (RCA), though it can also result from a blockage in the Left Circumflex Artery (LCx). On a standard 12-lead ECG, you can identify an inferior MI by looking for specific changes in the "inferior leads" (II, III, and aVF). 1. Primary Diagnostic Signs The hallmark of an acute inferior MI is ST-segment elevation in the leads that view the bottom of the heart: Lead II Lead III Lead aVF Diagnostic Criteria: Significant ST-elevation (usually ≥1 mm) in at least two of these contiguous leads. 2. Reciprocal Changes Reciprocal changes are "mirror images" of the injury that appear in leads on the opposite side of the heart. These are critical for confirming the diagnosis and ruling out mimics like pericarditis. Lead aVL: This is the most sensitive lead for reciprocal changes in an inferior MI. You will typically see * ST-segment depression* here. Lead I: Often shows ST-depression as well, though usually less pronounced than in aVL. 3. Identifying the Culprit Artery (RCA vs. LCx) While an angiogram is definitive, the ECG can offer clues about which artery is blocked: RCA Occlusion (Most Common): ST elevation in Lead III > Lead II. Reciprocal ST depression in Lead I and aVL. LCx Occlusion: ST elevation in Lead II ≥ Lead III. May have ST elevation in lateral leads (I, aVL, V5, V6) or absence of reciprocal depression in Lead I. 4. Associated "Danger" Signs Inferior MIs are unique because they often extend to other areas of the heart. You must check for these two common complications immediately, as they change treatment: A. Right Ventricular (RV) Infarction Because the RCA supplies the right ventricle in most people, roughly 40% of inferior MIs involve the RV. ECG Clue: Look at lead V1. If you see ST elevation in V1 (or if V1 is isoelectric while V2 is significantly depressed), suspect RV involvement. Action: Perform a Right-Sided ECG. Look for ST elevation in V4R. Clinical Relevance: These patients are very sensitive to preload-reducing medications (like nitrates/nitroglycerin), which can cause severe hypotension. B. Posterior Extension The infarction may extend to the back of the heart (posterior wall). ECG Clue: Look at the anterior leads (V1, V2, V3). Instead of ST elevation, you will see the "mirror image": Horizontal ST depression Tall, dominant R waves (the mirror of a Q wave) Upright, prominent T waves (the mirror of T-wave inversion) Action: Perform a Posterior ECG (leads V7, V8, V9) to confirm ST elevation. 5. Rhythm Disturbances The RCA supplies the AV node in ~90% of people. Consequently, inferior MIs are frequently associated with: Bradycardia (slow heart rate). AV Blocks (First-degree, Wenckebach/Type I, or even Complete Heart Block). Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (inferior STEMI): https://litfl.com/inferior-stemi-ecg-library/ Explanation for ventricular premature complex (PVC) On a standard 12-lead ECG, a Premature Ventricular Complex (PVC) is identified by a specific set of electrical features that distinguish it from normal sinus beats. These signs relate to timing, morphology (shape), and the pause that follows the beat. 1. Key Diagnostic Criteria The three most reliable signs of a PVC are: Prematurity: The complex occurs earlier than the next expected sinus beat. Wide QRS Complex: The QRS duration is prolonged, typically >120 ms (0.12 seconds). Because the impulse originates in the ventricle, it travels through the heart muscle (cell-to-cell) rather than the fast-conducting His-Purkinje system, resulting in a slower, wider depolarization. Absence of a P Wave: There is usually no P wave preceding the QRS complex because the impulse did not originate in the atria. (Note: A retrograde P wave may sometimes be seen after the QRS or buried within it). 2. Morphological Signs (Shape) Bizarre Appearance: The QRS complex often looks large, jagged, or distinctively different from the patient's normal sinus beats. Discordance: The ST segment and T wave are typically directed opposite to the main vector of the QRS complex. If the QRS is largely upright (positive), the T wave will be inverted (negative). If the QRS is largely downward (negative), the T wave will be upright. 3. Compensatory Pause Most PVCs are followed by a complete compensatory pause. This means the interval between the normal beat before the PVC and the normal beat after the PVC is exactly equal to two normal R-R intervals. Why? The PVC renders the ventricle refractory, blocking the next normal sinus impulse. However, the sinus node itself is usually not reset, so it fires the subsequent beat on schedule. 4. Localization Signs (Where the PVC comes from) The shape of the PVC on the 12-lead ECG can help identify its origin within the ventricles: Left Bundle Branch Block (LBBB) Pattern: If the PVC looks like an LBBB (dominant S wave in V1, wide upright R wave in V6), it likely originates from the Right Ventricle. Right Bundle Branch Block (RBBB) Pattern: If the PVC looks like an RBBB (dominant R wave in V1, wide S wave in V6), it likely originates from the Left Ventricle. 5. Patterns and Variants PVCs are often described by their frequency and pattern: Bigeminy: Every other beat is a PVC. Trigeminy: Every third beat is a PVC. Couplet: Two consecutive PVCs. Multifocal: PVCs with different shapes (morphologies), suggesting they originate from different irritable foci in the heart. Interpolated PVC: A rare variant where a PVC occurs specifically between two sinus beats without a compensatory pause, effectively "squeezing" in without disrupting the rhythm. R-on-T Phenomenon: A dangerous sign where the PVC occurs very early, landing on the T wave of the preceding beat. This can precipitate ventricular fibrillation. Read more on LITFL: https://litfl.com/premature-ventricular-complex-pvc-ecg-library/ Explanation for abnormal QRS (ABQRS) An abnormal QRS complex on a 12-lead ECG is typically identified by deviations in three main categories: Duration ( width), Amplitude (height/voltage), and Morphology (shape/contour). 1. Abnormal Duration (Wide QRS) A normal QRS complex lasts between 0.08 and 0.10 seconds (80–100 ms). A duration of > 0.12 seconds (>120 ms or 3 small squares) is considered abnormally wide. Bundle Branch Blocks (BBB): A blockage in the electrical conduction system causes one ventricle to depolarize later than the other, widening the QRS. Right Bundle Branch Block (RBBB): Characterized by an rSR' ("bunny ears") pattern in leads V1–V2 and a wide, slurred S wave in leads I and V6. Left Bundle Branch Block (LBBB): Characterized by a deep, broad S wave in V1 and a broad, notched, or " M-shaped" R wave in leads I, aVL, V5, and V6. Ventricular Rhythms: Rhythms originating from the ventricles (rather than the atria) do not use the fast conduction system, resulting in a wide QRS. Examples include Premature Ventricular Complexes (PVCs), Ventricular Tachycardia (VT), and Idioventricular rhythms. Hyperkalemia: High potassium levels can slow conduction, leading to a bizarrely wide QRS that may merge with the T wave (sine-wave pattern). Wolff-Parkinson-White (WPW) Syndrome: An accessory pathway allows early activation of the ventricles ( pre-excitation), causing a Delta wave (slurring of the initial upstroke) and a widened QRS. 2. Abnormal Amplitude (Voltage) The height of the QRS complex represents the electrical force generated by the ventricular muscle mass. High Voltage (Hypertrophy): Left Ventricular Hypertrophy (LVH): The muscle wall is thickened, generating stronger electrical forces. A common sign is the Sokolow-Lyon criteria: Depth of S wave in V1 + Height of R wave in V5 or V6 > 35 mm. Right Ventricular Hypertrophy (RVH): Often causes a dominant R wave in lead V1 (height > 7 mm) and a deep S wave in V5 or V6. Low Voltage: Defined as QRS amplitude < 5 mm in all limb leads and < 10 mm in all precordial (chest) leads. Causes: Anything that insulates the heart or dampens the signal, such as pericardial effusion (fluid around the heart), COPD (air trapping), obesity, or hypothyroidism (myxedema). 3. Abnormal Morphology (Shape) Even if the width and height are normal, the shape of the wave can indicate pathology. Pathological Q Waves: Q waves are the first downward deflection of the QRS. While small "septal" Q waves are normal in some leads, * pathological* Q waves indicate dead myocardial tissue (previous Myocardial Infarction). Signs: Duration > 0.04 s (40 ms) or depth > 25% of the following R wave height. Poor R Wave Progression: Normally, the R wave grows larger as you move from lead V1 to V6. If the R wave remains small or absent in leads V1–V3, it is termed "poor progression." Causes: Anterior Myocardial Infarction (old or new), LBBB, or lead misplacement. Fragmented QRS: Presence of additional spikes, notches, or slurs within the QRS complex (not fitting a typical BBB pattern). This often represents myocardial scarring or fibrosis. Electrical Alternans: The height of the QRS complex alternates between beats (large, small, large, small). This is a specific sign of a large pericardial effusion (cardiac tamponade) as the heart swings back and forth in the fluid. Read more: LITFL (interventricular conduction delay): https://litfl.com/intraventricular-conduction-delay-qrs-widening/ LITFL (low QRS voltage): https://litfl.com/low-qrs-voltage-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 10450.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: anterior myocardial infarction (AMI), abnormal QRS (ABQRS), sinus arrhythmia (SARRH) Explanation for anterior myocardial infarction (AMI) On a standard 12-lead ECG, an anterior myocardial infarction (MI) is primarily identified by changes in the precordial leads overlying the front of the heart. The following breakdown details the specific leads involved, the characteristic waveform changes, and the anatomical context. 1. Affected Leads The primary diagnostic changes for an anterior MI appear in the precordial chest leads: V3 and V4: These are the specific "anterior" leads. V1 and V2: Often involved as well, indicating septal involvement (Anteroseptal MI). V5 and V6: Involvement here suggests the infarct extends to the lateral wall (Anterolateral MI). Extensive Anterior MI: If ST elevation is present across V1–V6, it indicates a large infarction affecting a significant portion of the left ventricle. 2. Characteristic Waveform Changes The ECG signs evolve over time, but the classic findings of an acute anterior STEMI (ST-Elevation Myocardial Infarction) include: ST-Segment Elevation: This is the hallmark sign. You will see significant elevation of the ST segment (the flat line between the QRS complex and the T wave) in leads V3 and V4 (and often V1-V2). Note: The elevation is often convex (rounded upward) or obliquely straight. Severe cases may show " tombstoning," where the ST segment rises high enough to merge with the T wave. Hyperacute T Waves: In the very early stages (minutes after occlusion), T waves in the anterior leads may become tall, broad, and peaked. This often precedes visible ST elevation. Pathological Q Waves: As the infarction progresses and heart muscle tissue dies (necrosis), deep Q waves may develop in leads V1–V4. Poor R-Wave Progression: Normally, the R wave (the upward spike) gets taller moving from lead V1 to V6. In an anterior MI, the R waves in V1–V4 may remain small or disappear entirely (becoming QS complexes), indicating loss of viable muscle in the anterior wall. 3. Reciprocal Changes "Reciprocal changes" refer to ST depression that appears in leads electrically opposite to the infarct. Inferior Leads (II, III, aVF): You will frequently see reciprocal ST depression in these leads during an anterior MI. Significance: While not always present, reciprocal depression in the inferior leads strongly confirms that the ST elevation in the anterior leads is due to an acute MI rather than a benign mimic (like pericarditis or benign early repolarization). 4. Culprit Artery The presence of these signs almost always indicates an occlusion of the Left Anterior Descending (LAD) artery. Proximal LAD Occlusion: Tends to cause widespread elevation (V1–V6, plus leads I and aVL). Distal LAD Occlusion: Changes are often more localized to V3–V4. Summary Checklist for Anterior MI Primary Elevation: V3, V4 (often V1–V4) Reciprocal Depression: II, III, aVF T Waves: Hyperacute (early) or Inverted (late) QRS Complex: Loss of R-wave height; development of Q waves Culprit Artery: Left Anterior Descending (LAD) Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (anterior STEMI): https://litfl.com/anterior-myocardial-infarction-ecg-library/ Explanation for abnormal QRS (ABQRS) An abnormal QRS complex on a 12-lead ECG is typically identified by deviations in three main categories: Duration ( width), Amplitude (height/voltage), and Morphology (shape/contour). 1. Abnormal Duration (Wide QRS) A normal QRS complex lasts between 0.08 and 0.10 seconds (80–100 ms). A duration of > 0.12 seconds (>120 ms or 3 small squares) is considered abnormally wide. Bundle Branch Blocks (BBB): A blockage in the electrical conduction system causes one ventricle to depolarize later than the other, widening the QRS. Right Bundle Branch Block (RBBB): Characterized by an rSR' ("bunny ears") pattern in leads V1–V2 and a wide, slurred S wave in leads I and V6. Left Bundle Branch Block (LBBB): Characterized by a deep, broad S wave in V1 and a broad, notched, or " M-shaped" R wave in leads I, aVL, V5, and V6. Ventricular Rhythms: Rhythms originating from the ventricles (rather than the atria) do not use the fast conduction system, resulting in a wide QRS. Examples include Premature Ventricular Complexes (PVCs), Ventricular Tachycardia (VT), and Idioventricular rhythms. Hyperkalemia: High potassium levels can slow conduction, leading to a bizarrely wide QRS that may merge with the T wave (sine-wave pattern). Wolff-Parkinson-White (WPW) Syndrome: An accessory pathway allows early activation of the ventricles ( pre-excitation), causing a Delta wave (slurring of the initial upstroke) and a widened QRS. 2. Abnormal Amplitude (Voltage) The height of the QRS complex represents the electrical force generated by the ventricular muscle mass. High Voltage (Hypertrophy): Left Ventricular Hypertrophy (LVH): The muscle wall is thickened, generating stronger electrical forces. A common sign is the Sokolow-Lyon criteria: Depth of S wave in V1 + Height of R wave in V5 or V6 > 35 mm. Right Ventricular Hypertrophy (RVH): Often causes a dominant R wave in lead V1 (height > 7 mm) and a deep S wave in V5 or V6. Low Voltage: Defined as QRS amplitude < 5 mm in all limb leads and < 10 mm in all precordial (chest) leads. Causes: Anything that insulates the heart or dampens the signal, such as pericardial effusion (fluid around the heart), COPD (air trapping), obesity, or hypothyroidism (myxedema). 3. Abnormal Morphology (Shape) Even if the width and height are normal, the shape of the wave can indicate pathology. Pathological Q Waves: Q waves are the first downward deflection of the QRS. While small "septal" Q waves are normal in some leads, * pathological* Q waves indicate dead myocardial tissue (previous Myocardial Infarction). Signs: Duration > 0.04 s (40 ms) or depth > 25% of the following R wave height. Poor R Wave Progression: Normally, the R wave grows larger as you move from lead V1 to V6. If the R wave remains small or absent in leads V1–V3, it is termed "poor progression." Causes: Anterior Myocardial Infarction (old or new), LBBB, or lead misplacement. Fragmented QRS: Presence of additional spikes, notches, or slurs within the QRS complex (not fitting a typical BBB pattern). This often represents myocardial scarring or fibrosis. Electrical Alternans: The height of the QRS complex alternates between beats (large, small, large, small). This is a specific sign of a large pericardial effusion (cardiac tamponade) as the heart swings back and forth in the fluid. Read more: LITFL (interventricular conduction delay): https://litfl.com/intraventricular-conduction-delay-qrs-widening/ LITFL (low QRS voltage): https://litfl.com/low-qrs-voltage-ecg-library/ Explanation for sinus arrhythmia (SARRH) On a standard 12-lead ECG, sinus arrhythmia is characterized by a normal sinus rhythm that has an irregular rate. The "irregularity" is usually patterned and predictable, corresponding to the patient's breathing cycle. Here are the specific signs and criteria used to identify it: 1. The Core Criterion: R-R Interval Variation The hallmark sign of sinus arrhythmia is a variation in the timing between heartbeats (the R-R interval) that originates from the sinus node. Variation Threshold: The difference between the longest and shortest R-R interval (or P-P interval) is greater than 0.12 seconds (which equals 3 small squares on standard ECG paper). Pattern: The rhythm typically speeds up and slows down in a cyclical pattern. 2. Normal P-Wave Morphology Because the electrical impulse still originates from the sinoatrial (SA) node, the atrial depolarization looks normal. Appearance: P-waves are upright in leads I and II. Consistency: The P-waves have a constant shape and structure beat-to-beat. There is one P-wave for every QRS complex. 3. Constant PR Interval The time it takes for the impulse to travel from the atria to the ventricles remains stable. The PR interval is constant and typically within the normal range of 0.12 to 0.20 seconds. 4. Normal QRS Complex The ventricular contraction is unaffected, so the QRS complex is narrow (usually < 0.12 seconds) and follows every P-wave. 5. Correlation with Respiration (Respiratory Sinus Arrhythmia) This is the most common form, especially in children and young, healthy adults. You can often see this "phasic" change on the rhythm strip at the bottom of the 12-lead page: Inspiration (Breathing in): The heart rate increases (R-R intervals get shorter). Expiration (Breathing out): The heart rate decreases (R-R intervals get longer). Note on Clinical Context: While usually benign and respiratory in nature, non-respiratory sinus arrhythmia (where the irregularity does not align with the breathing cycle) can occur. This is more commonly seen in older adults and can be associated with underlying heart disease or digitalis toxicity. Read more on LITFL: https://litfl.com/sinus-arrhythmia-ecg-library/

[image: 8063.png] Diagnostic Code Explanations (2) Diagnoses for this EKG: left anterior fascicular block (LAFB), sinus rhythm (SR) Explanation for left anterior fascicular block (LAFB) Left anterior fascicular block (LAFB), also known as left anterior hemiblock, is a conduction abnormality where the anterior fascicle of the left bundle branch is blocked. This forces electrical impulses to depolarize the left ventricle via the posterior fascicle, altering the direction of the electrical vector. Diagnosing LAFB relies on specific changes in the QRS axis and morphology. Below are the primary signs seen on a standard 12-lead ECG. 1. Pathological Left Axis Deviation (LAD) This is the hallmark sign of LAFB. Because the anterior fascicle is blocked, the electrical wavefront travels from the posterior/inferior wall towards the anterior/superior wall. This creates a net electrical vector that points upward and to the left. Criterion: The axis is typically between -45° and -90°. Note: While an axis between -30° and -45° suggests possible LAFB, it is less specific and can be a normal variant or due to left ventricular hypertrophy. A shift beyond -45° is generally required for a definitive diagnosis. 2. Lead-Specific QRS Morphology The redirected electrical vector creates a "divergent" pattern between the lateral and inferior leads. Lateral Leads (I and aVL): Pattern: qR complex (small Q wave, tall R wave). Explanation: The initial forces go downwards/rightwards (creating the small 'q'), followed by the major forces going upwards/leftwards (creating the tall 'R'). Inferior Leads (II, III, and aVF): Pattern: rS complex (small R wave, deep S wave). Explanation: The initial forces move briefly towards these leads (small 'r'), but the bulk of the depolarization moves away from them (deep 'S'). Key Check: Lead aVF should be negative. Lead II should also be negative (which confirms the axis is more negative than -30°). 3. Normal or Minimally Prolonged QRS Duration Unlike a complete Left Bundle Branch Block (LBBB), the main conduction trunk is intact, so ventricular depolarization is not significantly delayed. Criterion: QRS duration is usually < 120 ms (typically 80–110 ms). Context: If the QRS is ≥ 120 ms, a complete bundle branch block is likely present (e.g., RBBB + LAFB). 4. Prolonged R-Wave Peak Time in Lead aVL This measures the time from the onset of the QRS to the peak of the R wave (intrinsicoid deflection), representing the time it takes for depolarization to reach the lateral wall. Criterion: R-wave peak time ≥ 45 ms in lead aVL. Comparison: This is often delayed compared to lead V6. Summary of Diagnostic Criteria (Rule of Thumb) To quickly identify LAFB, look for: Left Axis Deviation (Positive Lead I, Negative Lead aVF). Negative Lead II (Deep S wave). Small Q waves in I and aVL. Normal QRS width (< 120 ms). Important Clinical Note: LAFB is a diagnosis of exclusion. You must ensure that the Left Axis Deviation is not caused by other conditions, such as a preexisting inferior myocardial infarction (which causes deep Q waves in II, III, and aVF) or severe Left Ventricular Hypertrophy (LVH). Read more on LITFL: https://litfl.com/left-anterior-fascicular-block-lafb-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 15796.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: atrial fibrillation (AFIB) Explanation for atrial fibrillation (AFIB) On a standard 12-lead ECG, atrial fibrillation (AFib) is characterized by three primary diagnostic signs: an " irregularly irregular" rhythm, the absence of P waves, and the presence of fibrillatory waves. The following details explain these signs and how to identify them. 1. Irregularly Irregular Rhythm This is the hallmark of AFib. What to look for: Check the R-R intervals (the distance between the spikes of the QRS complexes). In AFib, these intervals are completely unpredictable. There is no pattern to the irregularity—it is chaotic. Why it happens: The AV node is bombarded by chaotic electrical impulses from the atria (often >300 per minute). It blocks many of them but lets others through at random intervals, causing the ventricles to beat irregularly. 2. Absence of P Waves What to look for: Look at the baseline immediately before each QRS complex. In a normal sinus rhythm, you would see a small, rounded "bump" (the P wave). In AFib, these discrete, organized bumps are missing. Lead Specifics: This is often most obvious in Lead II and Lead V1, which are typically the best leads for viewing atrial activity. 3. Presence of Fibrillatory (f) Waves Instead of P waves, you will often see a wavy, chaotic baseline between QRS complexes. Appearance: These can look like fine, low-amplitude tremors (fine AFib) or more distinct, jagged oscillations (coarse AFib). Best Leads: Fibrillatory waves are usually most visible in Lead V1 or the inferior leads (II, III, aVF). Note: In long-standing AFib, the f-waves may be so fine that the baseline simply looks flat (isoelectric). 4. Ventricular Rate (Heart Rate) While the atrial rate is extremely fast (300–600 bpm), the ventricular rate (what you feel as a pulse) varies based on how many impulses pass through the AV node. AFib with RVR (Rapid Ventricular Response): Heart rate >100 bpm. This is common in untreated, new-onset AFib. Controlled AFib: Heart rate between 60–100 bpm (often achieved with medication). Slow AFib: Heart rate <60 bpm (may indicate medication toxicity or intrinsic conduction disease). 5. QRS Morphology (Shape) Narrow QRS: typically, the QRS complexes remain narrow (<120 ms) because the electrical impulse still travels through the ventricles normally (via the His-Purkinje system). Ashman’s Phenomenon: Occasionally, you may see a wide, bizarre QRS complex that looks like a PVC (Premature Ventricular Complex). If this occurs after a long R-R interval followed immediately by a short R-R interval, it is likely a benign conduction aberration known as Ashman's phenomenon, rather than a true ventricular beat. Read more on LITFL: https://litfl.com/atrial-fibrillation-ecg-library/

[image: 11052.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 1639.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 15989.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: left anterior fascicular block (LAFB), incomplete right bundle branch block (IRBBB), sinus bradycardia (SBRAD) Explanation for left anterior fascicular block (LAFB) Left anterior fascicular block (LAFB), also known as left anterior hemiblock, is a conduction abnormality where the anterior fascicle of the left bundle branch is blocked. This forces electrical impulses to depolarize the left ventricle via the posterior fascicle, altering the direction of the electrical vector. Diagnosing LAFB relies on specific changes in the QRS axis and morphology. Below are the primary signs seen on a standard 12-lead ECG. 1. Pathological Left Axis Deviation (LAD) This is the hallmark sign of LAFB. Because the anterior fascicle is blocked, the electrical wavefront travels from the posterior/inferior wall towards the anterior/superior wall. This creates a net electrical vector that points upward and to the left. Criterion: The axis is typically between -45° and -90°. Note: While an axis between -30° and -45° suggests possible LAFB, it is less specific and can be a normal variant or due to left ventricular hypertrophy. A shift beyond -45° is generally required for a definitive diagnosis. 2. Lead-Specific QRS Morphology The redirected electrical vector creates a "divergent" pattern between the lateral and inferior leads. Lateral Leads (I and aVL): Pattern: qR complex (small Q wave, tall R wave). Explanation: The initial forces go downwards/rightwards (creating the small 'q'), followed by the major forces going upwards/leftwards (creating the tall 'R'). Inferior Leads (II, III, and aVF): Pattern: rS complex (small R wave, deep S wave). Explanation: The initial forces move briefly towards these leads (small 'r'), but the bulk of the depolarization moves away from them (deep 'S'). Key Check: Lead aVF should be negative. Lead II should also be negative (which confirms the axis is more negative than -30°). 3. Normal or Minimally Prolonged QRS Duration Unlike a complete Left Bundle Branch Block (LBBB), the main conduction trunk is intact, so ventricular depolarization is not significantly delayed. Criterion: QRS duration is usually < 120 ms (typically 80–110 ms). Context: If the QRS is ≥ 120 ms, a complete bundle branch block is likely present (e.g., RBBB + LAFB). 4. Prolonged R-Wave Peak Time in Lead aVL This measures the time from the onset of the QRS to the peak of the R wave (intrinsicoid deflection), representing the time it takes for depolarization to reach the lateral wall. Criterion: R-wave peak time ≥ 45 ms in lead aVL. Comparison: This is often delayed compared to lead V6. Summary of Diagnostic Criteria (Rule of Thumb) To quickly identify LAFB, look for: Left Axis Deviation (Positive Lead I, Negative Lead aVF). Negative Lead II (Deep S wave). Small Q waves in I and aVL. Normal QRS width (< 120 ms). Important Clinical Note: LAFB is a diagnosis of exclusion. You must ensure that the Left Axis Deviation is not caused by other conditions, such as a preexisting inferior myocardial infarction (which causes deep Q waves in II, III, and aVF) or severe Left Ventricular Hypertrophy (LVH). Read more on LITFL: https://litfl.com/left-anterior-fascicular-block-lafb-ecg-library/ Explanation for incomplete right bundle branch block (IRBBB) An Incomplete Right Bundle Branch Block (IRBBB) is a conduction delay where the electrical impulse travels to the right ventricle more slowly than normal, but not slowly enough to be classified as a "complete" block. On a standard 12-lead ECG, the diagnosis is made by looking for specific changes in the QRS duration and the wave shapes in leads V1 and V6. 1. QRS Duration (The Main Criteria) The primary difference between an incomplete and a complete block is the duration of the QRS complex. Incomplete RBBB: The QRS duration is prolonged but remains between 100 and 119 milliseconds (0.10 to < 0.12 seconds). Complete RBBB: The QRS duration is ≥ 120 milliseconds (≥ 0.12 seconds). 2. Signs in the Right Precordial Leads (V1 and V2) The most distinct sign of IRBBB appears in leads V1 and V2 (the leads placed over the right ventricle). RSR' Pattern ("M-Shaped" Complex): You will typically see a three-phase wave described as rSr', rsR', or * *rSR'**. The first wave (r) represents normal activation of the septum. The second wave (s) represents the normal activation of the left ventricle. The third wave (R') is a second upward spike that represents the delayed activation of the right ventricle. R' is often taller than r: The second upward spike (R') is usually larger than the initial one. 3. Signs in the Lateral Leads (V6 and Lead I) While V1 shows the delayed "push" of electricity toward the right, the leads on the left side of the chest (V6 and Lead I) show that same electrical force moving away from them. Slurred S Wave: You will see a wide, "slurred" S wave at the end of the QRS complex. This S wave is wider than the preceding R wave (or > 40ms) and looks "messy" or slow compared to the sharp upstroke of the R wave. Summary of Diagnostic Criteria | Feature | Incomplete RBBB Signs | QRS Duration: 100 – 119 ms (Narrower than complete block) Lead V1 / V2: rSR' pattern (Bunny ears / M-shape) Lead I / V6: Wide, slurred S wave Clinical Significance It is important to note that an incomplete RBBB is frequently a normal variant (benign finding), particularly in children, young adults, and well-conditioned athletes. However, it can occasionally be associated with conditions that strain the right side of the heart, such as an atrial septal defect (ASD) or right ventricular hypertrophy. Read more on LITFL: https://litfl.com/right-bundle-branch-block-rbbb-ecg-library/ Explanation for sinus bradycardia (SBRAD) On a standard 12-lead ECG, sinus bradycardia is identified by a heart rate of less than 60 beats per minute (bpm) while maintaining all the characteristics of a normal sinus rhythm. To diagnose it, you must confirm that the rhythm originates from the sinus node (the heart's natural pacemaker) and is conducting normally to the ventricles. Here are the specific signs and criteria to look for: 1. Heart Rate Criterion: Ventricular rate is < 60 bpm. Calculation: On standard ECG paper (25 mm/s), the distance between consecutive R-waves will be greater than 5 large squares (since $300 \div 5 = 60$). 2. Rhythm Criterion: The rhythm is regular. Check: The R-R intervals (distance between heartbeats) are consistent. Slight variation (respiratory sinus arrhythmia) is common and acceptable, but the overall pattern should be steady. 3. P-Wave Morphology (Crucial for "Sinus" Origin) To confirm the rhythm is sinus (coming from the sinoatrial node) rather than an atrial or junctional escape rhythm, the P-waves must have a specific vector: Leads I, II, and aVF: P-waves must be upright (positive). Lead aVR: P-waves must be inverted (negative). Consistency: All P-waves should look identical, indicating they originate from the same focus. 4. P-to-QRS Relationship Criterion: There is a 1:1 relationship. Check: Every P-wave is followed by a QRS complex, and every QRS complex is preceded by a P-wave. This rules out Complete Heart Block (3rd-degree AV block), where P-waves and QRS complexes beat independently. 5. Intervals PR Interval: Normal and constant (between 0.12 and 0.20 seconds; 3–5 small squares). This rules out 1st-degree AV block (where the PR is > 0.20s). QRS Duration: Usually normal (< 0.12 seconds; less than 3 small squares), unless there is a pre-existing bundle branch block. Note on Differential Diagnosis: If the rate is slow but the P-waves are inverted in lead II or absent entirely, the rhythm is likely a Junctional Escape Rhythm, not sinus bradycardia. If there are multiple P-waves for a single QRS, you are looking at an **AV Block **. Read more on LITFL: https://litfl.com/sinus-bradycardia-ecg-library/

[image: 2388.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 2585.png] Diagnostic Code Explanations (3) Diagnoses for this EKG: inferior myocardial infarction (IMI), first degree AV block (1AVB), sinus rhythm (SR) Explanation for inferior myocardial infarction (IMI) An inferior myocardial infarction (MI) is a heart attack affecting the inferior wall of the heart, which rests on the diaphragm. It is most commonly caused by an occlusion of the Right Coronary Artery (RCA), though it can also result from a blockage in the Left Circumflex Artery (LCx). On a standard 12-lead ECG, you can identify an inferior MI by looking for specific changes in the "inferior leads" (II, III, and aVF). 1. Primary Diagnostic Signs The hallmark of an acute inferior MI is ST-segment elevation in the leads that view the bottom of the heart: Lead II Lead III Lead aVF Diagnostic Criteria: Significant ST-elevation (usually ≥1 mm) in at least two of these contiguous leads. 2. Reciprocal Changes Reciprocal changes are "mirror images" of the injury that appear in leads on the opposite side of the heart. These are critical for confirming the diagnosis and ruling out mimics like pericarditis. Lead aVL: This is the most sensitive lead for reciprocal changes in an inferior MI. You will typically see * ST-segment depression* here. Lead I: Often shows ST-depression as well, though usually less pronounced than in aVL. 3. Identifying the Culprit Artery (RCA vs. LCx) While an angiogram is definitive, the ECG can offer clues about which artery is blocked: RCA Occlusion (Most Common): ST elevation in Lead III > Lead II. Reciprocal ST depression in Lead I and aVL. LCx Occlusion: ST elevation in Lead II ≥ Lead III. May have ST elevation in lateral leads (I, aVL, V5, V6) or absence of reciprocal depression in Lead I. 4. Associated "Danger" Signs Inferior MIs are unique because they often extend to other areas of the heart. You must check for these two common complications immediately, as they change treatment: A. Right Ventricular (RV) Infarction Because the RCA supplies the right ventricle in most people, roughly 40% of inferior MIs involve the RV. ECG Clue: Look at lead V1. If you see ST elevation in V1 (or if V1 is isoelectric while V2 is significantly depressed), suspect RV involvement. Action: Perform a Right-Sided ECG. Look for ST elevation in V4R. Clinical Relevance: These patients are very sensitive to preload-reducing medications (like nitrates/nitroglycerin), which can cause severe hypotension. B. Posterior Extension The infarction may extend to the back of the heart (posterior wall). ECG Clue: Look at the anterior leads (V1, V2, V3). Instead of ST elevation, you will see the "mirror image": Horizontal ST depression Tall, dominant R waves (the mirror of a Q wave) Upright, prominent T waves (the mirror of T-wave inversion) Action: Perform a Posterior ECG (leads V7, V8, V9) to confirm ST elevation. 5. Rhythm Disturbances The RCA supplies the AV node in ~90% of people. Consequently, inferior MIs are frequently associated with: Bradycardia (slow heart rate). AV Blocks (First-degree, Wenckebach/Type I, or even Complete Heart Block). Read more: LITFL (ST segment): https://litfl.com/st-segment-ecg-library/ LITFL (MI localization): https://litfl.com/mi-localization-ecg-library/ LITFL (inferior STEMI): https://litfl.com/inferior-stemi-ecg-library/ Explanation for first degree AV block (1AVB) First-degree atrioventricular (AV) block is characterized by a delay in the electrical conduction from the atria to the ventricles. Despite the name "block," the electrical impulse is not actually blocked; it is simply slowed down as it passes through the AV node. The hallmark sign on a standard 12-lead ECG is a prolonged PR interval that remains constant. Key Diagnostic Criteria To diagnose first-degree AV block, the ECG must show the following features: Prolonged PR Interval: Adults: The PR interval is consistently greater than 200 milliseconds (ms). On standard ECG paper (where 1 small square = 40 ms), this is wider than 5 small squares (or 1 large square). Pediatrics: The threshold is lower and age-dependent. For example, a PR interval >160 ms in an infant or >180 ms in a young child may qualify as first-degree block. 1:1 Relationship (One P for every QRS): Every P wave is followed by a QRS complex. Every QRS complex is preceded by a P wave. There are no dropped beats (unlike second-degree or third-degree heart blocks). Constant PR Interval: The duration of the PR interval is fixed and does not change from beat to beat. (If the PR interval progressively lengthens before a dropped beat, that is Second-Degree Type I / Wenckebach). How to Measure Properly Best Lead: Look at Lead II (or sometimes V1), as P waves are typically most visible there. Measurement: Measure from the beginning of the P wave (where it first leaves the isoelectric line) to the beginning of the QRS complex (the very first deflection, whether it is a Q wave or an R wave). Consistency: Verify the measurement across multiple beats to ensure it is fixed. Clinical Context & Nuances "Marked" First-Degree Block: If the PR interval exceeds 300 ms, it is often termed "marked" first-degree AV block. While usually asymptomatic, intervals this long can cause "pacemaker syndrome-like" symptoms (fatigue, dizziness) because the atria contract while the mitral/tricuspid valves are still closed from the previous beat. P Wave Location: In cases of marked block, the PR interval may be so long that the P wave is buried in the preceding T wave. You may notice a "camel hump" appearance on the T wave or a T wave that looks different than usual. Causes: Common causes include high vagal tone (common in athletes), medications (beta-blockers, calcium channel blockers, digoxin), electrolyte imbalances, or age-related degeneration of the conduction system. Summary PR Interval: > 0.20 seconds (more than 5 small boxes) Rhythm: Regular (assuming the underlying sinus rhythm is regular) P Waves: Normal upright P waves in Lead II; always present Conduction Ratio: 1:1 (Atria : Ventricles) Dropped Beats? None Read more on LITFL: https://litfl.com/first-degree-heart-block-ecg-library/ Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/