Residency · Residency · Cardiology · includes a quiz

Advanced ECG Interpretation and Arrhythmia Analysis

Systematic Approach to the 12-Lead ECG

Rate and Rhythm Assessment

The foundation of every electrocardiographic interpretation begins with a disciplined assessment of rate and rhythm. Heart rate can be calculated most precisely by dividing 300 by the number of large boxes between consecutive R waves, a method that works reliably for regular rhythms. When the rhythm is irregular, counting the number of R waves within a 10-second strip and multiplying by six provides a reasonable average ventricular rate. Before proceeding further, the interpreter should march out both the P-P and R-R intervals with calipers to determine whether the rhythm is regular or irregular.

An irregularly irregular rhythm should be considered atrial fibrillation until proven otherwise, although multifocal atrial tachycardia must also be considered when three or more distinct P-wave morphologies are identified. A regularly irregular rhythm, by contrast, raises a different differential. Grouped beating suggests Wenckebach periodicity, while variable block patterns or premature complexes with fixed coupling intervals can produce predictable irregularity within an otherwise organized rhythm.

Axis Determination and Bundle Branch Blocks

Axis determination provides critical information about the electrical vector of ventricular depolarization. The normal axis ranges from -30 to +90 degrees, with left axis deviation defined as -30 to -90 degrees and right axis deviation as +90 to +180 degrees.

Right bundle branch block presents with the classic rsR' pattern in leads V1 and V2, a wide slurred S wave in leads I and V6, and a QRS duration of at least 120 ms. Importantly, right bundle branch block does not obscure ST-segment analysis in contiguous leads, allowing ischemic evaluation to proceed without significant confounding. Left bundle branch block, by contrast, produces a broad notched R wave in leads I, aVL, V5, and V6, with absent septal Q waves and a QRS duration of at least 120 ms.

Bifascicular block describes the combination of right bundle branch block with either left anterior fascicular block (the most common pattern) or left posterior fascicular block. An alternating bundle branch block pattern, in which the ECG morphology shifts between right and left bundle branch block on serial tracings, strongly suggests diffuse infranodal conduction disease and is an indication for permanent pacing.

P-Wave Morphology and Atrial Abnormalities

Careful attention to P-wave morphology can reveal important information about atrial chamber enlargement and underlying cardiac pathology. Right atrial enlargement produces P pulmonale, characterized by peaked P waves exceeding 2.5 mm in amplitude in lead II, often with a rightward P-wave axis. Left atrial abnormality manifests as P mitrale, with notched P waves exceeding 120 ms in duration in lead II and a negative terminal deflection in lead V1 that is greater than 1 mm deep and more than 40 ms wide, known as the Morris index. When both features are present simultaneously, biatrial abnormality should be considered, a pattern commonly encountered in rheumatic heart disease and restrictive cardiomyopathy.

ST-Segment and T-Wave Analysis

ST-segment and T-wave analysis represents one of the most clinically consequential aspects of ECG interpretation. The differential diagnosis for ST elevation is broad and includes acute myocardial infarction, pericarditis (characteristically diffuse with a concave-upward morphology and PR depression), early repolarization, left ventricular aneurysm, the Brugada pattern, and hyperkalemia. ST depression may reflect subendocardial ischemia, reciprocal changes to ST elevation in a contralateral territory, the scooped "Salvador Dali mustache" morphology of digoxin effect, or the strain pattern of left ventricular hypertrophy.

T-wave inversions carry their own distinct differential. Symmetric deep T-wave inversions suggest ischemia, while asymmetric inversions in the context of voltage criteria are more consistent with LVH strain. Wellens syndrome warrants particular attention, as it identifies critical proximal left anterior descending artery stenosis. Type A Wellens manifests as biphasic T waves in leads V2 and V3, while Type B shows deep symmetric T-wave inversions in the same distribution. Post-pacing memory T waves can also produce inversions that may be confused with ischemia.

The de Winter T-wave pattern deserves special emphasis, as it represents an ST-elevation myocardial infarction equivalent caused by left anterior descending artery occlusion despite not meeting traditional STEMI criteria. This pattern is characterized by upsloping ST depression at the J-point with tall symmetric T waves in the precordial leads. Hyperacute T waves, appearing as tall, broad-based, symmetric T waves, may represent the earliest sign of acute myocardial infarction and can precede frank ST elevation by minutes.

<image> A comprehensive 12-lead ECG reference diagram showing six panels arranged in a 2x3 grid. Panel 1: Normal sinus rhythm with labeled intervals (PR 160ms, QRS 80ms, QTc 420ms). Panel 2: LBBB pattern with broad notched R in V5-V6 and deep S in V1-V2. Panel 3: RBBB with rsR' in V1 and wide S in V6. Panel 4: Wellens Type A (biphasic T waves in V2-V3) and Type B (deep symmetric T-wave inversions). Panel 5: de Winter T waves with upsloping ST depression and peaked T waves in V1-V4. Panel 6: Brugada Type 1 pattern with coved ST elevation in V1-V2. Each panel should have clean ECG tracings on a pink grid background with annotations in blue arrows pointing to key diagnostic features. Include lead labels for each tracing. </image>

Advanced Arrhythmia Analysis

Narrow Complex Tachycardias (QRS < 120 ms)

The differential diagnosis for narrow complex tachycardias requires systematic evaluation of the relationship between atrial and ventricular activity. Sinus tachycardia is characterized by gradual onset and offset, a rate that is appropriate for the clinical context, and P-wave morphology identical to sinus rhythm. Inappropriate sinus tachycardia represents a distinct entity in which resting heart rate exceeds 100 bpm with normal P-wave morphology and an exaggerated heart rate response to minimal exertion; it remains a diagnosis of exclusion.

Typical (slow-fast) atrioventricular nodal reentrant tachycardia is the most common regular supraventricular tachycardia, accounting for approximately 60% of all SVTs. Retrograde P waves are characteristically buried within the QRS complex or appear as a pseudo-r' in lead V1 or a pseudo-S wave in the inferior leads, with an RP interval of less than 70 ms. Atypical AVNRT presents as a long RP tachycardia with P waves visible before the QRS complex, reflecting either fast-slow or slow-slow conduction patterns.

Orthodromic atrioventricular reentrant tachycardia utilizes an accessory pathway in the retrograde direction, producing a narrow complex tachycardia with P waves visible in the ST segment and an RP interval shorter than the PR interval, qualifying it as a short RP tachycardia. Atrial tachycardia is distinguished by a P-wave morphology that differs from sinus, with possible warm-up and cool-down behavior. The P-wave morphology provides localization information: a positive P wave in lead V1 suggests a left atrial origin.

Typical atrial flutter involves a counterclockwise macro-reentrant circuit around the tricuspid annulus, producing the classic sawtooth pattern in leads II, III, and aVF with an atrial rate of approximately 300 bpm. With 2:1 atrioventricular conduction, the ventricular rate is characteristically around 150 bpm. Atypical flutter displays variable morphology and is frequently encountered after prior ablation or cardiac surgery. Junctional tachycardia presents as a narrow complex rhythm with a rate of 60 to 130 bpm, AV dissociation or retrograde P waves, and is commonly seen following cardiac surgery, in digitalis toxicity, or complicating inferior myocardial infarction.

Wide Complex Tachycardias (QRS >= 120 ms)

The evaluation of wide complex tachycardia demands a disciplined approach grounded in a fundamental principle: ventricular tachycardia should be assumed until proven otherwise, regardless of hemodynamic stability. Hemodynamic tolerance does not exclude VT, as patients with preserved ventricular function may remain stable in sustained VT.

The Brugada criteria provide a stepwise algorithm for differentiating VT from SVT with aberrancy. The first step evaluates whether an RS complex is absent in all precordial leads, which favors VT. If an RS complex is identified, an RS interval exceeding 100 ms in any precordial lead supports VT. The presence of AV dissociation at any point in the algorithm confirms VT. Finally, morphology criteria in leads V1-V2 and V6 are applied to further refine the diagnosis. The Vereckei aVR algorithm offers an alternative approach: an initial R wave in aVR favors VT, as does an initial r or q wave exceeding 40 ms, a notch on the descending limb of a predominantly negative QRS, or a Vi/Vt ratio of 1 or less.

AV dissociation is the single most reliable criterion for diagnosing VT, and its manifestations include independent P waves marching through the QRS complexes, fusion beats resulting from simultaneous supraventricular and ventricular activation, and capture beats in which a normally conducted supraventricular impulse momentarily normalizes the QRS morphology. Concordance, in which all precordial QRS complexes point in the same direction (either positive or negative), strongly suggests VT. Features that favor SVT with aberrancy include a typical RBBB or LBBB morphology, a preceding P wave, a prior ECG demonstrating the same bundle branch block pattern, and rate-related aberrancy consistent with the Ashman phenomenon.

Preexcitation Syndromes

The Wolff-Parkinson-White pattern on the resting ECG is defined by a short PR interval of less than 120 ms, a delta wave representing the slurred initial upstroke of the QRS complex, and a widened QRS. Delta wave polarity algorithms such as those developed by Arruda and Milstein allow localization of the accessory pathway based on the 12-lead ECG.

The combination of WPW with atrial fibrillation creates a uniquely dangerous situation. The ECG demonstrates an irregularly irregular wide complex tachycardia with variable QRS width and often extremely rapid rates exceeding 200 bpm. AV nodal blocking agents including adenosine, verapamil, and digoxin are strictly contraindicated in this setting because they may enhance conduction through the accessory pathway, potentially precipitating ventricular fibrillation. Treatment of preexcited atrial fibrillation requires intravenous procainamide or ibutilide, with electrical cardioversion performed promptly if the patient is hemodynamically unstable. Concealed accessory pathways conduct only in the retrograde direction, produce no preexcitation on the resting ECG, and present exclusively as orthodromic AVRT.

<image> A diagnostic algorithm flowchart for wide complex tachycardia (WCT) differentiation. Start with "Wide Complex Tachycardia QRS >= 120ms" at top. First branch: "Hemodynamically Unstable?" -- if yes, arrow to "Immediate Cardioversion." If no, proceed to stepwise analysis. Step 1: "AV dissociation, fusion beats, or capture beats?" -- if yes, "VT confirmed." Step 2: "Absence of RS complex in all precordial leads?" -- if yes, "VT." Step 3: "RS interval > 100ms?" -- if yes, "VT." Step 4: "Morphology criteria: RBBB-type -- monophasic R or qR in V1, R/S < 1 in V6 = VT; LBBB-type -- initial r > 30ms in V1, notched S in V1, any Q in V6 = VT." Final box: "If none of above, consider SVT with aberrancy but treat as VT if uncertain." Use red boxes for VT diagnosis, blue for SVT, yellow for decision points, green for clinical actions. </image>

Conduction Abnormalities and Heart Block

Sinoatrial Node Disease

Sinus bradycardia, defined as a rate below 60 bpm, is a physiologic finding in trained athletes and during sleep and does not inherently require intervention. Sinus arrest or sinus pause describes the complete absence of P waves for a duration that is not a simple multiple of the baseline PP interval, a feature that distinguishes it from sinoatrial exit block. Sinoatrial exit block manifests in two forms: Type I exhibits progressive shortening of PP intervals before a dropped P wave, analogous to Wenckebach periodicity at the AV node, while Type II produces a sudden absence of an expected P wave with a pause that is an exact multiple of the baseline PP interval.

Tachy-brady syndrome represents alternating episodes of atrial fibrillation or atrial flutter with symptomatic sinus pauses following arrhythmia termination. Management typically requires permanent pacemaker implantation to provide a safety net for bradycardia, which in turn permits the use of rate control medications that would otherwise exacerbate the bradycardic episodes.

Atrioventricular Block

First-degree AV block is defined by a PR interval exceeding 200 ms with all P waves successfully conducted, is usually benign, and localizes to the level of the AV node. Second-degree Type I block, or Wenckebach, is characterized by progressive PR prolongation with grouped beating and eventually a dropped QRS complex. It is typically located at the level of the AV node and is often benign. Second-degree Type II block, or Mobitz Type II, presents with a constant PR interval and sudden, unexpected failure of conduction without preceding PR prolongation. This pattern reflects infranodal disease within the His-Purkinje system and carries a high risk of progression to complete heart block, often necessitating permanent pacing.

A 2:1 AV block pattern presents a diagnostic challenge because it is impossible to differentiate Type I from Type II based on a single rhythm strip, as there is no opportunity to observe PR interval behavior across consecutive conducted beats. A narrow QRS complex favors Type I (AV nodal) block, while a wide QRS complex favors Type II (infranodal) block. High-grade AV block refers to the occurrence of two or more consecutive dropped QRS complexes with regular P-P intervals. Third-degree or complete heart block demonstrates complete AV dissociation with a regular escape rhythm. The characteristics of the escape rhythm provide information about its origin: a junctional escape produces a rate of 40 to 60 bpm with a narrow QRS, while a ventricular escape produces a rate of 20 to 40 bpm with a wide QRS complex.

Intraventricular Conduction Delay

Nonspecific intraventricular conduction delay describes a QRS duration of 110 to 119 ms without the characteristic morphology of either right or left bundle branch block. Rate-dependent BBB, also known as phase 3 block, occurs when bundle branch block appears above a critical heart rate because the preceding action potential has not fully repolarized, leaving the bundle branch refractory to the next impulse. Phase 4 block represents the opposite phenomenon, in which bundle branch block appears at slow heart rates because of spontaneous depolarization within a diseased bundle branch during the prolonged diastolic interval. Trifascicular block, a term describing the combination of RBBB, LAFB, and first-degree AV block, is technically a misnomer because this pattern does not necessarily imply disease in all three fascicles, but it remains a high-risk finding for progression to complete heart block.

Channelopathies and Inherited Arrhythmia Syndromes

Long QT Syndrome

QTc prolongation is defined as exceeding 470 ms in males and 480 ms in females, with borderline values ranging from 450 to 470 ms in males and 460 to 480 ms in females. Among the available QTc correction formulas, the Bazett formula (QT divided by the square root of the RR interval) is the most widely used but tends to overcorrect at high heart rates; the Fridericia formula (QT divided by the cube root of the RR interval) provides more accurate correction at extremes of heart rate.

The three major congenital long QT subtypes each have distinctive ECG signatures and clinical triggers. LQT1 is associated with KCNQ1 mutations and manifests with broad-based T waves. Arrhythmic events characteristically occur during exercise, particularly swimming, and beta-blockers are the most effective therapy. LQT2, caused by KCNH2 mutations, produces low-amplitude notched T waves, with events triggered by auditory stimuli or emotional stress. Beta-blockers combined with strict avoidance of QT-prolonging drugs form the management foundation. LQT3, resulting from SCN5A mutations, displays late-onset peaked T waves with a long isoelectric ST segment, and events characteristically occur during sleep or rest. Mexiletine may shorten the QT interval in this subtype, but beta-blockers are less effective, and ICD implantation warrants strong consideration.

Acquired long QT syndrome results from QT-prolonging medications such as sotalol, dofetilide, amiodarone, haloperidol, fluoroquinolones, and methadone, or from electrolyte abnormalities including hypokalemia, hypomagnesemia, and hypocalcemia. Torsades de pointes, the polymorphic ventricular tachycardia with a characteristic rotating QRS axis that occurs in the setting of prolonged QT, is treated acutely with intravenous magnesium 2 grams, overdrive pacing, and isoproterenol.

Brugada Syndrome

The Brugada syndrome diagnosis rests on the identification of the Type 1 (coved) pattern, which requires at least 2 mm of ST elevation followed by a negative T wave in at least one right precordial lead (V1 or V2). This is the only diagnostic pattern; the Type 2 saddleback and Type 3 patterns are not diagnostic without pharmacologic provocation testing. Sodium channel blocker challenge with ajmaline, procainamide, or flecainide can unmask a latent Type 1 pattern. Sensitivity for detecting the Brugada pattern can be improved by placing leads V1 and V2 in the second or third intercostal space rather than the standard fourth intercostal space. SCN5A mutations are identified in approximately 20 to 25% of cases, and risk stratification incorporates the presence of a spontaneous Type 1 pattern, history of syncope, and documented VF or VT.

Catecholaminergic Polymorphic VT (CPVT)

Catecholaminergic polymorphic ventricular tachycardia is a devastating inherited arrhythmia syndrome in which exercise or emotional stress triggers bidirectional VT, characterized by alternating QRS axis on a beat-to-beat basis, or polymorphic VT. The resting ECG is typically normal, and the heart is structurally normal, making this a challenging diagnosis that requires exercise stress testing for provocation. RYR2 mutations, inherited in an autosomal dominant pattern, are the most common genetic cause, while CASQ2 mutations are inherited in an autosomal recessive fashion. Treatment centers on beta-blockers, with nadolol preferred for its long-acting pharmacokinetics, and flecainide as an important adjunctive agent. An ICD is indicated for breakthrough events despite medical therapy, and left cardiac sympathetic denervation is considered for refractory cases.

<image> A four-panel comparison of inherited arrhythmia syndrome ECG patterns. Panel 1 (top left): Long QT Type 1 showing broad-based T waves with QTc 520ms labeled, lead II rhythm strip. Panel 2 (top right): Long QT Type 2 showing low-amplitude bifid/notched T waves with QTc 540ms, lead II rhythm strip. Panel 3 (bottom left): Brugada Type 1 showing coved ST elevation >= 2mm followed by negative T wave in V1-V2, with clear annotation of the coved morphology. Panel 4 (bottom right): CPVT showing bidirectional VT with alternating QRS axis during exercise, beats labeled with alternating red and blue colors. Each panel on standard ECG grid background with syndrome name, gene, and key feature labeled in a small box below each tracing. </image>

Pacemaker and ICD ECG Interpretation

Basic Pacing Modes

Understanding pacemaker ECG interpretation requires familiarity with the fundamental pacing modes. AAI mode provides atrial pacing and sensing, producing a single atrial pacing spike followed by a P wave. VVI mode delivers ventricular pacing and sensing, with a ventricular spike followed by a wide QRS complex that typically demonstrates LBBB morphology when pacing from the right ventricular apex, or variable morphology when pacing from the septum.

DDD mode represents dual-chamber pacing and sensing, and the ECG may display any of four possible states: atrial paced with ventricular paced (AV sequential pacing), atrial sensed with ventricular paced (tracking mode), atrial paced with ventricular sensed (inhibited ventricular output), or atrial sensed with ventricular sensed (fully inhibited mode with native conduction). CRT or biventricular pacing typically produces an LBBB morphology in lead V1 but with a dominant R wave, and the QRS should be narrower than the native LBBB if effective resynchronization has been achieved.

Pacemaker Malfunction on ECG

Pacemaker malfunction can manifest in several distinct patterns on the ECG. Failure to capture produces pacing spikes that are not followed by the expected subsequent depolarization, and may result from lead dislodgement, battery depletion, or elevated pacing thresholds due to fibrosis or electrolyte derangement. Failure to sense, or undersensing, is recognized by pacing spikes occurring despite the presence of intrinsic cardiac rhythm, resulting in inappropriate asynchronous pacing. Oversensing presents as the absence of expected pacing spikes because the device erroneously senses non-cardiac signals such as T waves, myopotentials, or electromagnetic interference, resulting in inappropriate inhibition of pacing output.

Pacemaker-mediated tachycardia is a specific complication of DDD pacing in which a retrograde P wave following ventricular pacing is sensed by the atrial lead, triggering another ventricular pacing event and creating an endless loop tachycardia. This can be terminated acutely by magnet application, which converts the device to an asynchronous mode, or by reprogramming the PVARP (post-ventricular atrial refractory period).

Key Clinical Pearls

• Always compare with prior ECGs; a "new" LBBB is only meaningful if a prior ECG shows narrow QRS -- many LBBBs are old and incidental
• In any regular narrow complex tachycardia at rate ~150 bpm, think atrial flutter with 2:1 block and look for hidden flutter waves, especially in V1 and inferior leads
• The single most reliable criterion for VT in wide complex tachycardia is AV dissociation -- look carefully for march-out P waves at a rate independent of QRS
• Never give IV verapamil or adenosine to a wide complex tachycardia of uncertain etiology -- if it is VT, these agents can cause hemodynamic collapse
• Drug-induced QT prolongation risk is synergistic: a single QT-prolonging drug may be safe, but combinations (e.g., antibiotic + antiemetic + antipsychotic) can precipitate torsades
• Hyperkalemia can mimic nearly any arrhythmia: peaked T waves, prolonged PR, widened QRS, sine wave pattern, asystole -- always check potassium in any unexplained ECG change
• High right precordial lead placement (2nd intercostal space) should be performed when Brugada is suspected and standard leads are non-diagnostic

References

• Kusumoto FM, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients with Bradycardia and Cardiac Conduction Delay. Circulation. 2019.
• Page RL, et al. 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients with Supraventricular Tachycardia. Circulation. 2016.
• Al-Khatib SM, et al. 2017 AHA/ACC/HRS Guideline for Management of Patients with Ventricular Arrhythmias and Prevention of Sudden Cardiac Death. Circulation. 2018.
• Priori SG, et al. 2015 ESC Guidelines for the Management of Patients with Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death. Eur Heart J. 2015.
• Brugada J, et al. 2019 ESC Guidelines for the Management of Patients with Supraventricular Tachycardia. Eur Heart J. 2020.

Read this lecture as Markdown