Cardiac Arrhythmias And Conduction Disorders - USMLE Step 2

SECTION 1

Historical Context & Motivation

The study of cardiac arrhythmias has evolved dramatically over more than a century, transforming from rudimentary observations of irregular pulses into a sophisticated discipline integrating electrophysiology, molecular biology, and advanced therapeutics. Early clinicians recognized that the heart could beat in irregular patterns, but they lacked tools to characterize these abnormalities systematically. The development of the electrocardiogram (ECG) fundamentally changed our capacity to diagnose and classify rhythm disturbances, laying the foundation for modern electrophysiology and interventional cardiology. Understanding this historical trajectory underscores why ECG interpretation remains the cornerstone of arrhythmia management on the wards and on standardized examinations such as USMLE Step 2.

1887 • First Human Electrocardiogram

Augustus Waller records the first human electrocardiogram using a capillary electrometer, demonstrating that the heart's electrical activity could be captured externally. This pioneering step proved that cardiac rhythm could be studied noninvasively.

1903 • Einthoven's String Galvanometer

Willem Einthoven develops the string galvanometer and defines the P, QRS, and T waves, establishing the nomenclature still used today. His work earned the 1924 Nobel Prize and provided the critical framework for identifying atrial and ventricular arrhythmias.

1947 • First Successful Defibrillation

Claude Beck performs the first successful open-chest defibrillation during cardiac surgery, introducing the concept that lethal arrhythmias such as ventricular fibrillation could be electrically terminated.

1969 • Intracardiac Electrophysiology Studies

Scherlag and colleagues demonstrate His bundle recording in humans, inaugurating the era of invasive electrophysiology (EP) studies. This technique enabled precise localization of conduction abnormalities and accessory pathways.

1998 • Catheter Ablation & Modern EP

Radiofrequency and cryoablation techniques become standard, allowing curative treatment for many supraventricular tachycardias and, increasingly, for atrial fibrillation. Today, implantable cardioverter-defibrillators (ICDs) and cardiac resynchronization therapy further refine arrhythmia management.

Despite these technological advances, the fundamental clinical question remains the same: when the heart beats too fast, too slow, or too irregularly, how do we identify the mechanism, assess hemodynamic significance, and select the optimal treatment? This lesson addresses that question comprehensively, emphasizing the clinical reasoning tested on USMLE Step 2.

SECTION 2

Core Principles & Definitions

Cardiac arrhythmias arise from disturbances in impulse formation, impulse conduction, or both. The heart's electrical activity originates in the sinoatrial (SA) node, propagates through the atria, is delayed at the atrioventricular (AV) node to allow atrial contraction to complete before ventricular filling, and then travels rapidly through the His-Purkinje system to depolarize the ventricles in a coordinated fashion. Disruption at any point along this pathway can produce clinically significant arrhythmias or conduction disorders.

Automaticity

The intrinsic ability of cardiac pacemaker cells to depolarize spontaneously. Enhanced automaticity in ectopic foci (e.g., atrial or ventricular myocytes) can override the SA node and generate tachyarrhythmias.

Re-entry

The most common mechanism of sustained tachyarrhythmias. Requires two functionally distinct pathways, unidirectional block in one, and slow conduction in the other, creating a self-perpetuating circuit.

Triggered Activity

Caused by afterdepolarizations — abnormal depolarizations during (early afterdepolarizations, EADs) or after (delayed afterdepolarizations, DADs) the action potential. EADs are associated with prolonged QT and torsades de pointes; DADs with digitalis toxicity.

Conduction Block

Failure or slowing of impulse propagation at specific anatomical sites — SA node, AV node, or bundle branches. AV blocks are classified by degree (first, second Mobitz I and II, and third) based on ECG findings.

Rate Classification

Arrhythmias are broadly divided into tachyarrhythmias (>100 bpm) and bradyarrhythmias (<60 bpm). Further classification by QRS width (narrow vs. wide) and regularity guides acute management.

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KEY TAKEAWAY

Think of the cardiac conduction system like a highway network. The SA node is the on-ramp that sets traffic speed, the AV node is a toll booth that intentionally slows traffic to prevent pile-ups, and the His-Purkinje system is the high-speed expressway distributing cars evenly across lanes. An arrhythmia is essentially a traffic jam: it can start because a rogue driver (enhanced automaticity) runs a red light, because cars circle a roundabout endlessly (re-entry), or because the toll booth breaks down and lets nobody through (conduction block).

SECTION 3

The Cardiac Conduction System & Normal ECG

Left panel: The conduction system begins at the SA node (intrinsic rate 60–100 bpm), traverses the AV node (40–60 bpm), proceeds through the bundle of His, right and left bundle branches (RBB/LBB), and terminates in the Purkinje fibers (20–40 bpm). Right panel: Normal ECG morphology with labeled P wave, QRS complex, T wave, and key intervals.

The diagram above illustrates the hierarchical nature of cardiac pacemaking. The SA node is the dominant pacemaker because its intrinsic rate of depolarization is the fastest; lower pacemaker sites are suppressed through overdrive suppression. When the SA node fails or conduction is interrupted, these subsidiary pacemakers emerge at progressively slower rates — a concept essential for understanding escape rhythms. On the ECG strip, the P wave corresponds to atrial depolarization, the PR interval reflects AV nodal delay, the QRS complex represents ventricular depolarization via the His-Purkinje system, and the T wave corresponds to ventricular repolarization. A widened QRS (≥120 ms) implies conduction is bypassing or delayed in the His-Purkinje system, as occurs in bundle branch blocks and ventricular tachycardia.

SECTION 4

Mechanisms of Arrhythmia — Deep Dive

Re-entry: The Most Testable Mechanism

Re-entrant circuits account for the majority of sustained tachyarrhythmias encountered in clinical practice, including AV nodal re-entrant tachycardia (AVNRT), AV re-entrant tachycardia (AVRT) via accessory pathways (e.g., Wolff-Parkinson-White syndrome), atrial flutter, and many cases of ventricular tachycardia. Three conditions must coexist for re-entry to occur: (1) two functionally distinct pathways, (2) unidirectional block in one pathway, and (3) slow conduction in the alternative pathway that allows the initially blocked pathway to recover excitability. Once the wavefront re-enters the recovered pathway, a self-sustaining loop is established.

Triggered Activity: Afterdepolarizations

Triggered activity results from oscillations in membrane potential during or after repolarization. Early afterdepolarizations (EADs) occur during phase 2 or 3 of the action potential and are promoted by conditions that prolong repolarization, including hypokalemia, hypomagnesemia, and QT-prolonging drugs (class IA and III antiarrhythmics, certain antibiotics and antipsychotics). EADs can initiate torsades de pointes, a polymorphic ventricular tachycardia associated with prolonged QTc. In contrast, delayed afterdepolarizations (DADs) occur after full repolarization (phase 4) and are triggered by intracellular calcium overload, classically from digitalis toxicity or catecholamine excess.

Enhanced Automaticity

Enhanced automaticity occurs when the rate of spontaneous phase 4 depolarization increases in either normal pacemaker tissue or in ectopic foci that do not normally exhibit automaticity. Clinical settings that promote this mechanism include ischemia, electrolyte imbalances, sympathetic stimulation, and hyperthyroidism. Examples include multifocal atrial tachycardia (MAT) — classically associated with decompensated COPD and characterized by at least three distinct P-wave morphologies — and accelerated idioventricular rhythm in the setting of myocardial reperfusion.

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High-Yield for Step 2

Always identify the mechanism when approaching an arrhythmia question: re-entry → responsive to cardioversion and adenosine (if circuit involves the AV node); triggered activity → correct the underlying trigger (stop the offending drug, replace Mg²⁺/K⁺); enhanced automaticity → treat the underlying condition (e.g., correct hypoxia in MAT).

SECTION 5

Classification of Arrhythmias & Conduction Disorders

A systematic approach to arrhythmia classification begins with two ECG-derived parameters: heart rate (tachycardia vs. bradycardia) and QRS width (narrow ≤ 120 ms vs. wide > 120 ms). This two-axis framework is the entry point for the USMLE's algorithmic management questions and mirrors the ACLS tachycardia and bradycardia protocols.

This algorithm stratifies tachyarrhythmias by QRS width (narrow vs. wide) and regularity. Memorize the four quadrants and their differential diagnoses for rapid ECG interpretation on Step 2.

Conduction Disorders: AV Blocks

Type	ECG Findings	Location	Management
1st Degree	PR > 200 ms (0.20 s), every P followed by QRS	AV node (most common)	Observation; usually benign
2nd Degree Mobitz I (Wenckebach)	Progressive PR prolongation → dropped QRS; grouped beating	AV node	Observation; atropine if symptomatic
2nd Degree Mobitz II	Constant PR with sudden dropped QRS; no progressive prolongation	Infra-nodal (His-Purkinje)	Pacemaker required — high risk of progression to complete block
3rd Degree (Complete)	AV dissociation: P waves and QRS march independently; escape rhythm	AV node or infra-nodal	Pacemaker required; temporary pacing for hemodynamic instability

Classification of AV Conduction Blocks

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Mobitz I vs. Mobitz II

A critical Step 2 distinction: Mobitz I (Wenckebach) is generally benign and occurs at the AV node, often reversible with atropine. Mobitz II occurs below the AV node (infra-nodal) and carries a high risk of progressing to complete heart block — it requires a pacemaker. The mnemonic: "Wenckebach = watch; Mobitz II = implant."

SECTION 6

Clinical Vignette: Step-by-Step ECG Interpretation

A 62-year-old man with a history of hypertension and prior MI presents to the ED with palpitations and lightheadedness. His heart rate is 150 bpm, blood pressure 90/60 mmHg. The ECG shows a regular, wide-complex tachycardia at 150 bpm with AV dissociation. No prior ECGs are available.

Systematic Approach to Wide-Complex Tachycardia

1
Step 1 — Assess Rate and Rhythm

The rate is 150 bpm and the rhythm is regular. Since the rate is >100 bpm, this is a tachyarrhythmia. Because the QRS is >120 ms, we enter the wide-complex tachycardia (WCT) arm of our algorithm.

Wide + regular tachycardia
2
Step 2 — Determine the Differential

A regular WCT has four main etiologies: ventricular tachycardia (VT), SVT with aberrancy (e.g., rate-related BBB), SVT with pre-existing BBB, and antidromic AVRT. However, in a patient with structural heart disease (prior MI), VT must be assumed until proven otherwise. AV dissociation is the most specific finding for VT.

AV dissociation present → VT is the diagnosis
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Step 3 — Assess Hemodynamic Stability

The patient is hypotensive (BP 90/60) and symptomatic with lightheadedness, indicating hemodynamic instability. Unstable tachyarrhythmias of any type warrant immediate synchronized cardioversion per ACLS protocols.

Hemodynamically unstable → immediate cardioversion
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Step 4 — If Stable, Consider Pharmacologic Options

If this patient were hemodynamically stable, the first-line pharmacologic agent for monomorphic VT would be amiodarone (150 mg IV bolus over 10 minutes). Procainamide is an alternative. Lidocaine may be used but is second-line. Do NOT give adenosine if VT is suspected (it is unlikely to terminate VT and can cause hypotension).

Stable monomorphic VT → amiodarone IV
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Step 5 — Long-Term Management

After acute stabilization, this patient with sustained VT and structural heart disease (prior MI) requires evaluation for an implantable cardioverter-defibrillator (ICD) for secondary prevention of sudden cardiac death. Echocardiography to assess LVEF and coronary angiography to evaluate for ongoing ischemia are essential components of the workup.

Secondary prevention → ICD implantation

SECTION 7

Antiarrhythmic Drug Classes & Management Strategies

The Vaughan-Williams classification organizes antiarrhythmic drugs by their primary mechanism of action. While this framework has limitations — many drugs exhibit properties spanning multiple classes — it remains the standard organizational scheme tested on USMLE Step 2. Selecting the appropriate antiarrhythmic requires integrating the arrhythmia type, the underlying substrate (e.g., structural heart disease, channelopathy), and the drug's side-effect profile.

Class	Mechanism	Key Agents	Primary Indications	Major Side Effects
IA	Na⁺ channel blockade (intermediate kinetics); ↑ AP duration	Procainamide, quinidine, disopyramide	SVT, WPW, VT	QT prolongation → TdP; procainamide: drug-induced lupus
IB	Na⁺ channel blockade (fast kinetics); ↓ AP duration	Lidocaine, mexiletine	Ventricular arrhythmias (post-MI)	CNS toxicity (seizures, confusion)
IC	Na⁺ channel blockade (slow kinetics); no effect on AP duration	Flecainide, propafenone	SVT, A-fib (structurally normal heart only)	Contraindicated in structural heart disease (CAST trial → ↑ mortality)
II	β-adrenergic blockade → ↓ SA/AV nodal automaticity and conduction	Metoprolol, atenolol, esmolol, propranolol	Rate control (A-fib, A-flutter); AVNRT/AVRT prevention	Bradycardia, hypotension, bronchospasm
III	K⁺ channel blockade → ↑ AP duration and refractory period	Amiodarone, sotalol, dofetilide, ibutilide	A-fib/flutter (rhythm control); VT	Amiodarone: pulmonary fibrosis, thyroid dysfunction, corneal deposits, hepatotoxicity. Sotalol: TdP
IV	Ca²⁺ channel blockade (non-dihydropyridine) → ↓ AV nodal conduction	Verapamil, diltiazem	Rate control (A-fib); AVNRT termination	Hypotension, constipation; avoid in HFrEF and WPW

Vaughan-Williams Classification of Antiarrhythmic Drugs

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MANAGEMENT PEARL

Think of antiarrhythmic drugs like tuning different knobs on a mixing board. Class I drugs turn down the sodium channel (the main beat), Class II turns down the sympathetic amplifier, Class III extends the time between beats by stretching the electrical pause (K⁺ channel blockade), and Class IV softens the calcium-driven AV node relay. Turning the wrong knob too far can paradoxically create worse rhythms — this is proarrhythmia, a crucial concept especially with Class IC agents (CAST trial) and QT-prolonging drugs.

SECTION 8

Advanced Topics: Atrial Fibrillation, Long QT, & Device Therapy

Atrial Fibrillation: Rate vs. Rhythm Control & Anticoagulation

Atrial fibrillation (A-fib) is the most common sustained arrhythmia, and its management involves three simultaneous considerations: (1) rate control, (2) rhythm control, and (3) stroke prevention with anticoagulation. The AFFIRM trial demonstrated that rate control is non-inferior to rhythm control for mortality in many patients, though newer data (EAST-AFNET 4) suggests early rhythm control may benefit recently diagnosed A-fib. The CHA₂DS₂-VASc score determines thromboembolic risk and guides anticoagulation decisions: scores ≥2 in men or ≥3 in women warrant oral anticoagulation, preferably with a direct oral anticoagulant (DOAC) such as apixaban, rivaroxaban, edoxaban, or dabigatran rather than warfarin, except in the setting of mechanical heart valves or moderate-to-severe mitral stenosis.

Feature	Rate Control Strategy	Rhythm Control Strategy
Goal	Control ventricular rate (<110 bpm at rest per RACE II)	Restore and maintain sinus rhythm
Agents	β-blockers, non-DHP CCBs (diltiazem/verapamil), digoxin	Flecainide/propafenone (no SHD), amiodarone, dofetilide, sotalol
Procedures	AV node ablation + pacemaker (refractory cases)	Pulmonary vein isolation (catheter ablation)
Preferred When	Older, asymptomatic, persistent A-fib, multiple comorbidities	Younger, symptomatic, paroxysmal A-fib, early after diagnosis
Anticoagulation	Still required based on CHA₂DS₂-VASc	Still required based on CHA₂DS₂-VASc (even if sinus restored)

Rate Control vs. Rhythm Control in Atrial Fibrillation

Long QT Syndrome & Torsades de Pointes

A prolonged QTc interval (>500 ms is high risk) predisposes to torsades de pointes (TdP), a polymorphic VT with a characteristic twisting morphology on ECG. Acquired long QT is far more common on exams and results from drugs (class IA/III antiarrhythmics, macrolides, fluoroquinolones, antipsychotics, methadone), hypokalemia, and hypomagnesemia. The acute treatment of TdP is IV magnesium sulfate regardless of serum magnesium level. Overdrive pacing or isoproterenol can be used to increase the heart rate and shorten the QT interval, suppressing EADs. Congenital long QT syndromes (Romano-Ward, Jervell and Lange-Nielsen) are treated with β-blockers, ICD placement, and avoidance of QT-prolonging medications.

Device Therapy: Pacemakers & ICDs

Permanent pacemakers are indicated for symptomatic bradycardia, Mobitz type II second-degree AV block, third-degree AV block, and certain sinus node dysfunction syndromes. Implantable cardioverter-defibrillators (ICDs) are indicated for secondary prevention (survivors of cardiac arrest or sustained VT) and for primary prevention in patients with LVEF ≤35% despite optimal medical therapy for at least 3 months. Cardiac resynchronization therapy (CRT) adds biventricular pacing for patients with HFrEF (LVEF ≤35%), NYHA class II–IV symptoms, and a wide QRS (≥150 ms), particularly with left bundle branch block morphology.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A 28-year-old woman presents with sudden-onset palpitations and a heart rate of 180 bpm. The ECG shows a narrow-complex, regular tachycardia with no visible P waves. Vagal maneuvers abruptly terminate the arrhythmia. What is the most likely mechanism and diagnosis?

Show answer

The most likely diagnosis is AV nodal re-entrant tachycardia (AVNRT), which is the most common regular narrow-complex SVT in young adults. The mechanism is re-entry within the AV node involving fast and slow pathways. P waves are typically buried in the QRS complex (pseudo-R' in V1 or pseudo-S in inferior leads) because atria and ventricles are activated simultaneously. The abrupt termination by vagal maneuvers confirms AV nodal dependence of the circuit, as vagal stimulation increases AV nodal refractoriness and interrupts the re-entrant loop. Adenosine would be the first-line pharmacologic agent if vagal maneuvers failed.

Question 2: BASIC CALCULATION

A 72-year-old man with atrial fibrillation has the following risk factors: hypertension, age 72, diabetes mellitus, and prior stroke. Calculate his CHA₂DS₂-VASc score and state the recommended anticoagulation strategy.

Show answer

The CHA₂DS₂-VASc score is calculated as follows: Congestive heart failure (0), Hypertension (1), Age ≥75 (0, he is 72), Diabetes (1), Stroke/TIA (2 points), Vascular disease (0), Age 65–74 (1), Sex category — male (0). Total = 1 + 1 + 2 + 1 = 5. With a score of 5, this patient has a high annual stroke risk (approximately 6.7% per year). Oral anticoagulation is strongly indicated, preferably with a DOAC (e.g., apixaban 5 mg BID or rivaroxaban 20 mg daily). Warfarin with target INR 2–3 is an alternative if DOACs are contraindicated (e.g., mechanical valve, severe renal impairment).

Question 3: INTERMEDIATE

A 55-year-old man presents with a syncopal episode. His ECG shows a regular rhythm at 38 bpm. P waves are present at a regular rate of 80 bpm, but there is no consistent relationship between P waves and QRS complexes. The QRS duration is 140 ms. What is the diagnosis, the likely anatomical location of the block, and the definitive management?

Show answer

This ECG demonstrates third-degree (complete) AV block with complete AV dissociation — the P waves march at 80 bpm independently of the QRS complexes at 38 bpm. The wide QRS escape rhythm (140 ms) indicates an infra-nodal (His-Purkinje) escape focus, which is slower and less reliable than a junctional escape. An AV nodal block would typically produce a narrow-complex junctional escape at 40–60 bpm. The patient is symptomatic (syncope), confirming hemodynamic compromise from the slow rate. Acute management includes atropine (though often ineffective for infra-nodal blocks), transcutaneous pacing, and possibly a temporary transvenous pacemaker. Definitive management is implantation of a permanent pacemaker. Third-degree AV block with syncope is a Class I indication for permanent pacing regardless of symptoms or rate.

Question 4: APPLIED

A 45-year-old man with Wolff-Parkinson-White syndrome develops atrial fibrillation with a rapid ventricular response (HR 220 bpm). Wide, irregular QRS complexes with varying morphology are seen on ECG. A resident proposes administering IV diltiazem for rate control. Explain why this is dangerous and identify the appropriate management.

Show answer

In WPW with atrial fibrillation, rapid atrial impulses can conduct antegradely down the accessory pathway (bundle of Kent) directly to the ventricles, bypassing the AV node. AV nodal blocking agents such as diltiazem, verapamil, β-blockers, adenosine, and digoxin are contraindicated because they slow or block AV nodal conduction without affecting the accessory pathway, thereby preferentially shunting all conduction through the bypass tract. This can paradoxically accelerate the ventricular rate and precipitate ventricular fibrillation and cardiac arrest. The appropriate management depends on hemodynamic stability. If unstable (as suggested by HR 220), immediate synchronized cardioversion is indicated. If stable, IV procainamide or ibutilide can be used to prolong the refractory period of the accessory pathway and slow or terminate the arrhythmia. Definitive treatment is catheter ablation of the accessory pathway.

Question 5: CRITICAL THINKING

A 68-year-old woman with ischemic cardiomyopathy (LVEF 30%) is on optimal medical therapy including β-blocker, ACE inhibitor, and spironolactone. Her ECG shows sinus rhythm with a QRS duration of 160 ms and left bundle branch block morphology. She reports NYHA class III symptoms despite medical optimization. Discuss the role of device therapy — specifically ICD and CRT — for this patient, including the physiologic rationale for each.

Show answer

This patient meets criteria for both an ICD and CRT. For primary prevention ICD implantation, the indication is LVEF ≤35% on guideline-directed medical therapy (GDMT) for ≥3 months with life expectancy >1 year — her LVEF of 30% qualifies. The ICD does not prevent arrhythmias but detects and terminates ventricular tachycardia or fibrillation via antitachycardia pacing or defibrillation shocks, reducing sudden cardiac death. For CRT (biventricular pacing), the indication is LVEF ≤35%, NYHA class II–ambulatory IV, QRS ≥150 ms with LBBB morphology, and persistent symptoms on GDMT — all of which she meets. LBBB causes dyssynchronous contraction: the septum activates before the lateral wall, reducing stroke volume and worsening mitral regurgitation. CRT resynchronizes ventricular contraction by pacing both ventricles simultaneously (or with optimized timing), thereby improving LVEF, reducing heart failure symptoms, and decreasing mortality. Most commonly, this patient would receive a CRT-D device, combining CRT with defibrillator capability in a single implant. The strongest evidence for CRT benefit exists in patients with true LBBB and QRS ≥150 ms — exactly this patient's profile.

SUMMARY

Cardiac Arrhythmias & Conduction Disorders — Review

Summary

Cardiac arrhythmias arise from three fundamental mechanisms: re-entry (the most common cause of sustained tachycardias, including AVNRT, AVRT, atrial flutter, and many forms of VT), triggered activity (EADs causing torsades de pointes in the setting of prolonged QT; DADs causing digitalis-toxic arrhythmias), and enhanced automaticity (e.g., MAT in COPD). The systematic ECG approach classifies tachyarrhythmias by QRS width (narrow vs. wide) and regularity, yielding four diagnostic quadrants. A wide-complex, regular tachycardia should be treated as ventricular tachycardia until proven otherwise, especially in patients with structural heart disease.

Conduction disorders range from benign first-degree AV block and Mobitz type I to potentially lethal Mobitz type II and third-degree AV block requiring permanent pacemakers. Atrial fibrillation management integrates rate control (β-blockers, CCBs), rhythm control (antiarrhythmics, ablation), and anticoagulation guided by the CHA₂DS₂-VASc score. The Vaughan-Williams classification organizes antiarrhythmic drugs by ion channel targets. Critical clinical pearls include: avoid AV nodal blockers in WPW with atrial fibrillation, treat torsades de pointes with IV magnesium, and consider ICD implantation for primary prevention in patients with LVEF ≤35% on optimal medical therapy.