Cardiac Conduction & Arrhythmias — Cardiac conduction system basics and arrhythmia generation concepts - Pathophysiology

SECTION 1

Historical Context & Motivation

The rhythmic beating of the heart has fascinated physicians and scientists for millennia, yet only within the last century and a half have we understood the electrical conduction system that coordinates atrial and ventricular contraction. Early anatomists recognized the heart's automaticity—its ability to beat independently of the nervous system—but the cellular and ionic mechanisms behind this property remained elusive until electrophysiology emerged as a formal discipline. The clinical imperative was always clear: sudden cardiac death, syncope, and heart failure frequently stemmed from disordered cardiac rhythm, making an understanding of normal conduction essential before pathological arrhythmias could be addressed.

1893 • Discovery of the Bundle of His

Wilhelm His Jr. identified specialized muscle fibers connecting the atria to the ventricles, providing the first anatomical evidence of a dedicated conduction pathway distinct from ordinary myocardium.

1906 • Einthoven's Electrocardiogram

Willem Einthoven refined the string galvanometer to record cardiac electrical activity from the body surface, giving clinicians the ECG/EKG—the single most important diagnostic tool in arrhythmia detection.

1907 • Sinoatrial Node Identified

Arthur Keith and Martin Flack described a small mass of specialized cells at the junction of the superior vena cava and right atrium—the sinoatrial (SA) node—establishing it as the heart's primary pacemaker.

1952 • Hodgkin–Huxley Model

Alan Hodgkin and Andrew Huxley mathematically described ion-channel kinetics in excitable membranes, providing the theoretical framework later applied to cardiac action potentials and the ionic basis of arrhythmogenesis.

1967 • First Successful Cardiac Ablation Concepts

Advances in intracardiac electrophysiology studies enabled the mapping and eventual ablation of accessory pathways and re-entrant circuits, translating conduction system knowledge into curative therapy.

This historical arc raises the central questions of the lesson: How does electrical activity originate in and propagate through the heart, and what happens at the cellular and tissue level when the system fails? By understanding normal conduction physiology, healthcare professionals can reason through arrhythmia mechanisms rather than merely memorizing ECG patterns.

SECTION 2

Core Principles of Cardiac Conduction

The cardiac conduction system is a network of specialized cardiac cells that generate and rapidly transmit electrical impulses, ensuring that atrial contraction precedes ventricular contraction by a precisely timed interval. Unlike skeletal muscle, which requires neural input to contract, the heart possesses intrinsic automaticity—the ability of certain cells to depolarize spontaneously. This property, combined with the ordered arrangement of gap junctions and specialized conduction fibers, produces the coordinated pump action that sustains systemic and pulmonary circulation.

Automaticity

Pacemaker cells in the SA node, AV node, and His-Purkinje system undergo spontaneous phase 4 depolarization due to the funny current (I_f), enabling them to initiate impulses without external stimulation.

Conductivity

Once initiated, action potentials spread cell-to-cell through gap junctions (connexins), with conduction velocity varying from 0.05 m/s in the AV node to 4 m/s in Purkinje fibers, ensuring rapid ventricular activation.

Refractoriness

Following depolarization, cardiac cells enter an effective refractory period (ERP) during which they cannot be re-excited. This prevents tetanic contraction and allows ventricular filling between beats.

Hierarchy of Pacemakers

The SA node fires fastest (60–100 bpm) and therefore suppresses lower pacemakers via overdrive suppression. If the SA node fails, the AV node (40–60 bpm) or ventricular foci (20–40 bpm) serve as escape pacemakers.

AV Nodal Delay

The atrioventricular (AV) node introduces a ~0.1 s delay, represented by the PR interval on the ECG. This ensures atrial contraction completes before ventricular systole begins, optimizing cardiac output.

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KEY TAKEAWAY

Think of the cardiac conduction system as a corporate hierarchy with a built-in succession plan. The SA node is the CEO, setting the pace for the entire organization. The AV node is the vice-president who deliberately slows communication to ensure the atrial 'departments' finish their work before the ventricular 'departments' begin. If the CEO is incapacitated, the VP steps in at a slower rate; if both fail, a department manager (Purkinje fiber) takes over at an even slower pace. This redundancy is critical for survival.

SECTION 3

The Conduction Pathway — Visual Overview

The cardiac conduction pathway from the SA node (cyan) through the AV node (violet), Bundle of His (amber), bundle branches (pink/emerald), and Purkinje fibers (orange). Note how conduction velocity increases dramatically in the His-Purkinje system to ensure near-simultaneous ventricular activation, while the AV node deliberately slows conduction to allow atrial emptying.

As shown in the diagram, the impulse originates in the SA node at the junction of the superior vena cava and the right atrium, then spreads across both atria via internodal pathways and through ordinary atrial myocardium. The wave front converges on the AV node, which is the only normal electrical connection between the atria and ventricles—the annulus fibrosus otherwise insulates them. After the critical AV delay, the impulse enters the rapidly conducting His-Purkinje system. The left bundle branch further divides into anterior and posterior fascicles, while the right bundle branch remains a single tract. Purkinje fibers ramify beneath the endocardium and activate the ventricular myocardium from apex to base, producing the efficient 'wringing' contraction that maximizes stroke volume.

SECTION 4

Ionic Mechanisms of the Cardiac Action Potential

Understanding arrhythmia generation requires familiarity with the cardiac action potential and its ionic underpinnings. Cardiac cells exhibit two fundamentally different action potential morphologies: the fast-response action potential of atrial and ventricular myocytes and Purkinje fibers, and the slow-response action potential of SA and AV nodal cells. The differences in phase 0 depolarization—sodium-dependent versus calcium-dependent—have profound implications for conduction velocity, refractoriness, and susceptibility to specific arrhythmia mechanisms.

Fast-Response Action Potential (Phases 0–4)

In ventricular myocytes, Phase 0 is a rapid upstroke driven by the opening of voltage-gated Na⁺ channels (I_Na), bringing the membrane potential from approximately −90 mV to +20 mV in about 1 ms. Phase 1 involves early repolarization via the transient outward K⁺ current (I_to). Phase 2 is the characteristic plateau sustained by a balance between inward Ca²⁺ current (I_Ca-L) and outward K⁺ currents. Phase 3 is final repolarization as K⁺ efflux (I_Kr, I_Ks, I_K1) dominates. Phase 4 is the stable resting membrane potential maintained by I_K1.

Slow-Response Action Potential (Pacemaker Cells)

In SA and AV nodal cells, Phase 4 is not stable—it exhibits gradual depolarization driven primarily by the funny current (I_f), a mixed Na⁺/K⁺ current activated by hyperpolarization. As the membrane potential drifts toward threshold (~−40 mV), T-type and then L-type Ca²⁺ channels open to produce a slower Phase 0 upstroke. The slower upstroke velocity explains why nodal conduction is much slower than in Purkinje fibers. Phase 3 repolarization is mediated by K⁺ efflux, after which Phase 4 depolarization begins again spontaneously—this cyclical process is the basis of cardiac automaticity.

RESTING MEMBRANE POTENTIAL (NERNST EQUATION FOR K⁺)

E_K = (RT / zF) × ln([K⁺]_o / [K⁺]_i) ≈ −90 mV

Where R = gas constant, T = temperature (K), z = ion valence (+1 for K⁺), F = Faraday's constant, [K⁺]_o = extracellular [K⁺], [K⁺]_i = intracellular [K⁺]. Hyperkalemia raises E_K (less negative), partially depolarizing the cell and inactivating Na⁺ channels—a common clinical cause of arrhythmia.

CONDUCTION VELOCITY DETERMINANTS

θ ∝ √(g_j × (dV/dt)_max × 1/C_m)

Conduction velocity (θ) depends on gap junction conductance (g_j), the maximum rate of rise of Phase 0 (dV/dt)_max reflecting Na⁺ channel availability, and inversely on membrane capacitance (C_m). Reduced gap junction coupling (e.g., fibrosis) or Na⁺ channel blockade slows conduction—a substrate for re-entry.

SECTION 5

Mechanisms of Arrhythmia Generation

Arrhythmias arise from three fundamental mechanisms, or combinations thereof: disorders of impulse formation (enhanced or abnormal automaticity, triggered activity), disorders of impulse conduction (re-entry, conduction block), or both. A clinician who can categorize an arrhythmia by mechanism can predict which pharmacological or interventional strategy is most appropriate.

The three fundamental arrhythmia mechanisms: (1) Automaticity—abnormal or enhanced spontaneous depolarization; (2) Triggered activity—early and delayed afterdepolarizations that reach threshold; (3) Re-entry—a circulating wavefront sustained by unidirectional block and slow conduction. Re-entry is the most common mechanism underlying clinically significant tachyarrhythmias.

Re-entry in Detail

Re-entry is the single most common mechanism responsible for sustained tachyarrhythmias in clinical practice, including AV nodal re-entrant tachycardia (AVNRT), AV re-entrant tachycardia (AVRT) via an accessory pathway (as in Wolff-Parkinson-White syndrome), atrial flutter, and many forms of ventricular tachycardia. A re-entrant circuit requires three conditions: the presence of two functionally or anatomically distinct pathways, unidirectional block in one pathway, and sufficiently slow conduction in the other pathway so that previously refractory tissue has recovered excitability by the time the circulating impulse returns. This concept is formalized as the excitable gap—the portion of the circuit that has fully repolarized and is available for re-excitation.

RE-ENTRY WAVELENGTH

λ = θ × ERP

The wavelength (λ) of a re-entrant circuit equals conduction velocity (θ) multiplied by the effective refractory period (ERP). Re-entry is sustained when λ < circuit path length, creating an excitable gap. Interventions that increase θ or prolong ERP (e.g., Class III antiarrhythmics) can terminate re-entry by eliminating the excitable gap.

Triggered Activity — Afterdepolarizations

Early afterdepolarizations (EADs) occur during Phase 2 or Phase 3, when the action potential duration is pathologically prolonged. Conditions that prolong repolarization—such as long QT syndrome, hypokalemia, hypomagnesemia, or certain drugs (e.g., sotalol, haloperidol)—allow L-type Ca²⁺ channels to recover from inactivation and re-open, generating a secondary depolarization that may reach threshold and trigger Torsades de Pointes. Delayed afterdepolarizations (DADs) occur after full repolarization and are caused by intracellular Ca²⁺ overload activating the Na⁺/Ca²⁺ exchanger (I_NCX), which generates a net inward current. DADs are the hallmark mechanism of digitalis toxicity and catecholaminergic polymorphic ventricular tachycardia (CPVT).

SECTION 6

Worked Example — Analyzing an Arrhythmia by Mechanism

Consider the following clinical scenario: A 28-year-old woman presents to the emergency department with sudden-onset palpitations and a heart rate of 180 bpm. She is hemodynamically stable. The narrow-complex tachycardia terminates immediately with administration of adenosine. No accessory pathway is identified on the post-conversion ECG. What arrhythmia mechanism best explains this presentation, and why did adenosine work?

Clinical Reasoning: AVNRT and the Re-entry Mechanism

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Step 1 — Identify the rhythm characteristics

The tachycardia is narrow-complex (QRS < 120 ms), indicating that ventricular activation occurs through the normal His-Purkinje system. This rules out ventricular tachycardia and pre-excited tachycardias as primary diagnoses. The regular rate of 180 bpm and abrupt onset (paroxysmal) are consistent with a supraventricular tachycardia (SVT).

Classification: Narrow-complex, regular SVT
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Step 2 — Determine the mechanism

The sudden onset and termination are hallmarks of re-entry rather than automaticity (which has a warm-up/cool-down pattern). The absence of an accessory pathway on the resting ECG makes AVRT less likely. The most common re-entrant SVT in young adults without pre-excitation is AVNRT, which involves dual pathways within the AV node—a fast pathway with a long refractory period and a slow pathway with a short refractory period.

Mechanism: Re-entry within the AV node (dual pathways)
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Step 3 — Apply the re-entry wavelength concept

In typical (slow-fast) AVNRT, a premature atrial complex encounters the fast pathway during its refractory period (unidirectional block), conducts anterograde down the slow pathway, and then returns retrograde up the now-recovered fast pathway. The circuit is sustained because the wavelength (λ = θ × ERP) is shorter than the anatomic circuit length, creating an excitable gap.

λ < circuit path length → excitable gap maintained → sustained re-entry
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Step 4 — Explain why adenosine terminates the arrhythmia

Adenosine activates A₁ receptors on AV nodal cells, opening K⁺-ACh channels (I_KACh) and hyperpolarizing the cell. This slows AV nodal conduction and prolongs its refractory period. By increasing the ERP, adenosine increases the wavelength (λ) to exceed the circuit length, collapsing the excitable gap and terminating the re-entrant circuit.

Adenosine ↑ ERP → ↑ λ → λ > circuit length → re-entry terminated

SECTION 7

Comparing Arrhythmia Mechanisms — Clinical Features

Distinguishing the underlying mechanism of an arrhythmia has direct therapeutic implications. Automaticity-driven arrhythmias often respond to rate control or removal of the inciting stimulus, triggered arrhythmias require correction of the precipitating condition (e.g., withdrawing the offending drug, repleting electrolytes), and re-entrant arrhythmias may be terminated acutely by breaking the circuit and prevented long-term by catheter ablation.

Feature	Automaticity	Triggered Activity	Re-entry
Onset	Gradual warm-up	Often triggered by a premature beat or pacing	Sudden (paroxysmal), often initiated by a PAC or PVC
Termination	Gradual cool-down	May self-terminate or respond to verapamil/magnesium	Abrupt; terminable by vagal maneuvers, adenosine, or overdrive pacing
Response to overdrive pacing	Transient suppression then resumes	May induce or worsen	Entrainment and termination
Typical examples	Sinus tachycardia, accelerated idioventricular rhythm, ectopic atrial tachycardia	Torsades de Pointes (EAD), digitalis-induced arrhythmias (DAD)	AVNRT, AVRT (WPW), atrial flutter, monomorphic VT
Key treatment strategy	Remove precipitant (sympathetic drive, hypoxia); β-blockers	Correct QT prolongation or Ca²⁺ overload; magnesium for Torsades	Break the circuit (adenosine, cardioversion, ablation); antiarrhythmics that alter θ or ERP

Clinical differentiation of the three arrhythmia mechanisms

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KEY TAKEAWAY

Determining the arrhythmia mechanism is akin to troubleshooting an electrical circuit. Automaticity is like a rogue power source that has been turned up too high—you need to turn it down or disconnect it. Triggered activity resembles a faulty relay that fires an extra signal when stressed—removing the stress (QT-prolonging drug, Ca²⁺ overload) resolves the issue. Re-entry is a short circuit where current loops endlessly through an unintended path—you must either cut the loop (ablation) or make it too long to sustain the circling current (drugs that prolong refractoriness).

SECTION 8

Connection to Advanced Electrophysiology & Pharmacology

The basic conduction and arrhythmia concepts covered in this lesson form the foundation for the Vaughan-Williams classification of antiarrhythmic drugs, advanced electrophysiology studies (EPS), and contemporary mapping and ablation techniques. Understanding which ion channels govern each phase of the action potential directly predicts the mechanism of action—and the pro-arrhythmic risks—of each drug class.

Concept (This Lesson)	Advanced Application
Phase 0 Na⁺ current (fast response)	Class I antiarrhythmics (Na⁺ channel blockers: IA, IB, IC) slow conduction to extinguish re-entry or paradoxically worsen it (pro-arrhythmia risk in structural heart disease)
Phase 4 automaticity ( $I_f$ current)	Ivabradine selectively blocks $I_f$ to reduce SA node rate without affecting contractility—used in HFrEF and inappropriate sinus tachycardia
Phase 2/3 K⁺ currents and ERP	Class III agents (amiodarone, sotalol, dofetilide) prolong APD and ERP, increasing wavelength to prevent re-entry—but risk EADs and Torsades if QT prolongation is excessive
AV nodal slow conduction (Ca²⁺-dependent)	Class IV agents (verapamil, diltiazem) block L-type Ca²⁺ channels, slowing AV conduction—used to terminate or rate-control SVTs involving the AV node
Re-entrant circuit anatomy	Catheter ablation uses radiofrequency or cryoenergy to create a lesion in a critical isthmus of the circuit, permanently eliminating the re-entry substrate

From basic conduction concepts to clinical electrophysiology and pharmacology

As you advance to clinical rotations and pharmacology coursework, you will encounter the Sicilian Gambit framework, which moves beyond the Vaughan-Williams scheme to classify antiarrhythmics by their specific channel, receptor, and pump targets. You will also study how structural remodeling in heart failure and atrial fibrillation alters connexin expression and creates fibrotic barriers that generate the substrate for complex, multi-loop re-entry circuits resistant to pharmacological therapy.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Explain why the SA node normally suppresses the intrinsic automaticity of the AV node and Purkinje fibers. What would happen to the heart rhythm if the SA node suddenly ceased firing?

Show answer

The SA node has the fastest intrinsic firing rate (60–100 bpm) and depolarizes lower pacemakers before they can reach threshold on their own—a phenomenon called overdrive suppression. If the SA node suddenly stops firing, there is typically a brief pause (reflecting the suppression of the subsidiary pacemaker), after which the AV node would take over as the escape pacemaker at a rate of approximately 40–60 bpm. If the AV node also fails, ventricular (Purkinje fiber) escape rhythms at 20–40 bpm may emerge, though these rates may be insufficient to maintain adequate cardiac output, potentially causing syncope or cardiac arrest.

Question 2: BASIC CALCULATION

A re-entrant circuit has a path length of 15 cm, a conduction velocity (θ) of 0.5 m/s, and an effective refractory period (ERP) of 200 ms. Calculate the wavelength (λ). Is re-entry sustained or terminated? Explain.

Show answer

Using λ = θ × ERP: λ = 0.5 m/s × 0.200 s = 0.10 m = 10 cm. The path length is 15 cm, so λ (10 cm) < path length (15 cm), meaning there is a 5 cm excitable gap. Since tissue ahead of the wavefront has had time to recover from refractoriness, the impulse can continue to propagate around the circuit. Re-entry is sustained. To terminate it, one would need to either increase θ or increase ERP so that λ ≥ 15 cm.

Question 3: INTERMEDIATE

A patient with long QT syndrome develops Torsades de Pointes. Identify the specific arrhythmia mechanism (including the type of afterdepolarization), the ionic current most likely responsible, and explain why intravenous magnesium is considered first-line therapy.

Show answer

Torsades de Pointes is initiated by early afterdepolarizations (EADs), a form of triggered activity. The prolonged QT interval reflects excessive action potential duration, which allows L-type Ca²⁺ channels (I_Ca-L) to recover from inactivation during Phase 2 or 3 and re-open, generating a secondary depolarization. If this EAD reaches threshold, it triggers a premature beat; in the setting of dispersion of refractoriness, this can degenerate into polymorphic VT. Intravenous magnesium suppresses EADs by stabilizing cell membranes and reducing Ca²⁺ influx through L-type channels, even when the QT interval remains prolonged. Magnesium does not shorten the QT interval itself but prevents the triggered beats that initiate Torsades.

Question 4: APPLIED

A patient on digoxin for heart failure presents with a new arrhythmia characterized by bidirectional ventricular tachycardia. Using your knowledge of ion channels and arrhythmia mechanisms, explain the pathophysiology from digoxin's mechanism of action through to the resulting arrhythmia.

Show answer

Digoxin inhibits the Na⁺/K⁺-ATPase, leading to intracellular Na⁺ accumulation. Elevated intracellular Na⁺ reduces the electrochemical gradient driving the Na⁺/Ca²⁺ exchanger (NCX), which normally extrudes Ca²⁺ in exchange for Na⁺ influx. The result is intracellular Ca²⁺ overload. Excessive Ca²⁺ triggers spontaneous Ca²⁺ release from the sarcoplasmic reticulum via ryanodine receptors. This Ca²⁺ is extruded by the NCX operating in forward mode, which generates a net inward (depolarizing) current—the basis of delayed afterdepolarizations (DADs). When DADs reach threshold, they trigger ectopic beats. The alternating QRS axis in bidirectional VT is thought to arise from triggered activity originating alternately from the left anterior and posterior fascicles, reflecting regional differences in Ca²⁺ handling and DAD threshold.

Question 5: CRITICAL THINKING

A Class IC antiarrhythmic drug (e.g., flecainide) is highly effective at suppressing premature ventricular complexes (PVCs). However, the CAST trial demonstrated increased mortality when flecainide was used in post-myocardial infarction patients. Using the re-entry wavelength equation and your understanding of how myocardial scar tissue alters conduction, explain the pro-arrhythmic mechanism that likely accounts for this finding.

Show answer

Flecainide is a potent Na⁺ channel blocker that markedly reduces (dV/dt)_max and conduction velocity (θ) with relatively little effect on the effective refractory period (ERP). In the re-entry wavelength equation λ = θ × ERP, reducing θ while ERP remains unchanged shortens the wavelength. In post-MI patients, scar tissue creates fixed anatomic barriers with narrow isthmuses of surviving myocardium—the substrate for re-entrant VT circuits. In normal hearts, the wavelength may exceed the potential circuit path length, preventing re-entry. Flecainide, by shortening λ, can cause λ to fall below the path length of circuits around the scar, thereby creating an excitable gap where none existed before and converting previously non-sustained or impossible re-entry into sustained, potentially fatal ventricular tachycardia. This paradox—an antiarrhythmic drug causing lethal arrhythmias—underscores why mechanism-based reasoning, not just empirical suppression of ectopy, must guide antiarrhythmic therapy.

SUMMARY

Lesson Summary — Cardiac Conduction & Arrhythmias

Summary

The cardiac conduction system consists of specialized cells—the SA node, AV node, Bundle of His, bundle branches, and Purkinje fibers—that generate and propagate electrical impulses in an ordered sequence. Automaticity allows pacemaker cells to fire spontaneously via Phase 4 depolarization driven by the funny current (I_f), with the SA node commanding the hierarchy through overdrive suppression. The AV nodal delay ensures mechanical coordination, while the rapid conduction velocity of the His-Purkinje system achieves near-simultaneous ventricular activation.

Arrhythmias arise from three mechanisms: enhanced or abnormal automaticity, triggered activity (EADs from prolonged repolarization and DADs from Ca²⁺ overload), and re-entry—the most common clinical mechanism—which requires two pathways, unidirectional block, and slow conduction to sustain a circulating wavefront. The re-entry wavelength equation (λ = θ × ERP) provides a quantitative framework for understanding both how re-entry is sustained and how antiarrhythmic drugs and ablation terminate it. Mastering these foundational concepts equips healthcare students to reason through ECG interpretation, pharmacological selection, and interventional electrophysiology with mechanistic clarity.