Cardiomyocyte action potential simulator — Na⁺ / Ca²⁺ / K⁺ channel dynamics and antiarrhythmic drug classes (Vaughan Williams I–IV)
Every heartbeat begins as a precisely choreographed sequence of ion channel openings and closings across the cardiomyocyte membrane. The ventricular action potential (AP) moves the transmembrane voltage from a resting −90 mV to a peak of roughly +20 mV and back again in about 250–350 ms, driven in turn by fast sodium current, L-type calcium current, and multiple potassium currents. This electrical waveform is the trigger for excitation–contraction coupling — without the correctly-shaped AP, the heart cannot contract in a coordinated, rhythmic way.
Phase 4 — Resting / diastolic potential: Maintained near −90 mV chiefly by the inward-rectifier potassium current (IK1), which holds the membrane close to the K⁺ equilibrium potential between beats.
Phase 0 — Rapid depolarization (upstroke): When threshold is reached, fast voltage-gated Na⁺ channels (Nav1.5) open explosively, driving a massive inward INa that flips the membrane from −90 mV to roughly +20 mV in under a millisecond. This is the steepest, most electrically "loud" event of the cycle and determines conduction velocity through the tissue.
Phase 1 — Early (notch) repolarization: Na⁺ channels rapidly inactivate while a transient outward K⁺ current (Ito) fires briefly, producing a small downward notch before the plateau.
Phase 2 — Plateau: A prolonged, near-flat plateau is sustained by a delicate balance between inward Ca²⁺ current through L-type calcium channels (ICaL) and outward repolarizing K⁺ currents (mainly the slow delayed rectifier IKs). This plateau — unique to cardiac muscle among excitable tissues — is what gives the ventricular AP its long duration and, critically, allows enough Ca²⁺ entry to trigger contraction while preventing the cell from re-firing too soon (protecting against tetany).
Phase 3 — Final repolarization: Ca²⁺ channels inactivate while the rapid delayed rectifier K⁺ current (IKr) and IKs drive the membrane rapidly back toward resting potential, aided again by IK1 as voltage approaches baseline.
Pacemaker cells of the SA and AV nodes generate their action potentials differently: instead of a fast Na⁺-driven upstroke, they rely on a slow, Ca²⁺-dependent upstroke and a spontaneously rising phase 4 ("funny current" If plus T-/L-type Ca²⁺ channels) that drives automaticity — the property that lets the SA node initiate every normal heartbeat.
Each phase of the AP is produced by specific channel proteins encoded by well-characterized genes: Nav1.5 (SCN5A) carries INa; Cav1.2 (CACNA1C) carries ICaL; hERG/Kv11.1 (KCNH2) carries IKr; KCNQ1 with KCNE1 carries IKs; and Kir2.1 (KCNJ2) carries IK1. Mutations in any of these genes underlie inherited arrhythmia syndromes (e.g. SCN5A loss-of-function in Brugada syndrome, KCNH2 loss-of-function in congenital long QT type 2).
The Ca²⁺ that enters during phase 2 does more than shape the AP — it triggers calcium-induced calcium release from the sarcoplasmic reticulum via ryanodine receptors, flooding the cytoplasm with the Ca²⁺ needed for actin–myosin cross-bridge cycling. This tight coupling between electrical signal and mechanical contraction is why any drug that alters ion channel behavior can, deliberately or otherwise, alter how forcefully and how rhythmically the heart beats.
Because fast Na⁺ channels are inactivated during most of the plateau, the cell is in an absolute refractory period — it cannot generate a second upstroke no matter how strong the stimulus. As repolarization proceeds through phase 3, a relative refractory period follows, during which a stronger-than-normal stimulus can still provoke a premature response. This built-in refractoriness is the heart's primary safeguard against re-entrant arrhythmias: it normally prevents a wave of depolarization from circling back and re-exciting tissue that has just fired.
In the healthy heart, the AP originates in the SA node, spreads across the atria, is deliberately delayed at the AV node (allowing the atria to finish contracting before the ventricles begin), and then races through the His-Purkinje system to activate the ventricles in a coordinated, apex-to-base sequence — the electrical basis of an efficient, synchronized heartbeat.
Class I antiarrhythmics act directly on the fast voltage-gated Na⁺ channel (Nav1.5) responsible for phase 0 of the action potential. By binding preferentially to open or inactivated channel states, they reduce the number of channels available to fire on the next beat, slowing the maximum upstroke velocity (dV/dt max) and, correspondingly, the speed at which the depolarization wave propagates through cardiac tissue. This conduction slowing can interrupt the re-entrant circuits that sustain many arrhythmias.
Class I drugs occupy the same pore region of Nav1.5 that Na⁺ ions must pass through, but their affinity for the channel varies with its conformational state: they bind weakly to the resting (closed) state and much more strongly to the open and inactivated states that occur during and immediately after an action potential.
This produces "use-dependence" (also called state-dependent block): the faster the heart rate, the more often channels cycle through open/inactivated states, the more drug accumulates bound to the channel, and the greater the resulting conduction slowing. This is therapeutically convenient — the drug preferentially suppresses fast, abnormal rhythms while having comparatively less effect at normal resting heart rates.
IA (e.g. quinidine, procainamide, disopyramide): intermediate-affinity Na⁺ channel block plus some K⁺ channel block — moderately slows conduction and moderately prolongs action potential duration and the QT interval.
IB (e.g. lidocaine, mexiletine): fast on/off binding kinetics, weak at normal heart rates but selectively effective on rapidly firing or depolarized (ischemic) tissue — tends to shorten APD slightly. Historically favored for acute ventricular arrhythmias in the setting of ischemia.
IC (e.g. flecainide, propafenone): slow dissociation kinetics producing the most pronounced conduction slowing of the three subclasses, with minimal direct effect on APD. Highly effective at suppressing arrhythmias, but the marked conduction slowing itself becomes a proarrhythmic liability in structurally diseased hearts.
Class I agents are used to suppress ventricular ectopy, terminate certain re-entrant tachycardias, and manage supraventricular arrhythmias in structurally normal hearts. Because they slow conduction, they can also widen the QRS complex and, paradoxically, create the conditions for new re-entrant circuits in diseased myocardium.
The Cardiac Arrhythmia Suppression Trial (CAST, 1989) found that Class IC agents (flecainide, encainide), despite effectively suppressing ventricular ectopy after myocardial infarction, significantly increased overall mortality compared to placebo. This landmark result reshaped antiarrhythmic prescribing: suppressing an ECG finding is not the same as improving survival, and Class I agents are now avoided in patients with structural heart disease or prior MI.
Unlike the other three Vaughan Williams classes, beta-blockers do not act directly on a cardiac ion channel pore. Instead, they block β1-adrenergic receptors on nodal and myocardial cells, removing the sympathetic nervous system's ability to accelerate the heart. Because sympathetic tone acts upstream of several ion currents that shape automaticity and conduction, beta-blockade produces electrophysiological effects — slower heart rate, slower AV conduction — through this indirect, receptor-mediated route.
Sympathetic stimulation of β1-adrenergic receptors activates a Gs-protein → adenylyl cyclase → cAMP → protein kinase A (PKA) cascade. PKA phosphorylates several key electrophysiological targets, including the pacemaker "funny current" channels (HCN, carrying If) and L-type Ca²⁺ channels in the SA and AV nodes — increasing their activity and steepening the spontaneous phase-4 diastolic depolarization slope that determines heart rate.
Beta-blockers competitively block the β1 receptor itself, preventing this cascade from being activated by circulating catecholamines or sympathetic nerve stimulation. The net effect is a shallower phase-4 slope in the SA node (slower spontaneous firing = negative chronotropy) and slower, more decremental conduction through the AV node (negative dromotropy) — without directly plugging any channel pore.
The Vaughan Williams classification groups drugs by their dominant electrophysiological mechanism. Classes I, III, and IV each map onto a specific channel (Na⁺, K⁺, Ca²⁺ respectively) that is blocked directly by the drug molecule. Class II instead removes an upstream regulatory input — sympathetic β-adrenergic drive — that would otherwise be pushing several of those same channels toward greater activity.
This distinction matters clinically: beta-blockers are particularly effective against arrhythmias that are triggered or sustained by high sympathetic tone (exercise-induced, catecholaminergic, or stress-related arrhythmias), while having comparatively modest effects on arrhythmias with a purely structural or re-entrant substrate that isn't sympathetically driven.
Beta-blockers are a first-line tool for ventricular rate control in atrial fibrillation and atrial flutter (by slowing AV nodal conduction, fewer atrial impulses reach the ventricles per minute), for suppressing sympathetically-triggered ventricular ectopy, and for reducing sudden cardiac death risk after myocardial infarction. They are also foundational therapy in congenital long QT syndrome type 1, where sympathetic surges are a major trigger for life-threatening polymorphic ventricular tachycardia.
Because beta-blockers act upstream of the channel machinery rather than plugging a pore directly, their antiarrhythmic effect is most pronounced precisely when it is needed most — during states of high adrenergic drive such as exercise or acute stress — making them uniquely suited to catecholamine-sensitive rhythm disorders.
Class III antiarrhythmics block repolarizing potassium currents — chiefly the rapid delayed rectifier IKr carried by the hERG channel — slowing phase 3 and lengthening the total action potential duration (APD). A longer APD means a longer effective refractory period, which can interrupt re-entrant circuits that depend on tissue recovering excitability quickly. The same mechanism, however, carries a well-known and clinically important risk: excessive QT prolongation and the polymorphic ventricular tachycardia known as torsades de pointes.
IKr is the dominant current driving the initial, rapid portion of phase 3 repolarization. When hERG channels are blocked, the outward K⁺ flux that normally terminates the plateau and pulls the membrane back to −90 mV is reduced, so repolarization takes longer. The plateau phase effectively lingers, extending total APD and, on the surface ECG, the QT interval (which approximates ventricular APD summed across the myocardium).
A longer APD directly extends the effective refractory period, since cardiac cells cannot generate a new action potential while still depolarized. This is the intended antiarrhythmic effect: re-entrant circuits that rely on tissue becoming excitable again quickly are disrupted when refractoriness is prolonged uniformly across the affected region.
Unlike Class I sodium blockers, most pure Class III agents show reverse use-dependence: their APD-prolonging effect is actually greater at slow heart rates and diminished at fast heart rates. This is problematic because it means the QT-prolonging (and pro-arrhythmic) effect is most pronounced exactly when the heart is beating slowly — for example during sleep or with bradycardia — rather than being self-limiting during tachycardia the way Class I use-dependence is protective.
Excessive APD prolongation can trigger early afterdepolarizations (EADs) — small secondary depolarizations occurring during phase 2/3 when repolarizing current is insufficient — which, if they reach threshold, can initiate the twisting, polymorphic ventricular tachycardia called torsades de pointes. This is why QTc monitoring is mandatory when initiating most Class III drugs.
Amiodarone is nominally a Class III agent but, unusually, blocks Na⁺, Ca²⁺, and K⁺ channels together and also has weak non-competitive beta-blocking activity — meaning it exhibits properties of all four Vaughan Williams classes simultaneously. This broad-spectrum, "balanced" channel blockade is thought to explain why amiodarone carries a substantially lower torsades risk than pure IKr blockers like dofetilide, despite producing marked QT prolongation on the ECG — its simultaneous Ca²⁺ and late-Na⁺ channel effects appear to suppress the very afterdepolarizations that IKr blockade alone tends to provoke.
Sotalol and dofetilide, both relatively "pure" IKr blockers, require in-hospital initiation with continuous ECG/QTc monitoring in many protocols precisely because their reverse use-dependent QT prolongation and torsades risk are less buffered than amiodarone's multi-channel profile.
The AV node is electrophysiologically distinct from surrounding atrial and ventricular muscle: its upstroke is generated by slow, L-type calcium current (ICaL) rather than the fast Na⁺ current that drives phase 0 elsewhere in the heart. Non-dihydropyridine Class IV calcium channel blockers exploit this difference, selectively slowing conduction through the AV node while having comparatively little direct effect on fast-response tissue such as atrial or ventricular myocardium.
Atrial and ventricular myocytes generate a "fast response" action potential: a steep phase 0 upstroke driven by Nav1.5-mediated INa. Nodal tissue (the SA and AV nodes) instead generates a "slow response" action potential: a shallower, slower upstroke carried predominantly by L-type Ca²⁺ current, because these cells have a relatively depolarized resting potential that keeps most fast Na⁺ channels permanently inactivated.
Because AV nodal conduction depends on ICaL rather than INa, Class IV drugs that block L-type Ca²⁺ channels selectively slow conduction through the AV node — increasing the PR interval and, more importantly during atrial fibrillation or flutter, reducing how many of the rapid atrial impulses are able to pass through to activate the ventricles, thereby controlling ventricular rate.
Not all calcium channel blockers are antiarrhythmics. The non-dihydropyridine subclass (verapamil, diltiazem) has relatively balanced effects on cardiac nodal tissue and vascular smooth muscle, giving it clinically useful cardiac electrophysiological activity. The dihydropyridine subclass (amlodipine, nifedipine, and related "-pine" drugs) is far more selective for vascular smooth muscle Ca²⁺ channels and has minimal effect on AV nodal conduction — these agents are used for hypertension and angina, not as antiarrhythmics, and should not be relied upon for rate control.
Non-dihydropyridine Class IV agents are used for ventricular rate control in atrial fibrillation/flutter and for terminating paroxysmal supraventricular tachycardias that depend on the AV node as part of a re-entrant circuit (AV nodal re-entrant tachycardia). Because they share the effect of slowing AV conduction, combining Class IV agents with intravenous beta-blockers carries a meaningful risk of severe bradycardia or high-degree AV block and is generally avoided.
Class IV agents (and AV-nodal blockers generally) are contraindicated in atrial fibrillation associated with Wolff-Parkinson-White syndrome: by slowing conduction through the normal AV nodal pathway, they can paradoxically favor conduction down an accessory pathway, potentially accelerating the ventricular rate and precipitating ventricular fibrillation.