Antiarrhythmics

Every heartbeat starts as an electrical impulse. In each cardiac cell, ions flow across the membrane in a set sequence that makes up the action potential: sodium (Na⁺) rushes in to fire the impulse (depolarization), calcium (Ca²⁺) sustains it and drives contraction, and potassium (K⁺) flows out to reset the cell (repolarization). The pacemaker (SA node) sets the rate; the AV node gates how fast impulses pass to the ventricles.

An arrhythmia is this orderly sequence going wrong — the heart beating too fast, too slow, or chaotically (as in atrial fibrillation), often from an abnormal focus firing or an impulse looping back on itself (re-entry). Antiarrhythmics work by adjusting the very ion channels that shape the action potential.

Antiarrhythmics are grouped by which ion channel they block — the Vaughan-Williams classification. Class I blocks sodium channels (slowing the initial upstroke; e.g. flecainide, lidocaine). Class II are the beta-blockers, which slow the SA/AV nodes via the sympathetic system. Class III blocks potassium channels, prolonging repolarization (e.g. sotalol, and amiodarone). Class IV are the non-dihydropyridine calcium channel blockers (diltiazem, verapamil), slowing AV conduction.

Amiodarone is the exception that proves the rule: it shows properties of all four classes at once. That broad action makes it highly effective — and, as the side effects show, uniquely toxic. Adenosine sits outside the classification: it briefly blocks the AV node to stop a re-entrant SVT.

Antiarrhythmics carry a built-in paradox: a drug that alters the heart’s electrical timing to *fix* one arrhythmia can create another. This is proarrhythmia. Class III agents (and others) prolong the QT interval, which can trigger a dangerous rhythm called torsades de pointes — so the QT is watched closely.

Amiodarone’s all-class potency comes at a cost seen nowhere else: because it is fat-soluble and stored in tissues with an extremely long half-life (weeks to months), it accumulates in the lungs (fibrosis), liver, thyroid (it is iodine-rich → hypo- or hyperthyroidism), eyes, and skin (blue-gray discoloration, photosensitivity). These slow, cumulative toxicities are the reason for its boxed warning and its intensive monitoring schedule.