Antiarrhythmic Drugs

Correct Answer

D — Verapamil and diltiazem

Rationale

Verapamil and diltiazem are the two non-dihydropyridine calcium channel blockers that constitute Vaughan Williams Class Four. They block L-type calcium channels in nodal tissue, slowing conduction through the sinoatrial and atrioventricular nodes. Amlodipine and nifedipine are dihydropyridine calcium channel blockers that act predominantly on vascular smooth muscle and are not used as antiarrhythmics. Adenosine and digoxin are unclassified agents that fall outside the Vaughan Williams system. Metoprolol and atenolol are Class Two beta-adrenergic receptor antagonists.

Correct Answer

B — A1 adenosine receptor

Rationale

Adenosine acts through the A1 adenosine receptor, a G protein-coupled receptor. Activation of A1 receptors in nodal tissue opens inwardly rectifying potassium channels and inhibits calcium current, producing hyperpolarization and slowing — or transiently blocking — conduction through the atrioventricular node. Adenosine does not act on beta-adrenergic receptors, muscarinic receptors, or calcium channels directly. It falls outside the Vaughan Williams classification because its mechanism is distinct from all four standard classes. Theophylline, a competitive adenosine receptor antagonist, blocks the effects of adenosine — which is why theophylline-treated patients may require higher adenosine doses.

Correct Answer

A — Classified outside the Vaughan Williams system; exerts antiarrhythmic effects through a vagotonic mechanism

Rationale

Digoxin is classified outside the Vaughan Williams system. Its antiarrhythmic effect on the atrioventricular node is mediated through vagotonic (parasympathomimetic) enhancement of nodal refractoriness rather than through direct ion channel blockade. It is not classified as Class Two (beta-blockade), Class Four (calcium channel blockade), or Class One (sodium channel blockade), even though it slows atrioventricular nodal conduction by a different mechanism.

Correct Answer

C — Adenosine

Rationale

Adenosine is classified outside the Vaughan Williams system. It acts on A1 adenosine receptors in the atrioventricular node and is the drug of choice for acute termination of atrioventricular nodal reentrant tachycardia and other nodal-dependent supraventricular tachycardias. Lidocaine is a Class One agent used for ventricular arrhythmias. Verapamil is a Class Four agent. Amiodarone is a multi-class agent primarily used for ventricular and atrial arrhythmias requiring sustained therapy.

Correct Answer

D — Verapamil and diltiazem

Rationale

Verapamil and diltiazem are both classified as Vaughan Williams Class Four antiarrhythmic agents — non-dihydropyridine calcium channel blockers that slow conduction through the atrioventricular node. Adenosine and digoxin are classified outside the Vaughan Williams system. Metoprolol and atenolol are Class Two beta-adrenergic receptor antagonists. Amiodarone and dronedarone are Class Three agents with additional multi-class activity.

Correct Answer

B — Adenosine

Rationale

Adenosine is classified outside the Vaughan Williams system and has an ultra-short half-life of less than 10 seconds, resulting from rapid cellular uptake and enzymatic degradation. This pharmacological property makes it uniquely suited for acute termination of supraventricular tachycardias, as its effects are transient and rapidly reversible. Digoxin is also classified outside the Vaughan Williams system but has a half-life of approximately 36 to 48 hours. Magnesium sulfate is used for torsades de pointes and is unclassified. Ibutilide is a Class Three agent with a half-life of several hours.

Correct Answer

A — Activation of A1 adenosine receptors opens inwardly rectifying potassium channels and inhibits calcium current in nodal cells, hyperpolarizing the membrane and slowing conduction

Rationale

Adenosine acts on A1 receptors, which are G protein-coupled receptors linked to inhibitory Gi proteins. Gi activation opens inwardly rectifying potassium channels (IKAdo), allowing potassium efflux and hyperpolarizing the nodal cell membrane. Simultaneously, Gi inhibits adenylyl cyclase, reducing cyclic adenosine monophosphate and decreasing L-type calcium channel activity. Both effects — hyperpolarization and reduced calcium current — slow phase 4 automaticity in the sinoatrial node and slow or block conduction through the atrioventricular node. The atrioventricular node depends on calcium current for depolarization, so calcium current inhibition is particularly effective at the node. This combination interrupts nodal-dependent reentrant circuits (such as atrioventricular nodal reentrant tachycardia), terminating the tachycardia.

Correct Answer

C — Theophylline — which competitively blocks A1 adenosine receptors, reducing adenosine's nodal effect

Rationale

Theophylline (and caffeine) are competitive antagonists at adenosine receptors. By occupying A1 receptors in the atrioventricular node, theophylline prevents adenosine from binding and exerting its hyperpolarizing, conduction-slowing effect. Patients taking theophylline for asthma or chronic obstructive pulmonary disease may therefore require larger doses of adenosine to terminate supraventricular tachycardia. The opposite interaction occurs with dipyridamole (an antiplatelet agent): dipyridamole blocks the cellular nucleoside transporter responsible for adenosine uptake into red blood cells and endothelial cells, prolonging adenosine's half-life and potentiating its effect — sometimes causing profound and prolonged heart block at standard adenosine doses. Dipyridamole is therefore a reason to use reduced adenosine doses or to avoid adenosine altogether.

Correct Answer

D — Potassium competes with digoxin for binding to the sodium-potassium ATPase pump; hypokalemia reduces this competition, increasing digoxin binding and toxicity

Rationale

Digoxin exerts its effects by binding to and inhibiting the sodium-potassium ATPase (sodium pump) on cardiac cell membranes. Potassium ions compete with digoxin for the same binding site on the pump. When serum potassium is low (hypokalemia), there is less potassium available to compete with digoxin, so digoxin binds the pump more effectively — increasing pump inhibition and worsening toxicity at the same plasma digoxin concentration. Digoxin toxicity presents with gastrointestinal symptoms (nausea, vomiting, anorexia), neurological symptoms (fatigue, confusion), characteristic visual disturbances (yellow-green halos, blurred vision), and cardiac manifestations (bradycardia, atrioventricular block, various arrhythmias). Digoxin has a narrow therapeutic index, and hypokalemia — commonly caused by diuretics used in the same heart failure patients taking digoxin — is a frequent precipitant of toxicity.

Correct Answer

B — Digoxin-specific antibody fragments (Fab fragments) to bind and inactivate circulating digoxin

Rationale

Digoxin-specific antibody fragments (Fab fragments, commercially available as Digibind or DigiFab) are the specific antidote for life-threatening digoxin toxicity. They are antibody fragments derived from sheep immunized against digoxin; each fragment binds digoxin with very high affinity, rapidly reducing the free plasma concentration and reversing cardiac toxicity. Indications include hemodynamically compromising bradycardia or arrhythmias, hyperkalemia (which reflects severe sodium pump inhibition), and markedly elevated digoxin levels. Atropine can provide temporary support for bradycardia but is not the definitive treatment. Intravenous calcium is actually contraindicated in digoxin toxicity — it can precipitate lethal ventricular arrhythmias by increasing intracellular calcium in an already calcium-overloaded myocardium. Magnesium is used for torsades de pointes, not specifically for digoxin-induced arrhythmias.

Correct Answer

A — Verapamil's negative inotropic effect and peripheral vasodilation can cause severe hemodynamic deterioration in a patient already compromised by ventricular tachycardia, and it may accelerate the arrhythmia

Rationale

Ventricular tachycardia already reduces cardiac output by shortening diastolic filling time and impairing ventricular contraction coordination. Administering verapamil in this setting adds powerful negative inotropic and vasodilatory effects that can precipitate hemodynamic collapse. Additionally, verapamil has proarrhythmic potential in ventricular tachycardia — it can accelerate the arrhythmia or precipitate degeneration to ventricular fibrillation. Wide-complex tachycardia should be treated as ventricular tachycardia until definitively proven otherwise, and verapamil should never be given empirically in this setting. The appropriate agents for stable ventricular tachycardia are procainamide (first-line) or amiodarone. Unstable ventricular tachycardia requires immediate electrical cardioversion.

Correct Answer

C — Verapamil blocks the atrioventricular node without affecting the accessory pathway, directing all rapid atrial impulses through the bypass tract and risking ventricular fibrillation

Rationale

Accessory pathways in Wolff-Parkinson-White syndrome are composed of fast-conducting myocardial fibers, not nodal tissue. They do not depend on calcium current for conduction — unlike the atrioventricular node — so calcium channel blockers such as verapamil have no effect on accessory pathway conduction. When verapamil is given during pre-excited atrial fibrillation, it blocks the atrioventricular node (reducing or eliminating this normal conduction route) while leaving the accessory pathway fully functional. All the rapid, chaotic atrial impulses — which in atrial fibrillation can fire at 400 to 600 beats per minute — are then funneled exclusively through the unblocked accessory pathway into the ventricles. This can produce extremely rapid ventricular rates and degenerate into ventricular fibrillation. The same danger applies to adenosine, digoxin, and beta-blockers in pre-excited atrial fibrillation. Correct treatment is procainamide (which slows accessory pathway conduction) or immediate cardioversion.

Correct Answer

D — Magnesium suppresses early afterdepolarizations — the triggered depolarizations during the prolonged action potential that initiate torsades de pointes — by reducing inward calcium and sodium currents

Rationale

Torsades de pointes is initiated by early afterdepolarizations — abnormal depolarizations that arise during the prolonged phase 2 or phase 3 of the action potential in QT-prolonged states. These early afterdepolarizations are driven by inward calcium current (through L-type channels and the sodium-calcium exchanger) and by the late inward sodium current that becomes prominent during prolonged repolarization. Magnesium acts as a physiological calcium antagonist and blocks these inward currents, suppressing the early afterdepolarizations that trigger torsades de pointes without requiring the patient to have hypomagnesemia. Magnesium does not directly shorten the QT interval or block sodium channels in the conventional sense. It does not replace the need to identify and correct the underlying cause (withdraw the offending drug, correct hypokalemia, increase heart rate).

Correct Answer

B — Digoxin's rate control depends on enhanced vagal tone, which is overridden by sympathetic activation during exercise or emotional stress, allowing the ventricular rate to escape control

Rationale

Digoxin slows the ventricular rate in atrial fibrillation indirectly by enhancing vagal tone, which increases atrioventricular nodal refractoriness. However, vagal tone is substantially reduced during physical activity, emotional stress, or illness — states when sympathetic activation dominates. During these states, digoxin loses much of its rate-control efficacy, and the ventricular rate can rise to poorly controlled levels. In contrast, beta-blockers and non-dihydropyridine calcium channel blockers (verapamil and diltiazem) directly block the atrioventricular node through mechanisms independent of vagal tone, maintaining rate control during activity. Digoxin has acceptable bioavailability (70 to 80 percent) via the oral route and is renally — not hepatically — cleared. Its role in rate control is primarily in sedentary patients or in combination with other rate control agents.

Correct Answer

C — The arrhythmia depended on the atrioventricular node as a critical part of the reentrant circuit; adenosine terminated it by transiently blocking nodal conduction

Rationale

Adenosine terminates arrhythmias that require the atrioventricular node for their perpetuation — specifically atrioventricular nodal reentrant tachycardia and atrioventricular reentrant tachycardia using an accessory pathway. By transiently blocking atrioventricular nodal conduction (via A1 receptor-mediated hyperpolarization), adenosine interrupts the reentrant circuit and terminates the tachycardia. Arrhythmias that do not depend on the atrioventricular node — such as atrial tachycardia or atrial flutter — may slow transiently when adenosine blocks the node (revealing the underlying atrial activity), but they are not terminated. Complete termination by adenosine is diagnostic of atrioventricular node-dependent reentry. This patient's presentation — young woman, sudden onset, narrow complex, no visible P waves — is classic for atrioventricular nodal reentrant tachycardia, the most common paroxysmal supraventricular tachycardia.

Correct Answer

A — If the rhythm is ventricular tachycardia — which accounts for the majority of wide-complex tachycardias — verapamil's negative inotropic effect and vasodilation can precipitate hemodynamic collapse and may accelerate the arrhythmia

Rationale

Wide-complex tachycardia must be treated as ventricular tachycardia until definitively proven otherwise — approximately 80 percent of wide-complex tachycardias in patients with structural heart disease are ventricular tachycardia. Administering verapamil under this assumption is dangerous because its potent negative inotropic and vasodilatory effects can cause rapid hemodynamic deterioration in a patient whose cardiac output is already reduced by ventricular tachycardia. Verapamil can also accelerate ventricular tachycardia or cause degeneration to ventricular fibrillation. Even if the rhythm turned out to be a supraventricular tachycardia with aberrant conduction, the consequences of that error — giving verapamil to a ventricular tachycardia patient — are unacceptable. Procainamide is safe in both ventricular tachycardia and wide-complex supraventricular tachycardia, making it the preferred choice when the diagnosis is uncertain.

Correct Answer

D — Potassium normally competes with digoxin for binding to the sodium-potassium ATPase pump; hypokalemia reduces this competition, allowing digoxin to bind the pump more effectively and intensifying its toxic effects

Rationale

Digoxin inhibits the sodium-potassium ATPase pump by binding to its extracellular potassium-binding site. Potassium ions are natural competitors for this same binding site — when extracellular potassium is normal, some pump sites are occupied by potassium rather than digoxin, limiting the degree of pump inhibition. When potassium falls (as commonly occurs with loop diuretic use), the competitive pressure is reduced, and digoxin binds a greater fraction of available pump sites at the same plasma concentration. The result is increased pump inhibition, rising intracellular sodium, increased sodium-calcium exchange, and elevated intracellular calcium — producing the bradycardia, atrioventricular block, and triggered arrhythmias characteristic of digoxin toxicity. This patient's digoxin level was within the therapeutic range, but the hypokalemia from furosemide-induced potassium wasting converted a safe digoxin level into a toxic one — a classic and clinically important interaction.

Correct Answer

B — Magnesium suppresses early afterdepolarizations — the triggered depolarizations during the prolonged action potential that initiate torsades de pointes — by reducing inward calcium and sodium currents, independent of serum magnesium level

Rationale

Torsades de pointes is not a re-entry arrhythmia but rather a triggered arrhythmia initiated by early afterdepolarizations arising during the prolonged action potential in QT-prolonged states. These early afterdepolarizations are driven by inward calcium current (through L-type channels and reverse-mode sodium-calcium exchange) and by the late inward sodium current. Magnesium acts as a physiological antagonist of these inward currents, reducing the magnitude of early afterdepolarizations and preventing them from reaching the threshold for a triggered action potential. This mechanism is not dependent on correcting magnesium deficiency — it is a pharmacodynamic effect that operates even when serum magnesium is normal. Intravenous magnesium is therefore the first-line treatment for torsades de pointes regardless of serum level, while the definitive management includes withdrawing sotalol, correcting hypokalemia (targeting potassium above 4.5 mEq/L), and increasing heart rate (which shortens the action potential and reduces early afterdepolarization risk).