Antiarrhythmic Drugs

Correct Answer

C — Multi-class agent with Class One, Two, Three, and Four activity

Rationale

Amiodarone is a multi-class antiarrhythmic with activity across all four Vaughan Williams classes: sodium channel blockade (Class One), beta-adrenergic receptor antagonism (Class Two), potassium channel blockade (Class Three), and calcium channel blockade (Class Four). Its dominant clinical effect is Class Three — prolongation of action potential duration and the effective refractory period — but the multi-class profile contributes to its broad antiarrhythmic efficacy and its extensive adverse effect profile. Dofetilide and ibutilide are pure Class Three agents. Amiodarone is not unclassified; it is formally assigned to Class Three as its primary classification while being recognized as multi-class.

Correct Answer

A — Class Two (beta-adrenergic receptor antagonist) and Class Three (potassium channel blocker)

Rationale

Sotalol combines Class Two beta-adrenergic receptor antagonism with Class Three potassium channel blockade. The beta-blocking component slows heart rate and reduces automaticity; the potassium channel blocking component prolongs action potential duration and the effective refractory period, increasing the QT interval. This dual activity makes sotalol useful for both rate and rhythm control, but it also means that QT prolongation is a prominent risk — particularly in patients with renal impairment, where the drug accumulates. Dofetilide and ibutilide are pure Class Three agents without beta-blocking activity. Amiodarone has Class Two activity among others but is not classified primarily as a dual Class Two/Three agent.

Correct Answer

D — Dofetilide

Rationale

Dofetilide is a pure Class Three potassium channel blocker. It selectively blocks the rapid delayed rectifier potassium current with no significant activity at sodium channels, beta-adrenergic receptors, or calcium channels. Amiodarone has Class One, Two, Three, and Four activity. Sotalol has both Class Two (beta-blocking) and Class Three (potassium channel blocking) activity. Dronedarone has multi-class activity similar to amiodarone but without the iodine moiety.

Correct Answer

B — Ibutilide

Rationale

Ibutilide is a Class Three potassium channel blocker available only for intravenous administration. It is used for acute pharmacological cardioversion of recent-onset atrial fibrillation and atrial flutter. It is particularly effective for atrial flutter conversion — more so than for atrial fibrillation. Ibutilide carries the highest risk of torsades de pointes (a dangerous polymorphic ventricular arrhythmia) among the Class Three agents, and patients must be monitored for at least four hours after administration. Dofetilide is oral and used for chronic rhythm control. Dronedarone and sotalol are also oral agents used for chronic rhythm control.

Correct Answer

C — Dronedarone

Rationale

Dronedarone is structurally related to amiodarone but was specifically designed without the iodine moiety, eliminating the thyroid and pulmonary toxicities associated with amiodarone's high iodine content. Ibutilide is an intravenous pure Class Three agent used for acute cardioversion. Dofetilide is a pure potassium channel blocker unrelated in structure to amiodarone. Sotalol combines Class Two and Class Three activity and is structurally distinct from amiodarone.

Correct Answer

A — Ibutilide

Rationale

Ibutilide carries the highest risk of torsades de pointes of any Class Three antiarrhythmic agent, with rates of approximately 4 to 8 percent in clinical use. For this reason, continuous electrocardiographic monitoring and resuscitation equipment must be immediately available during and for several hours after ibutilide infusion. Amiodarone, despite its potent QT-prolonging effects, has a paradoxically low torsades de pointes risk — likely due to its additional sodium and calcium channel blocking activities that suppress the triggered activity responsible for torsades de pointes. Sotalol has moderate torsades de pointes risk. Dronedarone has lower torsades de pointes risk than sotalol or ibutilide.

Correct Answer

D — Amiodarone-induced pulmonary toxicity — discontinue amiodarone; corticosteroids may be used in severe cases

Rationale

Amiodarone-induced pulmonary toxicity is one of the most serious adverse effects of long-term amiodarone therapy, occurring in 1 to 5 percent of patients per year. It presents as an interstitial pneumonitis with progressive dyspnea, dry cough, and bilateral infiltrates on imaging. Reduced diffusing capacity on pulmonary function testing is a sensitive early finding. The mechanism involves direct phospholipid accumulation in lung tissue and an immune-mediated inflammatory response. Management requires discontinuation of amiodarone; corticosteroids are added in severe or rapidly progressive cases. Because amiodarone has an extremely long half-life (40 to 55 days), pulmonary toxicity can persist or worsen for weeks after the drug is stopped. Annual chest imaging and pulmonary function monitoring are recommended during therapy.

Correct Answer

B — Amiodarone contains approximately 37 percent iodine by weight; excess iodine can suppress thyroid hormone synthesis (causing hypothyroidism) or trigger autonomous hormone production in susceptible glands (causing hyperthyroidism)

Rationale

Amiodarone is approximately 37 percent iodine by weight, and each 200 mg tablet releases far more iodine than the daily dietary requirement. This iodine excess produces two opposing effects depending on thyroid physiology. In most patients, high iodine concentrations transiently suppress thyroid hormone synthesis (the Wolff-Chaikoff effect), causing hypothyroidism — the more common of the two toxicities. In patients with pre-existing nodular or autonomous thyroid tissue (common in iodine-deficient regions), excess iodine provides substrate for unregulated hormone production, causing hyperthyroidism. Amiodarone also inhibits peripheral conversion of thyroxine to triiodothyronine, which contributes to the hypothyroid state and can also raise thyroid-stimulating hormone levels. Thyroid function must be monitored every 6 months during amiodarone therapy.

Correct Answer

A — Adverse effects such as pulmonary or thyroid toxicity can persist or worsen for weeks to months after amiodarone is discontinued

Rationale

Amiodarone's extraordinary half-life of 40 to 55 days means that the drug and its active metabolite desethylamiodarone persist in tissues long after the drug is stopped. This has two major clinical implications. First, when amiodarone is discontinued because of toxicity — such as pulmonary toxicity or thyroid dysfunction — the adverse effects can continue or even worsen for weeks to months because tissue drug concentrations fall slowly. Second, achieving steady-state plasma concentrations takes months, which is why loading doses are required at initiation. Amiodarone is dosed once daily (not twice daily) because of its long half-life. Drug interactions with warfarin, digoxin, and statins persist long after amiodarone is stopped, requiring continued dose monitoring of interacting drugs during the washout period.

Correct Answer

C — Sotalol is renally eliminated without hepatic metabolism; impaired clearance causes drug accumulation, excessive QT prolongation, and increased risk of torsades de pointes

Rationale

Sotalol is excreted unchanged by the kidneys with negligible hepatic metabolism. When creatinine clearance is below 40 milliliters per minute, renal clearance of sotalol is insufficient to prevent drug accumulation. Rising plasma concentrations produce increasingly marked potassium channel blockade, excessively prolonging the QT interval and substantially raising the risk of torsades de pointes (a dangerous polymorphic ventricular arrhythmia triggered by QT prolongation). Sotalol is therefore contraindicated at a creatinine clearance below 40 milliliters per minute. It also requires in-hospital initiation with continuous electrocardiographic monitoring for at least three days to detect early QT prolongation. Dofetilide similarly requires renal dose adjustment for the same pharmacokinetic reason.

Correct Answer

D — Verapamil, cimetidine, and trimethoprim

Rationale

Verapamil, cimetidine, and trimethoprim are absolutely contraindicated with dofetilide through distinct mechanisms. Verapamil raises dofetilide plasma levels by inhibiting its renal tubular secretion, and verapamil itself prolongs the QT interval — the combined effect markedly increases torsades de pointes risk. Cimetidine (an H2 receptor antagonist) and trimethoprim (an antibiotic) both inhibit the renal cation transport system responsible for dofetilide elimination, causing drug accumulation and excessive QT prolongation. Because dofetilide's therapeutic window is narrow and QT prolongation is directly dose-dependent, any agent that raises plasma dofetilide concentrations is potentially lethal. Warfarin, digoxin, and statins interact with amiodarone rather than dofetilide. Loop diuretics are a concern with dofetilide only indirectly — by causing hypokalemia, which potentiates QT prolongation.

Correct Answer

B — Reduce the warfarin dose by approximately one-third and halve the digoxin dose, because amiodarone inhibits warfarin metabolism and reduces renal and non-renal digoxin clearance

Rationale

Amiodarone inhibits the cytochrome P450 enzyme CYP2C9, which is responsible for hepatic metabolism of the S-enantiomer of warfarin (the more potent anticoagulant enantiomer). This inhibition reduces warfarin clearance and markedly elevates the international normalized ratio (a measure of anticoagulant effect). The warfarin dose typically requires reduction by approximately one-third, and the international normalized ratio must be monitored closely during the weeks of amiodarone initiation and again during its slow washout after discontinuation. For digoxin, amiodarone reduces both renal tubular secretion and non-renal clearance of digoxin, raising plasma digoxin levels into the toxic range. The digoxin dose should be halved and plasma levels monitored. Both interactions persist long after amiodarone is stopped, given its 40 to 55 day half-life.

Correct Answer

A — Clinical trials demonstrated increased mortality with dronedarone in patients with heart failure with reduced ejection fraction and in those with permanent atrial fibrillation

Rationale

Dronedarone's contraindications in heart failure with reduced ejection fraction and permanent atrial fibrillation are based directly on clinical trial evidence. The ANDROMEDA trial was stopped early when dronedarone increased mortality in patients with recent decompensated heart failure with reduced ejection fraction. The PALLAS trial similarly found increased rates of stroke, heart failure hospitalization, and cardiovascular death in patients with permanent atrial fibrillation. The mechanisms are not fully established, but dronedarone is thought to have proarrhythmic and potentially negative inotropic effects in these vulnerable populations that outweigh its antiarrhythmic benefit. Dronedarone is reserved for patients with paroxysmal or persistent atrial fibrillation and preserved or mildly reduced ventricular function. Amiodarone and dofetilide are the preferred rhythm control agents when heart failure with reduced ejection fraction is present.

Correct Answer

C — Amiodarone's negative inotropic effect is mild at standard doses and is clinically offset by its vasodilatory and rate-reducing effects, making it hemodynamically well tolerated in heart failure with reduced ejection fraction

Rationale

Although amiodarone blocks calcium channels, its negative inotropic effect is relatively weak at the doses used clinically. This contrasts with verapamil and diltiazem, which produce more pronounced negative inotropy at therapeutic doses. Additionally, amiodarone has peripheral vasodilatory properties (partly mediated by its alpha-adrenergic blocking activity) that reduce afterload, and its rate-reducing effects lower myocardial oxygen demand. Together, these effects are hemodynamically neutral or even favorable in the setting of heart failure with reduced ejection fraction. Multiple clinical trials have demonstrated that amiodarone does not increase mortality in heart failure with reduced ejection fraction, unlike dronedarone. Amiodarone is therefore the preferred rhythm control and antiarrhythmic agent when heart failure with reduced ejection fraction is present.

Correct Answer

D — Amiodarone and its metabolites accumulate in lung tissue, causing phospholipid accumulation in macrophages and an inflammatory interstitial pneumonitis

Rationale

Amiodarone-induced pulmonary toxicity results from the drug's high lipophilicity and its ability to accumulate in lung macrophages and type II pneumocytes. Amiodarone inhibits lysosomal phospholipase, causing phospholipid accumulation within cells (a process called phospholipidosis), combined with a direct immune-mediated inflammatory response that produces interstitial pneumonitis. The presentation — insidious dyspnea, bilateral infiltrates, and reduced diffusing capacity in the absence of infection — is classic for this toxicity. Management requires discontinuation of amiodarone; corticosteroids are added in severe cases. Because of amiodarone's 40 to 55 day half-life, drug levels and toxicity can persist for months after stopping the drug. Annual chest imaging and pulmonary function testing are used for surveillance during therapy.

Correct Answer

B — Amiodarone inhibits CYP2C9, the hepatic enzyme responsible for metabolism of the active S-enantiomer of warfarin, raising plasma warfarin levels

Rationale

Warfarin is administered as a racemic mixture of R- and S-enantiomers. The S-enantiomer is approximately four times more potent as an anticoagulant and is metabolized by the hepatic cytochrome P450 enzyme CYP2C9. Amiodarone is a potent inhibitor of CYP2C9 and reduces the clearance of S-warfarin, causing plasma warfarin concentrations — and anticoagulant effect — to rise substantially. A warfarin dose reduction of approximately one-third is typically required when amiodarone is added, and the international normalized ratio must be monitored closely during the weeks of amiodarone initiation. This interaction persists for months after amiodarone is discontinued because of its extremely long half-life. Failure to anticipate this interaction is a well-recognized cause of dangerous over-anticoagulation.

Correct Answer

A — Amiodarone — because dronedarone is contraindicated in heart failure with reduced ejection fraction due to increased mortality demonstrated in clinical trials

Rationale

Dronedarone is absolutely contraindicated in heart failure with reduced ejection fraction. The ANDROMEDA trial demonstrated increased mortality with dronedarone in patients with recent decompensated heart failure, and subsequent data confirmed unacceptable risk in the heart failure with reduced ejection fraction population. Amiodarone, by contrast, has been shown in multiple trials to be hemodynamically well tolerated in heart failure with reduced ejection fraction, with no increase in mortality. Although amiodarone carries substantial long-term toxicity risks (pulmonary, thyroid, hepatic, corneal, cutaneous), these are managed with surveillance monitoring and are outweighed by the need for rhythm control in a patient with symptomatic atrial fibrillation and reduced ejection fraction. Dronedarone's reduced toxicity profile is irrelevant if it cannot be safely used in this patient's substrate. Dofetilide is an alternative Class Three option safe in heart failure with reduced ejection fraction.

Correct Answer

C — Dofetilide prolongs the QT interval in a dose-dependent manner, and the degree of QT prolongation — which determines torsades de pointes risk — must be assessed after each dose titration before the patient is discharged

Rationale

Dofetilide's risk of torsades de pointes (a dangerous polymorphic ventricular arrhythmia triggered by QT prolongation) is directly proportional to its plasma concentration and the resulting degree of QT prolongation. Because the dose must be individualized based on creatinine clearance — this patient's dose will be adjusted using the four-tier renal dosing system — and because the actual QT response to the selected dose cannot be predicted precisely in advance, in-hospital initiation with continuous electrocardiographic monitoring for at least three days is mandated. If the QT interval prolongs beyond a threshold (greater than 500 milliseconds, or greater than 15 percent above baseline), the dose must be reduced or the drug discontinued. This in-hospital monitoring requirement applies to all patients started on dofetilide, regardless of renal function, and is a regulatory requirement in the United States. Dofetilide has no negative inotropic effect and does not require intravenous loading.