Antiarrhythmic Drugs

Correct Answer

A — Sotalol

Rationale

Sotalol is the antiarrhythmic agent most commonly associated with torsades de pointes as a class effect. Its Class Three potassium channel blocking activity prolongs the QT interval in a dose-dependent manner, and its torsades de pointes risk is the highest among the commonly used antiarrhythmic agents. Lidocaine (Class Ib) shortens rather than prolongs action potential duration. Metoprolol (Class Two) does not prolong the QT interval in a clinically meaningful way. Flecainide (Class Ic) prolongs the PR and QRS intervals but does not substantially prolong the QT interval.

Correct Answer

C — Fluoroquinolone antibiotics, macrolide antibiotics, and antipsychotic medications

Rationale

Many non-antiarrhythmic drugs prolong the QT interval by blocking cardiac potassium channels — the same mechanism as Class Three antiarrhythmic agents. Key classes include fluoroquinolone antibiotics (such as ciprofloxacin and levofloxacin), macrolide antibiotics (such as azithromycin and clarithromycin), azole antifungals (such as fluconazole), antipsychotic medications (such as haloperidol and ziprasidone), and antiemetic agents (such as ondansetron and methadone). When these drugs are combined with antiarrhythmic agents that also prolong the QT interval, or with each other, the risk of torsades de pointes is additive. The other options list drugs that do not primarily prolong the QT interval — antihypertensives, statins, and diuretics affect other aspects of cardiac physiology but are not QT-prolonging agents.

Correct Answer

B — Magnesium sulfate

Rationale

Magnesium sulfate is classified outside the Vaughan Williams system and is the first-line pharmacological treatment for acute torsades de pointes, regardless of the patient's serum magnesium level. It suppresses the early afterdepolarizations that initiate torsades de pointes by reducing inward calcium and sodium currents. Amiodarone is a multi-class agent used for other arrhythmias but is not first-line for torsades de pointes. Lidocaine and procainamide are sodium channel blockers used for ventricular tachycardia but are not first-line for torsades de pointes.

Correct Answer

D — Amiodarone

Rationale

Amiodarone has an elimination half-life of 40 to 55 days — among the longest of any drug in routine clinical use. This extraordinary half-life results from extensive tissue distribution and accumulation, particularly in fat, lung, liver, and thyroid. Clinical consequences include a delayed onset of full effect despite loading doses, persistence of adverse effects for weeks to months after discontinuation, and prolonged drug interactions after the drug is stopped. Sotalol, dofetilide, and lidocaine all have substantially shorter half-lives.

Correct Answer

A — Sotalol

Rationale

Sotalol is predominantly renally eliminated and requires dose adjustment based on creatinine clearance. Accumulation in renal impairment causes excessive QT prolongation and substantially increases the risk of torsades de pointes. Amiodarone is hepatically metabolized and does not require renal dose adjustment. Lidocaine is hepatically metabolized. Propafenone is also hepatically metabolized with minimal renal excretion of the active drug.

Correct Answer

C — Sotalol

Rationale

Sotalol is classified as a non-selective beta-adrenergic receptor antagonist (Class Two) with additional potassium channel blocking activity (Class Three). This dual classification distinguishes it from pure beta-blockers such as metoprolol, bisoprolol, and atenolol, which are cardioselective Class Two agents without Class Three activity. Sotalol's potassium channel blockade prolongs the QT interval and the effective refractory period, making it effective for both rate and rhythm control but also conferring significant torsades de pointes risk.

Correct Answer

D — Female sex, hypokalemia, bradycardia, and a QT interval above 500 milliseconds

Rationale

Torsades de pointes risk is greatest when multiple factors converge. Female sex confers intrinsically longer baseline QT intervals and greater sensitivity to QT-prolonging drugs. Hypokalemia reduces competing potassium efflux during repolarization, exaggerating drug-induced QT prolongation. Bradycardia allows more time for the late inward currents that drive early afterdepolarizations to accumulate — torsades de pointes is a bradycardia-dependent arrhythmia, which explains why increasing heart rate is a treatment strategy. A corrected QT interval above 500 milliseconds is a widely used threshold associated with substantially increased risk. Torsades de pointes can and does occur at therapeutic drug concentrations when these risk factors are present — it is not confined to toxic drug levels. Combining multiple QT-prolonging drugs amplifies risk additively or synergistically.

Correct Answer

B — Halve the digoxin dose because amiodarone reduces both renal tubular secretion and non-renal clearance of digoxin, raising plasma digoxin levels into the toxic range

Rationale

Amiodarone reduces digoxin clearance through two routes: it inhibits P-glycoprotein-mediated renal tubular secretion of digoxin (reducing renal elimination) and also reduces non-renal clearance pathways. Both mechanisms cause digoxin to accumulate in plasma. Since digoxin has a narrow therapeutic index, the resulting rise in plasma concentration can quickly produce toxicity — manifesting as nausea, visual disturbances, bradycardia, and atrioventricular block. The standard recommendation is to halve the digoxin dose when amiodarone is initiated, then monitor digoxin levels and clinical status closely. This interaction persists for months after amiodarone is stopped because of amiodarone's extremely long half-life. The same principle applies when other P-glycoprotein inhibitors are added to digoxin regimens.

Correct Answer

A — Cimetidine inhibits the renal cation transport system responsible for dofetilide elimination, causing drug accumulation and excessive QT prolongation

Rationale

Dofetilide is eliminated primarily through renal tubular secretion via the organic cation transporter system. Cimetidine inhibits this transport system, blocking dofetilide's elimination and causing plasma concentrations to rise substantially. Since dofetilide prolongs the QT interval in a directly dose-dependent manner, accumulation produces excessive QT prolongation and a markedly elevated risk of torsades de pointes (a dangerous polymorphic ventricular arrhythmia). Trimethoprim (an antibiotic) causes the same interaction through the same transport pathway and is equally contraindicated. Verapamil's contraindication with dofetilide involves a different mechanism — verapamil both inhibits renal tubular secretion of dofetilide and prolongs the QT interval itself, making the combination doubly dangerous. Cimetidine does not directly block cardiac potassium channels and is not primarily a CYP3A4 inhibitor in this context.

Correct Answer

C — Furosemide causes hypokalemia, which reduces competition for potassium channel binding and amplifies sotalol's QT-prolonging effect, increasing torsades de pointes risk

Rationale

Loop diuretics such as furosemide cause urinary potassium wasting, producing hypokalemia. Low extracellular potassium reduces the outward potassium current during cardiac repolarization and also reduces the competition between potassium ions and sotalol at the delayed rectifier potassium channel — allowing sotalol to produce greater channel blockade at the same plasma concentration. The result is more pronounced QT prolongation and an increased risk of torsades de pointes. Electrolyte monitoring — particularly potassium and magnesium — is mandatory in patients taking sotalol (or any QT-prolonging antiarrhythmic) who are also on diuretics. Potassium should be maintained above 4.0 mEq/L, and ideally above 4.5 mEq/L, in these patients. Furosemide does not directly block potassium channels or reduce sotalol clearance through tubular competition.

Correct Answer

D — Amiodarone — because it is hepatically metabolized and does not depend on renal clearance, making it safe to use at standard doses in renal impairment

Rationale

Amiodarone is extensively metabolized by the liver and excreted in bile; renal clearance plays no meaningful role in its elimination. It can therefore be used at standard doses regardless of renal function — making it a preferred antiarrhythmic option in patients with significant renal impairment who require rhythm or rate control. Lidocaine and mexiletine are similarly hepatically metabolized and safe in renal impairment. By contrast, sotalol is renally cleared and contraindicated at creatinine clearance below 40 milliliters per minute. Dofetilide requires a four-tier renal dose reduction based on creatinine clearance. Digoxin accumulates in renal impairment due to reduced renal elimination — its narrow therapeutic index means that even modest accumulation can produce toxicity, requiring dose reduction and careful monitoring in this patient.

Correct Answer

B — Metoprolol — a beta-blocker that is considered relatively safe in pregnancy for rate control, though fetal heart rate monitoring is recommended

Rationale

Beta-blockers, particularly metoprolol, are considered relatively safe for rate control in pregnancy and are widely used when the maternal benefit outweighs the fetal risk. Beta-blockers do cross the placenta and can cause fetal bradycardia, hypoglycemia, and intrauterine growth restriction — effects that warrant monitoring but do not preclude use when rate control is necessary. Digoxin is also considered relatively safe in pregnancy and is an alternative for rate control. Amiodarone is avoided in pregnancy because it causes neonatal hypothyroidism, bradycardia, and QT prolongation due to its high iodine content and placental transfer. Sotalol carries QT prolongation risk and is not a preferred agent in pregnancy. Verapamil does cross the placenta and can cause fetal bradycardia and heart block, making it a less preferred option.

Correct Answer

A — Age-related decline in renal function reduces drug clearance; standard doses produce higher plasma levels in elderly patients, increasing the risk of toxicity and QT prolongation

Rationale

Renal function declines with age — a phenomenon that is not always apparent from serum creatinine alone because muscle mass also decreases with age, reducing creatinine production. Formal calculation of creatinine clearance (using formulas such as the Cockcroft-Gault equation) is therefore required before initiating renally cleared antiarrhythmics in elderly patients. Reduced clearance of drugs such as sotalol, dofetilide, digoxin, and procainamide causes accumulation at standard doses, producing higher plasma concentrations than intended. For QT-prolonging agents this means greater QT prolongation and higher torsades de pointes risk; for digoxin it means toxicity at doses that would be safe in younger patients. Starting at half the standard dose and titrating slowly, monitoring the QT interval, and targeting a lower digoxin plasma level (typically below 0.8 nanograms per milliliter rather than the standard upper limit of 2.0) are the key adjustments in elderly patients.

Correct Answer

C — Amiodarone inhibits CYP3A4, reducing simvastatin metabolism and raising statin plasma levels, increasing the risk of statin-induced myopathy

Rationale

Amiodarone is a broad inhibitor of cytochrome P450 enzymes, including CYP3A4. Simvastatin (and lovastatin) are metabolized primarily by CYP3A4; when amiodarone inhibits this enzyme, statin clearance falls and plasma statin concentrations rise. Elevated statin levels increase the risk of myopathy — muscle injury ranging from mild myalgia to severe rhabdomyolysis with kidney injury. The practical response is to switch the patient to a statin with less CYP3A4 dependence (such as rosuvastatin or pravastatin) or to cap the simvastatin dose at a lower level. This interaction is a practical consequence of amiodarone's multi-enzyme inhibition profile and is a clinically encountered problem given how frequently patients with atrial fibrillation are also taking statins for cardiovascular risk reduction. Statins do not prolong the QT interval.

Correct Answer

D — Furosemide caused hypokalemia, which amplified sotalol's potassium channel blocking effect and produced excessive QT prolongation, triggering torsades de pointes

Rationale

Furosemide causes urinary potassium wasting, producing hypokalemia. When extracellular potassium falls, there is less potassium available to flow outward through delayed rectifier potassium channels during repolarization — both because the concentration gradient is reduced and because potassium normally competes with drug binding at the channel. The result is that sotalol produces greater channel blockade and more QT prolongation at the same plasma concentration. This converted a previously tolerated sotalol dose into a proarrhythmic combination. Torsades de pointes (a dangerous polymorphic ventricular arrhythmia caused by QT prolongation) followed. This is a clinically common and preventable interaction: any patient taking a QT-prolonging antiarrhythmic who is started on a diuretic requires close electrolyte monitoring and potassium replacement to maintain levels above 4.0 to 4.5 mEq/L. Furosemide does not directly block potassium channels or inhibit sotalol's tubular secretion.

Correct Answer

B — Trimethoprim inhibits the renal cation transporter responsible for dofetilide elimination, causing dofetilide to accumulate and producing dangerous QT prolongation

Rationale

Dofetilide is eliminated primarily through active renal tubular secretion via the organic cation transporter system. Trimethoprim inhibits this transporter, blocking dofetilide's elimination and causing plasma concentrations to rise substantially. Since dofetilide prolongs the QT interval in a directly dose-dependent manner, accumulation produces excessive QT prolongation and a markedly elevated risk of torsades de pointes (a dangerous polymorphic ventricular arrhythmia). This interaction is especially dangerous in this patient because his existing renal impairment has already reduced baseline dofetilide clearance — adding trimethoprim's transporter inhibition compounds the effect. Cimetidine causes the same interaction through the same renal transport pathway. Dofetilide is not hepatically metabolized through CYP2C9, and trimethoprim's effect on serum creatinine (which it can raise without affecting true glomerular filtration rate) is a separate consideration that would not directly cause dofetilide accumulation through the mechanism described here.

Correct Answer

C — Adenosine — because its ultra-short half-life of less than 10 seconds means it is degraded before reaching the fetus in significant concentrations

Rationale

Adenosine is the first-line agent for acute termination of supraventricular tachycardia during pregnancy. Its half-life of less than 10 seconds — due to rapid uptake by red blood cells and endothelial cells and degradation by adenosine deaminase — means the drug is cleared from maternal blood before it can reach the placenta and fetal circulation in meaningful amounts. This pharmacokinetic property makes it the antiarrhythmic with the most favorable safety profile for acute use in pregnancy. Adenosine terminates atrioventricular node-dependent re-entry circuits (the mechanism of most supraventricular tachycardias) by transiently blocking nodal conduction. Amiodarone is avoided in pregnancy due to neonatal thyroid and cardiac effects. Verapamil can cause fetal bradycardia and heart block. Sotalol carries QT prolongation risk and is not preferred for acute supraventricular tachycardia termination in any population.

Correct Answer

A — Elderly patients have reduced muscle mass and produce less creatinine, so serum creatinine appears normal even when renal clearance is substantially reduced; calculated creatinine clearance reveals the true reduction in sotalol elimination capacity

Rationale

Serum creatinine is a product of muscle metabolism. In elderly patients, age-related loss of muscle mass (sarcopenia) reduces creatinine production, so serum creatinine values remain within the normal laboratory range even when the kidneys are filtering at a substantially reduced rate. A serum creatinine of 1.4 mg/dL in an 82-year-old man with low muscle mass may correspond to a creatinine clearance of only 30 milliliters per minute — well below the 40 milliliters per minute threshold at which sotalol is contraindicated. The Cockcroft-Gault equation accounts for age, weight, and sex in addition to serum creatinine, providing a far more accurate estimate of renal drug clearance. Sotalol is renally eliminated and produces QT prolongation in direct proportion to its plasma concentration; when clearance is reduced and the drug accumulates, excessive QT prolongation and torsades de pointes (a dangerous polymorphic ventricular arrhythmia caused by QT prolongation) follow. Formal creatinine clearance calculation before initiating any renally cleared QT-prolonging antiarrhythmic in elderly patients is therefore a mandatory safety step.