Antiarrhythmic Drugs

Correct Answer

B — Metoprolol and atenolol

Rationale

Metoprolol and atenolol are both cardioselective beta-1 adrenergic receptor antagonists. At standard doses they preferentially block beta-1 receptors in the heart rather than beta-2 receptors in the airways and peripheral vasculature. Propranolol and nadolol are non-selective beta-blockers that block both beta-1 and beta-2 receptors. Carvedilol and labetalol are non-selective beta-blockers that also block alpha-1 adrenergic receptors. Sotalol is classified as a Class Three antiarrhythmic agent with additional beta-blocking properties.

Correct Answer

D — Non-selective beta-adrenergic receptor antagonist with additional alpha-1 adrenergic receptor blockade

Rationale

Carvedilol blocks beta-1, beta-2, and alpha-1 adrenergic receptors. The combined beta and alpha-1 blockade reduces both cardiac workload and peripheral vascular resistance, which is particularly useful in heart failure with reduced ejection fraction. Carvedilol is one of three beta-blockers proven to reduce mortality in heart failure with reduced ejection fraction, alongside metoprolol succinate and bisoprolol. Metoprolol and atenolol are selective beta-1 antagonists. Propranolol and nadolol are non-selective beta-blockers without alpha-1 activity. No clinically used antiarrhythmic agent selectively blocks beta-2 receptors.

Correct Answer

A — Propranolol

Rationale

Propranolol is a non-selective beta-adrenergic receptor antagonist that blocks both beta-1 receptors (in the heart) and beta-2 receptors (in the airways, peripheral vasculature, and other tissues) without alpha-1 blockade. Nadolol shares this non-selective profile. Metoprolol and bisoprolol are cardioselective beta-1 antagonists. Carvedilol is non-selective but also adds alpha-1 receptor blockade. The non-selective profile of propranolol is clinically relevant because beta-2 blockade can precipitate bronchoconstriction in patients with asthma or chronic obstructive pulmonary disease.

Correct Answer

C — Class Two

Rationale

Beta-adrenergic receptor antagonists constitute Vaughan Williams Class Two. Their antiarrhythmic mechanism is blockade of beta-adrenergic receptors, which reduces sympathetic stimulation of the heart, slows automaticity in pacemaker tissue, decreases conduction velocity through the atrioventricular node, and suppresses catecholamine-driven ectopic beats. Class One contains sodium channel blockers; Class Three contains potassium channel blockers; Class Four contains non-dihydropyridine calcium channel blockers. Sotalol has Class Two properties but is classified primarily as Class Three because of its dominant potassium channel blocking activity.

Correct Answer

B — Nadolol

Rationale

Nadolol is the preferred beta-blocker for catecholaminergic polymorphic ventricular tachycardia. It is non-selective (blocks both beta-1 and beta-2 receptors) and has a long half-life, providing sustained suppression of the catecholamine surges that trigger this arrhythmia. In catecholaminergic polymorphic ventricular tachycardia, physical or emotional stress causes adrenergic stimulation that provokes abnormal calcium release from the sarcoplasmic reticulum, leading to delayed afterdepolarizations and triggered ventricular arrhythmias. Nadolol is combined with flecainide for patients with breakthrough arrhythmias on beta-blocker monotherapy. Metoprolol, atenolol, and bisoprolol are cardioselective beta-1 agents less commonly used for this indication.

Correct Answer

D — Bisoprolol

Rationale

Bisoprolol is a cardioselective (beta-1 selective) beta-blocker. Cardioselective agents preferentially block beta-1 receptors in the heart at standard doses, with less effect on beta-2 receptors in the lungs and peripheral vasculature. Propranolol and nadolol are non-selective beta-blockers that block both beta-1 and beta-2 receptors. Carvedilol blocks alpha-1, beta-1, and beta-2 receptors.

Correct Answer

A — Non-dihydropyridine calcium channel blockers have a negative inotropic effect that can worsen ventricular function, whereas beta-blockers improve long-term outcomes in heart failure with reduced ejection fraction

Rationale

Verapamil and diltiazem (non-dihydropyridine calcium channel blockers) reduce myocardial contractility in addition to slowing atrioventricular nodal conduction. In patients with heart failure with reduced ejection fraction, this negative inotropic effect can precipitate acute decompensation and is therefore contraindicated. Beta-blockers, by contrast, have proven mortality benefit in stable heart failure with reduced ejection fraction — carvedilol, metoprolol succinate, and bisoprolol all reduce sudden cardiac death and hospitalizations. The preferred agent for rate control in atrial fibrillation with heart failure with reduced ejection fraction is therefore a beta-blocker, or digoxin if beta-blockers are contraindicated.

Correct Answer

A — Beta-blockers reduce sympathetic activation of the post-infarction heart, suppressing catecholamine-driven triggered activity and lowering the threshold for ventricular fibrillation

Rationale

After myocardial infarction, sustained sympathetic nervous system activation is a major driver of arrhythmic death. Elevated catecholamine (epinephrine and norepinephrine) levels accelerate phase 4 depolarization in ectopic foci, promote delayed afterdepolarizations in calcium-overloaded cells, and lower the threshold for ventricular fibrillation. Beta-blockers counter all of these effects by blocking beta-adrenergic receptors. They also slow heart rate, reduce myocardial oxygen demand, and limit the extent of ischemic injury. This combination of antiarrhythmic and anti-ischemic effects is why beta-blockers are a cornerstone of post-infarction therapy and are associated with a reduction in sudden cardiac death.

Correct Answer

C — Beta-blockers reduce catecholamine-driven triggered activity and lower the threshold for ventricular fibrillation in the post-infarction heart

Rationale

Beta-blockers reduce sudden cardiac death after myocardial infarction primarily by suppressing catecholamine-driven arrhythmias. The post-infarction heart is exposed to elevated sympathetic tone, which promotes triggered activity and lowers the threshold for ventricular fibrillation. Beta-1 receptor blockade directly opposes these effects. Beta-blockers do not block sodium channels, do not meaningfully prolong the QT interval in this context, and their antiplatelet effect is not the mechanism of arrhythmia prevention.

Correct Answer

B — Beta-blockers — because exercise and stress trigger arrhythmias via catecholamine-mediated adrenergic activation, which beta-blockers directly suppress

Rationale

Long QT syndrome type 1 results from loss-of-function mutations in the slow delayed rectifier potassium channel. Patients have prolonged ventricular repolarization at baseline, and adrenergic stimulation during exercise or emotional stress further impairs repolarization and generates early afterdepolarizations (abnormal depolarizations arising during the prolonged action potential). These early afterdepolarizations can trigger torsades de pointes (a dangerous polymorphic ventricular arrhythmia). Because catecholamine-mediated adrenergic activation is the proximate trigger, beta-blockers are first-line therapy. They suppress the adrenergic stimulation that initiates the cascade. Patients are also advised to avoid competitive sports. Amiodarone would further prolong the QT interval and is avoided in long QT syndromes.

Correct Answer

D — In acute decompensation the patient depends on sympathetic drive to maintain cardiac output; beta-blockade removes this support and risks further hemodynamic deterioration

Rationale

In acute decompensated heart failure, the heart relies on elevated sympathetic tone to maintain adequate cardiac output. Beta-blockers remove this compensatory support by blocking beta-1 adrenergic receptors, which can worsen hemodynamics and precipitate cardiogenic shock. This is why beta-blockers that provide long-term mortality benefit in stable chronic heart failure with reduced ejection fraction are held or withheld during acute decompensation until the patient is stabilized.

Correct Answer

C — Beta-blockers further slow atrioventricular nodal conduction by reducing sympathetic support, potentially causing complete heart block or asystole

Rationale

The atrioventricular node is the sole electrical bridge between the atria and ventricles, and its conduction velocity depends partly on sympathetic tone. In high-degree atrioventricular block, conduction is already severely impaired — only some impulses make it through, or the node relies on a ventricular escape rhythm to maintain cardiac output. Sympathetic stimulation helps maintain whatever residual conduction and heart rate remains. Beta-blockers, by blocking this sympathetic support, can precipitate complete heart block (no impulses through the node) or dangerous bradycardia. This is why high-degree atrioventricular block and severe symptomatic bradycardia are absolute contraindications to beta-blocker therapy. A permanent pacemaker is required before beta-blockers can be safely used in such patients if the drug is otherwise indicated.

Correct Answer

D — In decompensated heart failure the patient depends on sympathetic drive to maintain cardiac output; acutely withdrawing that support with a beta-blocker can cause further hemodynamic deterioration

Rationale

In acute decompensated heart failure, the already-failing ventricle cannot maintain adequate cardiac output without sympathetic compensation. Elevated catecholamine (epinephrine and norepinephrine) levels increase heart rate and contractility to sustain perfusion. Initiating a beta-blocker in this state removes that compensatory drive, acutely reducing heart rate and contractility and potentially causing cardiogenic shock. In stable chronic heart failure with reduced ejection fraction, the hemodynamic situation is compensated and the long-term harms of sustained sympathetic activation (remodeling, arrhythmia) outweigh the acute negative inotropic risk — which is why chronic beta-blocker therapy improves outcomes. The clinical rule is: start beta-blockers in stable, dry, euvolemic heart failure; hold or reduce them during acute decompensation.

Correct Answer

B — Chronic beta-blockade causes upregulation of beta-adrenergic receptors; when the drug is withdrawn, the increased receptor density produces exaggerated responses to normal catecholamine levels

Rationale

During chronic beta-blocker therapy, the body compensates for persistent receptor blockade by increasing the number of beta-adrenergic receptors on cell surfaces — a process called receptor upregulation. When the beta-blocker is abruptly discontinued, the elevated receptor density is suddenly exposed to circulating catecholamines (epinephrine and norepinephrine) at normal concentrations. The result is an exaggerated adrenergic response: tachycardia, hypertension, increased myocardial oxygen demand, and a heightened risk of angina or myocardial infarction in patients with underlying coronary artery disease. Beta-blockers should always be tapered gradually rather than stopped abruptly, particularly in patients with ischemic heart disease.

Correct Answer

C — Verapamil's negative inotropic effect can precipitate acute decompensation in a patient already depending on maximal contractility to maintain cardiac output

Rationale

Verapamil and diltiazem (non-dihydropyridine calcium channel blockers) block L-type calcium channels not only in the atrioventricular node but throughout the myocardium, reducing contractility. In a patient with heart failure with reduced ejection fraction whose ventricle is already pumping at the limits of its reserve, this negative inotropic effect can cause acute hemodynamic deterioration. Verapamil is therefore contraindicated in heart failure with reduced ejection fraction. Beta-blockers, used at appropriate doses in stable (not acutely decompensated) heart failure with reduced ejection fraction, provide both rate control and proven long-term mortality benefit through suppression of adverse adrenergic remodeling. If beta-blockers are contraindicated, digoxin is the alternative for rate control in this setting.

Correct Answer

A — A beta-blocker — because arrhythmias in long QT syndrome type 1 are triggered by adrenergic activation during exercise, which beta-blockers directly suppress

Rationale

Long QT syndrome type 1 has a characteristic trigger: physical exertion and swimming in particular. The adrenergic surge during exercise enhances calcium entry and impairs repolarization in cells already compromised by the potassium channel mutation. This creates early afterdepolarizations (abnormal depolarizations during the prolonged action potential) that can initiate torsades de pointes (a dangerous polymorphic ventricular arrhythmia). Beta-blockers are first-line therapy because they directly counter the adrenergic trigger. They reduce the risk of syncope and sudden cardiac death, though breakthrough events can still occur. Amiodarone would further prolong the QT interval and is generally avoided. Mexiletine is used in long QT syndrome type 3, not type 1. Activity restriction, particularly avoidance of competitive swimming, is also recommended.

Correct Answer

D — Metoprolol reduces catecholamine-driven arrhythmias and has proven mortality benefit after myocardial infarction; verapamil has negative inotropic effects that are harmful in a patient with reduced ejection fraction

Rationale

Metoprolol succinate is the appropriate choice. Beta-blockers reduce mortality after myocardial infarction by suppressing catecholamine-driven triggered activity and ventricular fibrillation. Non-dihydropyridine calcium channel blockers such as verapamil have significant negative inotropic effects that are contraindicated when the ejection fraction is reduced, as they can worsen ventricular function and increase heart failure risk. Verapamil does not meaningfully prolong the QT interval and does not block sodium channels.

Correct Answer

B — In acute decompensation the patient depends on sympathetic drive to maintain cardiac output; carvedilol's beta-blockade would remove this compensatory support and risk further hemodynamic collapse

Rationale

In acute decompensated heart failure, the failing ventricle cannot sustain adequate cardiac output without maximal sympathetic compensation. Elevated catecholamines (epinephrine and norepinephrine) increase heart rate and contractility to keep perfusion pressure above critical thresholds. Continuing or initiating a beta-blocker in this state removes that compensatory drive — reducing heart rate and contractility acutely before any long-term benefit can accrue — and risks precipitating cardiogenic shock. The standard approach is to hold beta-blockers during acute decompensation and restart them at a low dose once the patient is stabilized, euvolemic, and off inotropic support. This contrasts with their use in stable heart failure with reduced ejection fraction, where the balance shifts decisively in favor of long-term benefit.