Antiarrhythmic Drugs

Correct Answer

B — Procainamide

Rationale

Procainamide is a Class Ia sodium channel blocker with a broad antiarrhythmic spectrum, used for both supraventricular and ventricular arrhythmias. Lidocaine and mexiletine are Class Ib agents used primarily for ventricular arrhythmias. Flecainide is a Class Ic agent used predominantly for supraventricular arrhythmias in patients without structural heart disease. The Class Ia designation reflects intermediate sodium channel unbinding kinetics and additional potassium channel blocking activity.

Correct Answer

D — Amiodarone

Rationale

Amiodarone is classified as a multi-class antiarrhythmic agent with Class One (sodium channel blockade), Class Two (beta-adrenergic receptor antagonism), Class Three (potassium channel blockade), and Class Four (calcium channel blockade) activity. This broad spectrum of channel blockade contributes to both its efficacy across multiple arrhythmia types and its complex toxicity profile. Sotalol has Class Two and Three activity. Propafenone has Class One and weak Class Two activity. Quinidine has Class Ia and some Class Three activity.

Correct Answer

A — Quinidine

Rationale

Quinidine is the Class Ia agent used in Brugada syndrome. In addition to sodium channel blockade, quinidine blocks the transient outward potassium current that becomes abnormally dominant in Brugada syndrome when sodium channel function is reduced, partially restoring the balance of inward and outward currents in the right ventricular epicardium. Procainamide and disopyramide are also Class Ia agents but are not used for Brugada syndrome. Lidocaine is a Class Ib agent.

Correct Answer

C — Mexiletine

Rationale

Mexiletine is a Class Ib sodium channel blocker used as adjunctive therapy in long QT syndrome type 3. Class Ib agents bind preferentially to inactivated sodium channels. In long QT syndrome type 3 — caused by a gain-of-function sodium channel mutation producing a pathological late inward sodium current — mexiletine suppresses this abnormal current, shortening the prolonged action potential duration. Quinidine is Class Ia. Flecainide is Class Ic. Propafenone is Class Ic.

Correct Answer

B — Nadolol

Rationale

Nadolol is a non-selective beta-adrenergic receptor antagonist (blocking both beta-1 and beta-2 receptors) and is the preferred beta-blocker in catecholaminergic polymorphic ventricular tachycardia. Its non-selective blockade and long duration of action provide more consistent suppression of catecholamine-triggered arrhythmias than cardioselective agents. Metoprolol, bisoprolol, and atenolol are all cardioselective (beta-1 selective) agents.

Correct Answer

D — Class Two beta-adrenergic receptor antagonists

Rationale

Class Two agents — beta-adrenergic receptor antagonists — are classified as first-line pharmacological therapy for both long QT syndrome type 1 and type 2. In these conditions, arrhythmias are triggered by adrenergic stimulation (exercise in type 1, sudden auditory stimuli in type 2). Beta-blockers directly suppress this adrenergic trigger at the receptor level. Class One, Three, and Four agents are not first-line for these inherited channelopathies.

Correct Answer

A — Amiodarone produced higher rates of hospital admission alive than lidocaine or placebo, supporting amiodarone as the preferred antiarrhythmic agent in this setting, though neither drug improved survival to discharge

Rationale

The ALPS trial (Amiodarone, Lidocaine, or Placebo Study), published in 2016, enrolled patients with shock-refractory out-of-hospital ventricular fibrillation or pulseless ventricular tachycardia. Amiodarone produced higher rates of survival to hospital admission compared to lidocaine and placebo, demonstrating a short-term antiarrhythmic benefit. However, neither amiodarone nor lidocaine improved survival to hospital discharge or neurologically intact survival compared to placebo — the outcomes that matter most clinically. The trial reinforced that antiarrhythmic drugs in cardiac arrest are temporizing measures that may facilitate initial resuscitation but do not independently determine long-term neurological outcome, which is dominated by the quality and speed of resuscitation efforts and post-arrest care. Amiodarone remains preferred over lidocaine based on the hospital admission survival advantage.

Correct Answer

C — Procainamide blocks sodium channels in ventricular tissue, slowing conduction within the re-entry circuit until the wavefront encounters refractory tissue and the circuit extinguishes

Rationale

Procainamide is a Class Ia sodium channel blocker that slows conduction velocity throughout ventricular myocardium by reducing the rate of rise of the phase 0 action potential. In sustained ventricular tachycardia — which is almost always a re-entry arrhythmia — slowing conduction in the re-entry circuit eventually causes the circulating wavefront to arrive at tissue that has not yet recovered (is still refractory), extinguishing the circuit. Procainamide also has mild Class Three activity through potassium channel blockade, extending the effective refractory period — this combined effect makes it particularly effective. It is also useful in wide-complex tachycardia of uncertain origin because it works in both ventricular tachycardia and supraventricular tachycardia with aberrant conduction, unlike verapamil (which is dangerous if the rhythm is ventricular tachycardia).

Correct Answer

D — Exercise triggers tachyarrhythmias through catecholamine-mediated adrenergic stimulation; beta-blockers block this trigger directly at beta-adrenergic receptors, reducing the risk of torsades de pointes

Rationale

In long QT syndrome type 1, the potassium channel mutation prolongs the action potential and creates a risk of early afterdepolarizations. During exercise, catecholamine (epinephrine and norepinephrine) release activates beta-adrenergic receptors, increasing intracellular calcium and further impairing repolarization in the already-vulnerable myocardium. This adrenergic surge is the proximate trigger for early afterdepolarizations and torsades de pointes (a dangerous polymorphic ventricular arrhythmia). Beta-blockers block beta-adrenergic receptors and prevent this catecholamine-mediated trigger — they do not repair the ion channel defect or directly shorten the QT interval, but by removing the trigger, they substantially reduce arrhythmic events. Activity restriction — particularly avoidance of competitive swimming, which is the highest-risk activity in type 1 — is also recommended alongside beta-blocker therapy.

Correct Answer

B — Trigger: sudden auditory stimuli (alarms, phones, sudden loud noises); Management: beta-blockers to blunt adrenergic response and strict avoidance of QT-prolonging drugs

Rationale

Long QT syndrome type 2 has a characteristic trigger: sudden auditory stimuli such as alarm clocks, ringing phones, or other sudden loud sounds. These stimuli produce a startle response with acute adrenergic activation that, in the setting of prolonged repolarization from the potassium channel mutation, generates early afterdepolarizations and triggers torsades de pointes (a dangerous polymorphic ventricular arrhythmia). Management has two key pharmacological pillars: beta-blockers to blunt the adrenergic startle response, and strict avoidance of any drug that further prolongs the QT interval — a long list including fluoroquinolones, macrolides, azole antifungals, antipsychotics, antiemetics, and many others. Patients are also advised to silence alarms gradually rather than abruptly. Long QT syndrome type 3 has a rest or sleep trigger (distinct from type 2). Fever is the characteristic trigger for Brugada syndrome, not long QT syndrome type 2.

Correct Answer

A — Trigger: rest or sleep (bradycardia-dependent); mexiletine suppresses the pathological late inward sodium current caused by the gain-of-function sodium channel mutation, shortening the action potential duration

Rationale

Long QT syndrome type 3 results from gain-of-function mutations in the cardiac sodium channel that prevent complete channel inactivation, producing a persistent late inward sodium current during the action potential plateau. This sustained inward current delays repolarization and is most pronounced at slow heart rates — explaining why arrhythmias characteristically occur during rest or sleep, when bradycardia allows more time for the late current to accumulate. Mexiletine, a Class Ib sodium channel blocker with high affinity for inactivated channels, suppresses this pathological late sodium current and shortens the QT interval in type 3 patients. Beta-blockers are also used but are less effective in type 3 than in types 1 and 2 because the trigger is not primarily adrenergic. This mechanistic distinction — late sodium current as the problem, mexiletine as the targeted treatment — is a key differentiator among the long QT syndrome subtypes.

Correct Answer

C — Quinidine blocks the transient outward potassium current that becomes abnormally dominant when sodium channel function is reduced, partially restoring the balance of inward and outward currents in the right ventricular epicardium

Rationale

In Brugada syndrome, loss-of-function sodium channel mutations reduce inward sodium current during the early action potential. The transient outward potassium current (Ito), which is normally balanced by sodium influx, becomes relatively unopposed — particularly in the right ventricular epicardium where Ito density is high. This imbalance creates the characteristic electrocardiographic pattern and arrhythmogenic substrate. Quinidine blocks Ito, partially restoring the balance of inward and outward currents and reducing arrhythmic risk. Quinidine does not restore sodium channel function, does not work through calcium channel blockade, and its QT-prolonging effect is a side effect rather than its mechanism in Brugada syndrome.

Correct Answer

D — Amiodarone provided no mortality benefit over placebo, whereas the implantable cardioverter-defibrillator reduced all-cause mortality by approximately 23 percent, establishing device therapy as superior for primary prevention in this population

Rationale

SCD-HeFT (Sudden Cardiac Death in Heart Failure Trial) randomized patients with heart failure with reduced ejection fraction and ejection fraction at or below 35 percent to amiodarone, implantable cardioverter-defibrillator, or placebo. Amiodarone showed no survival benefit over placebo — a finding that definitively established that pharmacological antiarrhythmic therapy with amiodarone cannot substitute for device therapy in primary prevention of sudden cardiac death in this population. The implantable cardioverter-defibrillator reduced all-cause mortality by approximately 23 percent compared to placebo, making it the standard of care. This trial is the foundational evidence behind the current guideline recommendation that patients with heart failure with reduced ejection fraction and ejection fraction at or below 35 percent on optimal medical therapy should receive an implantable cardioverter-defibrillator for primary prevention of sudden cardiac death, unless life expectancy is limited or the patient declines.

Correct Answer

B — Ejection fraction at or below 35 percent in patients with heart failure or following myocardial infarction, after at least 90 days of optimal medical therapy

Rationale

The primary implantable cardioverter-defibrillator eligibility threshold for primary prevention of sudden cardiac death is an ejection fraction at or below 35 percent, derived from the SCD-HeFT trial (heart failure with reduced ejection fraction) and the MADIT-II trial (post-myocardial infarction). The indication applies when patients are on optimal guideline-directed medical therapy and their ejection fraction remains at or below this threshold — a waiting period of at least 90 days after a myocardial infarction and at least three months of heart failure therapy is required, because some patients improve with medical treatment alone. An ejection fraction of 35 percent is also the threshold used in the handoff document for this chapter. The 35 percent threshold applies regardless of whether the reduced ejection fraction is ischemic or non-ischemic in origin. Non-sustained ventricular tachycardia with inducible ventricular tachycardia on electrophysiology study may trigger consideration at higher ejection fractions, but 35 percent is the primary landmark number.

Correct Answer

C — Procainamide — because sodium channel blockade slows conduction within the re-entry circuit, extinguishing it when the wavefront encounters refractory tissue

Rationale

This patient has hemodynamically stable ventricular tachycardia — confirmed by atrioventricular dissociation and wide complex — and is therefore a candidate for pharmacological cardioversion rather than immediate electrical cardioversion. Procainamide is the first-line pharmacological agent for stable ventricular tachycardia. As a Class Ia sodium channel blocker, it slows conduction velocity throughout ventricular myocardium, progressively impeding the circulating wavefront in the re-entry circuit until the wavefront encounters still-refractory tissue and the circuit terminates. Procainamide also has additional potassium channel blocking activity that extends the effective refractory period, further disrupting the circuit. Verapamil is contraindicated in ventricular tachycardia — its negative inotropic and vasodilatory effects can cause hemodynamic collapse. Adenosine acts on the atrioventricular node, which is not part of ventricular tachycardia circuits. Digoxin's vagotonic mechanism has no role in acute ventricular tachycardia management.

Correct Answer

A — Mexiletine preferentially blocks inactivated sodium channels, suppressing the pathological late sodium current produced by the gain-of-function mutation and shortening the prolonged action potential duration

Rationale

Long QT syndrome type 3 is caused by a gain-of-function mutation in the sodium channel that prevents complete inactivation, allowing a persistent late inward sodium current to flow throughout the action potential plateau. This sustained depolarizing current delays repolarization and prolongs the QT interval. At slow heart rates — as during sleep or rest — there is more time for this late current to accumulate, making bradycardia-dependent triggers characteristic of type 3. Mexiletine is a Class Ib sodium channel blocker with high affinity for the inactivated state of the channel. By binding to inactivated channels and prolonging their inactivation, mexiletine reduces the late sodium current produced by the gain-of-function mutation. This selectively shortens the QT interval in type 3 patients — an effect not seen with the same potency in types 1 or 2, where potassium channel mutations are the primary defect. Mexiletine does not activate potassium channels or block beta-adrenergic receptors. Transient outward potassium current blockade is the mechanism by which quinidine treats Brugada syndrome, not long QT syndrome type 3.

Correct Answer

D — The SCD-HeFT trial demonstrated that amiodarone provided no mortality benefit over placebo in patients with reduced ejection fraction, whereas the implantable cardioverter-defibrillator reduced all-cause mortality, establishing device therapy as the standard for primary prevention

Rationale

The SCD-HeFT trial is the direct evidence basis for this recommendation. It randomized patients with heart failure with reduced ejection fraction (ejection fraction at or below 35 percent) to amiodarone, implantable cardioverter-defibrillator, or placebo, and found that amiodarone offered no survival advantage over placebo while the implantable cardioverter-defibrillator reduced all-cause mortality by approximately 23 percent. This definitively established that pharmacological antiarrhythmic therapy cannot substitute for device therapy for primary prevention of sudden cardiac death in this population. This patient meets the standard implantable cardioverter-defibrillator criteria: ejection fraction at or below 35 percent after at least 90 days of optimal medical therapy and no reversible cause of reduced function. Amiodarone is not contraindicated after myocardial infarction — it is safe in structural heart disease — but it has not been proven to reduce mortality in this primary prevention setting. Secondary prevention with amiodarone (option C) is a separate and legitimate use but does not apply here since this patient has had no prior arrhythmic event.

Correct Answer

B — Quinidine blocks the transient outward potassium current that becomes abnormally dominant in Brugada syndrome when sodium channel function is reduced, restoring the balance of inward and outward currents in the right ventricular epicardium

Rationale

The apparent paradox of using a sodium channel blocker in a disease of sodium channel deficiency is resolved by understanding quinidine's additional pharmacological properties. In the right ventricular epicardium, the transient outward potassium current (Ito) is normally balanced during the early action potential by the inward sodium current. When the sodium channel carries less current (as in Brugada syndrome), Ito becomes relatively dominant, causing a pathological notch in the epicardial action potential and creating a voltage gradient between epicardium and endocardium. This gradient produces the characteristic ST elevation and, in susceptible patients, ventricular fibrillation. Quinidine reduces Ito, restoring a more balanced current relationship and eliminating the arrhythmogenic voltage gradient. Quinidine does not repair the sodium channel itself, does not primarily act on calcium channels in this context, and while it does prolong the QT interval through potassium channel blockade, the therapeutic effect in Brugada syndrome is specifically through Ito blockade rather than QT prolongation.