Antiarrhythmic Drugs

Correct Answer

C — Adenosine terminates atrioventricular nodal-dependent tachycardias by blocking the nodal circuit; it slows but does not terminate atrial tachycardias

Rationale

Adenosine's antiarrhythmic effect is concentrated at the atrioventricular node, where it produces transient hyperpolarization and conduction block through A1 receptor activation. Tachycardias that require the atrioventricular node as a critical limb of their circuit — such as atrioventricular nodal reentrant tachycardia and atrioventricular reentrant tachycardia using an accessory pathway — are terminated when adenosine blocks the node and interrupts the circuit. Tachycardias arising from atrial tissue — such as atrial tachycardia, atrial flutter, and atrial fibrillation — do not depend on the atrioventricular node to sustain themselves. When adenosine is given in these rhythms, the atrial rate transiently slows (revealing the underlying atrial activity on the electrocardiogram) but the tachycardia resumes as soon as the adenosine effect wears off. This differential response makes adenosine a useful diagnostic tool to distinguish nodal-dependent from atrial-origin tachycardias.

Correct Answer

A — Procainamide

Rationale

Procainamide is the pharmacological drug of choice for pre-excited atrial fibrillation in Wolff-Parkinson-White syndrome. As a Class Ia sodium channel blocker, procainamide slows conduction through the accessory pathway — the route through which rapid atrial impulses reach the ventricles in this dangerous arrhythmia. By blocking the accessory pathway, procainamide reduces the ventricular rate and prevents degeneration into ventricular fibrillation. Adenosine, verapamil, diltiazem, digoxin, and beta-blockers are all contraindicated in pre-excited atrial fibrillation because they block the atrioventricular node without affecting the accessory pathway, directing all atrial impulses through the bypass tract and potentially producing a dangerously rapid ventricular rate or ventricular fibrillation. Electrical cardioversion is the alternative when procainamide is unavailable or the patient is unstable.

Correct Answer

D — Ibutilide

Rationale

Ibutilide is the most effective pharmacological agent for acute cardioversion of atrial flutter, achieving conversion rates substantially higher than other agents for this particular arrhythmia. As a Class Three potassium channel blocker administered intravenously, ibutilide prolongs the effective refractory period of atrial tissue, interrupting the cavotricuspid isthmus-dependent re-entry circuit that sustains typical atrial flutter. It is somewhat less effective for atrial fibrillation than for flutter. Adenosine slows the ventricular response to atrial flutter transiently by blocking the atrioventricular node but does not terminate the flutter circuit and is not used for cardioversion. Amiodarone can be used for rate control or rhythm control but is not the preferred agent for acute pharmacological cardioversion of atrial flutter. Beta-blockers such as metoprolol provide rate control but do not cardiovert flutter.

Correct Answer

B — Adenosine, verapamil, diltiazem, digoxin, and beta-blockers

Rationale

All agents that block the atrioventricular node without also blocking the accessory pathway are contraindicated in pre-excited atrial fibrillation. This includes adenosine, non-dihydropyridine calcium channel blockers (verapamil and diltiazem), digoxin, and beta-blockers. By blocking the atrioventricular node, these agents redirect all rapid atrial impulses — which in atrial fibrillation can fire at 400 to 600 beats per minute — exclusively through the unblocked accessory pathway into the ventricles, potentially producing a dangerously rapid ventricular rate and degenerating into ventricular fibrillation. Procainamide slows accessory pathway conduction and is the correct pharmacological treatment. Flecainide also blocks accessory pathways and can be used in structurally normal hearts. Ibutilide and other Class Three agents may be used with caution. Magnesium, lidocaine, and mexiletine are not used for this indication but are not specifically contraindicated through the accessory pathway mechanism.

Correct Answer

C — Adenosine

Rationale

Adenosine is the first-line pharmacological agent for acute termination of atrioventricular nodal reentrant tachycardia. The re-entry circuit in this arrhythmia runs through the atrioventricular node using two functionally distinct pathways — a slow pathway and a fast pathway. Adenosine transiently blocks conduction through the atrioventricular node, interrupting this circuit and restoring sinus rhythm within seconds. Its ultra-short half-life of less than 10 seconds means any side effects (flushing, chest discomfort, transient dyspnea) resolve almost immediately. If adenosine fails or is contraindicated, non-dihydropyridine calcium channel blockers (verapamil or diltiazem) or beta-blockers are alternatives. Procainamide is used for stable ventricular tachycardia, not for atrioventricular nodal reentrant tachycardia. Ibutilide is used for atrial flutter cardioversion. Amiodarone is used for chronic arrhythmia management, not for acute termination of supraventricular tachycardia.

Correct Answer

A — Treat as ventricular tachycardia until proven otherwise; verapamil is contraindicated

Rationale

Wide-complex tachycardia — a tachycardia with a QRS duration above 120 milliseconds — should always be treated as ventricular tachycardia until definitively proven otherwise, because approximately 80 percent of wide-complex tachycardias in patients with structural heart disease are ventricular tachycardia. Verapamil is contraindicated because its potent negative inotropic and vasodilatory effects can cause hemodynamic collapse if the rhythm is ventricular tachycardia; it is also potentially lethal if the rhythm is pre-excited atrial fibrillation from Wolff-Parkinson-White syndrome. Procainamide is the preferred agent for stable wide-complex tachycardia because it is effective in both ventricular tachycardia and supraventricular tachycardia with aberrant conduction, making it safe regardless of which diagnosis proves correct. Lidocaine is used for ventricular arrhythmias in ischemic tissue and is not specifically contraindicated in wide-complex tachycardia — it is simply a second-line option.

Correct Answer

D — Adenosine terminates nodal-dependent tachycardias completely; in atrial tachycardias it transiently slows the ventricular rate by blocking the atrioventricular node, revealing the underlying atrial activity without terminating the rhythm

Rationale

Adenosine's effect is concentrated at the atrioventricular node. Tachycardias that require the node as part of their re-entry circuit — atrioventricular nodal reentrant tachycardia and atrioventricular reentrant tachycardia — are terminated when nodal conduction is blocked. Tachycardias that originate in atrial tissue and use the node only to conduct to the ventricles — atrial tachycardia, atrial flutter, atrial fibrillation — continue uninterrupted in the atria when the node is blocked. The clinical result is a transient slowing of the ventricular rate (as fewer atrial impulses conduct through the blocked node), which makes the underlying atrial rhythm visible on the electrocardiogram (a tracing of the heart's electrical activity) — for example, revealing the sawtooth flutter waves of atrial flutter or the irregular atrial activity of atrial fibrillation. When adenosine wears off and nodal conduction resumes, the tachycardia continues. This differential response guides the diagnosis and subsequent management.

Correct Answer

B — Regular narrow-complex tachycardia, often with a pseudo-R prime deflection in lead V1 (representing retrograde P waves buried near the QRS); treated with adenosine

Rationale

In atrioventricular nodal reentrant tachycardia, the re-entry circuit runs within or immediately around the atrioventricular node using a slow pathway (antegrade conduction) and a fast pathway (retrograde conduction). Because retrograde atrial activation occurs nearly simultaneously with ventricular activation, P waves are buried within or just after the QRS complex rather than preceding it — producing the characteristic pseudo-R prime deflection in lead V1 on the electrocardiogram (a tracing of the heart's electrical activity). The tachycardia is a regular narrow-complex rhythm (unless aberrant conduction is present). Adenosine is first-line treatment and terminates the arrhythmia by blocking the atrioventricular nodal limb of the circuit. The delta wave and short PR interval describe Wolff-Parkinson-White syndrome at baseline. Sawtooth flutter waves describe atrial flutter. Irregular wide-complex tachycardia with varying morphology describes torsades de pointes.

Correct Answer

A — The impulse travels antegrade through the atrioventricular node (producing a narrow QRS) and retrograde through the accessory pathway; adenosine blocks atrioventricular nodal conduction, interrupting the antegrade limb and terminating the circuit

Rationale

In orthodromic atrioventricular reentrant tachycardia, the re-entry circuit uses the atrioventricular node for antegrade (atria to ventricles) conduction and the accessory pathway for retrograde (ventricles to atria) conduction. Because conduction to the ventricles travels through the normal His-Purkinje system, the QRS complex is narrow — ventricular activation is normal. Adenosine blocks the atrioventricular node, interrupting the antegrade limb of the circuit and terminating the tachycardia. At baseline, this patient's electrocardiogram shows the characteristic delta wave (a slurred upstroke at the beginning of the QRS) and short PR interval of Wolff-Parkinson-White syndrome — evidence of pre-excitation through the accessory pathway during sinus rhythm. Option D describes antidromic atrioventricular reentrant tachycardia (antegrade through the accessory pathway, producing a wide QRS), which is the less common form and in which adenosine carries more risk.

Correct Answer

C — Nodal blocking agents block the atrioventricular node but not the accessory pathway, forcing all rapid atrial impulses through the bypass tract and potentially producing ventricular fibrillation; procainamide slows accessory pathway conduction, reducing the ventricular rate

Rationale

In pre-excited atrial fibrillation, chaotic atrial impulses firing at 400 to 600 beats per minute are conducted to the ventricles through both the atrioventricular node and the accessory pathway. When a nodal blocking agent (adenosine, verapamil, diltiazem, digoxin, or a beta-blocker) is given, the atrioventricular node is blocked but the accessory pathway — which is composed of ordinary myocardial tissue, not nodal tissue — is unaffected. All rapid atrial impulses are then routed exclusively through the unblocked accessory pathway. Because accessory pathways can conduct at very high rates, the ventricular rate may reach 250 to 300 beats per minute or more, causing hemodynamic collapse and potentially degenerating into ventricular fibrillation. Procainamide, a Class Ia sodium channel blocker, slows conduction through the accessory pathway by reducing the sodium current on which it depends, increasing its effective refractory period, and reducing the ventricular rate. Electrical cardioversion is the definitive treatment when the patient is unstable.

Correct Answer

D — Atrial flutter; mechanism is a single large re-entry circuit cycling at approximately 300 beats per minute with 2:1 atrioventricular block producing a ventricular rate of 150 beats per minute; converted most effectively with ibutilide

Rationale

The sawtooth flutter waves and a ventricular rate of exactly 150 beats per minute are the classic electrocardiographic signature of typical atrial flutter. The flutter circuit — usually a cavotricuspid isthmus-dependent macro-re-entry circuit in the right atrium — cycles at approximately 300 beats per minute. The atrioventricular node typically conducts every second flutter impulse (2:1 block), producing a ventricular rate of approximately 150 beats per minute. Rate control is often harder to achieve in atrial flutter than in atrial fibrillation because of the organized, regular atrial activity. Ibutilide, a Class Three potassium channel blocker given intravenously, is the most effective pharmacological agent for acute cardioversion of atrial flutter — more effective than for atrial fibrillation. Verapamil can slow the ventricular rate but does not cardiovert flutter. Catheter ablation of the cavotricuspid isthmus is curative in over 95 percent of typical atrial flutter cases.

Correct Answer

B — Atrioventricular dissociation — the atria and ventricles beating independently — with fusion beats or capture beats

Rationale

Atrioventricular dissociation — where the atria continue firing at their own independent rate while the ventricles fire at a different rate driven by the ventricular ectopic focus — is the most reliable electrocardiographic sign confirming ventricular tachycardia. In supraventricular tachycardia, the atria and ventricles are electrically linked through the atrioventricular node or accessory pathway, so dissociation cannot occur. Two additional confirming signs are fusion beats (a QRS complex intermediate in morphology between the normal QRS and the ventricular tachycardia QRS, produced when a sinus impulse partially captures the ventricles at the same time as the ventricular focus fires) and capture beats (a normal-appearing QRS produced when a sinus impulse completely captures the ventricles during a brief pause in the ventricular tachycardia). Wide QRS during tachycardia alone and absent P waves are consistent with ventricular tachycardia but can also occur in supraventricular tachycardia with aberrant conduction and do not confirm the diagnosis.

Correct Answer

A — In ventricular tachycardia, verapamil's negative inotropic effect and vasodilation cause hemodynamic collapse; in pre-excited atrial fibrillation, blocking the atrioventricular node forces all impulses through the accessory pathway and risks ventricular fibrillation

Rationale

Verapamil is dangerous in wide-complex tachycardia through two distinct mechanisms. First, if the arrhythmia is ventricular tachycardia — which accounts for the large majority of wide-complex tachycardias in patients with structural heart disease — verapamil's potent negative inotropic effect and peripheral vasodilation reduce cardiac output in a patient who is already hemodynamically compromised by the tachycardia. This can rapidly cause cardiogenic shock or accelerate the ventricular tachycardia. Second, if the wide-complex tachycardia is pre-excited atrial fibrillation from Wolff-Parkinson-White syndrome, verapamil blocks the atrioventricular node without affecting the accessory pathway — redirecting all rapid atrial impulses through the bypass tract and potentially causing ventricular fibrillation. Because these two dangers apply to the two most important causes of wide-complex tachycardia, verapamil should never be given empirically in this setting. Verapamil does not prolong the QT interval and does not block accessory pathways.

Correct Answer

C — Atrioventricular nodal reentrant tachycardia and atrial flutter have anatomically defined single circuits that can be completely interrupted by ablating one critical site; atrial fibrillation involves multiple, diffuse, shifting triggers and substrates throughout the atria that are harder to eliminate with focal ablation

Rationale

The difference in ablation success rates reflects fundamental differences in arrhythmia substrate. Atrioventricular nodal reentrant tachycardia depends on a compact re-entry circuit within or immediately around the atrioventricular node — ablating the slow pathway at a precise anatomical location permanently interrupts the only circuit responsible for the tachycardia. Typical atrial flutter depends on a single macro-re-entry loop anchored to the cavotricuspid isthmus — a discrete anatomical bridge in the right atrium — and ablating a line across this isthmus permanently blocks the circuit. Both arrhythmias have one critical site. Atrial fibrillation, by contrast, is driven by multiple triggers (often arising from the pulmonary veins) and sustained by complex, distributed substrates throughout both atria, including areas of fibrosis and autonomic ganglia. Pulmonary vein isolation ablation addresses the most common trigger source but cannot eliminate all substrates, and new triggers or substrate changes can emerge over time, explaining the lower and less durable success rates.

Correct Answer

D — The arrhythmia depends on the atrioventricular node as a critical limb of its re-entry circuit; adenosine terminated it by transiently blocking nodal conduction, interrupting the circuit

Rationale

Termination of a tachycardia by adenosine is diagnostic of a nodal-dependent re-entry circuit. Adenosine transiently blocks conduction through the atrioventricular node; if the node is a required limb of the re-entry circuit, blocking it interrupts the circuit and terminates the arrhythmia. This patient's presentation — young woman, sudden onset, regular narrow-complex tachycardia, no visible P waves — is classic for atrioventricular nodal reentrant tachycardia, the most common paroxysmal supraventricular tachycardia. P waves are buried within or just after the QRS because atria and ventricles activate nearly simultaneously in this arrhythmia. Atrial flutter and atrial fibrillation do not depend on the atrioventricular node to sustain themselves and would not be terminated by adenosine — only transiently slowed. Adenosine has no direct effect on ventricular myocardium and does not terminate ventricular tachycardia through a calcium channel mechanism.

Correct Answer

B — Verapamil blocks the atrioventricular node but not the accessory pathway, so all rapid atrial impulses would be routed exclusively through the bypass tract, potentially increasing the ventricular rate to a level that causes ventricular fibrillation

Rationale

In pre-excited atrial fibrillation, rapid chaotic atrial impulses are conducted to the ventricles through both the atrioventricular node and the accessory pathway. The accessory pathway is composed of ordinary myocardial tissue and depends on sodium current rather than calcium current for conduction — it is therefore unaffected by calcium channel blockers such as verapamil. When verapamil blocks the atrioventricular node, it eliminates the normal filtering function of the node while leaving the accessory pathway wide open. All atrial impulses are then funneled exclusively through the bypass tract. Because accessory pathways can conduct at very high rates, the ventricular rate can reach 250 to 300 beats per minute or more, causing hemodynamic collapse and potentially triggering ventricular fibrillation. The correct pharmacological treatment is procainamide, which slows accessory pathway conduction by blocking sodium channels; definitive treatment is electrical cardioversion if the patient is unstable. Verapamil does not prolong the QT interval and does not block accessory pathways.

Correct Answer

A — Ibutilide prolongs the effective refractory period of atrial tissue by blocking potassium channels, interrupting the single cavotricuspid isthmus-dependent re-entry circuit that sustains typical atrial flutter

Rationale

Typical atrial flutter is a macro-re-entry circuit that circulates around the tricuspid annulus, with a critical isthmus of slow conduction between the vena cava and the tricuspid valve (the cavotricuspid isthmus). Ibutilide is a Class Three potassium channel blocker that prolongs the effective refractory period of atrial tissue. By extending the refractory period, ibutilide closes the excitable gap in the flutter circuit — the circulating wavefront encounters refractory tissue and the circuit extinguishes, restoring sinus rhythm. Ibutilide is more effective for atrial flutter cardioversion than for atrial fibrillation cardioversion because atrial flutter has a single defined circuit that is vulnerable to this approach. Ibutilide acts on atrial tissue, not the atrioventricular node, and does not slow the ventricular rate by nodal blockade. It does not suppress ectopic foci or enhance nodal conduction. Because this patient has been in flutter for 36 hours, anticoagulation considerations apply before and after cardioversion per the standard protocol.

Correct Answer

C — Verapamil is contraindicated because atrioventricular dissociation confirms ventricular tachycardia, and verapamil's negative inotropic effect would cause hemodynamic collapse; the narrow beats are capture beats confirming the ventricular tachycardia diagnosis

Rationale

Atrioventricular dissociation — where atria and ventricles fire independently — is a diagnostic hallmark of ventricular tachycardia. In ventricular tachycardia, the ventricular focus drives the ventricles at its own rate, independent of the sinoatrial node. Occasionally, a sinus impulse finds the atrioventricular node and His-Purkinje system transiently available and conducts to the ventricles normally, producing a narrow QRS complex that looks identical to a normal sinus beat amidst the wide-complex tachycardia — this is a capture beat, and its presence confirms ventricular tachycardia. Once ventricular tachycardia is confirmed by atrioventricular dissociation and capture beats, verapamil is absolutely contraindicated because its negative inotropic effect and vasodilation would cause severe hemodynamic deterioration in a patient already compromised by reduced cardiac output. The correct management for this hemodynamically unstable ventricular tachycardia is immediate synchronized electrical cardioversion.