In which tachyarrhythmias are adenosine most reliably effective for termination?

• A. Atrial fibrillation and atrial flutter
• B. Ventricular tachycardia
• C. AVNRT and AVRT
• D. Junctional tachycardia

Answer: (D)

Adenosine works by briefly blocking AV nodal conduction. Tachyarrhythmias that rely on the AV node as part of a reentrant circuit are most reliably terminated because interrupting AV nodal flow breaks the reentry. The classic examples are AV nodal reentrant tachycardia and AV reentrant tachycardia that use the AV node in their circuit. When the AV node is silenced for a moment, the reentrant loop can't continue, so the heart returns to a normal rhythm or a slower atrial rhythm.

This mechanism explains why adenosine is not consistently curative in other tachycardias. Atrial fibrillation or flutter originate in the atria, and shortening AV nodal conduction does not stop the atrial activity itself. Ventricular tachycardia is driven by a ventricular circuit independent of the AV node, so adenosine rarely terminates it. Junctional tachycardia, often automatic rather than reentrant, is typically not terminated by adenosine, though the AV node block it causes can transiently affect the rhythm.

So the most reliable termination occurs in AV nodal-dependent reentrant tachycardias (AVNRT and AVRT) because the key action is transient AV nodal block that interrupts the reentrant circuit.