Adenosine is contraindicated in which condition?

• A. Atrial fibrillation
• B. Second-degree AV block or sick sinus syndrome (unless patient has a functional artificial pacemaker)
• C. Ventricular tachycardia
• D. Sinus tachycardia

Answer: (B)

Adenosine works by briefly blocking AV nodal conduction to interrupt certain narrow complex supraventricular tachycardias. If a patient already has dysfunction in the conduction system—specifically second-degree AV block or sick sinus syndrome—the additional AV nodal block caused by adenosine can precipitate dangerous bradycardia or even asystole. That risk is the reason this situation is a contraindication unless a functional artificial pacemaker is present to maintain a rhythm.

In other scenarios, adenosine isn’t used to treat atrial fibrillation and isn’t helpful for ventricular tachycardia, though these facts don’t constitute the same immediate contraindication. Sinus tachycardia isn’t a contraindication either; adenosine would simply transiently slow AV conduction but isn’t routinely useful for that rhythm.