In a patient with atrial flutter and 2:1 AV conduction, which antiarrhythmic is contraindicated due to the risk of 1:1 AV conduction and potential hemodynamic collapse?

• A. Amiodarone
• B. Ibutilide
• C. Quinidine
• D. Diltiazem

Answer: (C)

The situation hinges on how the AV node handles impulses from the atrial flutter. In atrial flutter with 2:1 AV conduction, the AV node is partly blocking every other impulse, keeping the ventricular rate around 150. Some drugs can change this balance by altering AV nodal conduction. Quinidine, a class IA sodium channel blocker with notable anticholinergic (vagolytic) effects, can remove some of that protective AV nodal block and increase conduction through the AV node. This can allow each atrial impulse to reach the ventricles, producing 1:1 AV conduction. When the atrial rate is about 300 bpm, 1:1 conduction can drive a dangerously high ventricular rate, risking sudden hemodynamic collapse from pump failure or severe hypotension.

The other drugs listed don’t have the same tendency to promote 1:1 conduction in this scenario. Amiodarone and ibutilide are more likely to slow or interrupt conduction and can be used for rhythm control with a lower risk of triggering 1:1 conduction. Diltiazem slows AV nodal conduction, which typically reduces ventricular rate rather than enabling 1:1 conduction.