Adenosine is an example of a class ___ antiarrhythmic

• A. Class I
• B. Class II
• C. Class III
• D. Class V/ Non Classed

Answer: (D)

Adenosine works by activating A1 receptors in the AV node, which increases potassium conductance and causes rapid hyperpolarization. This briefly blocks AV nodal conduction, abruptly interrupting reentrant circuits that rely on the AV node and often terminating certain supraventricular tachycardias. This mechanism is not about blocking the classic ion channels described in the Vaughan Williams system (sodium channels for Class I, beta receptors for Class II, potassium channels for Class III, or calcium channels for Class IV). Instead, its antiarrhythmic effect comes from receptor-mediated modulation of nodal tissue, so it falls into Class V (non-classified) antiarrhythmic. Because its effects are extremely short-lived, it’s given as a rapid IV bolus with close monitoring. Side effects can include transient bradycardia, AV block, flushing, chest discomfort, and, in sensitive patients, bronchospasm.