Adenosine as an antiarrhythmic is categorized as which class?

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

Answer: (D)

Adenosine is placed in Class V (often labeled non-classified) because its mechanism doesn’t fit the four Vaughn Williams classes. The classic classes are defined by blocking specific ion channels or receptors: sodium channels (Class I), beta-adrenergic effects (Class II), potassium channels (Class III), and calcium channels (Class IV). Adenosine works differently—through A1 receptors in the AV node to open potassium channels, causing a brief hyperpolarization and temporary AV nodal block. This unique action short-circuits AV nodal conduction, which is how it terminates AV node–dependent tachycardias. Its effects are ultra-short-lived due to rapid metabolism, aligning with its use in acute settings.