Which condition markedly delays AV nodal conduction?

• A. Vagal activation
• B. Sympathetic activation
• C. Ischemia
• D. Exercise

Answer: (A)

Parasympathetic (vagal) activation slows conduction through the AV node. This happens because acetylcholine released by the vagus reduces cAMP and calcium entry in AV nodal cells, increasing the outward K+ current and prolonging the upstroke of the action potential. The result is a slower conduction velocity, a longer PR interval, and a greater likelihood of AV block in susceptible individuals.

That’s why this option is the best answer: vagal tone is the classic, most potent factor that markedly delays AV nodal conduction. In contrast, sympathetic activation speeds conduction by increasing calcium current and cAMP, so it shortens rather than delays conduction. Ischemia can impair conduction, but its effect on the AV node is less specific and typically context-dependent. Exercise raises sympathetic activity and heart rate, which again tends to enhance AV nodal conduction rather than delay it.