In a patient with atrial fibrillation at an atrial rate of 400 bpm, the ventricular rate is limited to around 150 bpm. Which property of the atrioventricular node explains this limitation?

• A Its supranormal conduction period that blocks alternate beats
• B Its long effective refractory period outlasting many atrial impulses ✓
• C Its reliance on fast sodium channel conduction, which fatigues rapidly
• D Its negative feedback inhibition by vagal discharge from the atria

Explanation

The AV node has a prolonged effective refractory period, so many of the chaotic atrial impulses arrive while the node is still refractory and are blocked. This protective property caps the ventricular rate and prevents 1:1 transmission of atrial flutter or fibrillation, which would be haemodynamically catastrophic. AV nodal cells depend on slow calcium channels, not fast sodium channels, which eliminates option C, and the limitation is intrinsic to nodal excitability rather than reflex vagal feedback.

Reference: Ganong Review of Medical Physiology, 26th ed.

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