If the sinoatrial node is destroyed, a junctional escape rhythm usually takes over pacing of the ventricles. What is the typical intrinsic discharge rate of the AV junctional pacemaker tissue?

• A 100 to 120 per minute
• B 70 to 90 per minute
• C 40 to 60 per minute ✓
• D 15 to 25 per minute

Explanation

Automaticity decreases down the conducting system because the slope of phase 4 depolarization becomes progressively shallower. The AV junction discharges intrinsically at 40 to 60 per minute, making it the usual backup after sinus arrest. Ventricular escape rhythms from the His-Purkinje system are much slower, around 20 to 40 per minute. Rates below 25 suggest a ventricular focus rather than junctional tissue.

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

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