Purkinje Fibers / Network rate as last escape pacemaker?

• A. Purkinje Fibers / Network = last escape pacemaker, 14-40 bpm
• B. SA Node, 60-100 bpm
• C. AV Node, 40-60 bpm
• D. Bundle of His, 40-60 bpm

Answer: (A)

The question tests the idea of the heart’s automaticity hierarchy and what happens when the main pacemakers fail. When both the SA node and the AV node aren’t pacing the heart, the Purkinje network in the ventricles can take over as an escape rhythm. These ventricular pacemakers have a very slow intrinsic rate, typically about 15–40 beats per minute, which is why they’re described as the last escape pacemaker. That slow pace explains why Purkinje-driven rhythms are markedly bradycardic and why they only kick in when the higher pacemakers fail.

In contrast, the SA node normally drives at 60–100 bpm, and if the AV node takes over, the typical rate is around 40–60 bpm. Those sites fire faster than the Purkinje network, so they serve as the earlier backup pacemakers. Therefore, Purkinje fibers being the last escape pacemaker with a rate in the low tens of bpm best fits the scenario described.