What is the normal firing rate for Purkinje fibers when everything else has failed?

Purkinje fibers, which are part of the cardiac conduction system, serve as a backup pacemaker for the heart when the higher pacemakers—such as the sinoatrial (SA) node and the atrioventricular (AV) node—fail. Under normal physiological conditions, the SA node has the highest intrinsic firing rate, typically 60-100 beats per minute, while the AV node fires at a rate of about 40-60 beats per minute.

When both of these fail, the Purkinje fibers take over with a lower intrinsic firing rate. The correct range for their firing rate is typically between 20 to 40 beats per minute. This is represented in the option that correctly identifies the range of 20-50 bpm. This value is crucial as it highlights the importance of the Purkinje fibers in maintaining some level of cardiac output even when the primary pacemakers are dysfunctional. While the heart will not function optimally at this rate, it provides a critical safety mechanism for the body in the event of a pacemaker failure.