Which structure has the lowest inherent pacing rate?

The structure with the lowest inherent pacing rate is the Purkinje fibers. In the hierarchy of cardiac pacemaking, the Purkinje fibers typically have the slowest intrinsic firing rate, usually around 20 to 40 beats per minute. This is significantly lower than the pacing rates of the other structures listed.

The SA node is the primary pacemaker of the heart, with an inherent rate of approximately 60 to 100 beats per minute. It leads the electrical conduction system that dictates the overall heart rhythm. The AV node serves as a secondary pacemaker with a pacing range of around 40 to 60 beats per minute, while the Bundle of His can also pace the heart at a rate similar to that of the AV node, slightly above that of the Purkinje fibers.

In contrast, the Purkinje fibers, while capable of initiating impulses to maintain heart rhythm when all else fails, do so at a rate that is sufficiently lower than that of the SA node and AV node, highlighting their role primarily as a backup pacemaking mechanism rather than a primary one.