In Junctional Rhythm, what is the primary pacing structure when the SA node is nonfunctional?

In Junctional Rhythm, when the SA node is nonfunctional, the primary pacing structure is the AV node. The AV node acts as a backup pacemaker when the SA node fails to initiate impulses. Normally, the SA node is responsible for generating electrical impulses that set the rhythm for the heart, but when its function is compromised, the AV node takes over this role.

The AV node has an intrinsic firing rate that is slower than that of the SA node, typically around 40 to 60 beats per minute, hence the heart rate in Junctional Rhythm is typically reduced compared to normal sinus rhythm. This is important because it indicates that while the heart is still able to beat, the efficiency and rate may be compromised due to the reliance on the AV node.

In contrast, while the Bundle of His and Purkinje fibers can also serve as pacing sites, they are generally considered to be secondary backups, kicking in at even lower rates than the AV node, usually when both the SA and AV nodes fail. Therefore, while they can contribute to heart activity, the primary structure responsible for pacing in the case of Junctional Rhythm is the AV node.