What is the only escape rhythm available below the bundle of His?

The correct choice identifies the ventricular escape rhythm as the only escape rhythm available below the bundle of His. This rhythm occurs when the primary pacemaker of the heart, the sinoatrial (SA) node, fails to initiate impulses, leading to the heart relying on slower pacemaker cells located in the ventricles for conduction.

In cases where conduction through the atrioventricular (AV) node is blocked, impulses can no longer be transmitted from the atria to the ventricles. As a result, the ventricles need a way to maintain a heartbeat, and this is where the ventricular escape rhythm is critical. The escape rhythm generated from ventricular pacemaker cells typically results in a slower heart rate, as these cells are inherently less rapid in firing compared to the SA node.

Other potential options mentioned, such as supraventricular, atrial, and junctional escape rhythms, all occur at or above the level of the AV node. These rhythms may provide a backup pacing mechanism but do not originate from the ventricle itself. Thus, the only escape rhythm available when looking strictly below the bundle of His is the ventricular escape rhythm, which plays a crucial role when higher-level pacemaking is impaired.