Cardiac Medicine Certification (CMC) Practice Exam

What happens in junctional rhythms when the SA node fails?

1. The ventricles pace the heartbeat at a fast rate.

2. The AV node takes over as the back-up pacemaker.

3. The heart goes into ventricular fibrillation.

4. P waves are consistently present.

The correct answer is: The AV node takes over as the back-up pacemaker.

When the SA node fails, the AV node becomes the back-up pacemaker of the heart. This physiological response occurs because the AV node has intrinsic pacing abilities, although at a slower rate than the SA node. Junctional rhythms specifically arise when the normal pacemaking activity of the SA node is compromised, leading to the AV node assuming control over the heart's rhythm. In a junctional rhythm, the impulses originate from the AV node, which may result in different characteristics on an ECG, including the absence of distinct P waves or inverted P waves, depending on the timing of atrial depolarization relative to ventricular depolarization. This is crucial in differentiation from other types of cardiac rhythms. The other choices do not accurately reflect the physiological response. Ventricles pacing the heartbeat at a fast rate would align more with a ventricular rhythm rather than a junctional rhythm, cardiac arrest scenarios like ventricular fibrillation would not typically occur solely due to SA node failure, and consistently present P waves would contradict the definition of junctional rhythms, where P waves are often absent or altered.