What happens if the third degree AV block is below the AV node?

What happens if the third degree AV block is below the AV node?

• A. The atria stop contracting
• B. The junctional focus takes over pacing
• C. The ventricular focus starts pacing
• D. The heart rate increases significantly

Answer: (C)

In the scenario where there is a third-degree AV block occurring below the AV node, the correct outcome is that the ventricular focus takes over pacing. In this type of block, there is a complete disconnection between the atria and ventricles, meaning the atria and ventricles are no longer communicating effectively.

When the block occurs below the AV node, the natural pacemaker above the block (the AV node) is no longer able to conduct impulses to the ventricles. As a result, an escape rhythm from a lower pacemaker, which is typically located in the ventricles, takes over to maintain ventricular contraction. This ventricular pacing is usually much slower than the normal heart rate since the intrinsic firing rate of ventricular pacemakers is lower than that of the sinus node.

This phenomenon ensures that the heart can still pump blood, albeit at a reduced rate, preventing asystole and providing a critical back-up mechanism when higher-level pacing fails. The junctional focus would be more involved in a scenario where the block occurs at or near the AV node itself, leading to junctional rhythms, but in cases of significant block below the AV node, the escape rhythm primarily originates from the ventricular focus.