What paces the ventricles in a third degree AV block?

What paces the ventricles in a third degree AV block?

• A. SA node
• B. AV node
• C. Automaticity foci
• D. Pacing device

Answer: (C)

In a third-degree AV block, also known as complete heart block, there is a complete disconnection between the atrial and ventricular electrical activity. This means that the normal conduction pathway through the AV node is disrupted, and the ventricles must rely on an intrinsic pacemaker to initiate contraction.

The primary pacemakers for the ventricles in the absence of normal conduction from the atria are the automaticity foci. These are areas within the ventricles that have the capability to generate electrical impulses independently, typically at a slower rate than the SA node or the AV node. In this situation, the ventricular automaticity foci will pace the heart, producing a slower ventricular escape rhythm while the atria continue to beat independently, often resulting in a dissociation between P waves and QRS complexes on the EKG.

This reliance on automaticity foci is crucial in third-degree AV block since the normal pacemaker function of the SA node is not supported by the AV node, which is blocked. As a result, the correct answer relates to the automaticity foci serving as the emergent pacemaker for the ventricles in this type of block.