Where do third degree AV blocks typically occur?

Where do third degree AV blocks typically occur?

• A. In the atrial node
• B. In the upper AV node
• C. Within the bundle of His
• D. Below the AV node

Answer: (B)

The third-degree AV block, also known as complete heart block, typically occurs in the conduction system of the heart, specifically within the AV node or just below it. In this condition, the electrical signals from the atria are completely blocked from reaching the ventricles, leading to an independent rhythm between the two chambers.

When considering the AV node, it is important to understand that the upper part of the AV node is primarily responsible for the transmission of impulses from the atria to the ventricles. A block in this area can disrupt this transmission, resulting in the separation of atrial and ventricular activity.

The lower sections of the AV node or the pathways immediately below, such as the bundle of His and Purkinje fibers, are also involved in heart block scenarios, but the characteristic presentation of a third-degree block begins at the AV node itself. This makes the upper AV node the most common site of the block in cases of complete heart block, where the connections between the atrial impulses and ventricular conduction are entirely disrupted. Understanding this helps in diagnosing and managing heart block effectively and knowing the implications for cardiac function.