What defines the heart's electrical conduction in a third-degree AV block?

In a third-degree atrioventricular (AV) block, there is complete dissociation between the electrical activity of the atria and the ventricles. This means that the impulses generated in the sinoatrial (SA) node, which initiate atrial contraction, do not reach the ventricles at all. As a result, the atria and ventricles beat independently of each other. The atria continue to contract at their regular pace, usually determined by the SA node, while the ventricles may contract at a slower rate, dictated by an escape rhythm from either the AV node or a lower pacemaker in the conduction system.

This dissociation is crucial in understanding the mechanical failure in this type of block and summarizes the pathophysiology. Patients with a third-degree AV block typically present with symptoms such as dizziness, fatigue, or syncope due to this lack of coordination and resultant decrease in cardiac output.

The other described options do not accurately depict the nature of a third-degree AV block. Normal conduction through the AV node is characteristic of a healthy heart rhythm, while partial conduction with varying degrees applies more to a second-degree AV block, where some impulses are conducted and some are not. Accelerated conduction from the atria to the ventricles