Which of the following best describes a third-degree AV block?

A third-degree AV block, also known as complete heart block, occurs when there is a complete dissociation between the atria and ventricles. In this scenario, the electrical impulses generated in the atria do not reach the ventricles, meaning that the atrial contractions and ventricular contractions occur independently of each other. This lack of communication leads to a slower ventricular response, often reliant on a junctional or ventricular escape rhythm for the heart to maintain cardiac output.

The term "complete" in relation to this block indicates that no impulses pass through the AV node, distinguishing it from partial blocks, which may still allow some impulses to be conducted to the ventricles. In contrast, first-degree heart block only involves a delay in conduction with every atrial impulse still being conducted to the ventricles, and sinus bradycardia refers to a slower than normal heart rate without conduction block characteristics. Thus, the best description of a third-degree AV block is indeed that it represents a complete interruption in the normal electrical conduction between the atria and ventricles.