What is the main characteristic of a third-degree AV block on an EKG?

The main characteristic of a third-degree AV block, also known as complete heart block, is the complete dissociation between atrial and ventricular activities. In this condition, the electrical signals from the atria do not conduct to the ventricles. As a result, the atrial rate and ventricular rate operate independently, which can be seen on the EKG as two separate rhythms. The P waves (representing atrial contractions) occur at a regular rate, while the QRS complexes (representing ventricular contractions) occur at a different, often slower, rate. This dissociation leads to the characteristic appearance of a third-degree AV block on an EKG, where the intervals between the atrial and ventricular events do not correlate in any way.

In contrast, a prolonged PR interval is associated with first-degree or second-degree AV blocks, indicating a delay in conduction rather than a total block. Normal QRS intervals can be seen in various situations, but they do not specifically indicate a third-degree AV block; in fact, they may present with wide QRS complexes depending on the escape rhythm. Paroxysmal atrial contractions refer to intermittent, early atrial beats and do not pertain to the characteristics of AV blocks at all.