What is the definition of third degree AV block?

The correct definition of third-degree AV block is independent pacing of the atria and ventricles. In this condition, there is a complete failure of conduction between the atria and the ventricles, meaning that the electrical impulses originating from the atria do not reach the ventricles at all. As a consequence, the atria and ventricles contract independently of one another.

In a typical presentation of third-degree AV block, atrial depolarization occurs, but the impulses do not reach the ventricles, which rely on a separate pacemaker (often the bundle of His or Purkinje fibers) for ventricular contractions. This results in a dissociation between the atrial rhythm and the ventricular rhythm, commonly referred to as "AV dissociation." The atria may continue to generate impulses at their normal rate, while the ventricles may beat at a slower intrinsic rate.

The other options describe different conduction abnormalities: a prolonged PR interval with constant conduction is indicative of first-degree AV block; a fixed PR interval with intermittent drops suggests second-degree AV block (specifically type I or Mobitz type I); and lengthening PR intervals with a dropped QRS is characteristic of second-degree AV block type II. Therefore, the definition of third-degree AV block specifically