What characterizes a first degree AV block?

A first-degree AV block is characterized by an extended PR interval, which indicates that while the electrical impulses are being conducted from the atria to the ventricles, this conduction occurs at a longer than normal duration. The PR interval represents the time taken for an electrical impulse to travel from the atria to the ventricles. In a first-degree AV block, this interval is consistently prolonged, typically measuring over 200 milliseconds, but it remains constant with each heartbeat.

The concept behind a first-degree AV block is that all impulses successfully reach the ventricles, but they do so more slowly than expected. This condition is often considered benign and does not usually require treatment, as the heart continues to function normally in terms of pumping blood.

The other answers involve varying degrees of impulse blockage or changes in conduction that do not apply to a first-degree AV block. For instance, complete blockage of impulses (as mentioned in one of the options) characterizes more severe types of AV blocks, while occasional failures to conduct relate to second-degree blocks. The option indicating that the PR interval shortens over time would describe a different kind of phenomenon altogether, observed in certain arrhythmias, but not in a first-degree block.