What characterizes a first degree AV block?

• A. Electrical impulses are completely blocked
• B. PR interval longer than 0.20 seconds
• C. Only some impulses are conducted
• D. Impulse conduction occurs without delay

Answer: (B)

A first degree AV block is characterized by a PR interval that exceeds 0.20 seconds. This specific prolongation indicates a delay in the electrical conduction through the atrioventricular (AV) node. In a healthy heart, the PR interval typically ranges from 0.12 to 0.20 seconds, and when it is prolonged beyond this range, it signifies that the conduction through the AV node is slower than normal, which is the hallmark of a first degree AV block.

In this condition, every atrial impulse is still conducted to the ventricles, but the delay is evident in the longer PR interval. Therefore, the first degree AV block is uniquely identified by this lengthening of the PR interval while maintaining a consistent one-to-one relationship between atrial and ventricular contractions. This helps clinicians determine the presence of the first degree block and assess the function of the AV node.