What does an increased duration of the PR interval in a heart block indicate?

An increased duration of the PR interval specifically indicates a first-degree atrioventricular (AV) block. In this condition, the PR interval is prolonged beyond the normal range (greater than 200 milliseconds) due to a slower conduction through the AV node. This results in every atrial impulse being conducted to the ventricles, albeit with a delay.

As for the other types of heart block:

In second-degree AV block Type I (Wenckebach), there is a progressive lengthening of the PR interval until a beat is dropped. This means that while the PR interval increases, it does not remain consistently prolonged.

In second-degree AV block Type II, the PR interval remains constant and is usually within normal limits when conducted, but the condition is characterized by the sudden dropping of ventricular beats regardless of the length of the PR interval.

Third-degree AV block is characterized by a complete dissociation between atrial and ventricular activity, meaning the PR interval is not relevant since the atria and ventricles function independently.

Therefore, the sustained increased duration of the PR interval that is consistently prolonged is a hallmark of first-degree AV block.