What characterizes the PR interval in first-degree AV block (1*AVB)?

In first-degree atrioventricular block (1° AVB), the PR interval is characterized as being prolonged, specifically greater than 0.20 seconds. This is a hallmark of first-degree AV block, where the electrical signal from the atria to the ventricles is delayed but still conducted.

When assessing the PR interval, which represents the time duration from the start of atrial depolarization to the start of ventricular depolarization, a measurement over 0.20 seconds indicates that the conduction through the AV node is slower than normal. In this condition, every atrial impulse successfully reaches the ventricles but with this prolonged duration.

In contrast, a PR interval that measures less than 0.12 seconds would be considered normal and not indicative of any block, while values between 0.12 and 0.20 seconds might suggest a normal variant or a lesser degree of delay. An exact value of 0.20 seconds also does not define first-degree AV block, as the characteristic feature is that the PR interval must exceed this duration for the diagnosis.