A first degree AV node block occurs when conduction through the AV node is slowed, thus delaying the time it takes for the action potential to travel from the SA node, through the AV node, and to the ventricles.

A first degree AV block is indicated on the ECG by a prolonged PR interval. Recall that the P wave indicates atrial depolarization (initiated by firing of the SA node). The atrial depolarization eventually spreads to the AV node where there is a slight delay before the electrical impulse is conducted to the ventricles. If the AV nodal conduction (dromotropy) is decreased, it will take longer for the impulse to reach the ventricles, so there will be a greater distance between the P wave and the QRS complex (remember the QRS complex indicates ventricular depolarization). Thus the PR interval will be prolonged.

The PR interval is normally 0.12-0.20 seconds or 120 to 200 milliseconds. A PR interval consistently longer than 0.20 seconds (greater than 5 small boxes) indicates a 1st degree AV block. Note: There is a 1:1 ratio between P waves and QRS complexes, unlike 2nd degree AV nodal block and 3rd degree AV nodal block.

In general, a 1st degree AV block is a benign finding that does not require any treatment, however it may be an indicator of higher degree AV block in the future. Higher doses of AV blocking medications should be avoided.