A first-degree atrioventricular node block occurs when conduction through the AV node is slowed, thereby delaying the time it takes for the action potential to travel from the sinoatrial node through the AV node, and to the ventricles.
A first-degree, or 1st degree, AV block is indicated on the ECG by a prolonged PR interval, as seen below.
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, meaning 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 between 0.12 and 0.20 seconds. A PR interval consistently longer than 0.20 seconds, or greater than five small boxes, indicates a first degree AV block.
There is a 1/1 ratio between P waves and QRS complexes in first-degree AV block, unlike second-degree, or 2nd degree, and
third-degree, or 3rd degree, AV nodal blocks.
In general, a first-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 and, depending on the PR interval, AV blocking medications may be avoided.
Note: A first-degree AV block is also part of a trifascicular block, as the AV node is sometimes considered the third fascicle.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009;doi:10.1161/CIRCULATIONAHA.108.191095.