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).

Note: A first-degree AV block is also part of a trifascicular block, as the AV node is sometimes considered the third fascicle. ECG Examples: First-Degree Atrioventricular (AV) Block ECG (Example 1) ...