atrioventricular block first degree atrioventricular block 2026-07-20T09:13:22Z MPheno.ontology AV block 1 first-degree AV block First-degree AV block is more accurately defined as an AV delay because no P waves are blocked. The delay is typically due to a minor AV conduction defect occurring at or below the AV node. In a "marked" first-degree AV block, the PR interval is excessively prolonged and the P waves may be buried in the preceding T wave. 1st-degree AV block MP:0031756 a delay in the propagation of electrical impulses from the atria to the ventricles characterized by a prolonged PR interval without any dropped beats; every P wave is followed by a QRS complex, i.e., all atrial impulses reach the ventricles first-degree atrioventricular block AV block I