AV block partial or complete block of electrical impulses originating in the atrium or sinus node, preventing them from reaching the AV node and ventricles. In first-degree AV block, there is prolongation of AV conduction time (PR interval); in second-degree AV block, some but not all atrial impulses fail to reach the ventricles, thus some ventricular beats are dropped; in complete AV block (third degree), complete atrioventricular dissociation (2) occurs; atria and ventricles beat independently. SYN: block (3), heart block.