partial or complete block of electric impulses originating in the atrium or sinus node preventing them from reaching the atrioventricular node and ventricles. In first degree A-V block, there is prolongation of A-V conduction time (P-R interval); in second degree A-V block, some but not all atrial impulses fail to reach the ventricles, thus some ventricular beats are dropped; in complete A-V block, complete atrioventricular dissociation (2) occurs; no impulses can reach the ventricles despite even a slow ventricular rate (under 45 per minute); atria and ventricles beat independently. Syn: block(3), heart block.