A heart block is an abnormality of conduction of the electrical heart impulse. Both anatomic or functional imapirment of atrial-ventricular (AV) conduction is contributory. Types: first degree AV block: P-R interval longer than 0.21 seconds in adults; all supraventricular impulses are conducted second degree AV block: Mobitz type I (Wenckebach) block: conduction time (P-R interval) is progressively prolonged prior to dropped beats. Mobitz type II conduction time (P-R interval) is not prolonged prior to dropped beats. third degree AV block: no impulses are conducted.