A conduction disturbance that consists of a delay (or complete inability) of a electrical impulse, generated in the atria, to reach the ventricles. Clinical types are divided into first (no serious), second and third degree (most serious). Some drugs may precipitate atrioventricular block (for example clonidine, methyldopa, verapamil). A permanent pacemaker may be required for a third degree (complete) heart block.