sinoatrial (SA) block [L, sinus, hollow, atrium, hall; Fr, bloc] , a conduction disturbance in the heart during which an impulse formed within the sinus node is blocked or delayed from depolarizing the atria. There are two types of SA block. Type I (SA Wenckebach) is characterized on the electrocardiogram by group beating, shortening of P-P intervals, and pauses that are less than twice the shortest cycle. The P-R intervals are not affected unless there is also an atrioventricular conduction defect. In the case of a 3:2 conduction ratio, a bigeminal sinus rhythm (two sinus-conducted beats and a pause) is noted. Type II SA block is identified on the electrocardiogram by absent P waves without shortening P-P intervals. Causes include excessive vagal stimulation, sinoatrial block acute infections, and atherosclerosis. SA block also may be an adverse reaction to quinidine or digitalis. Treatment for symptomatic SA block includes the use of atropine and isoproterenol; an electronic pacemaker is used if drug therapy is ineffective. See also atrioventricular block, heart block, intraatrial block, intraventricular block.
