Autonomic blockade and sick sinus syndrome. New concept in the interpretation of electrophysiological and Holter data.

Autonomic blockade is commonly employed as a test of sinus node dysfunction. We compared primary and secondary atrial postpacing pauses and postextrasystolic sino-atrial responses before and after autonomic blockade in 56 patients with the clinical diagnosis of sick sinus syndrome. Pharmacological autonomic blockade was achieved by atropine 0.04 mg kg-1 and propranolol 0.2 mg kg-1, i.v. In a group of patients with a normal intrinsic heart rate the number of positive electrophysiologic variables associated with sinus node dysfunction declined after autonomic blockade. In 91% of these patients, sinus node function was characterized by a normal intrinsic recovery time, gradual exponential return to the constant sinus cycle length, and biphasic postextrasystolic return responses. Three patients in this group had intrinsic SA-block revealed by atrial pacing and verified by Holter monitoring. Besides normal intrinsic pacemaker properties in 53% of patients, rhythm monitoring revealed severe sinus node dysfunction as manifested by bradycardia and the tachycardia-bradycardia syndrome. SA-block and sinus arrest up to 29120 ms. In the abnormal intrinsic heart rate group, disturbed intrinsic rhythmicity was characterized in all by a prolonged corrected intrinsic recovery time (2320 +/- 2740 ms [+/- SD]), arrhythmia and/or bradycardia in the secondary postpacing cycles, chaotic postextrasystolic patterns, or prolonged sinoatrial conduction times. Significantly slow minimal heart rates during sleep significantly prolonged average sinus cycle lengths and positive ECGs for sinoatrial disorders in the waking period were present on the 24-h rhythm recording. It is concluded that intrinsic heart rate obtained by autonomic blockade is the best and most simple method for the diagnosis of intrinsic sinus node dysfunction. Combined autonomic blockade and electrophysiological tests can be of great value in unmasking the severity and degree of intrinsic dysfunction and analyzing the abnormality of secondary pacemaker function. These investigations, however, are rather insensitive and therefore ineffective in detecting autonomic sinus node dysfunction. To assess the role and significance of the autonomous neurovegetative tone in the genesis of sinoatrial disorders, rhythm monitoring is required.