Differential effects of autonomic blockade on sinus and atrioventricular nodal function in normals and in intrinsic sinus node dysfunction.

The effect of pharmacologic total autonomic blockade on sinus and atrioventricular nodes was studied in 10 normals and 21 patients with sick sinus syndrome with abnormal intrinsic corrected sinus node recovery time. In normals the intrinsic heart rate (113.3 +/- 11.6 beats/min) was higher than the resting heart rate (87.3 +/- 12 beats/min; P less than 0.001). The AH interval at an identical paced rate decreased from 119 +/- 36 msec to 93 +/- 17.6 msec after autonomic blockade (P less than 0.05). Mean atrial paced cycle length at AH Wenckebach block was not different during control and after drugs (319 +/- 46 msec vs. 311.5 +/- 39 msec; P = NS). Although sinus cycle length shortened in all cases after autonomic blockade, paced cycle length at AH Wenckebach increased (4) or remained unchanged (3) in 7 cases. Maximum normal "intrinsic" paced cycle length at AH Wenckebach was 390 msec (mean +/- 2 SD). In sick sinus syndrome, resting heart rate (66.3 +/- 18.8 beats/min) and intrinsic heart rate (74.6 +/- 16.4 beats/min) were similar (P = NS); AH at identical paced rate: control 136.6 +/- 54 msec, after drugs 130.5 +/- 35 msec (P = NS); cycle length at AH Wenckebach: control 380.5 +/- 73 msec, after autonomic blockade 383 +/- 49 msec (P = NS). Two of 3 cases with abnormal atrioventricular nodal response to atrial pacing during control normalized after autonomic blockade; 9/21 (42.8%) cases developed AH Wenckebach at cycle length greater than 390 msec after autonomic blockade. The data suggest that the autonomic nervous system has differential effects on sinus and atrioventricular nodes. Patients with sick sinus syndrome frequently have abnormalities of "intrinsic" atrioventricular nodal conduction unmasked by autonomic blockade.