Differential effects of functional autonomic blockade on the variables of sinus nodal automaticity in sick sinus syndrome

To study the pathophysiologic mechanism of sick sinus syndrome and to establish the relation of intrinsic heart rate, corrected sinus nodal recovery time and sinoatrial conduction time in this syndrome, electrophysiologic studies were conducted in 22 men (mean age 60 ± 12 years) with the clinical diagnosis of sick sinus syndrome. Measurements were determined before and after autonomic blockade with propranolol (0.2 mg/kg body weight) and atropine sulfate (0.04 mg/kg). Fifty-nine percent of patients (Group I) had an abnormal intrinsic heart rate, suggesting intrinsic abnormality of sinus nodal automaticity; 41 percent (Group II) had a normal intrinsic heart rate after autonomic blockade, suggesting disturbed autonomic regulation. One patient with an observed intrinsic heart rate higher than the upper limit of predicted intrinsic heart rate was also included in Group II. The mean corrected sinus nodal recovery time before autonomic blockade was 751 ± 502.8 ms and was abnormal (more than 450 ms) in 10 of the 13 patients in Group I and 2 of the 9 patients in Group II. After autonomic blockade this interval was 694 ± 638.7 ms and was abnormal in 12 of the 13 patients in Group I and in 2 of the 9 patients in Group II. The patients in each group could be further classified into three groups on the basis of normal or abnormal corrected sinus nodal recovery time before or after autonomic blockade. Not all patients with abnormal intrinsic heart rate (Group I) had abnormal corrected sinus nodal recovery time and vice versa. Patients in Group II were younger in age, had a lesser incidence of organic heart disease and were more severely symptomatic.Mean sinoatrial conduction time during control studies was 210.4 ±96.3 ms and decreased significantly (143.2 ± 59.6 ms, p < 0.005) after autonomic blockade. This interval was abnormal in 3 of the 13 patients in Group I and in 6 of the 9 patients in Group II during control studies; after autonomic blockade it remained abnormal in 3 patients in Group I and in 1 patient in Group II.It is concluded that determination of heart rate and corrected sinus nodal recovery time after autonomic blockade increases the sensitivity of electrophysiologic testing and offers some insight into the pathophysiology of sick sinus syndrome. Patients with sick sinus syndrome who have a normal intrinsic heart rate have a greater incidence of abnormal sinoatrial conduction time than do those with an abnormal intrinsic heart rate. Thus, abnormal sinoatrial conduction time is usually due to extrinsic autonomic influences.