Role of autonomic regulatory mechanism in sinoatrial conduction and sinus node automaticity in sick sinus syndrome.

Publisher Website
Google Scholar
Abstract
To assess the role of autonomic regulatory mechanisms on sinoatrial conduction and automaticity in patients with clinical evidence of sick sinus syndrome, electrophysiologic studies were conducted in 12 male patients, mean age 64 .+-. 7 yr. Heart rate [HR], sinus node recovery time (SNRT) and sinoatrial conduction time (SACT) were determined before and after autonomic blockade with atropine (0.04 mg/kg) and propranolol (0.2 mg/kg). The mean sinus cycle length was 1033 .+-. 208 ms before autonomic blockade and 1016 .+-. 188 ms after autonomic blockade. The observed intrinsic heart rate (IHR) after autonomic blockade was abnormal in 9 of 12 patients. The mean corrected SNRT [SNRTc] before autonomic blockade was 606.5 .+-. 432 ms and was abnormal (> 450 ms) in 6 of 12 patients (50%). After autonomic blockade the mean SNRT was 661.8 .+-. 604.8 ms and was abnormal (> 450 ms) in 8 of 12 (67%). Six of 9 patients (66%) with an abnormal observed IHR had an abnormal SNRTc before autonomic blockade. After autonomic blockade, 8 of these 9 (88%) had an abnormal SNRTc. SACT was estimated by continuous pacing and by premature stimulation. The mean SACT estimated by continuous pacing before autonomic blockade was 190.3 .+-. 99.6 ms and decreased significantly after autonomic blockade (140.6 .+-. 53.4 ms, P < 0.05). The mean SACT estimated by premature stimulation before autonomic blockade was 218 .+-. 87.6 ms and decreased significantly after autonomic blockade (143.7 .+-. 49.9 ms, P < 0.01). The correlation coefficient between the 2 methods was 0.8 before autonomic blockade and 0.85 after autonomic blockade. SACT was abnormally prolonged (> 206 ms) in 5 of 12 patients (41%) during control studies and in 2 of 12 patients (16%) after autonomic blockade. SACT by either method decreased in 8 of 12 patients (67%) and increased in 4 of 12 patients (33%) after autonomic blockade. Changes in SACT after autonomic blockade could not be predicted from symptoms, ECG findings or determinations of IHR. Assessment of IHR, SNRT and SACT after autonomic blockade can differentiate patients with sick sinus syndrome, whether due to intrinsic or extrinsic abnormality of sinus node automaticity or conduction. The significant decrease in SACT in most patients with sick sinus syndrome after autonomic blockade suggests enhanced basal parasympathetic tone and normal intrinsic conduction in the perinodal region. The increase in SACT in a minority of patients after autonomic blockade suggests that enhanced basal sympathetic tone can mask an underlying intrinsic abnormality of sinoatrial conduction.