Influences of autonomic changes on the sinus node recovery time in patients with sick sinus syndrome

To assess the relative contribution of sympathetic and vagal influences on diurnal variation of sinus node recovery time (SNRT) in sick sinus syndrome (SSS), the diurnal changes of SNRT and the effects of propranolol and subsequent atropine on SNRT were examined in 39 patients with SSS. SNRT was measured before and after intravenous propranolol (0.1 mg/kg), and after subsequent intravenous atropine (0.02 mg/kg) in the cardiac catheterization laboratory. After completion of the electrophysiologic studies in the laboratory, SNRT was measured at 0 a.m. (midnight), 6 a.m. and 12 noon on the following day in the ward. After propranolol, SNRT was prolonged in 22 of 26 patients and shortened in 4 patients. After subsequent atropine, SNRT was prolonged in 5 of 26 patients and shortened in 21 patients. The patients with SNRT longer than 3 sec had a tendency to have greater diurnal variation of SNRT than those with SNRT less than 3 sec. A strong correlation (r = 0.98) was found between SNRT after propranolol and the longest SNRT in a 24-hour period. A difference of SNRT between after propranolol and after subsequent atropine was significantly correlated (r = 0.88) with a difference between the longest and the shortest SNRT in a 24-hour period. These results suggest that the diurnal changes in SNRT are regulated by the autonomic nervous system in SSS. SNRT after propranolol may be useful in estimating the longest SNRT in a day.