The effects of negative chronotropic interventions on sinus node recovery time

The effects of multiple increases in sinus cycle length on sinus node recovery time (SNRT) were examined in 5 dogs. Pacing was performed from the left atrial appendage for 30 and 60 seconds using at least 4 different pacing cycle lengths selected between 230 and 620 msec. Each dog received propranolol (1 mg/kg, IV) prior to any measurements. The effects of increases in sinus cycle length on SNRT were first assessed during 2 levels (4 and 8 Hz) of continuous vagal stimulation. From a control cycle length of 439±28 msec (mean±SE), the vagal stimulations lengthened the sinus cycle lengths to 604±10 msec and 758±16 msec respectively. Sinus cycle length was then prolonged by combined muscarinic and beta-receptor blockade resulting in a sinus cycle length of 549±9 msec. Autonomic blockade plus verapamil (3–10 mg IV) resulted in sinus cycle lengths of 612±14 and 721±18 msec respectively, which were not significantly different from those obtained with vagal stimulation. Data relating SNRT to the sinus cycle length, pacing cycle length, duration of pacing and the negative chronotropic interventions used to achieve the changes in the sinus cycle length were analyzed via covariance analysis. The results demonstrate that the single most important determinant of SNRT is the sinus cycle length. Furthermore, equivalent increases in sinus cycle length whether obtained by vagal stimulation, autonomic blockade or intravenous verapamil results in SNRTs that are not significantly different. Therefore, in the sinus node, changes in the rate of pacemaker activity, regardless of how they are achieved, will largely determine the changes in SNRT.