Altered Autonomic Mechanisms in the Sick Sinus Syndrome and Diurnal Variations in the Parameters of Sinus Node Function : Symposium on Sinus Node Function and Its Abnormality

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Abstract
The role of 2 divisions of autonomic nervous system was investigated and the variations of the sinus cycle length after autonomic blockade (the intrinsic cycle length, ICL) and the sinus node recovery time (SNRT) were tested in 26 patients with symptomatic sinus bradycardia. Three groups of patients were recognized from the responses of sinus rate to autonomic blocking with propranolol followed by atropine, comparing with the healthy controls. In 9 patients (group A) with normal responses to cholinergic and .beta.-adrenergic blockades, reactions to the autonomic stimulations with baroreflex and Valsalva maneuver and to isoproterenol were all normal. Reduced automaticity of sinus node and normal autonomic regulation are conceivable in these patients. In some of this group, sinoatrial disease was evident from abnormal ICL. In other 17 patients with reduced response to atropine, reactions to the autonomic stimulations and to isoproterenol were blunted. With propranolol enhanced prolongation of sinus cycle length was observed in 7 patients (group BI) and normal or reduced response in 10 (group BII). Severe clinical symptoms were more prevalent in group BI and BII patients than group A and SNRT and ICL was abnormal in most of Group B. Sinoatrial disease is evident in these patients; reduced cholinergic control and partial .beta.-adrenergic compensation in group BI and disturbed regulation by cholinergic and .beta.-adrenergic system in group BII are suggested. Difference between ICL at 1600 h and 0000 h was > 200 ms in 8 and exceeded 300 ms in 5 of 17 patients whose ICL were determined twice. Measurement of SNRT was repeated > 3 times at 0000 h, 0600 h and 1000 h and 1600 h and 2200 h in 18 patients. SNRT fluctuated > 2 s in 10 and the ratio of the longest SNRT to the shortest was 2 or more in 7 patients. Different and altered autonomic mechanisms exist in patients with sick sinus syndrome. Sinus node automaticity free from autonomic influence is fluctuating and the clinical parameters of sinus node function are non-reproducible in some of the sick sinus patients.