[Vagal effect on sino-atrial node frequency and A-V overdrive in hypersensitive carotid sinus reflex and sick sinus...

Two syndromes, hypersensitive carotid sinus reflex (HCSR) and "sick sinus syndrome" (SSS) were studied in 28 patients, 6 of whom served as controls. Carotid sinus pressure was performed during sinus rhythm (CSP 1) and atrial stimulation just above the sinus rate (CSP 2). Overdrive suppression was also tested. Vagal stimulation provoked different effects in HCSR and in SSS. The vagal reaction to CSP 1 was more manifest in HCSR than in both SSS and the controls. After CSP 2 A-V conduction was delayed longer in HCSR than in SSS and the controls. Following atropine, the vagal effect on HCSR was more pronounced than on SSS and the controls. With HCSR, the duration of vagal influence on R-R intervals (A-V conduction) was significantly shorter than on P-P intervals (sinus rate). After atropine, there was less vagal effect on A-V conduction than on the sinus rate in HCSR, meaning that A-V conduction was shorter and less affected than the sinus rate by carotid sinus pressure. Prolonged suppression following overdrive established the diagnosis of SSS. This suppression was found not to be affected by atropine. A dependency of suppression on the rate and duration of overdrive could not be ascertained for SSS. The newly formulated "postdrive P-P interval recovery time" subdivided SSS in such a way that two different degrees of severity could be illustrated. The conclusion was that HCSR was caused by an excessive reaction to vagal stimulation rather than by a dysfunction of the sinus node or A-V conduction system. SSS might be based on tissue damage in the sinus nodal region but it did not show increased sensitivity to vagal influence.