Circadian rhythm of cardiac responses to vagal stimulation tests

To evaluate the existence of a circadian rhythm in cardiac responses to vagal stimulation tests, 10 children without neurologic or cardiopulmonary disease, were studied using a standardized procedure. A trigeminal airstream stimulation test (TAS) and an ocular compression test (OCT) were subsequently performed at 4 nonequidistant hours. These 2 vagal stimulation tests were performed according to well-established, previously published techniques. Prolongation of the RR-interval during TAS and OCT was calculated and compared to the mean control RR-interval preceding the stimulation period. Prolongation of the RR-interval during the stimulation period was obtained in all instances during TAS and OCT. Chronobiologic analysis was determined by the cosinor method and bootstrap analysis. A significant circadian rhythm was found in cardiac responses to vagal stimulation for both TAS and OCT. The maximal response hour for TAS was between 9:06 p.m. and 5:18 a.m. and for OCT between 10:18 p.m. and 4:24 a.m. The mean value and 95% intervals for the cardiac responses to both vagal stimulation tests revealed, when using the bootstrap analysis, an important overlap in the hour of appearance of the maximal responses to these procedures; the bootstrap analysis of the peak of the acrophase was at 2:50 a.m. for TAS and 1:15 a.m. for OCT. Our results indicate that there is a circadian rhythm in cardiac responses to vagal stimulation tests and demonstrate a preponderance of these responses during the early morning hours.