Vasovagal reactions induced by head-up tilt and tests of vagal cardiac function.

The vagal cardiac activity was compared in two groups of patients with a history of syncope. Sixty-one patients (Group A) experienced vasovagal reactions induced by head-up tilt, 61 sex and age-matched patients (Group B) did not. No significant differences in vagal cardiac activity between two groups were found. Group A was further subdivided into two subgroups: A1 (14 patients) with stronger, and A2 (47 patients) with smaller cardioinhibition. Again, vagal cardiac activity was not different between two subgroups. Thus, the baseline vagal cardiac activity ('tone') seemed not to be helpful in predicting the susceptibility to vasovagal orthostatic syncope and in discriminating the patients with stronger cardioinhibition.