It is well known that there is wide individual variation of responsiveness when parasympathetic nervous system is stimulated physiologically, pathologically, and therapeutically. But no appropriate explanation is present for that yet. So the purpose of this study is to verify individual diversity of vagal tone during resting state and after vagal stimulation and to find factor that may significantly contribute to it.

We serially measured variation of heart period(VHP : sec) which had been known to be an accurate index of cardiac parasympathetic tone during resting state(VHP_b) and after vagal stimulation by ice water application to face(VHP_v). And we analyzed correlation between VHP_b, VHP_v and ratio of VHP_v to VHP_b(ratio).

There was wide variation of vagal tone both in resting state (VHP_b : 0.025-0.161) and after vagal stimulation (VHP_v : 0.087-0.661). VHP_b could be classified into 3 groups according to stem and leaf distribution(group A : VHP_b could be classified into 3 groups according to stem and leaf distribution(group A : VHP_b<0.07, B : 0.07-0.1, C : >0.1). The low VHP_b group A showed dramatic increase in vagal tone after vagal stimulation (ratio 2.21-20.24 mean 7.7), on the other hand high VHP_b group C did not (ratio 0.80-2.55 mean 1.39). There ws also a significant negative correlation between VHP_b and ratio (r=0.69, p<0.001).

These results suggest that there is wide individual diversity of responsiveness to vagal stimulation and magnitude of vagal tone in resting state is an important factor to determine the responsiveness to vagal stimulation.