Clinical Study on Accentuated Antagonism in the Regulation of Heart Rate in Children.

Facial immersion testing in cold water (< 4 degrees C) was performed to study the responses of sinus cycle length to increased parasympathetic tone before and 5 min after exercise testing in 27 children. There were no episodes of sinus arrest or extrasystole during the facial immersion testing. The resting sinus cycle lengths were significantly shorter after (539 +/- 68 msec) than before (597 +/- 96 msec) exercise testing (p < 0.001). The maximal sinus cycle lengths before and after exercise testing during cold water facial immersion testing did not differ significantly (928 +/- 167 msec and 909 +/- 128 msec, respectively). Vagal chronotropic responses were calculated from the control sinus cycle lengths and the maximal sinus cycle lengths during facial immersion testing. Facial immersion caused greater prolongation of sinus cycle length after than before exercise (73 +/- 27% and 54 +/- 26%, respectively; p < 0.005). We speculate that this augmentation of vagal activity represents accentuated antagonism in these children, i.e., the same parasympathetic stimulus causes a greater response in the presence of a stronger background sympathetic activity.