[Effects of head-up tilting on vagal nerve activity in man]

To evaluate the contribution of the vagal nerve activity in the cardiovascular postural adaptation, effects of decremental head-up tilting (90 degrees, 64 degrees, 53 degrees, 44 degrees, 37 degrees, 30 degrees, 24 degrees, 17 degrees, 12 degrees, 6 degrees and 0 degree) on time- and frequency-domain heart rate variability variables were analyzed in healthy young female.

During head-up tilting, a hydrostatic venous pooling in the extremities occurs owing to gravity. To pump up the blood toward the upper body, the sympathetic nerve activity has been shown to play an important role. So, to date, few studies evaluated the effects of vagal nerve activity to stabilize the cerebral blood flow during head-up tilting.

Eight young female volunteers (age, 23.3 +/- 0.8 years; mean +/- SD) were evaluated. The electrocardiogram (ECG) by bipolar chest leads was recorded continuously during procedures, and the bed was tilted at 0.1 interval of sine function of tilting angle from upright position (90 degrees) to supine position (0 degree). The time domain measurements of cycle length variability (co-efficient of variance in percent for R-R intervals [CVRR], number of differences between adjacent R-R intervals that are > 50ms [RR50]) and the frequency domain measurements of low (0.08 to 0.15Hz, LF), high (0.15 to 0.40Hz, HF) and total (0.08 to 0.40, TF) power were performed to assess the cardiac sympathetic and vagal nerve activity.

The CVRR showed no significant change during decremental head-up tilting, whereas the RR50 and the square root of HF power, more specific indices of cardiac parasympathetic tone, showed significant negative linear correlations to the sine of the tilting angle. In markers of cardiac sympathetic tone, there were significant positive correlations between the sine of the tilting angle and the normalized LF power or the LF-to-HF power ratio (LF/HF).

These findings suggest that, in healthy young female, not only cardiac sympathetic nervous system but also cardiac vagal nervous system respond linearly to the change in body axis component of gravity, and they may contribute reciprocally and coordinately to cardiovascular postural adaptation.