Spectral characteristics of heart rate and blood pressure variabilities during head-out water immersion.

To elucidate the roles of the autonomic nervous system in cardiovascular adaptation to microgravity, spectral analysis of heart rate and blood pressure variabilities were conducted during thermoneutral head-out water immersion in 8 healthy young subjects aged 23 to 31 years. In all subjects immersed up to the shoulder, the low-frequency (LF: 0.04-0.15 Hz) component in blood pressure variability and the ratio of LF power to high-frequency power (LF/HF ratio) in heart rate variability decreased, while the high-frequency (HF: 0.15-0.5 Hz) component of heart rate variability increased. Concomitantly, a decrease in heart rate was observed without significant changes in blood pressure during immersion. These results suggest that cardiac and vasomotor sympathetic nerve activities are suppressed and cardiac parasympathetic (vagal) nerve activity is activated during head-out water immersion.