We have been proceeding with studies on the effects of water immersion on autonomic nerve activity using the power spectral analysis of heart rate variability. The results obtained so far suggest that cardiac parasympathetic nerve activity is enhanced and sympathetic nerve activity is suppressed during immersion at temperatures between 25°C and 34°C and that parasympathetic nerve activity is suppressed and sympathetic nerve activity is enhanced during immersion at temperatures around 38°C. However, water immersion affects the respiration rate and tidal volume, and though the change in the respiration rate does not affect the real cardiac autonomic nerve activity, it affects the index of autonomic nerve activity as assessed by the power spectral analysis of heart rate variability. Therefore, this study examined the changes in cardiac autonomic nerve activity during water immersion with the tidal volume measured and its changes considered while controlling the respiration to a certain level. Eight healthy young males (ages: 19 to 28) sat calmly for 20 minutes before immersion and then soaked in water at the subaxillary level in sitting position for 15 minutes while controlling their respiration rate to 15cycles/min. Autonomic nerve activity was estimated by the power spectral analysis of the heart rate together with the Fast Fourier Transformation. Integral values of power were obtained in the high frequency (HF; 0.15 to 0.50Hz) and low frequency (LF; 0.04 to 0.15Hz) component areas. HF was used as the index of cardiac parasympathetic nerve activity, and the ratio of LF to HF (LF/HF), as the index of cardiac sympathetic nerve activity. During immersion at 34°C, HF increased significantly and the heart rate and LF/HF decreased slightly though not at a statistically significant level. During immersion at 27°C, HF increased significantly and the heart rate and LF/HF decreased significantly. During immersion at 38°C, the heart rate increased significantly while HF decreased and LF/HF varied slightly with no statistical significance. The tidal volume increased significantly during immersion at 27°C and 34°C, and it increased during immersion at 38°C though it was not statistically significant.
These results suggest that cardiac parasympathetic nerve activity is enhanced while sympathetic nerve activity is suppressed during immersion at 27°C, because the remarkable increase in HF that occurred during immersion cannot be accounted for by the increase in the tidal volume per breathing cycle alone. However, it is possible that the increase in the tidal volume enhanced the increase in HF. It was suggested, however, that autonomic nerve activities did not change significantly during water immersion at 38°C though there is possibility that the changes in HF were underestimated due to the increase in the tidal volume.

The purpose of this study was to assess an alteration of cardiac autonomic nerve activity during water immersion. Ten healthy young males (age : 21-28 yr.) were immersed in water at the temperatures of 25°C, 30°C and 34°C. Subjects sat calmly for 20 minutes in sitting position before water immersion, and then were immersed in water at subaxillary level in sitting position for 15 minutes, performing controlled breathing (15 cycle/min.) . Electrocardiograms were recorded continuously. Autonomic nerve activity was estimated with the analysis of power spectral by using the Fast Fourier Transformation (FFT) . High (HF ; 0.15-0.50 Hz) and low (LF ; 0.04-0.15 Hz) frequency areas and the ratio of LF to HF (LF/HF) were calculated as the indices of cardiac parasympathetic nerve activity, sympathetic nerve activity with parasympathetic modulation, and sympathetic nerve activity, respectively. During the water immersion at 25°C, 30°C and 34°C, HF was significantly increased, while the heart rate and LF/HF were significantly decreased. There were no statistically significant differences among both of HF and LF/HF during the immersion at 25°C, 30°C and 34°C, although the rate of change in HF at the temperature of 25°C appeared to be prominent compared to those at 30°C and 34°C and some subjects showed an exaggerated change in HF immediately after immersion. These results suggest that cardiac parasympathetic nerve activity is enhanced and cardiac sympathetic nerve activity is suppressed during a short time water immersion at the thermo-neutral temperature (34°C) and the temperatures of 25°C and 30°C, which are the usual temperatures found in indoor pools.

This study was conducted to assess the effects of bathing in still water and in flowing water on the heart rate variability. Eight healthy young males (age 20 to 28) bathed in still water at temperatures of 34°C, 38°C, and 41°C for 20 minutes each. The other eight healthy young males (age 22 to 28) bathed in flowing water at a temperature of 36°C for 30 minutes. Electrocardiograms were recorded before, during, and after the bathing. Subjects sat still for 20 minutes before bathing, and then bathed in water to the axilla in a sitting position. Subjects breathed freely during the experiment. Heart rate variability was estimated with the power spectral analysis using FFT. The power densities in the high frequency (0.15 to 0.50Hz) and low frequency (0.04 to 0.15Hz) areas as obtained from this frequency analysis (HF and LF) as well as the ratio of LF/HF were calculated, and HF was used as index of cardiac parasympathetic activity, LF as index of sympathetic activity with parasympathetic modulation, and LF/HF as index of sympathetic activity.
During bathing in still water at 34°C and 36°C, no significant change from the value before the bathing was found in heart rate, HF, LF, or LF/HF. HF and LF significantly decreased during the bathing in still water at 38°C and 41°C, LF/HF significantly increased during the bathing in still water at 38°C, During the bathing in still water at 41°C, we could not calculate LF/HF for many subjects because HF disappeared. During the bathing in water flowing at a moderate speed (1.0m/sec), LF/HF increased significantly. During the bathing in water flowing at a high speed (2.0m/sec), heart rate and LF/HF increased significantly while LF decreased significantly.
These results suggest that parasympathetic nervous activities are suppressed and sympathetic nervous activities are enhanced during bathing in still water at temperatures higher than the neutral temperature (34°C), and sympathetic nervous activity is enhanced during the bathing in flowing water at 36°C, However, the effects of respiration rate and tidal-volume on HF, and the validity of the HR variabilities as an index of autonomic nervous activities should be examined in further detail.