水温27℃, 34℃, 38℃における短時間水浸が心臓自律神経系活動に及ぼす影響

呼吸の影響を考慮した検討

抄録

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.