This study we aimed i) to clarify the time courses of HR changes during apneic facial immersion in water, and ii) to compare them to those observed on land. Six healthy Japanese adult males participated in this study. HR were measured under 4 different conditions on land and in water (Figure1): (1) standing (S-condition), (2) standing with breath-holding (SB-condition), (3) forward bending with breath-holding (FB-condition), and (4) apneic facial immersion (AFI-condition). Each participant immersed his face for 1 minute while standing at leaning forward position on land or in water. In the water condition, the water surface was set at the xiphisternal level. The water temperature for both facial and body immersion were kept at 36 degrees Celsius. HR was calculated from R-R interval of the ECG. In the SB-condition in water, HR decreased within a few seconds right after the onset of breath holding and was stable till to the end of the trial. The patterns of HR changes between on land and in water were different in the FB-condition. The patterns of HR changes in the AFI-condition, were very similar between on land and in water, i.e. decrease -> increase -> decrease -> staying. Our results indicated that time courses of HR changes during apneic facial immersion "in water" is similar to those observed "on land".
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