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A series of experiments were carried out to clarify the cause of death and the risk factors related to sudden death in the Japanese senior population while bathing in a Japanese style "hot bath." The biodynamic changes while bathing were carefully monitored under actual bathing situations occurring in both the winter and summer seasons. We observed double product (DP), total peripheral blood vessel resistance (TPR), cardiac output (CO), and blood vessel compliance (COMP) by measuring blood pressure, heart rate, pulse wave, and electrocardiogram (ECG). The finding of a high level of DP in the elderly suggests that more myocardial oxygen consumption is needed than for young adults, particularly in subjects with arrhythmia. Although the values for TPR and CO changed somewhat during bathing, the changes were considered normal and to be expected. However, more significant and substantial changes were observed during the winter experiment than during the summer experiment, no doubt owing to lower temperature of the bathing room. The value of COMP did not vary significantly between winter and summer subjects. Twelve subjects in the elderly developed ECG changes while bathing such as supraventricular extrasystole or ventricular tachycardia. No clinical significance was found in the biochemical analyses of the blood obtained before and after bathing. In conclusion, some subjects in the elderly showed risky changes in the above parameters and ECG, factors which may partially explain some of the causes of the many reported cases of lapse of consciousness and unexpected sudden death in the elderly while bathing especially in the winter season. Cold climate, hot water immersion, and hydrostatic pressure may affect their physiological compensation along with existing of coronary stenosis or weakness of respiratory function as a normal consequence of advanced age.

About 100-120 cases of 'sudden death in bathroom' are reported every year in our prefecture, which is 8-10% of the total number of what is considered unnatural deaths. For reasons that are still unclear, the victims die while bathing in the bathtub. This baffling form of death occurs mostly in the winter season and 80-% of these are elderly persons. However, despite a full autopsy, it is sometimes still very difficult to determine the cause of death or the direct cause of drowning in the bathtub. To prevent others from this mysterious form of death and to determine the risk factors for the elderly while bathing, we examined several parameters such as physiological changes and biochemical blood analysis in a total of 54 volunteers during both the winter and summer seasons in Japan. We found that some of the elderly developed cardiac arrhythmia such as ventricular tachycardia, ventricular extrasystole, supraventricular extrasystole, and bradycardia. Also, decreases of systolic blood pressure during bathing were observed during this experiment. We conclude that the development of extrasysytole or ventricular tachycardia can be a stress involved in bathing. These changes might contribute to cardiac arrest or to drowning in the bathtub. Moreover, a sudden decrease of blood pressure is considered to be a risk factor of cerebrovascular accident.