Experimental investigation in rats to identify the cause of sudden death during bathing in J apan

Aim Approximately 14,000 cases of sudden death during bathing are reported annually in J apan. The cause of the deaths is still unclear. Because J apanese‐style bathing is characterized by whole‐body bathing in high temperature water (41–45°C), we hypothesized that heat exposure causes sudden death as a result of hyperthermia. Methods The aim was to clarify the relationship between heat exposure and mortality in an animal experiment. In the experiment, 44 male Wistar rats weighing 180–255 g were anesthetized with sodium pentobarbital, and their body temperature was measured with a thermometer inserted into the esophagus. The animals were immersed up to their shoulder in 40–45°C water until respiratory arrest occurred or until they awoke from the anesthesia. Results All animals immersed in water heated to ≥41°C died, whereas all animals immersed in 40°C water awakened from the anesthetic. The mean interval between the start of immersion and respiratory arrest in 41°C, 42°C, 43°C, 44°C, and 45°C water was 105 min, 54 min, 34 min, 27 min, and 22 min, respectively. Regression analysis revealed a significant relationship between the interval and water temperature (r 2  = 0.96, P  < 0.01), and a Cox proportional hazard analysis revealed that water temperature was a significant predictor (adjusted odds ratio, 23.9; 95% confidence interval, 7.4–77.2). The body temperature of the animals that died during hot water immersion was above 41°C (median, 42.9°C). Conclusions Water temperature was correlated with the interval between the start of immersion and respiratory arrest. The results suggest that hyperthermia contributes to the cause of sudden death during hot water bathing.