Objective: Warm water immeresion (WWI) has been customary in Japan as useful thermal therapy. However, a comprehensive investigation of the effects of WWI on internal organs has never been undertaken. The purpose of this study was to examine the effects of single WWI on indocyanine green (ICG) excretion in healthy humans. Subjects: 23 healthy males. (32.6±1.9 [mean±SEM] years) Methods: 1CG was administered intravenously (0.5mg/kg) to calculate excretion rate (ER). ICG injection was given before and after WWI (10min at 41°C). Sublingual temperature (ST), blood pressure (BP), heart rate (HR), and cardiac output (CO) were simultaneously measured by noninvasive methods. Results: Significant changes were observed after WWI. These included an increase in ST, HR, and CO and a decrease in systolic BP (p<0.01). ER significantly decreased from 0.210±0.015 to 0.168±0.009 (p<0.01). Front-back ratio of ICG-ER which was calculated in order to evaluate the effect of aging increased significantly with age (R=0.582, p<0.0001). Conclusion: These results indicate that although CO increased due to the vasodilating effects of WWI, hepatic blood flow decreased after WWI and its response reduced with age even in before middle age.

Treadmill exercise tolerance test(Modified Bruce Method) was performed with and without warm water bathing (WWB, 41°C, 10min) in 14 healthy aged men and women over 65y. o. (68.6±6.0y.o).
Increase in HR, BP and PRP during exercise was reduced after WWB. Duration of exercise and ST depression and occurrence of arrythmia during treadmill exerecise were significantly improved after WWB. Fatigability of the legs and Borg's index were also decreased aftr WWB. These results indicated the improvement of exercise tolerance after WWB was probably due to the increase in cardiac functions and collagenous viscosity (extensivility) of the musculoskeletal system.

The effects of the bathing on renal function were studied in 12 healthy men (32.3±7.7y. o.). The subjects took 41°C, 10min bathing and kept warm by a blanket for 30min, and then a mixture of 10% para-aminohippurate (PAH, 0.3ml/kg) and 10% sodium thiosulfate (NTS, 80ml) was infused intravenously. Clearance of PAH and NTS was calculated as the index of renal plasma flow (RPF) and glomerular filtration rate (GFR). PAH and NTS clearance test was also done at rest without bathing in another day. Blood Pressure (BP), Heart rate (HR), cardiac output (CO), sublingual temperature by electric thermista as deep body temperature were measured during the experiment. 30min after bathing, sublingual temperature was significantly increased by 0.9°C, and CO by +40%. After bathing, renal plasma flow (PAH clearance) significantly increased from 388.5±158.9ml/min to 572±170.7ml/min. Glomerular filtration rate (NTS clearance) was, however, unchanged from 115.6±37.3ml/min to 119.3±51.3ml/min. Filtration fraction (GFR/RPF) was significantly decreased. These results indicated that GFR was not improved by bathing although CO and RPF was increased by thermal vasodilation effect. The mechanism of unchanged GFR, despite of increased RPF, is probably that glomerular filtration pressure unchanged by thermal vasodilation of glomerular efferent and afferent arterioles.

The effects of single bathing of the artificial mineral spring (Basukurin: Tsumura Juntendo Inc. Japan), equivalent mixture of sodium sulfate and sodium bicarbonate, were studied in 13 normal and 22 hypertensive subjects comparing with natural simple hot spring (Kirisima spa).
Blood pressure (BP: automatic sphygmomanometer), heart rate (HR), cardiac output (CO: earpiece type dye-dilution densisometer), total peripheral resistance (TPR: mean BP/cardiac index), forehead deep body temperature (DBT: Core Temp CTM 204) plasma renin activity (PRA: RIA by Habar's method), plasma norepinephrine (NE) and blood gas partial pressure (pO₂ and pCO₂) and pH were measured before and after bathing. Mineral concentration of simple hot spring and Basukurin bath (120g powder/360l simple hot spring) were 0.019% and 0.052%, respectively. The temperature and duration of bathing were both 41°C for 10min.
In normotensives, only systolic BP and TPR were significantly decreased 30min after the simple and Basukurin bathing by the same degree. HR was significantly increased throughout 30min after both bathing. CO was increased only after 10min of Basuku rin bathing. In hypertensives, systolic and diastolic BP and TPR were significantly decreased throughout 30min after simple and Basukurin bathing. The decrement in BP and TPR, however were significantly greater in Basukurin bathing. HR and CO were significantly increased after both bathing. Forehead DBT was significantly elevated after both bathing and gradually decreased throughout 30min of observation. In Basukurin bathing, although the increment in DBT was greater than simple hot spring bathing, the decrease was slower than simple hot spring bathing suggsting the protection from heat radiation by Basukurin bathing. PRA and plasma NE concentration examined in hypertensive subjects after 30min of bathing were significantly elevated only in Basukurin bathing. Although arterial gas concentration and pH were not changed after bathing, venous blood pO₂ and pH were significantly increased and pCO₂ was significantly decreased after 10min of Basukurin bathing.
These data indicates that single Basukurin bathing has more potent hypotensive effect due to vasodilation than simple hot spring. Enhanced vasodilation by Basukurin bathing is probably derived from the protection of heat radiation from the skin by the formtion of thin mineral film on the skin. Increased venous blood pO₂ and pH and decreased pCO₂ represents also the improvement of peripheral circulation assisted by vasodilation and elevated CO.