Peripheral and central blood flow in man during cold, thermoneutral, and hot water immersion.

Cardiovascular reflexes were studied during immersion in water to the chest. Cardiac output (CO) was determined by acetylene rebreathing; forearm muscle and subcutaneous blood flow by 133Xe-clearance; and cutaneous blood flow by laser Doppler. Measurements were taken in a) control situation (CTR) (subject sitting dry); b) immersed in thermoneutral (NWI); c) in cold (CWI); and d) in hot water (HWI). The overall trend was that water immersion per se increased stroke volume (SV), but mostly during NWI and CWI, where heart rate (HR) was decreased by 15%; during HWI, HR increased by 32%, the temperature effect evidently overriding the immersion effect. Insignificant increases in CO were seen in NWI and HWI (18% and 44%), and no effect in CWI. Arterial pressure and total peripheral resistance (TPR) increased significantly in CWI due to an increase in peripheral vascular resistance, while significant decreases in TPR and CPR were observed in HWI and tendencies to decreases were found in NWI.