CHANGES IN BODY CORE TEMPERATURES AND HEAT-BALANCE AFTER AN ABRUPT RELEASE OF LOWER-BODY NEGATIVE-PRESSURE IN HUMANS

Changes in body core temperature (T(cor)) and heat balance after an ab rupt release of lower body negative pressure (LBNP) were investigated in 5 volunteers under the following conditions: (1) an ambient tempera ture (T(a)) of 20-degrees-C or (2) 35-degrees-C, and (3) T(a) of 25-de grees-C with a leg skin temperature of 30-degrees-C or (4) 35-degrees- C. The leg skin temperature was controlled with water perfusion device s wound around the legs. Rectal (T(re)), tympanic (T(ty)) and esophage al (T(es)) temperatures, skin temperatures (7 sites) and oxygen consum ption were measured. The intensity of LBNP was adjusted so that the am ount of blood pooled in the legs was the same under all conditions. Wh en a thermal balance was attained during LBNP, application of LBNP was suddenly halted. The skin temperatures increased significantly after the release of LBNP under all conditions, while oxygen consumption har dly changed. The release of LBNP caused significant falls in T(cor) s under conditions (1) and (3), but lowered T(cor) s very slightly under conditions (2) and (4). The changes in T(es) were always more rapid a nd greater than those of T(ty) and T(re). The falls in T(ty) and T(re) appeared to be explained by changes in heat balance, whereas the shar p drop of T(es) could not be explained especially during the first 8 m in after the release of LBNP. The results suggest that a fall in T(cor ) after a release of LBNP is attributed to an increase in heat loss du e to reflexive skin vasodilation and is dependent on the temperature o f venous blood returning from the lower body. It is presumed that T(es ) may not be an appropriate indicator for T(cor) when venous return ch anges rapidly.