Elevated skin and core temperatures both contribute to reductions in tolerance to a simulated hemorrhagic challenge

Abstract

Tolerance to a simulated haemorrhagic insult, such as lower-body negative pressure (LBNP), is profoundly reduced when accompanied by whole-body heat stress. The aim of this study was to investigate the separate and combined influence of elevated skin (Tskin) and core temperatures (Tcore) on LBNP tolerance. We hypothesized that elevations in Tskin as well as Tcore would both contribute to reductions in LBNP tolerance and that the reduction in LBNP tolerance would be greatest when both Tskin and Tcore were elevated. Nine participants underwent progressive LBNP to presyncope on four occasions, as follows: (i) control, with neutral Tskin (34.3 ± 0.5°C) and Tcore (36.8 ± 0.2°C); (ii) primarily skin hyperthermia, with high Tskin (37.6 ± 0.2°C) and neutral Tcore (37.1 ± 0.2°C); (iii) primarily core hyperthermia, with neutral Tskin (35.0 ± 0.5°C) and high Tcore (38.3 ± 0.2°C); and (iv) combined skin and core hyperthermia, with high Tskin (38.8 ± 0.6°C) and high Tcore (38.1 ± 0.2°C). The LBNP tolerance was quantified via the cumulative stress index (in millimetres of mercury × minutes). The LBNP tolerance was reduced during the skin hyperthermia (569 ± 151 mmHg min) and core hyperthermia trials (563 ± 194 mmHg min) relative to control conditions (1010 ± 246 mmHg min; both P < 0.05). However, LBNP tolerance did not differ between skin hyperthermia and core hyperthermia trials (P = 0.92). The lowest LBNP tolerance was observed during combined skin and core hyperthermia(257±106mmHgmin;P<0.05 relative to all other trials). These data indicate that elevated skin temperature, as well as elevated core temperature, can both contribute to reductions in LBNP tolerance in heat-stressed individuals. However, heat stress-induced reductions in LBNP tolerance are greatest in conditions when both skin and core temperatures are elevated.