Indexed on: 17 Dec '16Published on: 17 Dec '16Published in: Experimental Physiology
Tolerance to a simulated hemorrhagic 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 (Tcore) temperatures 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 pre-syncope on four occasions: 1) Control: neutral Tskin (34.3 ± 0.5°C) and Tcore (36.8 ± 0.2 °C), 2) Primarily skin hyperthermia: high Tskin (37.6 ± 0.2°C) and neutral Tcore (37.1 ± 0.2 °C), 3) Primarily core hyperthermia: neutral Tskin (35.0 ± 0.5°C) and high Tcore (38.3 ± 0.2 °C) and 4) Combined skin and core hyperthermia: high Tskin (38.8 ± 0.6°C) and high Tcore (38.1 ± 0.2 °C). LBNP tolerance was quantified via the cumulative stress index (CSI; mmHg×min). LBNP tolerance was reduced during the skin hyperthermia (569 ± 151 mmHg×min) and core hyperthermia (563 ± 194 mmHg×min) trials relative to control (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 ± 106 mmHg×min; 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 under conditions when both skin and core temperatures are elevated. This article is protected by copyright. All rights reserved.