Active and passive heat stress similarly compromises tolerance to a simulated hemorrhagic challenge (1104.22)

Abstract

Passive heat stress increases core and skin temperatures and reduces tolerance to simulated hemorrhage, as assessed via lower body negative pressure (LBNP). It is unknown whether exercise induced heat stress reduces LBNP tolerance to a similar extent relative to passive heat stress. Eight males (32 ± 7 yrs, 176 ± 8 cm, 77.0 ± 9.8 kg) underwent LBNP to pre‐syncope on three separate and randomized occasions: passive heat stress and two occasions of exercise induced heat stress where skin temperature was moderate (~36°C, Active 36) or warm (~38°C, Active 38). LBNP tolerance was quantified using the cumulative stress index (CSI). Prior to LBNP, increases in core temperature were similar in all trials (1.18 ± 0.20°C; P < 0.05), and while mean skin temperature was similar between passive heat stress (38.2 ± 0.5°C) and Active 38 (38.2 ± 0.8°C; P = 0.90) both trials were greater compared to Active 36 (36.6 ± 0.5°C; P < 0.05). LBNP tolerance was not different between passive heat stress and Active 38 (383 ± 223 and 322 ± 178 CSI; P = 0.12) but both were similarly reduced relative to the Active 36 (516 ± 147 CSI, both P < 0.05). At pre‐syncope mean arterial pressure and heart rate were not different between trials. These data suggest that tolerance to a simulated hemorrhagic challenge is similar following passive and exercise induced heat stress when skin temperatures are similarly elevated.Grant Funding Source: DOD: W81XWH‐09‐2‐0194 and NIH: HL61388