Modest Reductions in Skin Temperature After the Onset of a Simulated Hemorrhagic Challenge Improve LBNP Tolerance Following Exercise Heat Stress

Abstract

Soldiers are often deployed to warm environments where they are exposed to exercise and heat stress whilst wearing body armor, resulting in increased mean skin (Tskin) and core (Tcore) temperatures. Our aim was to investigate whether modestly lowering Tskin 60 seconds after the onset of a simulated hemorrhagic challenge (Lower Body Negative Pressure; LBNP) would improve LBNP tolerance following exercise heat stress. Ten healthy subjects (Age: 26 ± 5 yrs; Ht: 179.3 ± 7.2 cm; Wt: 81.4 ± 18.1 kg) completed two blinded and counterbalanced trials (Trial A and Trial B) where they cycled at 55.7 ± 4.5% of their predetermined VO2Max whilst wearing a warm water perfused suit. Participants then underwent LBNP to pre syncope. The increase in Tcore from the onset of exercise to pre‐LBNP was not different between trials (+1.1 ± 0.2 °C; P > 0.05). In Trial A, Tskin remained elevated throughout LBNP while in Trial B, Tskin was modestly lowered 60 seconds after the onset of LBNP. LBNP tolerance was quantified as cumulative stress index (CSI; mmHg*min). Mean arterial pressure (MAP) was lowered following exercise (pre‐LBNP) relative to baseline in both trials (Trial A from: 90 ± 7 to: 81 ± 9 mmHg and Trial B from: 89 ± 11 to: 79 ± 5 mmHg; both P < 0.05). Tskin was not different between Trial A and Trial B at baseline (32.5 ± 0.3 vs. 32.9 ± 0.7 °C), pre‐LBNP (38.0 ± 0.6 vs. 38.0 ± 0.6 °C), and 60 seconds after the onset of LBNP (37.7 ± 0.5 vs. 37.9 ± 0.6 °C) (all P > 0.05). In Trial A, Tskin remained elevated throughout LBNP (40% CSI: 37.5 ± 0.5 °C and pre syncope: 37.4 ± 0.5 °C). In Trial B, Tskin was lower at 40% CSI and pre syncope (35.1 ± 1.2 and 34.8 ± 0.4 °C, respectively) relative to pre‐LBNP but remained higher relative to baseline (all P < 0.05). In Trial B, Tskin was lower at 40% CSI and pre syncope relative to those respective time points in Trial A (both P < 0.05). CSI was higher in Trial B (760 ± 400 mmHg*min) relative to Trial A (481 ± 272 mmHg*min; P = 0.033). MAP was not different between trials at pre syncope (Trial A: 63 ± 10 vs. Trial B: 64 ± 8 mmHg; P > 0.05). These data indicate that modestly reducing skin temperature shortly after the onset of a simulated hemorrhagic challenge improves LBNP tolerance following an exercise heat stress. Importantly, LBNP tolerance was improved by a slight reduction in Tskin, such that mean skin temperatures were reduced relative to pre‐LBNP yet remained higher than at baseline. This suggests that following an exercise heat stress a modest reduction in skin temperature applied after the onset of a hemorrhagic challenge may improve an individual's tolerance to a hemorrhagic insult. This may have important implications regarding immediate treatment of an exercise and heat stressed individual (e.g. soldier or firefighter) who has experienced a hemorrhagic injury.Support or Funding InformationFunded by UCCS CRCW awardThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.