Dehydration during whole‐body heat stress contributes to subsequent reductions in lower‐body negative pressure (LBNP) tolerance

Abstract

This study tested the hypothesis that dehydration, which accompanies heat stress, further compromises the maintenance of blood pressure during a simulated hemorrhagic challenge via LBNP. Seven individuals (34±9 y; 77±4 kg) underwent heat stress on two separate days (randomized), one with and one without intravenous (IV) infusion of Lactated Ringers solution sufficient to replace sweat loss. Hemorrhage was simulated via progressive LBNP to pre-syncope after core body (intestinal) temperature was raised ~1.4°C using a water perfused suit. LBNP tolerance was quantified with a cumulative stress index (CSI). Blood samples were obtained at normothermia and just prior to LBNP to estimate changes in plasma volume during heat stress. Body core temperature increases and sweat losses (~1.7% body mass deficit) were similar (P>0.05) between trials. IV infusion (1.3±0.5 L) prevented the reduction in plasma volume that occurred during the dehydration trial (IV infusion trial: +3.1±7.1%; dehydration trial: −6.5±5.5%, P=0.01). LBNP tolerance during the dehydration trial (407±126 CSI units) was lower compared to the IV trial (641±64 CSI units, P<0.01). These data indicate that dehydration during heat stress reduces an individual's tolerance to a subsequent hemorrhagic challenge, therefore, increasing the likelihood of syncope. Supported by NIH Grant HL061388