title: Plasma hyperosmolality improves tolerance to combined heat stress and central hypovolemia in humans.
abstract: Heat stress profoundly impairs tolerance to central hypovolemia in humans via a number of mechanisms including heat-induced hypovolemia. However, heat stress also elevates plasma osmolality; the effects of which on tolerance to central hypovolemia remain unknown. This study examined the effect of plasma hyperosmolality on tolerance to central hypovolemia in heat-stressed humans. With the use of a counterbalanced and crossover design, 12 subjects (1 female) received intravenous infusion of either 0.9% iso-osmotic (ISO) or 3.0% hyperosmotic (HYPER) saline. Subjects were subsequently heated until core temperature increased ~1.4°C, after which all subjects underwent progressive lower-body negative pressure (LBNP) to presyncope. Plasma hyperosmolality improved LBNP tolerance (ISO: 288 ± 193 vs.
  
  
  
  382 ± 145 mmHg × min,P= 0.04). However, no differences in mean arterial pressure (P= 0.10), heart rate (P= 0.09), or muscle sympathetic nerve activity (P= 0.60,n= 6) were observed between conditions. When individual data were assessed, LBNP tolerance improved ≥25% in eight subjects but remained unchanged in the remaining four subjects. In subjects who exhibited improved LBNP tolerance, plasma hyperosmolality resulted in elevated mean arterial pressure (ISO: 62 ± 10 vs.
  
  
  
  72 ± 9 mmHg,P< 0.01) and a greater increase in heart rate (ISO: +12 ± 24 vs. HYPER: +23 ± 17 beats/min,P= 0.05) before presyncope. No differences in these variables were observed between conditions in subjects that did not improve LBNP tolerance (allP≥ 0.55). These results suggest that plasma hyperosmolality improves tolerance to central hypovolemia during heat stress in most, but not all, individuals.
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas.|Cardiovascular Prevention and Rehabilitation Centre, Montreal Heart Institute, Montréal, Québec, Canada; and.|Département de pharmacologie et physiologie, Faculté de médecine, Université de Montréal, Montréal, Québec, Canada.
authors.forename: Daniel
authors.initials: D
authors.lastname: Gagnon
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas.
authors.forename: Steven A
authors.initials: SA
authors.lastname: Romero
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas.
authors.forename: Hai
authors.initials: H
authors.lastname: Ngo
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas.
authors.forename: Paula Y S
authors.initials: PY
authors.lastname: Poh
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas; craigcrandall@texashealth.org.
authors.forename: Craig G
authors.initials: CG
authors.lastname: Crandall
mesh_terms: D000328:Adult; D001833:Body Temperature Regulation; D060467:Disease Resistance; D005260:Female; D018882:Heat Stress Disorders / Q000503:physiopathology*; D018869:Heat-Shock Response; D006801:Humans; D020896:Hypovolemia / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D009994:Osmolar Concentration; D010953:Plasma Volume; D013564:Sympathetic Nervous System / Q000503:physiopathology*; D055815:Young Adult
keywords: blood pressure; heart rate; lower-body negative pressure; muscle sympathetic nerve activity