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.

HYPER:

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.

HYPER:

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 (all P ≥ 0.55). These results suggest that plasma hyperosmolality improves tolerance to central hypovolemia during heat stress in most, but not all, individuals.

• Authors: Daniel Gagnon, Steven A Romero, Hai Ngo, Paula Y S Poh, Craig G Crandall
• Journal: American journal of physiology. Regulatory, integrative and comparative physiology (Am J Physiol Regul Integr Comp Physiol) Vol. 312 Issue 3 Pg. R273-R280 (03 01 2017) ISSN: 1522-1490 [Electronic] United States
• PMID: 28003210 (Publication Type: Journal Article, Research Support, N.I.H., Extramural, Research Support, U.S. Gov't, Non-P.H.S., Research Support, Non-U.S. Gov't)
• Copyright: Copyright © 2017 the American Physiological Society.

Topics

• Adult
• Body Temperature Regulation
• Disease Resistance
• Female
• Heat Stress Disorders (physiopathology)
• Heat-Shock Response
• Humans
• Hypovolemia (physiopathology)
• Male
• Middle Aged
• Osmolar Concentration
• Plasma Volume
• Sympathetic Nervous System (physiopathology)
• Young Adult