Active and passive heat stress similarly compromise tolerance to a simulated hemorrhagic challenge

Abstract

dall CG. Active and passive heat stress similarly compromise tolerance to a simulated hemorrhagic challenge. Am J Physiol Regul Integr Comp Physiol 307: R822-R827, 2014. First published July 30, 2014 doi:10.1152/ajpregu.00199.2014.-Passive heat stress increases core and skin temperatures and reduces tolerance to simulated hemorrhage (lower body negative pressure; LBNP). We tested whether exerciseinduced heat stress reduces LBNP tolerance to a greater extent relative to passive heat stress, when skin and core temperatures are similar. Eight participants (6 males, 32 Ϯ 7 yr, 176 Ϯ 8 cm, 77.0 Ϯ 9.8 kg) underwent LBNP to presyncope on three separate and randomized occasions: 1) passive heat stress, 2) exercise in a hot environment (40°C) where skin temperature was moderate (36°C, active 36), and 3) exercise in a hot environment (40°C) where skin temperature was matched relative to that achieved during passive heat stress (ϳ38°C, active 38). LBNP tolerance was quantified using the cumulative stress index (CSI). Before LBNP, increases in core temperature from baseline were not different between trials (1.18 Ϯ 0.20°C; P Ͼ 0.05). Also before LBNP, mean skin temperature was similar between passive heat stress (38.2 Ϯ 0.5°C) and active 38 (38.2 Ϯ 0.8°C; P ϭ 0.90) trials, whereas it was reduced in the active 36 trial (36.6 Ϯ 0.5°C; P Յ 0.05 compared with passive heat stress and active 38). LBNP tolerance was not different between passive heat stress and active 38 trials (383 Ϯ 223 and 322 Ϯ 178 CSI, respectively; P ϭ 0.12), but both were similarly reduced relative to active 36 (516 Ϯ 147 CSI, both P Յ 0.05). LBNP tolerance is not different between heat stresses induced either passively or by exercise in a hot environment when skin temperatures are similarly elevated. However, LBNP tolerance is influenced by the magnitude of the elevation in skin temperature following exercise induced heat stress. exercise; heat stress; orthostatic tolerance PASSIVE HEAT STRESS increases core and skin temperatures and is accompanied with profound reductions in tolerance to central hypovolemia [e.g., lower body negative pressure (LBNP)], which simulates a hemorrhagic state (2,26,29,31,48,51). This is due, in part, to a large displacement of blood to the cutaneous circulation and associated reductions in systemic vascular resistance (42) and central blood volume (12, 13), coupled with inadequate cutaneous vasoconstriction during the hypotensive challenge (11, 38). Such tolerance is likewise reduced following short-term exercise in a thermoneutral environment that is not accompanied by profound increases in skin and core temperatures (6). This response may be due to postexercise reductions in baroreflex sensitivity (40, 49), lowered arterial blood pressure (9,16,27,39), and an impaired transduction of sympathetic outflow into vasoconstriction (20), coupled with elevations in vascular conductance in the previously active limb (34).Blood pressure and vascular alterations following exercise may be exacerbated if the exercise is performed under ...

Cerebral Vascular Control and Metabolism in Heat Stress

“…8) (16,52,225,282,350,481). Fundamentally, loss of consciousness during orthostatic stress is attributed to inadequate CBF (461)-driven primarily by reductions in CPP (271,348).…”

Mechanisms of orthostatic intolerance during heat stress

“…Notably, this does not infer that passive heat stress data cannot be applied to the active (i.e., exercising) state. For instance, the orthostatic responses to passive and active heat stress are virtually indistiguishable when skin temperatures are similar (Pearson et al, 2014). During passive heat stress skin temperatures will be profoundly elevated (37–40 °C).…”

Pushing the limits of blood pressure control under severe heat stress. Focus on “Active and passive heat stress similarly compromise tolerance to a simulated hemorrhagic challenge”

“…the elevated core temperatures can be paralleled by markedly elevated skin temperatures. Thus, based on the study findings by Pearson et al (8), such individuals are likely at greater risk for heat-related syncope due to a significant leftward shift in the survival curve for the point at which they experience cardiovascular failure. It is important to consider, however, that their study employed LBNP in the supine posture (8).…”

“…Thus, based on the study findings by Pearson et al (8), such individuals are likely at greater risk for heat-related syncope due to a significant leftward shift in the survival curve for the point at which they experience cardiovascular failure. It is important to consider, however, that their study employed LBNP in the supine posture (8). While this position would likely parallel that of a wounded soldier on the battlefield, it does not take into consideration any postural influences that may present an added stress for the athlete or occupational worker who may work and/or recover in the upright posture for an extended period of time.…”

“…These results were specifically applied in context of soldiers who are at risk for a hemorrhagic injury during simultaneous exposure to passive (i.e., warm environments) and/or active heat stresses (i.e., foot patrols) while wearing body armor. Based on their findings, the authors recommended the implementation of measures to reduce skin temperature in an attempt to delay the point of cardiovascular failure as long as possible (8).…”