Orthostatism and heat acclimation

Abstract

Three groups of subjects (6 subj in each group) underwent the following precedures: group A was given a 20-min head-up tilt at 21 degrees C followed by 4 h of exercise at 33.9 degrees C DB, 32.2 degrees C WB, and a repetition of tilting after exercise in heat; group B underwent the same procedure at 21 degrees C; group C was tilted at 21 degrees C, rested in heat for 4 h and was retilted in heat. The above procedures were repeated for 8 days, and on the last day groups B and C underwent the same treatment as group A. Group A showed the usual decreases in heart rate and rectal temperature and an increase in sweat rate on acclimation. This corresponded to marked improvements in heat-orthostatism. While five subjects in group A fainted during post-exposure tilting on the first exposure, none fainted on the last day. Resting in heat (group C) did not cause any acclimation to work in heat. This corresponded to poor heat-orthostatism after the work-heat procedure when five subjects fainted. Mild training at 21 degrees C (group B) resulted in minor improvements to work in heat as evident by some improvements in heart rate responses after the 1st and 2nd h of exposure. This corresponded to better heat-orthostatism and fewer men fainting than in group C. The results indicated that heat-orthostatism improves on acclimation to the same extent as exercise heart rate and rectal temperature.

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

“…Given that both passive heat stress (2,26,29,31,48,51) and exercise in a thermoneutral environment (6) impair tolerance to a hypotensive challenge, we hypothesized that the combination of exercise in the heat would further reduce LBNP tolerance, relative to passive heat stress alone, when controlling for internal and mean skin temperatures. Counter to that hypothesis, LBNP tolerance was not different between these two perturbations when mean skin temperatures were clamped at similar levels.…”

“…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).…”

Effect of thermal stress on the vestibulosympathetic reflexes in humans

“…Orthostatic tolerance is reduced during heat stress compared with normothermia (1,15,27). The precise mechanism for this increased intolerance is currently unknown, but the vestibular system could be contributing to this intolerance if heat stress altered autonomic responses to the engagement of the otolith organs.…”

“…ORTHOSTATIC TOLERANCE IS REDUCED during heat stress compared with normothermia (1,14,15,27). Sympathetic nervous system reflexes are imperative in maintaining blood pressure during orthostatic stress.…”

Heat stress and orthostatic tolerance

“…In addition, among fatalities due to heat stroke in the work place, 42% of the total number of incidents (n=72) in 2008 to 2010 occurred within the first 3 days of work initiated in a warm environment 114) . When six unacclimated subjects were repeatedly given a prolonged exercise load followed by a 20-min HUT at 33.9˚C for 8 days, fainting episodes during HUT occurred in 80% (n=5) of subjects on the first day of repeated exercise-heat exposure, and tended to decrease after that (2 subjects on the 2nd day, and 1 subject on the 3rd day) 115) . These reports indicate the need for physical acclimation to heat for the prevention of heat stroke and orthostatic hypotension.…”