This study tested the hypothesis that sweat rate during passive heat stress is limited by baroreceptor unloading associated with heat stress. Two protocols were performed in which healthy subjects underwent passive heat stress that elicited an increase in intestinal temperature of ~1.8°C. Upon attaining this level of hyperthermia, in protocol one (n=10, 3 females) a bolus (19 ml/kg) of warm (~38°C) isotonic saline was rapidly (5-10 min) infused intravenously to elevate central venous pressure (CVP), while in protocol two (n=11, 5 females) phenylephrine was infused intravenously (60-120 μg/min) to return mean arterial pressure (MAP) to normothermic levels. Protocol one: Heat stress reduced CVP from 3.9 ± 1.9 mmHg (normothermia) to -0.6 ± 1.4 mmHg (P<0.001), while saline infusion returned CVP to normothermic levels (5.1 ± 1.7 mmHg; P>0.999). Sweat rate was elevated by heat stress (1.21 ± 0.44 mg/cm2/min) but remained unchanged during rapid saline infusion (1.26 ± 0.47 mg/cm2/min, P=0.5), whereas CVC increased from 77 ± 10 to 101 ± 20% of local heating max (P=0.029). Protocol two: MAP was reduced with heat stress from 85 ± 7 mmHg to 76 ± 8 mmHg (P=0.048). Although phenylephrine infusion returned MAP to normothermic levels (88 ± 7 mmHg; P>0.999), sweat rate remained unchanged during phenylephrine infusion (1.39 ± 0.22 mg/cm2/min vs. 1.41 ± 0.24 mg/cm2/min; P>0.999). These data indicate that both cardiopulmonary and arterial baroreceptor unloading do not limit increases in sweat rate during passive heat stress.
- Central venous pressure
- Skin blood flow
- Sweat rate
- Copyright © 2014, Journal of Applied Physiology