Does Attenuated Skin Blood Flow Lower Sweat Rate and Thereby the Critical Environmental Limit for Heat Balance?

Abstract

Attenuated skin blood flow (SkBF) is often assumed to critically impair core temperature (Tc) regulation. Recent evidence has shown that a profound pharmacologically‐induced attenuation of SkBF leads to a reduction in local sweat rate. Whether attenuated SkBF to a level more commonly observed in aged or diseased populations leads to decrements in sweat rate, and consequently, a reduction in heat loss potential is presently unknown. Therefore, the purpose of the present study was to test the hypothesis that attenuated SkBF would lead to lower sweat rate and heat loss potential, defined as the critical ambient vapour pressure (Pcrit) at which core temperature (Tc) inflected, while maintenance of a high SkBF would preserve sweat rate and heat loss potential. Seven healthy males (28±4 y) completed the protocol, which consisted of non‐encapsulated passive heat exposure (41°C), while ambient vapour pressure increased by 0.17 kPa (~2% RH) from 2.57 to a maximum of 5.95 kPa every 6 min until an upward inflection in Tc occurred. Three separate treatments elicited changes in plasma volume (ΔPV) with the goal of achieving three distinct levels of SkBF: control (CON), diuretic‐induced iso‐osmotic dehydration to lower SkBF (DEH), and continuous saline infusion to maintain SkBF (SAL). Tc, mean skin temperature (Tsk), heart rate (HR), mean laser‐Doppler flux (forearm, thigh; LDFmean), mean local sweat rate (LSRmean), and metabolic rate were measured. In DEH, a −14.2 ± 5.7% ΔPV resulted in a ~20% lower LDFmean (DEH: 139 ± 23, CON: 176 ± 22, SAL: 186 ± 22 PU; P = 0.034). However, LSRmean and whole‐body sweat losses were unaffected by treatment (P > 0.482). Pcrit for Tc was similar between treatments (CON: 5.05 ± 0.30, DEH: 4.93 ± 0.16, SAL: 5.12 ± 0.10 kPa; P = 0.166). Further, no differences were observed in the Tsk‐Ta gradient, metabolic rate, and changes in Tc (P > 0.197). In conclusion, a physiological reduction in SkBF consistent with those reported in the elderly and diseased populations alters neither local and whole‐body sweating nor heat loss potential during non‐encapsulated passive heat stress.Support or Funding InformationFunded by National Institutes of Health GM068865 and Department of Defense W81XWH‐12‐1‐0152 (CGC), and a Natural Sciences and Engineering Research Council Discovery Grant #386143‐2010 (OJ).