title: Does attenuated skin blood flow lower sweat rate and the critical environmental limit for heat balance during severe heat exposure?
abstract: What is the central question of this study? Does attenuated skin blood flow diminish sweating and reduce the critical environmental limit for heat balance, which indicates maximal heat loss potential, during severe heat stress? What is the main finding and its importance? Isosmotic hypovolaemia attenuated skin blood flow by ∼20% but did not result in different sweating rates, mean skin temperatures or critical environmental limits for heat balance compared with control and volume-infusion treatments, suggesting that the lower levels of skin blood flow commonly observed in aged and diseased populations may not diminish maximal whole-body heat dissipation. Attenuated skin blood flow (SkBF) is often assumed to impair core temperature (Tc) regulation. Profound pharmacologically induced reductions in SkBF (∼85%) lead to impaired sweating, but whether the smaller attenuations in SkBF (∼20%) more often associated with ageing and certain diseases lead to decrements in sweating and maximal heat loss potential is unknown. Seven healthy men (28 ± 4 years old) completed a 30 min equilibration period at 41°C and a vapour pressure (Pa) of 2.57 kPa followed by incremental steps in Paof 0.17 kPa every 6 min to 5.95 kPa. Differences in heat loss potential were assessed by identifying the critical vapour pressure (Pcrit) at which an upward inflection in Tcoccurred. The following three separate treatments elicited changes in plasma volume to achieve three distinct levels of SkBF: control (CON); diuretic-induced isosmotic dehydration to lower SkBF (DEH); and continuous saline infusion to maintain SkBF (SAL). The Tc, mean skin temperature (Tsk), heart rate, mean laser-Doppler flux (forearm and thigh; LDFmean), mean local sweat rate (forearm and thigh; LSRmean) and metabolic rate were measured. In DEH, a 14.2 ± 5.7% lower plasma volume resulted in a ∼20% lower LDFmeanin perfusion units (PU) (DEH, 139 ± 23 PU; CON, 176 ± 22 PU; and SAL, 186 ± 22 PU; P = 0.034). However, LSRmeanand whole-body sweat losses were unaffected by treatment throughout (P > 0.482). The Pcritfor Tcwas similar between treatments (CON, 5.05 ± 0.30 kPa; DEH, 4.93 ± 0.16 kPa; and SAL, 5.12 ± 0.10 kPa; P = 0.166). Furthermore, no differences were observed in the skin-air temperature gradient, metabolic rate or changes in Tc(P > 0.197). In conclusion, a ∼20% reduction in SkBF alters neither sweat rate nor the upper limit for heat loss from the skin during non-encapsulated passive heat stress.
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and the University of Texas Southwestern Medical Center, Dallas, TX, USA.|School of Human Kinetics, Faculty of Health Sciences, University of Ottawa, ON, Canada.
authors.forename: Matthew N
authors.initials: MN
authors.lastname: Cramer
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and the University of Texas Southwestern Medical Center, Dallas, TX, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Gagnon
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and the University of Texas Southwestern Medical Center, Dallas, TX, USA.
authors.forename: Craig G
authors.identifier: 0000-0001-7792-9713
authors.initials: CG
authors.lastname: Crandall
authors.affiliation: Thermal Ergonomics Laboratory, Faculty of Health Sciences, University of Sydney, NSW, Australia.
authors.forename: Ollie
authors.identifier: 0000-0002-6076-6337
authors.initials: O
authors.lastname: Jay
mesh_terms: D000328:Adult; D001833:Body Temperature Regulation / Q000502:physiology*; D003681:Dehydration / Q000503:physiopathology; D005542:Forearm / Q000098:blood supply; D006339:Heart Rate / Q000502:physiology; D018882:Heat Stress Disorders / Q000503:physiopathology*; D006358:Hot Temperature; D006801:Humans; D008297:Male; D010953:Plasma Volume / Q000502:physiology; D012039:Regional Blood Flow / Q000502:physiology*; D012867:Skin / Q000098:blood supply*; D012881:Skin Temperature / Q000502:physiology*; D013542:Sweat / Q000502:physiology*; D013546:Sweating / Q000502:physiology*
keywords: core temperature; cutaneous vascular conductance; heat loss potential; heat stress; sweat