Does attenuated skin blood flow lower sweat rate and the critical environmental limit for heat balance during severe heat exposure?

Attenuated skin blood flow (SkBF) is often assumed to impair core temperature (T_c) regulation. Profound pharmacologically-induced reductions in SkBF (∼85%) lead to impaired sweating but whether the smaller attenuations in SkBF (∼20%) more associated with ageing and certain diseases lead to decrements in sweating and maximum heat loss potential is unknown. Seven healthy males (28 ± 4y) completed a 30-min equilibration period at 41°C and a vapour pressure (P_a) of 2.57 kPa followed by incremental steps in P_a of 0.17 kPa every 6-min to 5.95 kPa. Differences in heat loss potential were assessed by identifying the critical vapour pressure (P_crit) at which an upward inflection in T_c occurred. Three separate treatments elicited changes in plasma volume to achieve three distinct levels of SkBF: control (CON), diuretic-induced iso-osmotic dehydration to lower SkBF (DEH), and continuous saline infusion to maintain SkBF (SAL). T_c, mean skin temperature (T_sk), heart rate, mean laser-Doppler flux (forearm, thigh; LDF_mean), mean local sweat rate (forearm, thigh; LSR_mean), and metabolic rate were measured. In DEH, a 14.2 ± 5.7% lower plasma volume resulted in a ∼20% lower LDF_mean (DEH: 139 ± 23, CON: 176 ± 22, SAL: 186 ± 22 PU; P = 0.034). However, LSR_mean and whole-body sweat losses were unaffected by treatment throughout (P > 0.482). P_crit for T_c 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 T_sk-T_a gradient, metabolic rate, or changes in T_c (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.

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