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

Abstract

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 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 (Pa) of 2.57 kPa followed by incremental steps in Pa of 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 Tc 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). Tc, mean skin temperature (Tsk), heart rate, mean laser-Doppler flux (forearm, thigh; LDFmean), mean local sweat rate (forearm, thigh; LSRmean), and metabolic rate were measured. In DEH, a 14.2±5.7% lower plasma volume 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 throughout (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, 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.

Folic acid supplementation does not attenuate thermoregulatory or cardiovascular strain of older adults exposed to extreme heat and humidity

“…However, interventions or conditions that affect local measures of cutaneous blood flow do not always similarly affect whole-limb measures. For example, we have observed that iso-osmotic dehydration (Cramer, Gagnon, Crandall, & Jay, 2017) and age reduce local cutaneous, but not whole-limb, vascular conductance during heat exposure. Along these lines, Stanhewicz et al (2013) reported that a dose of oral sapropterin increases both local cutaneous vascular conductance and forearm vascular conductance during passive heating in older adults.…”

“…The critical humidity inflection point is an indicator of maximum heat loss potential (Kamon & Avellini, ; Kenney & Zeman, ; Kenney, Mikita, Havenith, Puhl, & Crosby, ). We have previously demonstrated that, in young adults, furosemide‐induced reductions in cutaneous blood flow do not affect the critical humidity inflection point for core temperature (Cramer et al., ). As such, conditions or interventions that elicit physiologically relevant reductions or increases in cutaneous blood flow have little to no effect upon maximum heat loss potential under extreme heat conditions.…”

“…However, interventions or conditions that affect local measures of cutaneous blood flow do not always similarly affect whole‐limb measures. For example, we have observed that iso‐osmotic dehydration (Cramer, Gagnon, Crandall, & Jay, ) and age (Gagnon et al., ) reduce local cutaneous, but not whole‐limb, vascular conductance during heat exposure. Along these lines, Stanhewicz et al.…”

Free-living, continuous hypo-hydration, and cardiovascular response to exercise in a heated environment

“…; Cramer et al. ). Interestingly, an interaction was found between time and trial in T re and T sk , suggesting thermoregulatory mechanisms that are prompted during exercise manage heat strain more efficiently when euhydrated compared to dehydrated.…”

“…Although not significant, SBF was lower during the DEH trial, which DEH and heat stress may independently and concomitantly reduce SBF and heat dissipation (Gonzalez-Alonso 1998) and ultimately may impair aerobic performance (Kenefick et al 2010). However, reductions in SBF do not necessarily result in impaired thermoregulation (Vroman et al 1983;Cramer et al 2017). Interestingly, an interaction was found between time and trial in T re and T sk , suggesting thermoregulatory mechanisms that are prompted during exercise manage heat strain more efficiently when euhydrated compared to dehydrated.…”