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Exercising in hot conditions may increase the risk for exertional heat-related illness due to reduction in cerebral blood flow (CBF); however, the acute effect of exercise-induced changes on CBF during compensable and uncompensable heat stress remain unclear. We tested the hypothesis that exercising in hot dry and humid conditions would have different CBF responses. Nine healthy active males completed a 30 min baseline rest then 60 min of low intensity self-paced exercise (12 rating of perceived exertion) in a 1) control compensable neutral dry (CN; 23.7 ± 0.7 °C; 10.7 ± 0.8%R h) and 2) compensable hot dry (CH; 42.3 ± 0.3 °C; 10.7 ± 1.8%R h) that progressively increased to an uncompensable hot humid (UCH; 42.3 ± 0.3 °C; 55.2 ± 7.7%R h) environment in random order separated by at least 4 days. We observed that during CN environments from rest through 60 min of exercise, middle cerebral velocity (MCAv mean) and conductance (MCAv mean CVC) remained unchanged. In contrast, during CH, MCAv mean , MCAv mean CVC, and cardiac output (Q) increased and systemic vascular resistance (SVR) decreased. However, under UCH, MCAv mean , MCAv mean CVC, and Q was reduced. No difference in mean arterial pressure or ventilation was observed during any condition. Only during UCH, end-tidal P O 2 increased and P CO 2 decreased. The redistribution of blood to the skin for thermoregulation (heart rate, skin blood flow and sweat rate) remained higher during exercise in UCH environments. Conclusions : Collectively, exercise cerebral blood flow is altered by an integrative physiological manner that differs in CN, CH, and UCH environments. The control of CBF may be secondary to thermoregulatory control which may provide an explanation for the cause of exertional heat illness. • Mean cerebral artery velocity remains unchanged from rest to exercise during low-intensity self-paced exercise in a neutral dry compensable environment • Mean cerebral artery velocity increases from rest to exercise during low-intensity self-paced exercise in a hot dry compensable environment • The increase in mean cerebral artery velocity in hot dry occurs as cardiac output increase and systemic vascular resistance decreases • Mean cerebral artery velocity decreases from during low-intensity self-paced exercise in a hot humid uncompensable environment and this occurs as cardiac out decreases and systemic vascular resistance increases. • Skin blood flow and sweat rate increases to a greater extent in hot conditions and remains higher in uncompensable conditions as cerebral blood flow decreases. • Our findings show that cerebral blood flow is altered in an integrative physiologically response that is dependent on the magnitude of heat stress during compensable and uncompensable environmental conditions. [ABSTRACT FROM AUTHOR]

Cerebral blood flow and thermal perception during physical exercise under hyperthermia conditions in females are poorly understood. Because sex differences exist for blood pressure control, resting middle cerebral artery velocity (MCA Vmean), and pain, we tested the hypothesis that females would have greater reductions in MCAv mean and increased thermal perceptual strain during exercise hyperthermia compared to males. Twenty-two healthy active males and females completed 60 min of matched exercise metabolic heat production in a 1) control cool (24.0 ± 0.0 °C; 14.4 ± 3.4% R h) and 2) hot (42.3 ± 0.3 °C; 28.4 ± 5.2% R h) conditions in random order, separated by at least 3 days while MCAv mean , thermal comfort, and preference was obtained during the exercise. Compared to 36 °C mean body temperature (Mbt), as hyperthermia increased to 39 °C Mbt, females had a greater reduction in absolute (MCAv mean), and relative change (%Δ MCAv mean) and conductance (%Δ MCAv mean conductance) in MCA Vmean compared to males (Interaction: Temperature x Sex, P ≤ 0.002). During exercise in cool conditions, absolute and conductance MCAv mean was maintained from rest through exercise; however, females had greater MCA Vmean compared to males (Main effect: Sex, P < 0.0008). We also found disparities in females' perceptual thermal comfort and thermal preference. These differences may be associated with a greater reduction in partial pressure of end-tidal CO 2 , and different cardiovascular and blood pressure control to exercise under hyperthermia. In summary, females exercise cerebral blood flow velocity is reduced to a greater extent (25% vs 15%) and the initial reduction occurs at lower hyperthermia mean body temperatures (~38 °C vs ~39 °C) and are under greater thermal perceptual strain compared to males. • Because cerebral blood flow response to exercise under hyperthermia has only been examined in males, this study examined sex differences in middle cerebral artery blood flow velocity (MCAv mean) and thermal comfort and preference in hot and cool conditions. • This study found that both females and males maintain MCAv mean during exercise in cool conditions; however, females' velocity is greater than males. • During exercise hyperthermia, females have a greater relative change in the reduction of MCAv mean compared to males. • Females had lower mean arterial pressures during rest and exercise in hot compared to cool whereas men were able to maintain blood pressure between conditions. When normalizing for blood pressure differences, sex disparities for relative change for MCAv mean conductance remained. • Females also reported greater thermal comfort and had preference for cooler environments at similar mean body temperatures during exercise hyperthermia. [ABSTRACT FROM AUTHOR]