Passive and exercise-heat stress have been reported to cause reductions in cerebral blood flow (CBF); however, it is unknown if CBF velocity is different under hot dry and humid conditions. PURPOSE: This project tested the hypothesis that upright rest and exercise-heat stress in hot dry and humid conditions result in similar CBF reductions. METHODS: Seven healthy active (7 day activity: 9887±3564 steps/day; VO2max: 52±10 mL/kg/min) subjects (5M/2F, 24±9y, 171±6cm, 68±7kg) completed a 30 min baseline rest followed by 60 min of exercise at a rate of perceived exertion (RPE) of 12 (between light and somewhat hard) on a 20-point rating scale. Subjects were blinded to the ergometer (Watt) in a hot dry to humid (42.3±0.3℃; initial 10.4±0.4% relative humidity [Rh] that increased to 62.2±5.3%Rh) and control neutral dry (22.9±1.0℃; 11.5±1.9%Rh) condition in random order separated by at least 7 days. Heart rate (HR), intestinal temperature (Ti), absolute (cm/s) and relative (% of rest) middle cerebral artery (MCA) blood flow (CBF) velocity during systole, diastole and mean (MCA Vsys, MCA Vdia, MCA Vmean) were analyzed utilizing a 2-way repeated measures analysis of variance for interaction and main effects for condition x time. RESULTS: CBF or Ti at rest was not different between conditions (P>0.05). HR (∆14±11bpm) was elevated at rest and further increased (∆26±28 bpm) during exercise in hot compared to the neutral condition (Interaction: Condition x Time; Ph conditions; however, increased (∆1.2±0.8℃) to a greater during the hot humid >30-60% Rh condition (Interaction: Condition x Time; Ph) conditions compared to neutral dry (Interaction: Condition x Time; Ph) condition, CBF was reduced to similar velocities as in the neutral dry condition; however, it was reduced to a greater extent (∆-16±9%cm/s) during hot humid (>60%Rh) compared to the neutral condition (∆0±19% cm/s). Exercise intensity was slightly greater (54±1 vs 48±3%VO2max, P CONCLUSION:Exercise heat-stress during hot dry conditions increase MCA Vmean, which is primarily due to a rise in blood flow velocity during systole. However, exercising in hot and high humidity attenuates MCA Vmean, due to reduced diastole blood flow velocity.



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