Post-Exercise Hyperthermia Cerebral Blood Flow Hemodynamics Are Similar Between Males And Females

Abstract

PURPOSE: Females have greater orthostatic intolerance and increased adrenergic sensitivity to passive heat stress compared to males. It is unknown how cerebral blood flow is affected during post-exercise heat stress. We tested the hypothesis that females would have lower middle cerebral artery blood flow velocity (MCAvmean) during post-exercise hyperthermia. METHODS: Twenty-two healthy active adult (7 day activity: 8620±3008 steps/day; VO2max: 49±10 mL/kg/min ) subjects (11M/11F, 22.4±4.9y, 169±7.55cm, 68.3±13kg) exercised at a similar metabolic heat production (M: 7.1±1.5 W/kg and F: 6.9±1.4 W/kg; P=0.32) for 60 minutes (cycle ergometer) in cool (24±0.00C; 14.4±3.6%Rh) and hot (42.3±0.20C; 27.9±5.5%Rh) conditions in random order with a 7 day washout. During 1-hour post-exercise recovery, Transcranial Doppler examined MCAvmean, pulsatility index (PI) and intracranial pressure (ICP). Systemic vascular responses for mean arterial pressure (MAP), augmentation index (Alx), pulse wave velocity (PWV), systemic vascular resistance (SVR), and change in intestinal temperature (ΔTin) and heart rate (ΔHR) and were measured during 1-hour recovery. Area under the curve (AUC) variables were analyzed using a mixed model 2-way repeated-measures analysis of variance for interaction (I) and main effects (ME) for Condition x Sex. Alpha priori was set at P<0.05. RESULTS: Exercise thermal (ΔTin, Cool: Δ0.5±0.10C, Hot: Δ1.5±0.60C; ME: Condition; P<0.0001) and cardiovascular strain (ΔHR Cool: 58±15 b/min, Hot: 71±15 b/min; ME: Condition; P<0.01) was similar between groups that increased in hot compared to cool condition. During recovery both sexes had a similar AUC MCAvmean; however, MCAvmean was lower in hot compared to cool (ME: Condition; P<0.03). Females also showed reduced stiffness (AUC PWV and Alx) compared to males (ME: Condition; P<0.0001; ME: Sex, P<0.0008). Females had greater AUC SVR compared to males in both conditions (ME: sex; P<0.01). There were no differences for MAP, PI, or ICP between the Condition or Sex. CONCLUSIONS: These data suggest that no sex difference exists for MCAvmean during recovery. Both sexes have lower MCAvmean in hot compared to cool conditions. However, the peripheral vascular mechanisms for this attenuation may differ as females have lower arterial stiffness and higher SVR.