Sex Difference In Cerebral Blood Flow During Exercise Hyperthermia

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PURPOSE: Young healthy females are more prevalent to orthostatic hypotension compared to males. It is unknown if females show similar cerebral blood flow hemodynamics (middle cerebral artery, MCAvmean) compared to males during exercise hyperthermia. We tested the hypothesis that females have an attenuated MCAvmean response to exercising in hyperthermic conditions. METHODS: Twenty-two healthy active adults (7 day activity: 8620±3008 steps/day; VO2max: 49±10 mL/kg/min ) adults (11M/11F, 22.4±4.9y, 169±7.6cm, 68.3±13kg) exercised at similar metabolic heat production (M: 7.1±1.5 W/kg and F: 6.9±1.4 W/kg; P=0.32) for 60 min (cycle ergometer) in cool (24.0±0.00C; 14.4±3.6%Rh) and hot (42.3±0.20C; 27.9±5.5%Rh) conditions in random order with 7 days washout. MCAvmean absolute and percent change from rest, MCAvmean conductance (MCAvmeanCVC), cardiac output index (COi) and systemic vascular resistance index (SVRi), and the difference between conditions (hot minus cool) were examined by a mixed model 2-way repeated-measures analysis of variance for Interaction (I) and Main Effect (ME) for time and sex. Significance was set at P<0.05. RESULTS: 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) were similar between sex and increased in hot compared to cool condition. During the cool condition, absolute MCAvmean and MCAvmeanCVC were higher in females compared to males (ME: sex P<0.005). In contrast, MCAvmean and MCAvmeanCVC increased then decreased over time in hot compared to the cool condition (ME: time; P<0.0001). COi increased in hot compared to cool condition (ME: condition; P<0.008). Females had greater SVRi compared to males in both conditions (ME: sex; P<0.008). However, the %change and the difference between conditions, females compared to males had an attenuated MCAvmean (ME: sex; P<0.03) and MCAvmeanCVC (I: time × sex; P<0.01). CONCLUSIONS: These data indicate that sex differences exist for exercise cerebral blood flow. Females have higher MCAvmean in cool and have an attenuated MCAvmean response during exercise hyperthermia compared to males. This sex difference may be due to differences in blood pressure and systemic vascular resistance.

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