Sex differences in cerebral blood flow and cardiac function in response to exercise in the heat

We investigated the effect of exercising in hot conditions on cerebral blood flow and systolic left ventricular (LV) function in males and females, to explore sex differences. The experimental condition consisted of walking on a treadmill at 5 km/h and 2% incline, inside a heat chamber at 40°C (50% relative humidity), for 90 min. Middle cerebral artery velocity (MCAv) and LV global longitudinal strain (GLS) were assessed at baseline and every 30 min by means of transcranial Doppler and speckle-tracking echocardiography, respectively. Thirty-eight individuals (19♀, 19♂) were recruited. Both males and females exhibited non-significant increases in MCAv from baseline at 30 min (♂ Δ = 2.55 ± 2.15, P > 0.05; ♀ Δ = 0.54 ± 2.53 cm s^-1, P > 0.05; interaction P = 0.63). This was followed by a significant decrease at 60 (♂ Δ = -4.0 ± 1.23 P = 0.04; ♀ Δ = -5.41 ± 1.56 cm s^-1, P = 0.03) and 90 min (♂ Δ = -6.08 ± 1.37 P < 0.01; ♀ Δ = -7.39 ± 1.40 cm s^-1, P < 0.01). In males, there was a significant decrease in GLS from baseline at 60 (Δ = 2.17 ± 0.66%, P = 0.049) and 90 (Δ = 2.60 ± 0.77%, P = 0.036) min; no significant changes were observed in females. The correlation between changes in GLS and MCAv was higher for males (r = -0.631, P = 0.069) than for females (r = 0.252, P = 0.513). Males and females exhibited similar patterns of change in MCAv in response to a heat and exercise challenge. An exercise-related reduction in GLS, and a higher correlation between changes in GLS and MCAv, were more apparent in males. These data suggest that sex differences may exist in the relationships between cerebrovascular and cardiac responses to exercise in the heat in humans.