Heat stress exacerbates the reduction in middle cerebral artery blood velocity during prolonged self‐paced exercise

This study examined the influence of hyperthermia on middle cerebral artery mean blood velocity ( MCA V mean). Eleven cyclists undertook a 750 kJ self‐paced time trial in HOT (35 °C) and COOL (20 °C) conditions. Exercise time was longer in HOT (56 min) compared with COOL (49 min; P  < 0.001). Power output in HOT was significantly lower from 40% of work completed onward ( P  < 0.01). Rectal temperature increased to 39.6 ± 0.6 °C ( HOT ) and 38.8 ± 0.5 °C ( COOL ; P  < 0.01). Skin temperature, skin blood flow, and heart rate were higher throughout HOT compared with COOL ( P  < 0.05). A similar increase in ventilation ( P  < 0.05) and decrease in end‐tidal partial pressure of CO 2 ( PETCO 2 ; P  < 0.05) occurred in both conditions. Arterial blood pressure and oxygen uptake were lower from 50% of work completed onward in HOT compared with COOL ( P  < 0.01). MCA V mean increased at 10% in both conditions ( P  < 0.01), decreasing thereafter ( P  < 0.01) and to a greater extent in HOT from 40% of work completed onward ( P  < 0.05). Therefore, despite a comparable ventilatory response and PETCO 2 in the HOT and COOL conditions, the greater level of thermal strain developing in the heat appears to have exacerbated the reduction in MCA V mean, in part via increases in peripheral blood flow and a decrease in arterial blood pressure.