Hyperthermic-induced hyperventilation and associated respiratory alkalosis in humans

The purpose of this study was to determine if increased environmental heat leads to hyperthermic-induced hypocapnia and associated alkalosis during prolonged self-paced cycling. Nine male cyclists completed three 100 km stochastic time trials in hot (34°C), neutral (22°C) and cold (10°C) environments. Intermittent measurements of rectal and skin temperature, expired gases, blood pH, PaCO₂, PaO₂, and bicarbonate were made throughout. Rectal temperature increased significantly throughout all trials (P < 0.001) and was significantly correlated with increases in the ventilatory equivalent for carbon dioxide (\({\dot{V}_{\rm E}/\dot{V}\hbox{CO}_{2}};\) r = 0.77; P < 0.001) and blood pH (r = 0.69; P < 0.05). Rectal temperature was also negatively correlated with a reduction in PaCO₂ (r = −0.80; P < 0.001). PaO₂ and bicarbonate concentration remained constant throughout all trials. This study has shown that prolonged self-paced cycling is associated with a hyperthermic-induced hyperventilation, causing a decrease in arterialized carbon dioxide tension and consequential respiratory alkalosis.