Enhanced cerebral CO2 reactivity during strenuous exercise in man

Light and moderate exercise elevates the regional cerebral blood flow by similar to 20% as determined by ultrasound Doppler sonography (middle cerebral artery mean flow velocity; MCA V (mean)). However, strenuous exercise, especially in the heat, appears to reduce MCA V (mean) more than can be accounted for by the reduction in the arterial CO2 tension (P aCO2). This study evaluated whether the apparently large reduction in MCA V (mean) at the end of exhaustive exercise relates to an enhanced cerebrovascular CO2 reactivity. The CO2 reactivity was evaluated in six young healthy male subjects by the administration of CO2 as well as by voluntary hypo- and hyperventilation at rest and during exercise with and without hyperthermia. At rest, P aCO2 was 5.1 +/- 0.2 kPa (mean +/- SEM) and MCA V (mean) 50.7 +/- 3.8 cm s(-1)supercript stop and the relationship between MCA V (mean) and P aCO2 was linear (double-log slope 1.1 +/- 0.1). However, the relationship became curvilinear during exercise (slope 1.8 +/- 0.1; P < 0.01 vs. rest) and during exercise with hyperthermia (slope 2.3 +/- 0.3; P < 0.05 vs. control exercise). Accordingly, the cerebral CO2 reactivity increased from 30.5 +/- 2.7% kPa(-1)supercript stop at rest to 61.4 +/- 10.1% kPa(-1) stop during exercise with hyperthermia (P < 0.05). At exhaustion P aCO2 decreased 1.1 +/- 0.2 kPa during exercise with hyperthermia, which, with the determined cerebral CO2 reactivity, accounted for the 28 +/- 10% decrease in MCA V (mean). The results suggest that during exercise changes in cerebral blood flow are dominated by the arterial carbon dioxide tension.