Cardiorespiratory and cerebrovascular responses to hyperoxic and hypoxic rebreathing: effects of acclimatization to high altitude.

We examined the cardiorespiratory and cerebrovascular responses to hyperoxic and hypoxic rebreathing at low attitude and high altitude. We measured ventilation, middle cerebral artery blood flow velocity (MCAv) and arterial blood pressure in conditions of eucapnia, hypocapnia (voluntary hyperventilation) and during hyperoxic and hypoxic rebreathing firstly at low altitude (1400 m) then again at high altitude (3840 m) in five individuals, following 9 days at altitude >5000 m. High altitude was associated with elevations in the blood pressure response to hyperoxic rebreathing, whilst cerebrovascular reactivity was reduced and ventilatory sensitivity was unchanged. During hypoxic rebreathing, whilst ventilatory and blood pressure reactivity were increased (vs. low altitude and hyperoxic rebreathing), cerebrovascular reactivity was preserved. In conclusion, at high altitude there was an enhancement in peripheral but not central chemosensitivity to CO(2); and during hypoxic rebreathing, marked elevations in blood pressure may restore some of the reduction in cerebrovascular CO(2) reactivity, potentially reflecting a pressure-passive relationship in the brain offsetting sympathetic-induced cerebral vasoconstriction.