Cerebral blood flow and oxygen delivery at high altitude.

During acclimatization to moderate altitudes, a simple calculation from data of others shows that the rise in cerebral blood flow (CBF) is sufficient that oxygen delivery to brain (DaO2) is constant as arterial oxygen content (CaO2) falls. This balance occurs on average even though the hypocapnia caused by hypoxic hyperventilation causes cerebral vasoconstriction, conflicting with hypoxic cerebral vasodilation. The relative strengths of the ventilatory and cerebral vascular sensitivities may affect this balance in individual subjects. There is no evidence for a mechanism to detect or respond directly to DaO2. Hypoxic cerebral vasodilation is believed to depend upon tissue and capillary PO2 and content, not arterial. Despite these reservations it is of interest that the average resultant DO2 remains constant. I speculate here that this match may relate to the well-known local hyperemic response to neuronal activity which now has been shown to initially overcompensate, in that tissue PO2 and pH rise in the first few seconds after neural activity. Analysis of results from the paper by Severinghaus et al. (Circ. Res. 1966;19:274-282) shows that in their subjects, despite approximately 20% reductions in arterial oxygen content at 3,810 m altitude, the data does not show any significant fall in DaO2 as a result of increased cerebral blood flows.