Cerebral artery blood velocity in normal subjects during acute decreases in barometric pressure

To investigate the effect of acute changes in barometric pressure on region al cerebral perfusion Lye studied the middle cerebral artery (MCA) blood ve locity in five healthy male volunteers by means of a low-pressure chamber. The MCA blood velocity, arterial blood and respiratory gases were measured at the barometric pressures of 1, 0.8, 0.65, and 0.5 atmospheres. The obser ved blood velocity (V0) showed no systematic changes. Decreases in barometr ic pressure induced hypoxia and hypocapnia. When normalizing the MCA blood velocity (Vn) to a standard PCO2 (5.3 kPa), thereby correcting for the hypo xic induced hypocapnia, we obtained an inverse relationship between cerebra l artery blood velocity and arterial blood oxygen content (CaO2). The oxyge n supply to the brain, estimated as the product of V0 and CaO2, decreased W ith lowering of the barometric pressure. However, the product of Vn and CaO 2 remained constant. This suggests the existence of a regulatory mechanism attempting to maintain a constant oxygen supply to the brain during acute c hanges in CaO2, if the hyperventilation induced decrease in PCO2 can be omi tted. In the artificial situation of a low pressure chamber, our findings a re quite similar to those obtained at sea level. This indicates that the un derlying mechanisms of control of cerebral blood tow do not change during a cute exposure to altitude.