Cerebral Blood Flow In Man at High Altitude

Cerebral blood flow was determined by an N 2 O method in 7 normal men at sea level and after 6 to 12 hr and 3 to 5 days at 3810 m altitude. An infrared N 2 O analyzer was used both to measure end-tidal PN 2 O so that it could be kept constant for 15 min and to determine blood N 2 O, for which a simple gas extraction method was devised. In addition, acute changes in cerebral blood flow were estimated from cerebral A-V O 2 differences. Control cerebral blood flow was 43 ml per 100 g per min; it increased 24% at 6 to 12 hours and 13% at 3 to 5 days at altitude. After 3 to 5 days, pH of cerebrospinal fluid was normal (7.31) in four subjects while arterial blood pH was alkaline (7.47); arterial blood Pco 2 had fallen from 41 to 30 mm Hg. Acute correction of hypoxia restored cerebral blood flow to control while mean Pco 2 was still 31 mm Hg. Addition of O 2 and CO 2 to inspired air raised cerebral blood flow 34% above control at Pao 2 = 170, Paco 2 = 35 mm Hg. Values obtained by extrapolation suggest that if arterial Pco 2 was raised to control (41 mm Hg), cerebral blood flow would have been 60% above control. Cerebral blood flow thus appears to return to normal at the prevailing Paco 2 , probably because the pH of cerebrospinal fluid and of the extracellular fluid of cerebral vascular smooth muscle is kept normal by active transport across the ‘blood-brain’ barrier. It is postulated that an ion-impermeable barrier separates the blood stream from extracellular fluid of the smooth muscle of cerebral arterioles.