Cerebral blood flow in man at high altitude. Role of cerebrospinal fluid pH in normalization of flow in chronic hypocapnia.

Cerebral blood flow was determined by an N2O 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 N2O analyzer was used both to measure end-tidal PN2O SO that it could be kept constant for 15 min and to determine blood N2O, for which a simple gas extraction method was devised. In addition, acute changes in cerebral blood flow were estimated from cerebral A-V O2 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 Pco2 had fallen from 41 to 30 mm Hg. Acute correction of hypoxia restored cerebral blood flow to control while mean Pco2 was still 31 mm Hg. Addition of O2 and CO2 to inspired air raised cerebral blood flow 34% above control at Pao2 = 170, PacOo = 35 mm Hg. Values obtained by extrapolation suggest that if arterial Pco2 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<>, 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. ADDITIONAL