Cerebral blood flow and oxygen consumption in the rat in hypoxic hypoxia.

In order to measure cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRo(2)) at pronounced degrees of hypoxic hypoxia the Pao(2) of artificially ventilated and normocapnic rats was reduced to between 47 and 22 mm Hg for 15-25 min with subsequent measurements of CBF, using a -133Xenon modification of the Kety and Schmidt technique, and of the arteriovenous difference in oxygen content, the venous blood being obtained from the superior sagittal sinus. When the Pao(2) was reduced to minimal values of 22 mm Hg CBF increased 4- to 6-fold, the increase in CBF being unrelated to changes in blood pressure or Paco(2). The CMRo(2) remained unchanged at all levels of hypoxia. It is concluded that the maintenance of a normal, or near-normal, cerebral energy state even at extreme degrees of hypoxic hypoxia depends solely on a homeostatic increase in CBF.