Cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRo2) were measured in rats under nitrous oxide anaesthesia, using a 133Xenon modification of the Kety and Schmidt inert gas technique with sampling of cerebral venous blood from the retroglenoid vein. Extracerebral contamination of the venous blood sampled was studied by comparing the rates at which the activity of 133Xenon decreased in blood and tissues. Contamination was avoided by gentle compression of the contralateral retroglenoid vein during sampling. CBF and CMRo2 of the rat brain were 80+/-2 and 7.6+/-0.2 ml-(100g)-1-min-1, respectively. These values are about 25% lower than those previously obtained for cerebral cortical tissue under similar conditions. Induced hypercapnia (Paco2 about 70 mm Hg) or hypocapnia (Paco2 15-20 mm Hg) gave rise to expected changes in CBF but did not alter CMRo2. The CMRo2 of the rat brain is at least twice that of the human brain. This species difference, which is similar to that previously reported for the oxygen uptake of cerebral tissue in vitro, probably reflects on inverse relationship between brain weight and neuronal packing density.

It has been known for a long time that hypercapnia has rather striking effects on the human central nervous system (Brown, 1930; Sieker & Hickam, 1956). Although a variable amount of excitation may be seen, the predominant effect is loss of consciousness. According to Brown (1930), most subjects cannot tolerate more than 10-12 % of C02 for longer than 2 min.

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Abstract— The regional oxygen consumption and carbon dioxide production of a small area of the cerebral cortex (somato‐sensory cortex) in anesthetized dogs at steady state were determined quantitatively by means of a new method for measuring the regional cerebral blood flow (Lassen and Ingvar 1961, 1962) and by determination of the cortical arteriovenous difference of oxygen and carbon dioxide. Samples of cortical venous blood were obtained from the anterior part of the superior sagittal sinus after elimination of the diploic anastomoses. There were 26 complete sets of determinations. The a. v. differences (rAVDo1, and rAVDco2,) were found to be fairly stable (averages 11.3 and 11.7 vol. per cent). Large variations were found in the regional cortical blood flow (rCBF; 0.19 –1.06 ml per g per minute, average 0. 65 ± S.D. 0.23). The regional cortical oxygen consumption (rCMRo2 = rCBF × rAVDo2) hence varied between 2.3 and 13.4, average 7.0 (± S. D. 2.8), ml per 100 g tissue per minute. Studies of the EEG patterns prevailing at the time of the determinations showed that an EEG pattern dominated by faster frequencies (5–12 cycles per second) had an average rCMRo2, of 8.6 ml per 100 g per minute. A pattern dominated by high voltage slow waves (0.5 to 4 cycles per second) showed an average of 5.0, and a mixed EEG pattern of fast and slow waves an average rCMRo2 of 6.8 ml per 100 g per minute.