THE DETERMINATION OF CEREBRAL BLOOD FLOW IN MAN BY THE USE OF NITROUS OXIDE IN LOW CONCENTRATIONS

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Abstract
The proposed method is an application of the familiar Fick principle to the brain during its absorption of an indifferent gas from the blood passing through it. The relationships implicit in this principle may be expressed thus: cerebral blood flow (as ml. of blood/unit mass of brain tissue over any time period) is equal to the quantity of any gas taken up by unit mass of brain tissue during the same interval, divided by the quantity of that gas lost/ml. of blood during that period. The exptl. procedure involves the adm. by inhalation of a gas mixture containing N2O (15%), in O2 or a mixture of O2 (21%) and N (64%). From needles inserted in the internal jugular vein and the femoral artery simultaneous pairs of blood samples are taken at 2, 4, 6 and 10 mins. after the onset of inhalation of the mixture. From the respective N2O concs. it is possible to calculate the integral of the N2O arteriovenous difference over the 10 min. period which represents the quantity of gas lost to the brain/unit vol. of blood during that time. The quantity of N2O taken up by the brain in that time is calculated from the internal jugular conc. of N2O in the 10-min. sample and from the partition coefficient of NjO between brain and blood. The theoretical steps involved in this derivation have been subjected to exptl. verification in monkeys, dogs and human subjects. Final proof of the validity of the method has been obtained by a comparison with values for cerebral blood flow obtained simultaneously in rhesus monkeys by direct measurement using the bubble flow meter [see B. A. 18(2): entry 2751]. In 9 such comparisons the mean deviation between the 2 methods was [plus or minus] 10%. By means of this method cerebral blood flow and cerebral O2 consumption are reported for 11 human subjects with various minor diseases. The values have fallen within a fairly close range. Mean cerebral blood flow was 62 ml./100 g./min. and cerebral O2 consumption 3.7 ml./lOO g./min. respectively. These values are in excellent agreement with the avgs. recently reported for the rhesus monkey by the use of the bubble flow meter: cerebral blood flow 47 ml./lOO g./min. and cerebral O2 consumption 3.7 ml./lOO g./min. (Am. J. Med. Sci. 207: 813, 1944). For a normal human brain weighing 1300 grams the values obtained by the N2O method would yield a total cerebral blood flow of 800 ml./min. and a cerebral O2 consumption of 48 ml./min. Even 15% N2O is not completely inert physiologically and might conceivably influence cerebral blood flow. This disadvantage may be overcome by the use of a radioactive inert gas instead of N2O.

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