The Determination of Cerebral Blood Flow in Man by the -use of Nitroijs Oxide in Low’ Concentr~attons1

Quantitative measurements of the total blood flow to the brain of man have not previously been reported and the few procedures which have been used for the purpose of obtaining relative or qualitative values have recently been subjected to considerable criticism (1). The method to be described is based on the principle that the rate at which the cerebral venous blood content of an inert gas approaches the arterial blood content depends upon the volume of blood flowing through the brain. Certain aspects of this principle were recognized by Haggard (2) who postulated that the rate at which an anesthetic gas is taken up by the brain depends in part on the rate of blood flow to that organ, and similarly by Ferris, Molle a,nd Ryder (3) who stated that the clearance of nitrogen from the brain during the inhalation of 100 per cent oxygen may be markedly influenced by the magnitude of cerebral blood flow. As far as we are aware, however, the principle has not previously been subjected to mathematical analysis and applied to actual measurements of cerebral blood flow in animals or man. The experiments described in the preceding report (4) a,fforded an opportunity for calibrating this procedure against direct measurements of cerebral blood flow and thus for refining the technical and theoretical fact,ors to a degree that would not otherwise have been possible. METHODS. The specific substance which is employed is not of consequence provided that it is physiologically inert in the concentrations employed, capable of diffusing rapidly across the blood-brain barrier, and susceptible of accurate analysis in the blood. It need not$ even be a gas if it meets all these requirements. In the present experimentIs nitrous oxide has been employed although ot#her gases may be found to be more suittable. Radioact,ive gases offer certain unique advantages and their use is being considered. In the animal experiments t,he inhaled gas consisted of 40 per cent! nitrous oxide in oxygen which was inhaled through the inspiratory valve attached to the tracheal cannula. In the early experiments on man 15 per cent nitrous oxide and 85 per cent oxygen was employed and was inhaled tlhrough the inlet, t,ube of an anesthesia mask. Because the presence of nit,rogen in t/he blood imroduces cer-