The solubility of nitrous oxide in blood and brain.

Since the formulation of the lipide theory of anesthesia (1) there has been some interest in the solubilities of various volatile anesthetics in the brain (2-6). Unfortunately the results obtained in viva have been equivocal and inexact because of the technical difficulties of obtaining -and analyzing without loss samples of brain containing volatile gas. Even the studies on solubility in vitro, on which the theory was largely based, were made on peanut, olive, or similar oils rather than on brain lipides. Our attention was drawn to the problem of l;he solubility of gases in brain in the course of the development of a method for measurement of cerebral blood flow by means of the blood-brain exchange of an inert gas, nitrous oxide (7). It can be shown that cerebral blood flow may be calculated from the arterial (A) and cerebral venous (V) blood nitrous oxide concentrations over a time interval (u) measured from the onset of inhalation of a comparatively low tension of nitrous oxide. The interval u must be of sufficient length to insure practically complete equilibrium between brain and blood draining the brain with respect to mean nitrous oxide tension. If the results are to be expressed in flow per unit weight of brain rather than in terms of flow per unit of nitrous oxide capacity, a factor (8) must be introduced, representing the brain-blood partition coefficient of nitrous oxide or the ratio of nitrous oxide dissolved per gm. of brain to that dissolved per cc. of blood at a constant nitrous oxide tension and at 37”. The formula for calculation of cerebral blood flow, which is derived elsewhere may be expressed as follows :