The Rate of Uptake of Nitrous Oxide in Man1

During inhalational anesthesia, an inert gas or vapor diffuses across the pulmonary membrane, dissolves in the blood and is carried by the circu-lation to all of the tissues of the body. The rate at which an anesthetic dissolves from the gaseous phase in the alveoli into the pulmonary capillary blood is termed the rate of uptake of that gas. The uptake of any inert gas continues for many hours at a slowly decreasing rate until the entire body is saturated with the gas at the inspired ten-sion. For example, Behnke, studying nitrogen gas exchange, found that about 5 hours are re-quired to reach 90 per cent of complete saturation at the inspired gas tension (1). This slow process of body saturation should be differentiated from the process of saturation of the arterial blood, to which the depth of anesthesia is closely related. A normal denitrogenated sub-ject, beginning to breathe air, requires about 3 minutes to reach 90 per cent of complete arterial saturation at the inspired nitrogen tension. If he breathes nitrous oxide, Kety showed that about 20 minutes are required to reach 90 per cent of com-plete arterial saturation at the inspired nitrous oxide tension (2). Both arterial and body satura-tion occur more rapidly with a relatively insoluble gas such as nitrogen, than with the soluble anes-thetics, since large quantities of the soluble gases are carried away from the lung by the pulmonary blood, delaying the rise of alveolar gas tension. As the anesthetic gas is taken up in solution in the blood, the resulting decrease in gas volume in a breathing reservoir can be measured by means of a spirometer, just as oxygen consumption is meas-ured in the determination of basal metabolic rate. The method herein reported permits the determina-tion of the volume rate of uptake of the anesthetic gas (in this case nitrous oxide)-, and oxygen, while