Diffusion Hypoxia

The uptake of large volumes of nitrous oxide on induction of anesthesia gives rise to the concentration effect and the second gas effect. On recovery from anesthesia, the outpouring of large volumes of nitrous oxide can produce what Fink ^[96] called diffusion anoxia. These volumes may cause hypoxia ( Fig. 5-23 ) in two ways. First, they may directly affect oxygenation by displacing oxygen.^{[96] [97] [98]} Second, by diluting alveolar carbon dioxide, they may decrease respiratory drive and ventilation.^[98] Both of these effects require the release of large volumes of nitrous oxide into the alveoli. Because large volumes of nitrous oxide are released only during the first 5 to 10 minutes of recovery, this is the period of greatest concern. The concern is enhanced by the fact that the first 5 to 10 minutes of recovery also may be the time of greatest respiratory depression. For these reasons, many anesthetists administer 100% oxygen for the first 5 to 10 minutes of recovery. This procedure may be particularly indicated in patients with preexisting lung disease or in those in whom postoperative respiratory depression is anticipated (e.g., after a nitrous oxide-narcotic anesthetic).

Figure 5-23 At time zero, the inspired gas was changed from 21% oxygen/79% nitrous oxide to 21% oxygen/79% nitrogen. Arterial oxygen subsequently fell in association with the outpouring of nitrous oxide. (Adapted from Sheffer L, Steffenson JL, Birch AA: Nitrous oxide-induced diffusion hypoxia in patients breathing spontaneously. Anesthesiology 37:436–439, 1972.)