Mechanisms of abnormal gas exchange during anesthesia

A review of the literature and of our recent data (obtained by computer-based analysis of multiple inert gas elimination) re-emphasizes the significant gas exchange disturbances found to occur during general anesthesia which develop for probably several reasons. In this report we suggest firstly that the reduction in functional residual capacity (FRC) may well be an effect of anesthesia just as is abnormal gas exchange. In other words, the reduction in FRC is not per se the cause of gas exchange disturbances, but rather occurs alongside them. Secondly, the causes of abnormal gas exchange may have more to do with local ventilatory reserve in lung regions, be that reserve via the existence of collateral ventilation as in dogs or via acute or chronic airway pathology in disease. Thus, species without adequate collateral ventilation or with some degree of airway disease would seem to be at greatest risk of developing highly significant gas exchange disturbances during general anesthesia. We speculate that this vulnerability is probably due to multiple factors that change as a result of the particular anesthetic agents used. Such factors include altered hypoxic vasoconstriction, a change in airway secretions and clearance of those secretions, changes in bronchomotor tone, changes in surfactant activity, and alveolar volume loss due to rapid uptake of soluble anesthetic gases like nitrous oxide. To qualitatively and quantitatively distinguish amongst these various potential factors will require experimental protocols and techniques of a highly controlled and accurate nature. That in 1983 we still do not understand the basic mechanisms behind abnormal gas exchange during anesthesia attests to the difficulty of mounting such an experiment.