British Journal of Anaesthesia 1990; 64: 507-514 GAS EXCHANGE DURING ANAESTHESIA

The lungs oxygenate blood less efficiently in the anaesthetized than the awake subject. Defective arterial oxygenation after pulmonary resection was reported as early as 1943 by Maier and Cournand [25], and in 1958 Campbell and co-workers [7] showed that anaesthesia was often associated with increased alveolar-arterial oxygen partial pressure difference. However, despite almost 50 years ' awareness of impaired gas exchange during anaesthesia, the causes under-lying this impairment have remained unclear until recently. This review considers the different mechanisms by which arterial oxygenation may be impaired. Compliance and functional residual capacity (FRC) In 1955, Nims and co-workers [29] found that the static compliance of the total respiratory system (lungs and chest wall) was reduced in the anaesthetized subject, and in succeeding studies lung compliance was shown to be lower in the anaesthetized than in awake subjects (for a review, see [9]). Bendixen and co-workers [1] reported continuously decreasing compliance during an-aesthesia, accompanied by a similarly decreasing arterial partial pressure of oxygen (Pa^). Fol-lowing large inflations of the lungs, compliance and PZQ increased. The authors considered that this indicated slow development of absorption atelectasis. However, other research groups were unable to confirm the findings of Bendixen's group [31, 41]; and atelectasis could not be seen by conventional chest x-ray. Atelectasis as a mechanism of impaired gas exchange was rejected, therefore. In the same year as Bendixen and co-workers proposed the concept of atelectasis, Bergman [2