Oxygen breathing and QS-QT during postoperative pain relief in man.

POSTOPERATTVE HYPOXAEMIA following anaesthesia has been attributed mainly to two causes; venous admixture (shunt component) and variation in ventilationperfusion ratios (distribution component). In order to assess quantitatively the role played by the latter factor, the method of breathing 100 per cent oxygen has been employed by many investigators.This method is based on the fact that the diffusion and distribution components become negligible and AaDo2 can then result only from direct venous admixture, either through normally existing channels or through direct pulmonary arterio-venous shunts, or perfusion of alveoli which cannot receive any of the inspired oxygen because they are atelectatic or completely occluded by exudate or thickened walls. Griffo reported that breathing pure oxygen at atmospheric pressure for several hours does not enhance the development of pulmonary atelectasis in normal subjects. Several workers," however, have postulated that oxygen breathing of relatively short duration might lead to the development of diffuse pulmonary atelectasis. Such atelectasis has been suggested to result from the absorption of trapped gas distal to temporarily occluded airways. The main purpose of the present study was to seek further evidence for or against these views by extending the observations to patients breathing air and 100 per cent oxygen during postoperative pain relief with meperidine or epidural analgesia and to determine the possibility of difference of the increase in QS/QT between the epidural and meperidine analgesia groups.