Changes in lung volume and (PAO2-PaO2) during anaesthesia.

The theory, that the decrease in lung volume which occurs after induction of general anaesthesia is the cause of the higher (PAO2-PaO2) during anaesthesia, was examined in 18 patients. Lung volume was increased during anaesthesia by changing the posture of the patient to 30 degrees head-up, but there was no improvement in (PAO2-PaO2). There were no correlations between change in (PAO2-PaO2) and in cardiac output between different postures. It is concluded that gas exchange is independent of change in lung volume during anaesthesia, at least over the range examined. It remains possible that restoration of lung volume to pre-anaesthetic values may not restore regional distribution of ventilation to pre-anaesthetic values.