Lung aeration and pulmonary gas exchange during general and epidural anaesthesia : Influenced by inspired oxygen concentration and diaphragm shape

Abstract: Induction of general anaesthesia causes a rapid decrease in functional residual capacity, which can promote alveolar collapse (atelectasis) and airway closure in dependent lung regions. Mismatching of ventilation and perfusion (VA/Q) in the lungs is accompanied by impairment of pulmonary gas exchange. Inspired oxygen concentration and changes in diaphragmatic function and geometry may play a major role in the formation of alveolar collapse and pulmonary shunting. Epidural anaesthesia, in contrast to general anaesthesia, does not appear to influence FRC or airway closure and has minor effects on the VA/Q match.Using spiral computed tomography (CT), the lungs and the diaphragm were investigated either before or during anaesthesia in human adults (withoutpulmonary disease) scheduled for elective surgery. CT scans were analysed according to the following parameters: overaeration, normal aeration, reduced aeration, poor aeration, and atelectasis. Ventilation/perfusion (VA/Q) distribution was estimated using the multiple inert gas technique. General anaesthesia, in contrast to epidural anaesthesia, causes significant increases in poorly aerated lung regions and in `dependent densities. The perceived benefits of pre-anaesthetic oxygenation as well as intraoperative hyperoxygenation contrast with the recognised rapid development of atelectasis in dependent lung regions. In addition, our findings do not provideevidence for a simple direct relationship between the anaesthesia-induced, cranial displacement of the diaphragm and amount of lung collapse.In conclusion, no or little impairment of lung aeration and VA/Q matching is caused by the lithotomy position and/or epidural anaesthesia. Our findings do not provide evidence for a simple direct relationship between lung collapse and positioning. The minor movement of the diaphragm during anaesthesia may play an additional role in atelectasis formation. In addition, the findings call for a re-evaluation of the oxygenation technique during anaesthesia.