Atelectasis and gas exchange impairment during enflurane/nitrous oxide anaesthesia

The development of atelectasis and effects on gas exchange during enflurane anaesthesia in nitrogen/oxygen or nitrous oxide/oxygen (inspired oxygen fraction 0.4) were studied in 16 lung-healthy patients (mean age 49 years). Awake, no subject displayed atelectasis as assessed by computed x-ray tomography of the thorax. Pulmonary gas exchange, studied by multiple inert gas elimination technique, and blood gases were normal. After 10 min of enflurane anaesthesia in nitrogen/oxygen, 14 of 16 subjects had developed atelectasis. After 30 min of enflurane anaesthesia in nitrogen/oxygen or nitrous oxide/oxygen, all patients had developed atelectasis, and a further increase was observed after 90 min of anaesthesia to approximately 5% of the intrathoracic area. There was no difference between the two anaesthesia groups. In the nitrogen group, shunt rose to a maximum of 5.8% at 30 min of enflurane anaesthesia, with a significant reduction to the initial anaesthesia level after 90 min of anaesthesia (3.4%). Perfusion of poorly ventilated lung regions (low VA/Q) averaged 4-5% and did not vary significantly during the anaesthesia. In the nitrous oxide group, shunt increased to 6.3% after 90 min of anaesthesia, and there was a parallel decrease in perfusion of low VA/Q regions. The findings suggest that besides prompt collapse of lung tissue during induction of anaesthesia, absorption of gas from closed-off or poorly ventilated regions takes place and further increases the atelectatic area.