Atelectasis and pulmonary shunting during induction of general anaesthesia - Can they be avoided?

Background: Gas exchange is regularly impaired during general anaesthesia with mechanical ventilation. A major cause of this disorder appears to be atelectasis and consequently pulmonary shunt. After re-expansion, atelectasis reappears very slowly if 30% oxygen in nitrogen is used, but much faster if 100% oxygen is used. The aim of the present study- was to evaluate if early formation of atelectasis and pulmonary shunt may be avoided if the lungs are ventilated with 30% oxygen in nitrogen instead of 100% oxygen during the induction of general anaesthesia. Methods: Twenty-four adult patients with healthy lungs scheduled for elective surgery were investigated. During induction of anaesthesia, the lungs were manually ventilated via a face mask, using either 30% oxygen in nitrogen (group 1, n=12) or 100% oxygen (group 2, n=12). Atelectasis was estimated by computed x-ray tomography and ventilation-perfusion distribution with the multiple inert gas elimination technique, both awake and during general anaesthesia with mechanical ventilation. Results: No atelectasis was present in the awake subjects. After induction of anaesthesia, the mean amount of atelectasis was minor (0.2+/-0.4 cm(2)) in group 1 and considerably greater (8.0+/-8.2 cm(2)) in group 2 (P<0.001). The pulmonary shunt was 0.3+/-0.7% of cardiac output in the awake subjects. This value increased to 2.1+/-3.8% in group 1 and to 6.5+/-5.2% in group 2 (P<0.05). The indices of V-A/Q mismatch showed no difference between the two groups. Conclusion: During induction of general intravenous anaesthesia in patients with healthy lungs, gas composition plays an important role for atelectasis formation and the establishment of pulmonary shunt. By using a mixture containing 30% oxygen in nitrogen, the early formation of atelectasis and pulmonary shunt may, at least in part, be avoided. (C) Acta Anaesthesiologica Scandinavica 40 (1996)