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

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.

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.

No atelectasis was present in the awake subjects. After induction of anaesthesia, the mean amount of atelectasis was minor (0.2 +/- 0.4 cm2) in group 1 and considerably greater (8.0 +/- 8.2 cm2) 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 VA/Q mismatch showed no difference between the two groups.

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.