Airway closure, atelectasis and gas exchange during anaesthesia.

Pulmonary gas exchange is regularly impaired during general anaesthesia with mechanical ventilation. This results in decreased oxygenation of blood. Major causes are collapse of lung tissue (atelectasis) and airway closure. Collapsed lung tissue is present in 90% of all subjects, both during spontaneous breathing and after muscle paralysis, and whether intravenous or inhalational anaesthetics are used. Airway closure is also common and increases in magnitude with increasing age of the patient. There are correlation between the amount of atelectasis and pulmonary shunt and between airway closure and perfusion of poorly ventilated lung regions (low VA/Q). Atelectasis and airway closure explain as much as 74% of gas exchange impairment in routine anaesthesia. A major cause of atelectasis is the pre-oxygenation during induction of anaesthesia. Lowering the inspired O2 concentration to 80% suffices to avoid almost all atelectasis. Airway closure and low VA/Q can only be prevented by raising the FRC level by PEEP or by other means.