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Atelectasis formation and gas exchange impairment during anaesthesia.

Anaesthesia is accompanied by impaired oxygenation of the blood, and sometimes hypoxaemia may develop despite an increased oxygen fraction of the inspired gas. The major cause of this derangement is shunt, an effect of prompt atelectasis formation in dependent lung regions. An additional cause is ventilation/perfusion (V/Q) mismatch, possibly produced by intermittent airway closure. The magnitude of shunt and size of atelectasis are independent of the age of the patient, whereas V/Q mismatch increases with age, explaining the age dependent impairment of oxygenation.