Postoperative hypoxaemia: mechanisms and time course.

Postoperative hypoxaemia results predominantly from two mechanisms. Gas exchange is impaired during anaesthesia as a result of reduced tone in the muscles of the chest wall and probably alterations in bronchomotor and vascular tone, and the resulting changes persist into the postoperative period. In addition, there is an abnormality of control of breathing, which results in episodic obstructive apnoea. These episodes continue for several days after operation and are related to sleep pattern and analgesic administration, although the precise effects of different analgesic regimens have not been evaluated. Oxygen administration is effect in reducing the degree of hypoxaemia.