Diagnostic approach to patients with unexplained hypoxaemia

This article describes a systematic approach to analysing patients with hypoxaemia. The article is intended for anyone dealing with patients with reduced oxygen saturation during rest or exercise. Hypoxaemia has various causes. The underlying cause of hypoxaemia can be determined using the alveolar-arterial oxygen tension difference and the diffusion capacity for carbon monoxide. The administration of 100% oxygen corrects hypoxaemia in the case of a relative shunt or a diffusion disorder, but not in an absolute shunt.