Hypoxia: The Four Types

Key points

• • Hypoxia means too little oxygen reaching the tissues; hypoxaemia means a low oxygen level in arterial blood. The two are related but not the same.
• • Hypoxic hypoxia: the arterial oxygen tension itself is low, as at altitude, or with hypoventilation, ventilation-perfusion mismatch, shunt or impaired diffusion.
• • Anaemic hypoxia: arterial oxygen tension is normal but carrying capacity is reduced, as in anaemia, carbon monoxide poisoning and methaemoglobinaemia.
• • Stagnant or circulatory hypoxia: content is adequate but flow is not, as in shock, heart failure or a local arterial occlusion.
• • Histotoxic hypoxia: oxygen arrives but the cell cannot use it, the classic example being cyanide inhibiting cytochrome oxidase.
• • Cyanosis depends on the absolute amount of deoxygenated haemoglobin, so it appears readily in polycythaemia and may never appear in severe anaemia.