Hypoxic, anaemic, stagnant and histotoxic
Mnemonic
Four types, defined by where the oxygen delivery chain fails:
- Hypoxic (hypoxaemic) - low arterial oxygen tension. Altitude, lung disease, hypoventilation, shunt
- Anaemic - normal PaO2 but reduced carrying capacity. Anaemia, carbon monoxide, methaemoglobinaemia
- Stagnant (circulatory) - normal content but inadequate flow. Shock, heart failure, local arterial occlusion
- Histotoxic - oxygen delivered but the cell cannot use it. Cyanide, which poisons cytochrome oxidase
“Not enough in the blood, not enough carriers, not enough flow, or the cell cannot use it.”
The distinction is practical: in anaemic and histotoxic hypoxia the PaO2 and saturation are normal, so pulse oximetry is falsely reassuring, and in cyanide poisoning the venous oxygen saturation is characteristically high because tissues cannot extract it.
Expansion
- Hypoxic hypoxia - low arterial oxygen tension. Altitude, hypoventilation, lung disease, shunt
- Anaemic hypoxia - normal tension but reduced content. Anaemia, carbon monoxide poisoning, methaemoglobinaemia
- Stagnant (circulatory) hypoxia - normal content but inadequate flow. Shock, heart failure, local arterial occlusion
- Histotoxic hypoxia - adequate delivery but the tissue cannot use oxygen. Cyanide, hydrogen sulphide
The classification maps onto the oxygen cascade: uptake, carriage, delivery and utilisation.
A useful discriminator is mixed venous saturation. It is low in anaemic and stagnant hypoxia, because tissues extract more to compensate. It is characteristically high in histotoxic hypoxia and in sepsis, because oxygen is delivered but not consumed; a high value in a shocked patient therefore suggests impaired utilisation rather than adequate resuscitation.