Types of hypoxia: hypoxaemic, anaemic, ischaemic and histiotoxic
Hypoxia means too little oxygen reaching or being used by a tissue, and its four types depend on where the oxygen supply chain breaks down.
Remember
Hypoxaemic hypoxia is the type with a low arterial oxygen pressure (the pressure of oxygen dissolved in arterial blood). In the other three types that pressure can be normal, since the fault lies in how oxygen is carried, delivered or used.
Memory aid
lungs, blood, pipes, cells
Sort the four types by the place where the oxygen supply fails: the lungs, the blood, the pipes (blood flow) or the cells.
Hypoxia is classed into four types. Ask where the chain breaks as the oxygen makes its journey from the air to the cell.
Hypoxaemic hypoxia is a fault at the very start. The oxygen pressure in the blood is too low, so haemoglobin is left inadequately saturated and the blood carries less oxygen to the tissues. It happens at high altitude and in lung diseases that reduce oxygen absorption.
Anaemic hypoxia is a fault in the carrier: the blood cannot carry enough oxygen. Anaemia (reduced haemoglobin) does this, and so does carbon monoxide, a colourless, odourless gas made when fuel burns incompletely. Carbon monoxide binds to haemoglobin more tightly than oxygen does, so oxygen is displaced. Lungs and blood flow may be working normally, yet each portion of blood delivers less oxygen.
Ischaemic hypoxia (also called stagnant hypoxia) is a fault in delivery: too little blood reaches the tissue, for example through a blocked vessel or in heart failure. The blood itself can be well supplied with oxygen, because the problem is how much of it arrives.
Histiotoxic hypoxia (histiotoxic means poisonous to tissue, and the word is also spelt histotoxic) is a fault in the cells themselves: the oxygen arrives, but the cells cannot use it as their enzymes are disabled. Cyanide is the classic poison that does this, by blocking an enzyme in the mitochondria.
The cells then make less ATP (their energy currency), the ATP-driven pumps in the cell membrane fail, and the cell is injured.
Key facts
- Hypoxia
- too little oxygen reaching or being used by a tissue.
- Hypoxaemic hypoxia
- the oxygen pressure in the blood is too low to saturate haemoglobin, as at high altitude or in some lung diseases.
- Anaemic hypoxia
- the blood cannot carry enough oxygen, as in anaemia or carbon monoxide poisoning.
- Ischaemic hypoxia
- too little blood reaches the tissue, for example from a blocked vessel or in heart failure.
- Histiotoxic hypoxia
- the cells cannot use oxygen because their enzymes are disabled, as in cyanide poisoning.
- Pulse oximeter
- a finger probe that estimates how much haemoglobin is carrying oxygen, but a standard one cannot separate oxyhaemoglobin (haemoglobin carrying oxygen) from carboxyhaemoglobin (haemoglobin carrying carbon monoxide), so a normal reading does not exclude carbon monoxide poisoning.
- Clue for histiotoxic hypoxia
- when blood flow, haemoglobin function, arterial oxygen pressure and the pulse oximeter reading are all normal, yet the tissues are starved of oxygen, think of cells that cannot use it.
Exam traps
- Histiotoxic hypoxia is tempting for carbon monoxide because it can also disturb mitochondria, but the mechanism of binding to haemoglobin is a fault of oxygen transport rather than of oxygen use.
- Hypoxaemic hypoxia does not fit when the arterial oxygen pressure is normal, because oxygen is then crossing the lungs into the blood normally; with carbon monoxide the fault is in what the blood can carry.
- A normal carboxyhaemoglobin level argues against anaemic hypoxia from carbon monoxide, even though carbon monoxide is the classic cause of anaemic hypoxia with a normal arterial oxygen pressure and a falsely reassuring pulse oximeter reading.
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