Hypoxemia

Hypoxaemia (or hypoxemia) is an abnormal deficiency in the concentration of oxygen in arterial blood (Mosby's Medical Dictionary). A frequent error is made when the term is used to describe poor tissue diffusion as in hypoxia. It is possible to have a low oxygen content (eg due to anaemia) but a high PO2 in arterial blood so incorrect use can lead to confusion.

Hypoxaemia is different from hypoxia, which is an abnormally low oxygen availability to the body or an individual tissue or organ.

The type of hypoxia that is caused by hypoxaemia is referred to as hypoxemic hypoxia. Because of the frequent incorrect use of hypoxaemia, this is sometimes erroneously stated as hypoxic hypoxia.

Causes
Causes are classified into 5 groups:
 * 1) Low inspired fractional concentration of oxygen (low FiO2)
 * 2) Alveolar hypoventilation
 * 3) Impairment of diffusion across blood-gas membrane
 * 4) Shunt
 * 5) Ventilation-perfusion inequality

Conditions that result in hypoxaemia act via one or more of these primary causes.

Low inspired oxygen fraction (low FiO2)
See also: FiO2

If the concentration of oxygen in the inspired gas is low, then a reduced amount of oxygen is delivered to the gas exchanging parts (alveoli) of the lung each minute. This can result in hypoxaemia even if the lungs are normal. It is the inspired oxygen concentration that is important rather than the atmospheric concentration as the person may not be breathing atmospheric gas (eg during an anesthetic).

Alveolar hypoventilation
If the alveolar ventilation is low, there may be insufficient oxygen delivered to the alveoli each minute. This can cause hypoxaemia even if the lungs are normal, as the cause may be outside the lungs (eg airway obstruction, depression of the brain's respiratory center, or muscular weakness).

Impaired diffusion
Impaired diffusion across the blood-gas membrane in the lung can cause hypoxaemia. However this is a very rare cause as it is only in extremely unusual circumstances that actually does cause a problem. Most of the past cases once thought to be due to a diffusion problem are now recognised as being due to ventilation-perfusion inequality.

Shunt
Shunt of blood from the right side to the left side of the circulation (right-to-left shunt) is a powerful cause of hypoxaemia. The shunt may be intracardiac or may be intrapulmonary. This cause can be readily distinguished from the others as it is the only cause that cannot be corrected by the administration of 100% oxygen.

Ventilation-perfusion inequality
Ventilation-perfusion inequality (or ventilation perfusion mismatch) is a common cause of hypoxaemia in people with lung disease. It is the areas of the lung with V/Q ratios that are less than one (but not zero) that cause hypoxaemia by this mechanism. (A V/Q ratio of zero is actually shunt so does not contribute to this cause).