Definition

Hypoxemia (or hypoxaemia) is an abnormally low partial pressure of oxygen (PO₂) in arterial blood (West J. "Pulmonary Pathophysiology: The Essentials" p22). A frequent error is to use the term hypoxemia to mean low oxygen content in arterial blood. It is possible to have a low oxygen content (eg due to anemia) but a high PO₂ so incorrect use can lead to confusion.

Hypoxemia is different from hypoxia as that is an abnormally low oxygen availability to the body or an individual tissue or organ. Hypoxia may refer to: Hypoxia (medical), the lack of oxygen in tissues Hypoxia or Oxygen depletion, a reduced concentration of dissolved oxygen in a water body leading to stress or even death in aquatic organisms This is a disambiguation page: a list of articles associated with the same title. ...

The type of hypoxia that is caused by hypoxaemia is referred to as hypoxemic hypoxia. Because of the frequent incorrect use of hypoxemia, 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 hypoxemia 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 hypoxemia 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). FiO2, in the field of medicine, is the fraction of inspired oxygen in a gas. ... The lungs flank the heart and great vessels in the chest cavity. ...

Alveolar hypoventilation

If the alveolar ventilation is low, there may be insufficient oxygen delivered to the alveoli each minute. This can cause hypoxemia 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).

References

1. West J. "Pulmonary Pathophysiology: The Essentials" 1977 (Williams & Wilkins)