Definitions

Hypoxemia

Hypoxemia

[hahy-pok-see-mee-uh]
Hypoxaemia (or hypoxemia) is a deficiency in the concentration of dissolved oxygen in arterial blood. 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 (e.g., 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 groups:

  1. Low inspired partial pressure of oxygen (low PiO2)
  2. Alveolar hypoventilation
  3. Impairment of diffusion across blood-gas membrane
  4. Ventilation-perfusion inequality
  5. Shunt
  6. Reduced mixed venous oxygen content in the context of a Shunt

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

Low inspired oxygen partial pressure (low PiO2)

If the partial pressure 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. The reduced oxygen partial pressure can be a result of reduced fractional oxygen content (low FiO2) or simply a result of low barometric pressure, as can occur at high altitudes. This reduced PiO2 can result in hypoxaemia even if the lungs are normal. Additionally, it is the inspired oxygen content that is important in this case rather than the atmospheric concentration as the person may not be breathing atmospheric gas (e.g., during an anesthetic). NOTE: People will often simplify this concept and state low FiO2 as one of the 6 principal causes of hypoxemia, but this fails to account for important circumstances such as high altitude induced hypoxemia, where indeed FiO2 is normal.

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 (e.g., 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

Shunting 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. It has been traditionally thought that this cause could be readily distinguished from the others as the only cause that cannot be corrected by the administration of 100% oxygen. However, this may be an oversimplification as some types of intrapulmonary shunts can be closed with 100% O2 and the hypoxemia remedied.

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 ventilation/perfusion ratios that are less than one (but not zero) that cause hypoxaemia by this mechanism. A ventilation/perfusion ratio of zero is considered a shunt.

References

  1. West, John B. (1977). Pulmonary Pathophysiology: The Essentials. Williams & Wilkins.
  2. Anderson, Kenneth N. (ed.) (2002). Mosby's Medical, Nursing & Allied Health Dictionary. 6th ed., C.V. Mosby.

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