Hypoxemia

Hypoxemia - What is Hypoxemia?

Hypoxemia (or Hypoxaemia) is generally defined as decreased partial pressure of oxygen in blood, sometimes specifically as less than which, without further specification, would include both concentration of dissolved oxygen and oxygen bound to hemoglobin.

Inclusion of the latter would include anemia as a possible cause of hypoxemia (which, however, is not the case generally).

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

Still, hypoxia can be caused by hypoxemia, and such hypoxia is referred to as ''hypoxemic hypoxia'', which is distinguished from e.g. anemic hypoxia.

Because of the frequent incorrect use of hypoxemia, this is sometimes erroneously stated as ''hypoxic hypoxia''.

This article is licensed under the Creative Commons Attribution-ShareAlike License. It uses material from the Wikipedia article on "Hypoxemia" All material adapted used from Wikipedia is available under the terms of the Creative Commons Attribution-ShareAlike License. Wikipedia® itself is a registered trademark of the Wikimedia Foundation, Inc.

What Causes Hypoxemia?

Hypoxemia 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

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

In contrast, primary hemoglobin deficiency, e.g. anemia is generally not considered as a cause, since it doesn't decrease the partial pressure of oxygen in blood.

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 hypoxemia 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 hypoxemia 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 hypoxemia. 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 hypoxemia.

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 hypoxemia 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 hypoxemia by this mechanism. A ventilation/perfusion ratio of zero is considered a shunt.

This article is licensed under the Creative Commons Attribution-ShareAlike License. It uses material from the Wikipedia article on "Hypoxemia" All material adapted used from Wikipedia is available under the terms of the Creative Commons Attribution-ShareAlike License. Wikipedia® itself is a registered trademark of the Wikimedia Foundation, Inc.