Hypoxemia pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Pathophysiology
Causes are classified into 5 groups:
- Low inspired fractional concentration of oxygen (low FiO2)
- Alveolar hypoventilation
- Impairment of diffusion across blood-gas membrane
- Shunt
- 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).
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 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
Shunt 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. 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).