Hypoxemia is low blood oxygen content. It is usually defined as a hemoglobin oxygen saturation of under 90% or an oxygen partial pressure of under 60 mmHg. There are five different causes of hypoxemia that will be discussed below.
1. Hypoventilation
Also known as respiratory depression, this is a state in which breathing or lung function are reduced, and less air enters the alveoli in the lungs. The amount of oxygen entering the blood is therefore also reduced. It can be caused by injury or disease of the brain or nervous system, an obstruction in the airway, or as a side effect of drugs including barbiturates, benzodiazepines, opiates and alcohol.
2. Low inspired oxygen
The amount of oxygen reaching the alveoli and therefore the blood will be reduced if the amount of oxygen that is inspired is reduced. This can occur when breathing atmospheric gas if the oxygen partial pressure is reduced due to low pressure for example at high altitude. It may also occur when breathing non-atmospheric gas, for example if a problem occurs during an anesthesia or when inhaling helium from a balloon.
3. Right to left shunt
This is when a portion of blood does not get oxygenated in the lungs. It can be either anatomical, where the blood physically bypasses the lungs, for example due to the Tetralogy of Fallot congenital defect, or physiological, where the blood passes through the lungs but does not become oxygenated due to the alveolar spaces being filled with fluid, for example in drowning or pneumonia. Hypoxia caused by a shunt usually cannot be corrected using oxygen.
4. Ventilation-perfusion mismatch
This occurs when areas of the lungs are no longer able to transfer oxygen to the blood. This may be due to a lack of oxygen to transfer to the blood, for example, as a result of the alveolar damage caused by chronic obstructive pulmonary disease (COPD), the fluid buildup in pulmonary oedema or the lung collapsing. Conversely, it may also be due to a lack of blood to be perfused despite the alveoli being ventilated (known as physiological dead space) which may be due to a low cardiac output or compression or stretching of the alveolar capillary, which may occur in artificially ventilated patients. There can be different degrees of ventilation-perfusion mismatch with varying reductions in blood oxygen content. The most severe form is a shunt, as described above.
5. Impaired diffusion
This is when diffusion of oxygen across the blood gas membrane from the alveolar to the blood is impaired. It is the least common cause of hypoxemia as this rarely causes a problem with blood oxygen levels. It can occur when the time taken for the blood to pass through the lungs decreases, which can occur during exercise although in most cases presents no problem, and the physical separation between the blood and the oxygen in the alveoli increases, as seen in interstitial lung disease.
Sources:
www.sallyosborne.com/Med%20Lecture%20-20Causes%20of%20Hypoxemia.pdf
http://www.news-medical.net/health/What-Causes-Hypoxemia.aspx
