Hypoxemia - The Distinction Between Hypoxemia, Hypoxia and Anemia

The Distinction Between Hypoxemia, Hypoxia and Anemia

While the term hypoxemia is limited to low oxygen in the blood, the more general term is hypoxia, which is an abnormally low oxygen content in any tissue or organ. It will be seen that hypoxemia can cause hypoxia (the hypoxemic hypoxia) along with other mechanisms (e.g. anemic hypoxia or histotoxic hypoxia). Informally, hypoxemic hypoxia is sometimes given as hypoxic hypoxia.

Disagreements exist concerning the scope of the term hypoxemia. At one extreme, there is nearly universal agreement that a blood gas determination which shows that the partial pressure of oxygen in a good arterial sample of whole blood is lower than normal constitutes hypoxemia. One important condition that tests this rule is carbon monoxide poisoning, where the arterial partial pressure of oxygen is normal, but the content is much reduced. (The content is reduced because the hemoglobin is tightly bound by the carbon monoxide, which effectively excludes oxygen.)

There is also nearly universal agreement that an abnormally low percent saturation of arterial hemoglobin with oxygen constitutes hypoxemia. This concept has given rise to a ready measurement of percent saturation by pulse oximetry. However this measurement can be very misleading when blood flow is slowed or interrupted, leading to a local tissue hypoxia even though arterial blood in patent blood vessels is normal.

There is less agreement concerning whether the oxygen content of blood is relevant to hypoxemia, particularly because the measurement of oxygen content requires tonometry, a method that is not always available. Pulmonary medical specialists would say yes, as would the more technical dictionaries, but in so doing they include severe anemia as a cause of hypoxemia due to the disease's greatly reduced quantity of hemoglobin, the oxygen binding protein within the red blood cell. Trauma Critical Care specialists tend to say no, conforming to the simpler definition of hypoxemia being a low partial pressure of oxygen only, reserving the concept of oxygen content to discussions of oxygen delivery to the tissues.

Finally, the term was initially proposed to describe the low blood oxygen seen at high altitude and, had a general, non-technical definition - a defective oxygenation of the blood Current dictionaries and web sites track the original definition, generally defining the term as insufficient oxygenation of the (arterial) blood. Other sites speak of "level" of oxygen but this non-technical usage sidesteps the contentious details, and does not offer a definitive solution to the problem of whether to include anemia in the scope of hypoxemia. With this caveat, the following article will include low oxygen content as a cause of hypoxemia because it is a functionally and clinically important reason why tissues become hypoxic and must be considered when there are symptoms of tissue hypoxia.