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Hypoxemia

Hypoxemia

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Hypoxemia is generally defined as decreased partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

 of oxygen in blood, sometimes specifically as less than 60 mmHg (7,999.3 Pa) or causing hemoglobin oxygen saturation
Oxygen saturation
Oxygen saturation or dissolved oxygen is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water.It has particular significance in medicine and...

 of less than 90%.

Distinction from anemia and hypoxia


The hypoxemia definition as decreased partial pressure of oxygen excludes decreased oxygen content caused by anemia
Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...

 (decreased content of oxygen binding protein hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

) or other primary hemoglobin deficiency, because they don't decrease the partial pressure of oxygen in blood.

Still, some simply define it as insufficient oxygenation or total oxygen content of (arterial) blood, which, without further specification, would include both concentration of dissolved oxygen and oxygen bound to hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

. 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
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...

, 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.

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


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

Increased systemic oxygen consumption could also result in hypoxemia.

In contrast, primary hemoglobin deficiency, e.g. anemia
Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...

 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
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

 each minute. The reduced oxygen partial pressure can be a result of reduced fractional oxygen content (low FiO2
FiO2
FiO2, in the field of medicine, is the fraction of inspired oxygen in a gas mixture.The FiO2 is expressed as a number from 0 to 1 .The FiO2 of normal room air is 0.21 ....

) 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 5 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
Pulmonary alveolus
An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well...

 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
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 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 recognized 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 ratio
Ventilation/perfusion ratio
In respiratory physiology, the ventilation/perfusion ratio is a measurement used to assess the efficiency and adequacy of the matching of two variables: It is defined as: the ratio of the amount of air reaching the alveoli to the amount of blood reaching the alveoli.* "V" – ventilation – the air...

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