Hypoxemia

Hypoxemia is an abnormally low level of oxygen in arterial blood, usually assessed by measuring arterial oxygen partial pressure or oxygen saturation.

Ventilation–perfusion mismatch: A mismatch between airflow reaching alveoli and blood flow reaching their surrounding capillaries. Uneven matching of airflow and blood flow is a common cause of low arterial oxygen.
Arterial blood gas: A blood test measuring pH, carbon dioxide, and oxygen tensions, often with bicarbonate and calculated oxygen saturation. It directly measures arterial oxygen tension, a standard way to confirm hypoxemia.
Acute respiratory distress syndrome: A severe inflammatory lung injury causing acute hypoxemic respiratory failure and bilateral pulmonary opacities. This syndrome commonly produces severe hypoxemia through widespread lung injury and shunting.
Hypercapnia: An abnormally high level of carbon dioxide in arterial blood. Carbon dioxide retention can accompany hypoxemia, but the two abnormalities are not equivalent.
Right-to-left shunt: A pathway that lets blood enter systemic circulation without passing through ventilated lung regions. Shunted blood bypasses oxygenation and mixes with oxygenated blood.
Pulse oximetry: A noninvasive method that estimates arterial oxygen saturation from light absorption through tissue. It provides a rapid estimate of oxygen saturation, though it does not measure oxygen tension.
Pulmonary embolism: Obstruction of a pulmonary artery, usually by a blood clot that traveled from a vein. Blocked perfusion creates ventilation–perfusion mismatch and may reduce arterial oxygen.
Anemia: A reduced concentration of hemoglobin or red blood cells that lowers blood oxygen-carrying capacity. Anemia can impair oxygen delivery despite normal arterial oxygen tension and saturation.
Diffusion limitation: Failure of a gas to cross a biological membrane quickly enough to equilibrate between compartments. Impaired oxygen transfer across the alveolar membrane can lower arterial oxygen, especially during exertion.
Partial pressure of oxygen: The pressure contributed by oxygen in a gas mixture or dissolved in a liquid. Arterial oxygen partial pressure is a central laboratory measure of hypoxemia.

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