Hypoxemia

Hypoxemia is an abnormally low level of oxygen in arterial blood. It is most directly assessed by the arterial partial pressure of oxygen, or PaO₂, measured in an arterial blood gas sample. Oxygen saturation is closely related but is a different measurement: it describes the proportion of hemoglobin oxygen-binding sites occupied by oxygen. Arterial oxygen saturation can be estimated noninvasively as SpO₂ using pulse oximetry .^1,3

Hypoxemia can arise through several physiological mechanisms. These include breathing gas with a low oxygen partial pressure, inadequate ventilation, mismatch between ventilation and blood flow within the lungs, impaired diffusion of oxygen from the alveoli into the blood, and right-to-left shunting in which some blood reaches the arterial circulation without being adequately oxygenated. Ventilation-perfusion mismatch is a particularly common mechanism .^1,2

The effect of hypoxemia on the amount of oxygen carried in blood depends partly on the relationship between PaO₂ and hemoglobin oxygen saturation. Most oxygen in arterial blood is carried bound to hemoglobin rather than dissolved directly in plasma. Oxygen delivery to tissues also depends on factors beyond arterial oxygenation, particularly hemoglobin concentration and cardiac output .^1,3

Hypoxemia and hypoxia are therefore not synonymous. Hypoxemia specifically describes reduced oxygen in arterial blood, whereas hypoxia refers to inadequate oxygen availability at the tissue level. Hypoxemia can contribute to tissue hypoxia, but tissue hypoxia can also occur despite normal arterial oxygenation if oxygen delivery or utilization is impaired .^1,2