PaO2

PaO2, or the arterial partial pressure of oxygen, is the measure of the amount of dissolved oxygen present in arterial blood. It is a key parameter obtained from arterial blood gas analysis and is expressed in millimeters of mercury (mmHg). PaO2 reflects the efficiency of transferring oxygen from the alveoli into the bloodstream and is distinct from oxygen content, which also depends on hemoglobin.

Normal PaO2 values at sea level typically range from about 75 to 100 mmHg in healthy adults, with values tending to be slightly lower with advancing age or at higher altitudes. PaO2 is influenced by the inspired oxygen fraction (FiO2), pulmonary function, and the integrity of gas exchange across the alveolar-capillary membrane. It is measured from an arterial blood sample, usually obtained from the radial artery, as part of an arterial blood gas test. PaO2 is often interpreted together with the oxygen saturation (SaO2 or SpO2) and with the anteroposterior gradient between alveolar oxygen (PAO2) and PaO2, known as the alveolar-arterial gradient.

Hypoxemia is commonly defined as a PaO2 below 80 mmHg on room air, with severities sometimes categorized as mild (60–79 mmHg), moderate (40–59 mmHg), and severe (<40 mmHg). Gas exchange abnormalities such as shunt, diffusion limitation, and ventilation-perfusion mismatch can reduce PaO2. The relationship between PaO2 and SpO2 follows the oxygen dissociation curve and is influenced by factors like temperature and pH. PaO2 is essential for guiding oxygen therapy and assessing pulmonary function, though it does not measure total oxygen content, which also depends on hemoglobin concentration.