Figure 1.

The primary determinant of hypoxic pulmonary vasoconstriction is the precapillary pulmonary arteriole. The oxygen tension at this site is determined by alveolar PAO₂ and pulmonary arterial PO₂. In the absence of lung disease, pulmonary venous PO₂ and alveolar PAO₂ are usually similar. In the absence of significant shunts, preductal PaO₂ is reflective of pulmonary venous PO₂. Umbilical venous PO₂ is similar to mixed venous PO₂ and pulmonary arterial PO₂ in the absence of a left-to-right atrial/ductal shunt. Oxygen delivery to the brain and heart is based on preductal PaO₂ and SpO₂. Postductal PaO₂ (from an umbilical arterial line) can be low and does not predict PVR or oxygen delivery to vital organs. Copyright Satyan Lakshminrusimha.