What ultimately causes hypoxemia in PPHN?

In the context of Persistent Pulmonary Hypertension of the Newborn (PPHN), hypoxemia primarily arises from the shunting of blood away from the pulmonary circulation. In PPHN, the high resistance in the pulmonary vasculature leads to inadequate blood flow through the lungs, which interferes with the normal oxygenation processes. This results in the right-to-left shunting of blood, where deoxygenated blood bypasses the lungs and enters systemic circulation, causing low oxygen levels in the blood (hypoxemia).

The fundamental issue in PPHN is the failure of pulmonary blood vessels to relax, which is typically mediated by factors such as hypoxia, acidosis, or increased vascular resistance. As a result, the blood is diverted away from the lungs, preventing proper gas exchange and leading to significant hypoxemia in the newborn. This shunting effect is the critical mechanism underlying hypoxemia in PPHN, highlighting the importance of addressing pulmonary circulation to improve oxygenation in affected infants.