What occurs in PPHN when pulmonary artery (PA) pressure exceeds systemic pressure?

In persistent pulmonary hypertension of the newborn (PPHN), when the pulmonary artery (PA) pressure exceeds systemic pressure, right-to-left shunting occurs through any existing shunts, such as the foramen ovale or ductus arteriosus. This type of shunting happens because the higher pressure in the pulmonary artery creates a gradient that favors blood flow moving from the right side of the heart to the left side, bypassing the lungs.

This is problematic because it means that deoxygenated blood is entering systemic circulation, leading to decreased oxygen levels in arterial blood and further hypoxemia. This mechanism is an important consideration in managing infants with PPHN since it directly relates to the clinical manifestations of the condition, including cyanosis and respiratory distress.

In contrast, increased blood flow to the lungs is typically not seen in PPHN since the elevated PA pressures prevent adequate lung perfusion. Similarly, PPHN does not lead to decreased carbon dioxide levels, as the lungs are not effectively exchanging gases, and hypoxemia is worsened rather than reduced due to the right-to-left shunting that takes place. Understanding these dynamics is crucial in the care of neonates with PPHN to mitigate the risks associated with inadequate oxygenation.