Persistent pulmonary hypertension of the newborn (PPHN) occurs when pulmonary vascular resistance (PVR) remains abnormally elevated after birth, resulting in right-to-left shunting of blood through fetal circulatory pathways. This in turn leads to severe hypoxemia that may not respond to conventional respiratory support. The prevalence of PPHN has been estimated at 1.9 per 1000 live births .
The pathophysiology, clinical manifestations, diagnosis, and management of PPHN are discussed here.
Fetal and adult circulation — In the fetus, the pulmonary and systemic circuits operate in parallel. Both the right and left ventricles eject blood into the aorta with subsequent perfusion of the placenta, the fetal organ of respiration (figure 1). The right ventricle is dominant, and blood is shunted right-to-left through the foramen ovale and ductus arteriosus, mostly bypassing the lung, which is not participating in gas exchange.
In contrast, the postnatal (adult) circulation operates in series. All venous return passes through the right side of the heart and into the lung, where gas exchange occurs. The oxygenated blood returns to the left side of the heart and is pumped into the systemic circulation for oxygen delivery to the tissues. No mixing occurs between the two sides of the adult circulation.
Transitional circulation — Major circulatory adjustments occur at birth as the organ of gas exchange changes from the placenta to the lung. Under normal circumstances, a progressive fall in pulmonary vascular resistance (PVR) accompanies the immediate rise in systemic vascular resistance (SVR) that occurs after birth. For a short period, a transitional circulatory pattern exists that combines features of both the fetal and adult circulatory patterns. The decline in the PVR/SVR ratio results in a steady increase in pulmonary blood flow and oxygen uptake in the lung.