Definition. Persistent pulmonary hypertension of the newborn (PPHN) is a condition characterized by marked pulmonary hypertension resulting from elevated pulmonary vascular resistance (PVR) and altered pulmonary vasoreactivity, leading to right-to-left extrapulmonary shunting of blood across the foramen ovale and the ductus arteriosus, if it is patent. It is associated with a wide array of cardiopulmonary disorders that may also cause intrapulmonary shunting. When this disorder is of unknown cause and is the primary cause of cardiopulmonary distress, it is often called "idiopathic PPHN" or persistent fetal circulation.
Incidence is 2–6 per 1000 live births.
Pathophysiology. PPHN may be the result of underdevelopment of the lung together with its vascular bed (eg, congenital diaphragmatic hernia and hypoplastic lungs), maladaptation of the pulmonary vascular bed to the transition occurring around the time of birth (eg, various conditions of perinatal stress, hemorrhage, aspiration, hypoxia, and hypoglycemia); and maldevelopment of the pulmonary vascular bed in utero from a known or unknown cause. It is convenient to think in terms of this basic pathologic classification. However, the clinical manifestations of PPHN are often not attributable to a single physiologic or structural entity, and many disorders exhibit more than one underlying pathology. Often, even when there is evidence of perinatal or postnatal stress (eg, meconium aspiration), the underlying cause of PPHN had been secondary to an in utero process of some duration.
Preacinar arteries are already present in the lungs by 16 weeks' gestation; thereafter, respiratory units are added with further growth of the appropriate arteries. Muscularization of the peripheral pulmonary arteries is related to differentiation of pericytes and to recruitment of fibroblasts and is influenced by numerous trophic factors (eg, neuropeptides, fibroblast growth factors, and insulin-like growth factors). In addition, the growth, differentiation, and adaptation of the pulmonary vascular bed are also influenced by changes that occur in the connective tissue matrix (eg, elastin and collagen). The lungs of infants with PPHN contain many undilated precapillary arteries, and pulmonary arterial medial thickness is increased. There may be extension of muscle in small and peripheral arteries that are normally nonmuscular. After a few days, there is already evidence of structural remodeling with connective tissue deposition.
In the fetus, PVR is high, and only 5–10% of the combined cardiac output flows into the lungs, with most of the right ventricular output crossing the ductus arteriosus to the aorta. After birth, with expansion of the lungs, there is a sharp drop in PVR and pulmonary blood flow increases about 10-fold. The factors responsible for maintaining high PVR in the fetus and for effecting the acute reduction in PVR that occurs after birth are incompletely understood. Fetal and neonatal pulmonary vascular tone is modulated through a balance between vasoconstrictive and vasodilatory stimuli. Vasoconstrictive stimuli possibly include various products of arachidonic acid metabolism (eg, thromboxane, leukotrienes, and isoprostanes), Rho/Rho kinase, and the endothelins (ETs). The hemodynamic effect of the ETs are mediated by at least two receptors; ET-A and ET-B. There are at least three isoforms of ...
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