Clinically, PPHN is most often recognized in term or near term neonates, but clearly can occur in premature neonates as well (Table 1). PPHN is often associated with perinatal distress (e.g., asphyxia, low APGAR scores, meconium staining); however, idiopathic PPHN can lack signs of acute perinatal distress. PPHN often presents as respiratory distress and cyanosis within 6-12 hrs of birth. Laboratory findings include low glucose, hypocalcemia, hypothermia, polycythemia or thrombocytopenia. Radiographic findings are variable, depending upon the primary disease asociated with PPHN. Classically, the chest x-ray in idiopathic PPHN is oligemic, may appear slightly hyperinflated, and lacks parenchymal infiltrates. In general, the degree of hypoxemia is often disproportionate to the severity of radiographic evidence of lung disease.
Table I. Disorders Associated with Neonatal Pulmonary Hypertension
Not all term newborns with hypoxemic respiratory failure have PPHN-type physiology (5). Hypoxemia in the newborn can be due to several mechanisms, including: extrapulmonary shunt, in which high pulmonary artery pressure at systemic levels leads to right-to-left shunting of blood flow across the PDA or PFO; and intrapulmonary shunt or ventilation-perfusion mismatch, in which hypoxemia results from the lack of mixing of blood with aerated lung regions due to parenchymal lung disease, without the shunting of blood flow across the PDA and PFO. In the latter setting, hypoxemia is related to the amount of pulmonary arterial blood that perfuses non-aerated lung regions. Although PVR is often elevated in hypoxemic newborns without PPHN, high PVR does not contribute significantly to hypoxemia in these cases.
Several factors can contribute to high pulmonary artery pressure in neonates with PPHN-type physiology. Pulmonary hypertension can be due to vasoconstriction or structural lesions that directly increase PVR. Changes in lung volume in neonates with parenchymal lung disease can also be an important determinant of PVR. PVR increases at low lung volumes due to dense parenchymal infiltrate and poor lung recruitment, or with high lung volumes due to hyperinflation associated with overdistension or gas-trapping. Cardiac disease is also associated with PPHN. High pulmonary venous pressure due to left ventricular dysfunction can also elevate PAP (e.g., asphyxia, sepsis), causing right-to-left shunting, with little vasoconstriction. In this setting, enhancing cardiac performance and systemic hemodynamics may lower PAP more effectively than achieving pulmonary vasodilation. Thus, understanding the cardiopulmonary interactions are key to improving outcome in PPHN.
PPHN is characterized by hypoxemia that is poorly responsive to supplemental 02. In the presence of right-to-left shunting across the PDA, "differential cyanosis" is often present, which is difficult to detect by physical exam, and is defined by a difference in Pa02 between right radial artery versus descending aorta values £10 torr, or an 02 saturation gradient >5%. However, post-ductal desaturation can be found in ductus-dependent cardiac diseases, including hypoplastic left heart syndrome, coarctation of the aorta or interrupted aortic arch. The response to supplemental 02 can help to distinguish PPHN from primary lung or cardiac disease. Although supplemental oxygen traditionally increases Pa02 more readily in lung disease than cyanotic heart disease or PPHN, this may not be obvious with more advanced parenchymal lung disease. Marked improvement in Sa02 (increase to 100%) with supplemental oxygen suggests the presence of V/Q mismatch due to lung disease or highly reactive PPHN. Most patients with PPHN have at least a transient improvement in oxygenation in response to interventions such as high inspired oxygen and/or mechanical ventilation. Acute respiratory alkalosis induced by hyperventilation to achieve PaCo2 <30 torr and a pH >7.50 may increase Pa02 >50 torr in PPHN, but rarely in cyanotic heart disease.
The echocardiogram plays an essential diagnostic role and is an essential tool for managing newborns with PPHN. The initial echocardiography evaluation is important to rule-out structural heart disease causing hypoxemia (e.g., coarctation ofthe aorta and total anomalous pulmonary venous return). As stated above, not all term newborns with hypoxemia have PPHN physiology. Although high pulmonary artery pressure may be common, the diagnosis of PPHN is uncertain without evidence of bidirectional or predominantly right-to-left shunt ing across the PFO or PDA. Echocardiography signs suggestive of pulmonary hypertension (e.g., increased right ventricular systolic time intervals and septal flattening) are less helpful. In addition to demonstrating the presence of PPHN physiology, the echocardiogram is critical for the evaluation of left ventricular function and diagnosis of anatomic heart disease, including such "PPHN mimics" as coarctation of the aorta; total anomalous pulmonary venous return; hypoplastic left heart syndrome; and others. Studies should carefully assess the predominant direction of shunting at the PFO as well as the PDA. Although right-to-left shunting at the PDA and PFO is typical for PPHN, predominant right-to-left shunting at the PDA but left-to-right shunt at the PFO may help to identify the important role of left ventricular dysfunction to the underlying pathophysiology. In the presence of severe left ventricular dysfunction with pulmonary hypertension, pulmonary vasodilation alone may be ineffective in improving oxygenation. In this setting, efforts to reduce PVR should be accompanied by targeted therapies to increase cardiac performance and decrease left ventricular afterload. Thus, careful echocardiographic assessment provides invaluable information about the underlying pathophysiology and will help guide the course of treatment.
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