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INTRODUCTION

Pulmonary hypertension exists as a pathologic finding in newborn infants in several clinical settings. In the early newborn period, the syndrome of persistent pulmonary hypertension of the newborn (PPHN) can occur, consisting of failure of the fetal circulation to adapt to postnatal newborn physiology, mainly because of a persistent elevation in pulmonary vascular resistance. PPHN can occur as a primary pathology or secondary to pulmonary diseases such as meconium aspiration syndrome, pneumonia, pulmonary hypoplasia, or pulmonary/intrathoracic anomalies. Pulmonary hypertension can also occur after the early newborn period as a consequence of chronic pulmonary diseases such as bronchopulmonary dysplasia, or in association with congenital heart disease. At a physiologic level, pulmonary hypertension can be caused by increased pulmonary vascular resistance, by increased pulmonary blood flow, or by obstruction to pulmonary venous drainage. While the measurements of pulmonary artery pressure are similar no matter what the cause, some of the associated echocardiographic findings and the clinical scenarios will differ. The primary problem might not be easily discernible by echocardiography alone. However, assessment of the hemodynamics can often assist the clinician in choosing appropriate therapies.

Understanding the physiology of normal transition and of pulmonary hypertension is essential for interpreting the echocardiographic findings. Fetal circulation is characterized by high pulmonary vascular resistance and low pulmonary blood flow. Pulmonary pressures in the fetus are higher than those of the post-transitional newborn circulation. The normal transitional circulation involves a decreasing pulmonary vascular resistance and pulmonary artery pressure over the first 3 days of life. In a study of normal-term infants catheterized in the first 3 days of life, peak systolic pulmonary pressures were approximately equal to systemic pressures in the first 10 hours of life. Pulmonary pressures were lower in infants with increasing age over the first 3 days of life.1 PPHN was previously referred to as persistent fetal circulation (PFC) because the circulatory pattern does not transition from the fetal to newborn pattern along the normal timeline, and the baby becomes hypoxemic.

It is important to understand the expected findings in normal transition in order to interpret the echocardiogram in the clinical setting of PPHN. The physiology of PPHN includes elevated pulmonary pressures and decreased pulmonary blood flow due to high pulmonary vascular resistance. A higher force from the right ventricle is required to overcome the resistance of the pulmonary vessels. With decreased pulmonary blood flow, there will be decreased pulmonary venous return to the left atrium and therefore decreased left ventricular preload. Because of the increased pressure in the right ventricle, the interventricular septum can be pushed toward the left, potentially impairing function of the left ventricle. The systemic blood flow can be decreased as a result of the combination of these factors. The echocardiogram can be used to estimate the pulmonary artery pressure, provide information about the relative pulmonary to systemic pressure gradients, and evaluate the overall hemodynamic consequences. The echocardiogram is also critical in differentiating the diagnosis ...

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