الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Circulation in the fetus is characterized by high pulmonary vascular resistance (PVR) and low systemic vascular resistance (SVR). The placenta is the site of gas exchange. Pulmonary blood flow to fluid-filled lungs is low (approximately 8-10% of combined ventricular output in an ovine fetus).
Numerous factors contribute to the high pulmonary vascular tone in-utero, such as mechanical factors (compression of the small pulmonary arterioles by the fluid-filled alveoli and a lack of rhythmic distension), the presence of low-resting alveolar and arteriolar oxygen tensions, and a relative lack of vasodilators.
A series of circulatory events take place at birth to ensure a smooth transition from fetal to extra-uterine life. Clamping of the umbilical cord removes low resistance placental circulation, increasing systemic arterial pressure. Simultaneously, various mechanisms operate to rapidly reduce pulmonary arterial pressure and increase pulmonary blood flow. Of these, the most important stimuli appear to be ventilation of the lungs and an increase in oxygen tension.
Failure of the pulmonary circulation to undergo the normal transition after birth leads to PPHN, which is characterized by an elevated PVR/SVR ratio resulting from either vasoconstriction, structural remodeling of the pulmonary vasculature, intravascular obstruction or lung hypoplasia. There is right-to-left shunting of blood across the foramen ovale and ductus arteriosus, resulting in hypoxemia and labile oxygen saturations.
PPHN may be idiopathic or secondary to certain neonatal pulmonary diseases such as congenital diaphragmatic hernia (CDH), respiratory distress syndrome (RDS), pneumonia, meconium aspiration syndrome (MAS) and transient tachypnea of the newborn (TTN) and other conditions with genetic predisposition. PPHN was significantly associated with genetic variants in corticotropin- releasing hormone (cRh) receptor 1 (CRHR1) and cRh-binding protein (CRHBP).
Risk factors for PPHN include meconium stained amniotic fluid, perinatal acidosis and asphyxia, maternal risk factors such as prolonged rupture of membranes, maternal fever, or positive group B Streptococcal carrier status.
Resuscitation in the delivery room should focus on optimal lung recruitment and ventilation.
This diagnosis can be confirmed by measuring preductal (right extremity) and postductal (either lower extremity) arterial oxygenation. A difference in arterial PO2 ≥20 mmHg or oxygen saturation ≥5-10% should be considered suggestive of PPHN.
Echocardiography remains the gold standard diagnostic tool in PPHN.
Supportive measures are vital in successful management efforts.
Oxygen and optimal oxygen saturations providing adequate oxygenation forms the mainstay of PPHN therapy.
Ventilation and optimal lung expansion is essential for adequate oxygenation as well as the effective delivery of iNO. Conventional and high frequency ventilation (HFV) may be used to reduce the V/Q mismatch.
Exogenous surfactant therapy improved oxygenation and reduced the need for ECMO when PPHN was secondary to parenchymal lung disease.