Persistent pulmonary hypertension of the newborn (PPHN) is the inability to relax the pulmonary circulation arrangement at birth, which leads to non-oxygenated blood bypassing around the lungs and into the systemic circulation. Therefore, blood that has not gone to the lungs, where the oxygen is picked up from, is pushed into the left side of the heart which then is pumped throughout the body and causes many symptoms. The blood is shunted through the foramen ovale (PFO) and ductus arteriosus (PDA). These passageways are developed in utero to bypass the lungs so that circulation is possible for the infant while the mother provides the oxygen. Failure of one or both of these closing is a primary factor for causing PPHN (Persistent Newborn Pulmonary Hypertension, 2018).
The etiology of PPHN is varied and is accompanied by other neonatal diseases as well. According to Ostrea (2006), “any condition that leads to chronic hypoxia in utero or abnormal muscularization of the pulmonary vessels can lead to persistent pulmonary vasoconstriction and PPHN after birth” (Ostrea, 2006, p.179). Some of the conditions included in this category would be asphyxia, neonatal respiratory distress syndrome, and aspiration syndromes like meconium aspiration syndrome (MAS) (Raju, U., Sondhi, V., & Patnaik, S., 2010). Other conditions that are related to PPHN are ones that can result in declines in the pulmonary vascular bed such as pulmonary hypoplasia and congenital diaphragmatic hernia (CDH). Situations that can cause increases in pulmonary venous pressure such as a mitral obstruction, aortic stenosis, hypoplastic left heart syndrome, and endocardial fibroelastosis also are correlated with PPHN. With this information, it is now known that the diagnosis of PPHN can be problematic due to it being closely related with all the conditions listed. To properly diagnose PPHN, the condition must be fully understood through its pathophysiology.
Fetal circulation in utero has very distinguishing characteristics. Ostrea explains that some of those are (2006) “a high pulmonary vascular resistance, low systemic resistance in the placenta, and the presence of transient circulatory channels such as the ductus venosus, ductus arteriosus, and foramen ovale” (Ostrea, 2006, p. 180). Due to these structures, blood can bypass the lungs and move through the heart efficiently to provide systemic circulation. As an infant cannot breathe while in the womb, this is an important characteristic for the life of the infant. When the infant is birthed, an astonishing and scientific miracle takes place. These three pathways will constrict till closure over the first twenty-four hours after birth. The closure of these ducts forces the blood to begin moving through the lungs to become oxygenated and provide oxygen to the rest of the body. When an infant has PPHN, two of the pathways, such as the PFO and PDA, do not close during those first twenty-four hours. This creates a right to left shunt in the heart where deoxygenated blood moves from the right side of the heart to the left, bypassing the pulmonary system. That blood is then circulated throughout the body which limits perfusion into the tissues (Waknin, R.).
PPHN is most often seen in late preterm or term newborns. It consists of three different modalities: PPHN related to underdevelopment, mal-development, and mal-adaptation. All three types have numerous causes and risk factors, but according to Puthiyachirakkal and Mhanna (2013), “meconium aspiration syndrome is the most common cause of PPHN” (Puthiyachirakkal & Mhanna, 2013, p. 1). It is simply explained that PPHN in MAS results from obstructions in the airways, insufficient surfactant, and chemical pneumonitis (Greenspan, J. S., 2014). Some other risk factors include pneumothorax, asphyxia, and changes in the fetal heartbeat rhythm. With smooth muscle proliferation and adventitial thickening, the PVR rises which causes shunting from right-to-left through the PFO and PDA. The hypoxia and respiratory acidosis further exacerbate the constriction of the pulmonary vessels and generate a vicious cycle of shunting (Mhanna, Jean, M., & Ashraf, M., 2013).
Clinical manifestations for PPHN may differ with each case. Most patients diagnosed with PPHN are either term or late-term neonates, but it is not uncommon for preterm infants to also receive this same finding. Some patients may exhibit tachypnea, tachycardia, and low oxygen saturation levels. Another case might present with bradycardia, heart murmur, and cyanosis. A couple of instances might show meconium-stained amniotic fluid being discharged from the birthing canal. Not all symptoms need to be present for PPHN to be the diagnosis. Any combination or singular symptom could be the determining factor. The one thing the signs all have in common is that they tend to start presenting within the first 24 hours of life. Most have low APGAR scores and almost all the neonates received some sort of intervention in the delivery room such as oxygen therapy, bag and mask ventilation, and endotracheal intubation.
Most neonates will go through basic screenings and testing such as pulse oximetry, ABGs, and of course, chest radiographs. Although these tests can show the signs and symptoms for PPHN, the diagnosis of PPHN is primarily made by echocardiography. Echocardiography will show a normal cardiac structure with signs of pulmonary hypertension. That will be shown through a flattened or a displaced ventricular septum. It will also show ventricular function that is impaired. Echocardiography may also show the severity of PPHN that is present. Research by Stark and Eichenwald (2018) revealed, “the majority of infants also have other respiratory diagnoses associated with PPHN” (Stark & Eichenwald, 2018). MAS is seen in 41% of PPHN diagnoses. Another 14% goes to pneumonia and 13% to respiratory distress syndrome. A smaller percentage of the neonates are also associated with CDH at 10% and pulmonary hypoplasia at 4%. Only 17% of neonates with PPHN have no other respiratory condition association (Stark & Eichenwald, 2018).
The first steps in treating PPHN includes ventilation of the lungs, oxygenation of the lungs and tissues, maintaining proper blood pressure, and equilibrium of the body. Treating the underlying disease is critically important in the treatment of PPHN. The use of inhaled nitric oxide (iNO) was approved in 1999 for neonates diagnosed with PPHN (Inhaled nitric oxide, 2019), but only with near-term and term infants. Mortality in infants has significantly decreased with the use of iNO, although, about 40% of patients will not respond to it. Oxygen can be used as a vasodilator, but ventilating the lungs with 100% O2 has been linked with an uncertain reaction to iNO. Ventilation through a ventilator is used to maintain the PaCO2 between 40 and 60 mmHg to optimize lung volume. According to Lloyd and Smith (2016), “high-frequency oscillatory ventilation (HFOV) can minimize lung injury, but has not been proven to have a clear benefit over conventional ventilation, except when used in combination with iNO in the treatment of MAS” (Lloyd & Smith, 2016, p. 195).
Surfactant therapy has had mixed results in treating PPHN. As surfactant has not shown impressive results with improving lung compliance, it has presented better results when it comes to pulmonary morbidity, lower length of hospital stays with neonates and air leakage. Numerous medications are also being used in the treatment of PPHN. Some of those medications may include calcium channel blockers, phosphodiesterase type (PDE) 5 inhibitors, endothelin receptor antagonists, phosphodiesterase (PDE) type 3 inhibitors, and magnesium sulfate. In Puthiyachirakkal and Mhanna’s research (2013), “… studies have shown that intravenous magnesium sulfate can cause a reduction in pulmonary artery pressures” (Puthiyachirakkal & Mhanna, 2013, p. 1), however, observational studies are the only thing available for neonates.
PPHN is an emergency that needs premature involvement and managing to avoid serious hypoxemia and more than a few short to long-term illnesses. The most important treatment is the handling of the underlying disorder. These therapies include numerous useful methods like oxygen therapy, mechanical ventilation, iNO, and PDE inhibitors. However, the ideal method of managing PPHN remains debatable. Future high classed random studies of current and innovative beneficial procedures are desired to advance solid, document-based approaches for the handling of PPHN. Infants with PPHN need continuing follow-ups, subsequently because they are at risk for developmental disabilities and long-lasting health situations (Teng, R., & Wu, T., 2013).
References
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