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S E M I N A R S I N P E R I N A T O L O G Y 3 8 ( 2 0 1 4 ) 7 8 – 9 1

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1 Member of the s

Update on PPHN: Mechanisms and treatment

Jayasree Nair, MBBS, MDa,b, and Satyan Lakshminrusimha, MDa,b,n,1

aCenter for Developmental Biology of the Lung, State University of New York, Buffalo, NYbDivision of Neonatology, Department of Pediatrics, Women and Children's Hospital of Buffalo, 219 Bryant St, Buffalo,NY 14222

a r t i c l e i n f o

Keywords:

Pulmonary vascular resistance

Nitric oxide

Persistent fetal circulation

Hypoxic respiratory failure

Systemic vasodilators

front matter Published b1053/j.semperi.2013.11.00

hor.slakshmi@buffalo.edu (Speaker's bureau for Ikari

a b s t r a c t

Persistent pulmonary hypertension of the newborn (PPHN) is a syndrome of failed

circulatory adaptation at birth, seen in about 2/1000 live born infants. While it is mostly

seen in term and near-term infants, it can be recognized in some premature infants with

respiratory distress or bronchopulmonary dysplasia. Most commonly, PPHN is secondary to

delayed or impaired relaxation of the pulmonary vasculature associated with diverse

neonatal pulmonary pathologies, such as meconium aspiration syndrome, congenital

diaphragmatic hernia, and respiratory distress syndrome. Gentle ventilation strategies,

lung recruitment, inhaled nitric oxide, and surfactant therapy have improved outcome and

reduced the need for extracorporeal membrane oxygenation (ECMO) in PPHN. Newer

modalities of treatment discussed in this article include systemic and inhaled vasodilators

like sildenafil, prostaglandin E1, prostacyclin, and endothelin antagonists. With prompt

recognition/treatment and early referral to ECMO centers, the mortality rate for PPHN has

significantly decreased. However, the risk of potential neurodevelopmental impairment

warrants close follow-up after discharge for infants with PPHN.

Published by Elsevier Inc.

Introduction

Persistent pulmonary hypertension of the newborn (PPHN) isa syndrome characterized by sustained elevation of pulmo-nary vascular resistance (PVR) and is often associated withnormal or low systemic vascular resistance (SVR). This leadsto extrapulmonary shunting from right to left across persis-tent fetal channels - patent ductus arteriosus (PDA)and patent foramen ovale (PFO) leading to labile hypoxemia.This disorder was previously referred to as persistentfetal circulation (PFC) and is often secondary to an unsuc-cessful pulmonary transition at birth. This article gives a briefoverview of fetal circulation and transition at birth andfocuses on mechanisms of PPHN in various neonatal respi-ratory disorders and postnatal management of an infantwith PPHN.

y Elsevier Inc.4

. Lakshminrusimha).a.

Fetal circulation

Circulation in the fetus is characterized by high PVR and lowSVR. The placenta is the site of gas exchange. Pulmonaryblood flow to fluid-filled lungs is low (approximately 8–10% ofcombined ventricular output in an ovine fetus).1,2 However,more recent human fetal Doppler flow studies demonstrate amuch higher pulmonary blood flow (13% of combined ven-tricular output at 20 weeks gestation, increasing to 25% at30 weeks and 21% at 38 weeks).3

Numerous factors contribute to the high pulmonary vas-cular tone in utero, such as mechanical factors (compressionof the small pulmonary arterioles by the fluid-filled alveoliand a lack of rhythmic distension), the presence of low-resting alveolar and arteriolar oxygen tensions, and a relativelack of vasodilators.4 Low oxygen tension and elevated levels

Fig. 1 – Endothelium-derived vasodilators—prostacyclin(PGI2) and nitric oxide (NO)—and vasoconstrictor(endothelin, ET-1). The enzymes, cyclooxygenase (COX) andprostacyclin synthase (PGIS), are involved in the productionof prostacyclin. Prostacyclin acts on its receptor in thesmooth muscle cell and stimulates adenylate cyclase (AC) toproduce cyclic adenosine monophosphate (cAMP). CyclicAMP is broken down by phosphodiesterase 3A (PDE 3A) inthe smooth muscle cell. Milrinone inhibits PDE 3A andincreases cAMP levels in pulmonary arterial smooth musclecells and cardiac myocytes resulting in pulmonary (andsystemic) vasodilation and inotropy. Endothelin is a

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of vasoconstrictor mediators, such as endothelin-1 (ET-1) andthromboxane, play a crucial role in maintaining elevated fetalPVR.4 Serotonin increases fetal PVR5,6 and the use of seroto-nin re-uptake inhibitors (SSRI) during pregnancy has beenassociated with increased incidence of PPHN.7

Endothelin-1 synthesized by vascular endothelial cells is apotent vasoconstrictor8 and acts through two receptors, ETA

and ETB (Fig. 1). The ETA receptor plays a critical role invasoconstriction while the ETB receptor plays a significantrole in vasodilation. Selective blockade of the ETA receptorcauses fetal pulmonary vasodilation.9 Vasoconstrictioninduced by ET-1 is mediated by calcium.10 Pulmonary vaso-dilation to ETB receptor stimulation is mediated byendothelium-derived nitric oxide (NO).11,12

Vasoconstriction in response to low oxygen tension con-tributes to high PVR in the fetal lamb as it approachesterm.13,14 Basal production of vasodilator agents, such asprostacyclin (PGI2) and NO, is low in the fetus. Response toNO is dependent on activity of its target enzyme, solubleguanylate cyclase (sGC). In the ovine fetus (term gestation is145–147d), sGC mRNA levels are low during early pretermgestation (126d) and significantly increase during late pre-term and early term gestation (137d).15 There is abundant sGCactivity in the lung at late gestation and the early newbornperiod and gradually decreases in adult rats.16 Low levels ofpulmonary arterial sGC activity during late canalicular andearly saccular stages of lung development are likely respon-sible for the poor response to inhaled nitric oxide (iNO)observed in preterm infants o29 weeks GA.17

powerful vasoconstrictor and acts on ET-A receptors in thesmooth muscle cell and increases ionic calciumconcentration. A second endothelin receptor (ET-B) on theendothelial cell stimulates nitric oxide release andvasodilation. Endothelial nitric oxide synthase (eNOS)produces NO, which diffuses from the endothelium to thesmooth muscle cell and stimulates soluble guanylatecyclase (sGC) enzyme to produce cyclic guanosinemonophosphate (cGMP). Cyclic GMP is broken down by PDE5 enzyme in the smooth muscle cell. Sildenafil inhibits PDE5and increases cGMP levels in pulmonary arterial smoothmuscle cells. Cyclic AMP and cGMP reduce cytosolic ioniccalcium concentrations and induce smooth muscle cellrelaxation and pulmonary vasodilation. Nitric oxide is a freeradical and can avidly combine with superoxide anions toform a toxic vasoconstrictor, peroxynitrite. Hence, thebioavailability of NO in a tissue is determined by the localconcentration of superoxide anions. Hyperoxic ventilationwith 100% oxygen can increase the risk of formation ofsuperoxide anions in the pulmonary arterial smooth musclecells and limit the bioavailability of NO. (Copyright SatyanLakshminrusimha.)

Transition at birth

A series of circulatory events take place at birth to ensure asmooth transition from fetal to extra-uterine life. Clamping ofthe umbilical cord removes low resistance placental circula-tion, increasing systemic arterial pressure (Fig. 2). Simulta-neously, various mechanisms operate to rapidly reducepulmonary arterial pressure and increase pulmonary bloodflow. Of these, the most important stimuli appear to beventilation of the lungs and an increase in oxygen tension.With initiation of respiration, the fluid-filled fetal lungs aredistended with air.4 There is improved oxygenation of thepulmonary vascular bed, further decreasing PVR.18 There isan eight-fold increase in pulmonary blood flow, which raisesleft atrial pressure, closing the foramen ovale. As PVR dropslower than SVR, there is a reversal of flow across the ductusarteriosus. The increase in arterial oxygen saturation leads toclosure of the ductus arteriosus and ductus venosus withinthe first few hours after birth. In the final phase of neonatalpulmonary vascular transition, further decline in PVR isaccompanied by rapid structural remodeling of the entirepulmonary bed, from the main pulmonary arteries to thecapillaries.19

Vascular endothelium releases several vasoactive productsthat play a primary role in pulmonary transition at birth.Pulmonary endothelial NO production increases markedly atthe time of birth. Oxygen is believed to be an importantcatalyst for this increased NO production, although theprecise mechanism is not clear. It increases oxidative

phosphorylation and the release of red blood cell ATP, whichis a pulmonary vasodilator during fetal life and a potentialstimulus for endothelial NO production.20,21 In intrapulmo-nary arteries isolated from near-term fetal sheep, both basaland stimulated NO release increase with escalating oxygentension.22 The shear stress resulting from increased pulmo-nary blood flow and increased oxygenation also induceendothelial nitric oxide synthase (eNOS) expression, thus

Fig. 2 – Changes in pulmonary blood flow (secondary Y-axis on the right), mean systemic arterial pressure, and meanpulmonary arterial pressure (primary Y-axis on the left) in normal-term lambs ventilated with 21% oxygen over the first30 min of life. The pulmonary arterial pressure is higher than the systemic arterial pressure during fetal period. Clamping theumbilical cord at birth increases systemic blood pressure and ventilation reduces pulmonary arterial pressure and increasespulmonary blood flow. Data derived from 8 lambs from the author's laboratory.

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contributing to NO-mediated pulmonary vasodilation afterbirth.4 Nitric oxide exerts its action through sGC and cGMP(Fig. 1). Bloch et al.16 report that expression of sGC is higher inlate gestation and newborn rats than in adult rats, which mayexplain the better response to NO in neonates. There is asimilar developmental regulation of cGMP specific phospho-diesterase 5 (PDE5) expression and activity. Expression ofPDE5 in the lungs increases during gestation and in theimmediate newborn period.23,24

The arachidonic acid–prostacyclin pathway also plays animportant role in the transition at birth. The cyclooxygenaseenzyme acts on arachidonic acid to produce prostaglandinendoperoxides. Prostaglandins activate adenylate cyclase toincrease cAMP concentrations in vascular smooth musclecells. Inhibition of prostacyclin production by non-steroidalanti-inflammatory drugs (NSAIDs) during late pregnancy hasbeen associated with PPHN although this association hasbeen recently called into question.25 Atrial natriuretic peptide(ANP), B-type natriuretic peptide (BNP), and C-type natriureticpeptide (CNP) dilate fetal pulmonary vasculature by increas-ing cGMP through particulate guanylate cyclase (pGC)26 andmay play a role in pulmonary vascular transition at birth.

Etiology and pathophysiology of PPHN

Failure of the pulmonary circulation to undergo the normaltransition after birth leads to PPHN, which is characterized by anelevated PVR/SVR ratio resulting from vasoconstriction, struc-tural remodeling of the pulmonary vasculature, intravascularobstruction, or lung hypoplasia. There is right-to-left shunting ofblood across the foramen ovale and ductus arteriosus, resultingin hypoxemia and labile oxygen saturations (Fig. 3).

PPHNmay be idiopathic (10%) or secondary to certain neonatalpulmonary diseases, which leads to delayed relaxation of thepulmonary vascular bed. Common pulmonary conditions, suchas congenital diaphragmatic hernia (CDH), respiratory distresssyndrome (RDS), pneumonia, meconium aspiration syndrome(MAS), and transient tachypnea of the newborn (TTN), may beassociated with PPHN. Some of the rare causes of severe andintractable PPHN include alveolar capillary dysplasia,27 hyalinemembrane disease caused by mutations in surfactant protein B(SP-B) gene,28 and respiratory failure due to ATP-binding cassetteprotein member A3 (ABCA3) deficiency.29 Recently, Byers et al.30

noted a genetic association. PPHN was significantly associatedwith genetic variants in corticotropin-releasing hormone (cRh)receptor 1 (CRHR1) and cRh-binding protein (CRHBP).

Congenital diaphragmatic hernia

It is often associated with intractable PPHN. A combination ofpulmonary arterial hypertension, right ventricular hypertrophyand/or failure, and left ventricular hypoplasia with pulmonaryvenous hypertension results in severe PPHN unresponsive toconventional management. Associated pulmonary hypoplasiaand decreased cross-sectional area of the pulmonary vascularbed, contribute to hypoxemic respiratory failure in neonateswith CDH. Lung damage is exacerbated by volutrauma andhyperoxic mechanical ventilation. A combination of gentleventilation, reduced oxygen exposure, inodilators, and PGE1has been shown to improve outcome in CDH (Fig. 4). Thiscondition is discussed in detail in a later article in this issue.

Parenchymal diseases

Parenchymal diseases, such as meconium aspiration, peri-natal asphyxia, surfactant deficiency (RDS), and pneumonia,

Fig. 3 – Various etiological factors causing PPHN and hemodynamic changes in PPHN/HRF. Surfactant deficiency (RDS) orinactivation (MAS or pneumonia) results in parenchymal lung disease and ventilation-perfusion (V/Q) mismatch. Increasedpulmonary vascular resistance results in reduced pulmonary blood flow and right-to-left shunt through PDA and/or PFO.Pulmonary hypertension is often associated with systemic hypotension with septal deviation to the left. Cardiac dysfunctionsecondary to asphyxia, sepsis, or congenital diaphragmatic hernia (CDH) may complicate HRF. Parenchymal lung diseasesecondary to RDS, pneumonia, transient tachypnea of newborn (TTN), pneumonia, and atelectasis can result in V/Q mismatchand hypoxemia and PPHN. Idiopathic or “black-lung” PPHN is not associated with parenchymal lung disease and results inreduced pulmonary blood flow with pulmonary vascular remodeling. Pulmonary hypoplasia secondary to CDH or due tooligohydramnios (prolonged leakage of fluid or reduced production due to renal compromise) causes alveolar and vascularhypoplasia and PPHN. The right subclavian artery (and blood flowing to the right upper extremity) is always preductal. Theleft subclavian artery may be preductal, juxtaductal, or postductal. Hence, preductal oxygen saturations should be obtainedfrom the right upper extremity. PA, pulmonary artery; RV, right ventricle; LV, left ventricle; TR, tricuspid regurgitation; RA,right atrium; LA, left atrium; PDA, patent ductus arteriosus; PFO, patent foramen ovale. (Copyright Satyan Lakshminrusimha.)

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interfere with normal oxygenation and ventilation of thelungs. The ensuing acidosis, hypoxia, and hypercarbia cancause abnormal vasoconstriction in the lungs. Hypoxemiaensues from ventilation/perfusion (V/Q) mismatch as well asright-to-left intrapulmonary and extrapulmonary shuntingof blood.

Meconium aspiration syndrome

Meconium aspiration syndrome (MAS) is the most commoncause of PPHN, although its incidence has decreased in recentyears due to a reduced number of post-term deliveries.Meconium staining of amniotic fluid (MSAF) occurs in 5–24%of normal pregnancies, however only 5% of infants born withMSAF develop MAS. Meconium can partially or completelyobstruct the airway and also inactivate surfactant.31 Thisresults in decreasing V/Q ratios and increasing intrapulmo-nary right-to-left shunt. Other segments of the lungs may be

over-ventilated relative to perfusion, causing increased phys-iologic dead space and hypoxemia.The exact mechanism of PPHN associated with parenchy-

mal lung disease is unclear, but it is thought that pulmonaryartery vasoconstriction induced by fetal and neonatal hypo-xia may be a significant factor. Activation of inflammatorymediators, like thromboxane A2, angiotensin II, and cyto-kines, as well as abnormal pulmonary vascular musculariza-tion, possibly contribute to the pulmonary hypertensionobserved in these neonates. Levels of leukotrienes, platelet-activating factor, thromboxanes,32 and ET-133 are noted to beelevated in infants with PPHN.

Asphyxia

Multiple mechanisms contribute to hypoxemic respiratoryfailure and PPHN in asphyxiated newborns including fetalhypoxemia, ischemia, meconium aspiration, right and left

Fig. 4 – Pathophysiology of hemodynamic abnormalities incongenital diaphragmatic hernia. During fetal life, reducedpulmonary venous return contributes to left-sided cardiachypoplasia. In the immediate postnatal period, a shortperiod of better oxygenation is referred to as “honeymoon”period. Subsequently, a period of left ventricular dysfunctionresults in pulmonary venous hypertension. Progressivevolutrauma and oxygen toxicity leads to chronic lungdisease and contributes to pulmonary hypertension.Maintenance of ductal patency with intravenous PGE1 andenhanced left ventricular function with milrinone mayimprove oxygenation in CDH. (Copyright SatyanLakshminrusimha.)

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ventricular dysfunction, coagulation defects, hyperoxic resus-citation, and effects of mechanical ventilation.34 Acidosis andhypoxia increase PVR. In hypothermia trials for asphyxia,approximately 20% of asphyxiated infants in the controlgroup and 25% in the hypothermia group were diagnosedwith PPHN.35

Pulmonary hypertension in premature infants

Although PPHN is traditionally considered a disease of termand near-term infants, it is increasingly being diagnosed inpreterm infants.36 Some preterm infants present with PPHNin the first few days of life.17 Preterm infants with broncho-pulmonary dysplasia (BPD) may present with severe pulmo-nary hypertension later in the hospital course or afterdischarge from the Neonatal Intensive Care Unit (NICU).Preterm infants with fetal growth restriction are at high riskfor developing BPD with pulmonary hypertension.37 Pulmo-nary vascular disease contributes to poor outcomes in BPD.38

Idiopathic or “black-lung” PPHN

PPHN can occur in the absence of any parenchymal diseaseor lung hypoplasia due to abnormal muscularization ofpulmonary arterioles. There is severe hypoxemia with pul-monary vasoconstriction. Conditions, such as polycythemiaand hyperviscosity, may also increase PVR and contribute toPPHN in the absence of parenchymal lung disease. In uteroclosure of the ductus arteriosus due to antenatal exposure tonon-steroidal anti-inflammatory drugs, such as aspirin and

ibuprofen, has been associated with PPHN.39 This associationwas brought into question in a recent study.25 Antenatalligation of the ductus arteriosus in fetal lambs creates amodel of severe PPHN with clinical and pathological featuresof “black-lung” PPHN.40 There is some evidence to suggestthat the use of SSRI antidepressants after 20 weeks ofgestation is associated with PPHN.7,41 Prenatal exposure tofluoxetine (an SSRI) induced fetal pulmonary hypertension inrats.42 However, another study by Andrade et al.43 did notfind any increase in the incidence of PPHN when theycompared pregnant women who received an SSRI in thethird trimester with control mothers. Maternal physical andpsychological well-being should be the primary factor dictat-ing anti-depressant therapy during pregnancy and postpar-tum period.

Pathogenesis of PPHN

There is evidence suggesting that an alteration of the NOpathway contributes to PPHN. Activity and expression ofeNOS and sGC in lungs are decreased44–46 in the fetal lambmodel of PPHN. In these lambs, the vascular response to NOitself is also diminished,46 whereas the response to cGMP isnormal. Thus the decreased responsiveness appears to resultfrom decreased vascular smooth muscle sensitivity to NO atthe level of sGC. Decreased expression of eNOS47 and reducedlevels of NO metabolites in urine48 have also been noted ininfants with PPHN.Endothelin-1 through ETA stimulation is thought to con-

tribute to the pathogenesis of PPHN. ET-1 production isincreased49 in the lungs in the fetal lamb model of PPHN.Chronic intrauterine ETA receptor blockade following ductalligation decreases right ventricular hypertrophy and distalmuscularization of small pulmonary arteries and increasesthe fall in PVR at delivery in newborn lambs with PPHN.50 ET-1 has been shown to decrease eNOS expression and activitythrough ETA receptor-mediated generation of hydrogen per-oxide.51 In addition to hydrogen peroxide, superoxide,another reactive oxygen species may cause pulmonary vaso-constriction and play a role in the pathogenesis of PPHN.Superoxide may scavenge NO and disrupt its signalingpathway. In the fetal lamb model of PPHN, increased super-oxide levels have been demonstrated in the pulmonaryarteries.52,53

Antenatal and perinatal risk factors

Risk factors for PPHN include meconium-stained amnioticfluid, perinatal acidosis and asphyxia, maternal risk factorssuch as prolonged rupture of membranes, maternal fever andpositive group B Streptococcal carrier status. PPHN should beconsidered in the etiology of hypoxic respiratory failure interm or near-term infants. In addition to these known riskfactors, it has recently been postulated that the perinatalenvironment, including exposure to nicotine and certainmedications, maternal obesity and diabetes, epigenetics,painful stimuli, and birth by cesarean section may also affectthe maladaptation of the lung circulation at birth.54

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Management in the delivery room

Early recognition of PPHN and correction of factors thatprevent decrease in PVR are important to successfully man-age a late preterm or term infant with hypoxemic respiratoryfailure. One of the classic features of PPHN is labile hypo-xemia. These infants exhibit frequent desaturation episodesand wide swings in SpO2 and arterial PO2 without changes inventilator settings. A single, loud S2 and a systolic murmur oftricuspid regurgitation are often heard on auscultation.Neonates with PPHN are often cyanotic secondary to signifi-

cant right-to-left extrapulmonary shunting in the presence ofhigh PVR. Significant resuscitative efforts may be required inthe delivery room. Resuscitationmeasures in the delivery roomare mainly based on the Neonatal Resuscitation Program andthe American Academy of Pediatrics/American Heart Associa-tion guidelines.55–57 Resuscitation in the delivery room shouldfocus on optimal lung recruitment and ventilation. A preductalpulse oximeter is placed on the right upper extremity andsaturations are maintained in the range recommended by theneonatal resuscitation program. Supplementation with exces-sive oxygen during resuscitation results in rapid decrease inPVR but increases subsequent pulmonary arterial contractilityand reduces response to inhaled NO in animal studies.58,59

Postnatal management: General measures

Diagnosis

In a term or near-term infant with respiratory distress, aninitial evaluation would include a chest X-ray and an arterialblood gas. Hypoxemia disproportionate to the severity ofparenchymal disease on chest radiography should suggestidiopathic PPHN. Evidence of the underlying parenchymaldisease, such as RDS, and MAS, may be seen on chest X-ray insecondary PPHN. This diagnosis can be confirmed by meas-uring preductal (right extremity) and postductal (either lowerextremity) arterial oxygenation. A difference in arterial PO2 Z

20 mmHg or oxygen saturation Z5–10% should be consideredsuggestive of PPHN. Similar postductal desaturation may beobserved in ductal-dependent systemic blood flow lesions(such as hypoplastic left heart syndrome, critical aorticstenosis, interrupted aortic arch, and coarctation of aorta)and anatomic pulmonary vascular disease (pulmonaryvenous stenosis, total anomalous pulmonary venous return,and alveolocapillary dysplasia/malalignment of pulmonaryveins). Infants with a closed ductus arteriosus and shuntingat the atrial level and PPHN may not show differential oxygensaturation. The unusual occurrence of “reverse differential”oxygen saturation (lower preductal saturation compared withpostductal measurement) is often due to poor perfusion inthe right hand secondary to a peripheral intravenous armsplint but can be due to transposition of great arteriesassociated with PPHN or coarctation of aorta.Echocardiography remains the gold standard diagnostic

tool in PPHN. Right-to-left or bidirectional shunting of bloodat the foramen ovale and/or the ductus arteriosus isclassically seen, as well as high pulmonary arterial/right

ventricular systolic pressure estimated by Doppler velocitymeasurement of tricuspid regurgitation jet. The direction ofthe shunt at atrial and ductal level also provides clues tomanagement (Fig. 5). Left-to-right shunting at the foramenovale and ductus arteriosus with marked hypoxemia suggestspredominant intrapulmonary shunting, and interventionsshould focus on optimizing lung recruitment (increasingPEEP/mean airway pressure or intratracheal surfactant). Thepresence of right-to-left shunting at foramen ovale and/orductus arteriosus is suggestive of extrapulmonary shuntingand which may respond to pulmonary vasodilator therapy.Similarly, right-to-left shunting at the ductal level and left-to-right shunting at the atrial level suggest PPHN with leftventricular dysfunction with pulmonary venous hyperten-sion as seen in CDH, asphyxia, or sepsis. Ductal-dependentsystemic circulation syndromes listed in the previous para-graph may be associated with a similar shunt pattern. Vaso-dilator therapy with milrinone may be considered in thissetting. Echocardiography prior to initiation of therapy is alsonecessary to rule out cyanotic congenital heart disease,which may present with fixed hypoxemia and may beclinically indistinguishable from PPHN.60

Recently, B-type natriuretic peptide (BNP) was proposed asa biomarker in PPHN, especially to assess efficacy of treat-ment and to predict rebound PPHN.61,62 However, its value inthe practical management of PPHN is presently unclear. Somecenters obtain serial (monthly) echocardiograms with BNPlevels to diagnose pulmonary hypertension associated withBPD in preterm infants.

Supportive measures

Once PPHN is diagnosed, supportive measures are vital insuccessful management efforts. Care should be taken tomaintain normothermia and correct metabolic abnormalities,such as hypoglycemia, hypocalcemia, acidosis, and polycy-themia. Intravenous nutrition with adequate glucose infusionrate and appropriate calcium and amino acid supplementa-tion with an optimal chloride:acetate ratio should be admin-istered preferably through a central line.Covering eyes and ears and maintaining a low-noise

environment is a common practice. Minimal stimulation,along with judicious use of sedation and analgesia withnarcotic analgesics, like morphine and fentanyl or benzodia-zepines such as midazolam, is recommended. Paralysisshould be avoided as it has been associated with increasedmortality.63 Due to underlying shunts, any stimulus canresult in a precipitous drop in oxygen saturations.Systemic blood pressure should be maintained at normal

values for gestational age. If there is hypotension and/or poorperfusion indicating hypovolemia, volume replacement inthe form of 1–2 fluid boluses should be administered. Ifhypotension persists despite volume replacement, inotropicagents, such as dopamine, dobutamine, and epinephrine, areindicated. These agents are not selective to systemic circu-lation and may be associated with pulmonary vasoconstric-tion and elevation of PVR at high doses.Hyperventilation and alkali infusions to maintain an alka-

line pH were strategies previously in use but are nowconsidered outdated. There were concerns of impaired

Fig. 5 – Echocardiographic evaluation of neonatal hypoxemia based on ductal (black bar) and atrial (blue bar) shunts.Left-to-right shunt at the ductal and atrial level is considered normal but can also be seen in the presence of parenchymallung disease resulting in hypoxemia in the absence of PPHN (lower left quadrant). The presence of right-to-left shuntat the atrial and ductal level is associated with PPHN (upper right quadrant). Right-to-left shunt at the ductal level but aleft-to-right shunt at the atrial level is associated with left ventricular dysfunction, pulmonary venous hypertension, andductal-dependent systemic circulation (lower right quadrant) and is a contraindication for inhaled pulmonary vasodilators,such as iNO. In patients with right-sided obstruction (such as critical pulmonary stenosis, PS), right atrial blood flows to theleft atrium through the PFO. Pulmonary circulation is dependent on a left-to-right shunt at the PDA (upper left quadrant). PA,pulmonary artery; RV, right ventricle; LV, left ventricle; TR, tricuspid regurgitation; RA, right atrium; LA, left atrium; PDA,patent ductus arteriosus; Ao, aorta; PGE1, prostaglandin E1. (Copyright Satyan Lakshminrusimha.)

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cerebral perfusion and sensorineural deafness with respira-tory alkalosis.64,65 Similar or better outcomes with lesschronic lung disease were also observed in infants with PPHNmaintaining normal PCO2 (45–60 mmHg).66,67 Alkali infusionwas associated with increased use of ECMO and need foroxygen at 28 days.63 Thus, a lack of convincing data tosupport hyperventilation/alkali infusion therapy and bettertherapeutic options, such as inhaled vasodilators, have led todecreased use of alkalosis. Most centers avoid acidosis basedon animal studies demonstrating exaggerated hypoxic pul-monary vasoconstriction with pH o7.25.68

Inhaled therapies

Oxygen and optimal oxygen saturations

Providing adequate oxygenation forms the mainstay of PPHNtherapy. However, there are currently no randomized studiescomparing different PaO2 levels in the management of PPHNin a term infant. Hypoxia increases PVR58,68 and contributesto the pathophysiology of PPHN, although hyperoxia does notfurther decrease PVR and instead results in free-radicalinjury. It has been shown that brief exposure to 100% oxygenin newborn lambs results in increased contractility of

pulmonary arteries59 and reduces response to iNO.58,69 Inaddition to direct inactivation of NO, reactive oxygen speciescan decrease eNOS activity and sGC activity and increasePDE5 activity, resulting in decreased cGMP levels and poten-tiating pulmonary vasoconstriction. In the ovine ductal liga-tion model of PPHN, maintaining oxygen saturations in the90–97% range results in low PVR.58 We recommend main-taining preductal oxygen saturations in low to mid-90s duringmanagement of infants with PPHN with PaO2 levels between55 and 80 mmHg.

Ventilation

Optimal lung expansion is essential for adequate oxygenationas well as the effective delivery of iNO.70 Conventional andhigh-frequency ventilation (HFV)71 may be used to reduce theV/Q mismatch. In studies comparing the effectiveness of HFVwith conventional ventilation in infants with PPHN andrespiratory failure, neither mode of ventilation was moreeffective in preventing extracorporeal membrane oxygen-ation (ECMO).72,73 HFV in combination with iNO resulted inthe greatest improvement in oxygenation in some newbornswho had severe PPHN complicated by diffuse parenchymallung disease and underinflation.74 Infants with RDS and MASbenefit most from a combination of HFV and iNO therapy.75,76

Fig. 6 – Selective and micro-selective action of inhaled nitricoxide (NO); inhaled NO is a selective dilator of thepulmonary circulation without any significant systemicvasodilation as it combines with hemoglobin to formmethemoglobin. As it is an inhaled vasodilator, it selectivelyenters the well-ventilated alveoli and improves blood flowto these alveoli and reduces V/Q mismatch (micro-selectiveeffect). (Copyright Satyan Lakshminrusimha.)

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“Gentle” ventilation strategies with optimal PEEP, relativelylow PIP and some permissive hypercapnia are now beingrecommended to ensure adequate lung expansion withoutcausing barotrauma. In the presence of an indwelling arterialline, severity of PPHN is assessed by calculation of oxygen-ation index (OI).

OI ¼ Mean airway pressure (cmH2O) � FiO2 � 100 C

PaO2 (mmHg)

Surfactant

Exogenous surfactant therapy improved oxygenation andreduced the need for ECMO when PPHN was secondary toparenchymal lung disease, such as RDS, pneumonia/sepsis,or MAS.77 A multicenter trial demonstrated that this benefitwas greatest for infants with relatively mild disease and withan oxygenation index (OI) of 15–25.78 Over the past decade,the use of surfactant in treating secondary PPHN and respi-ratory failure has increased and might have contributed toimproved effectiveness of iNO with reduced need for ECMO.

Nitric oxide

In 1999, inhaled nitric oxide (iNO) was approved by the FDAfor use in near-term and term infants with PPHN. It has beenthe mainstay of PPHN treatment. It achieves potent andselective pulmonary vasodilation without decreasing sys-temic vascular tone. In the intravascular space, it combineswith hemoglobin to form methemoglobin, which preventssystemic vasodilation (selective effect). iNO reduces V/Qmismatch by entering only ventilated alveoli and redirectingpulmonary blood by dilating adjacent pulmonary arterioles(Fig. 6).Large multicenter trials79–81 have demonstrated that iNO

reduces the need for ECMO. A meta-analysis of sevenrandomized trials of iNO use in newborns with PPHN alsorevealed that 58% of hypoxic near-term and term infantsresponded to iNO within 30–60 min.82 While use of iNO didnot reduce mortality in any study analyzed, the need forrescue ECMO therapy was significantly decreased.There has been significant debate concerning the optimal

starting dose as well as time of initiation of iNO therapy.Inhaled NO has several potential side effects, includingplatelet dysfunction, pulmonary edema, methemoglobine-mia, and production of toxic byproducts such as nitrates. Incombination with superoxide, it further potentiates oxidativeinjury by forming peroxynitrites. Doses from 5–80 ppm havebeen studied; however, most randomized clinical trials sup-port a starting dose of 20 ppm. This was also the dose atwhich peak improvement in the pulmonary-to-systemicarterial pressure ratio was noted in a study involving directPAP measurements during cardiac catheterization.83 Doseshigher than 20 ppm have been associated with more adverseeffects like methemoglobinemia with only a minimal increasein response rate.79,84

Controversy exists over the appropriate timing of initiationof iNO in hypoxic respiratory failure. An OI of 25 is associatedwith a 50% risk of requiring ECMO or mortality,85 while an OIof 40 is generally accepted as an indication for ECMO. Konduri

et al.86 demonstrated that earlier initiation of iNO with an OIof 15–25 did not reduce the need for ECMO but may have atendency to reduce the risk of progression to severe hypo-xemic respiratory failure. Based on current available evi-dence, an acceptable indication for treatment with iNOwould be an OI 415–25 with echocardiographic evidence ofPPHN or a higher OI with or without evidence of right-to-left shunt.Due to rebound vasoconstriction and resultant pulmonary

hypertension on abrupt withdrawal, iNO needs to be weanedgradually.87 Weaning in steps from 20 ppm gradually over aperiod of time before its discontinuation has been shown toprevent the rebound effect.88 If there is oxygenationresponse, inspired oxygen concentration is first weanedbelow 60% and then iNO is weaned at the rate of 5 ppmevery 4 h. Once iNO dose is 5 ppm, gradual weaning at therate of 1 ppm q 4 h is performed at our institution.Almost 40% of infants with PPHN do not respond or sustain

a response to iNO. Adequate lung expansion should beestablished by increasing PEEP, surfactant therapy, and useof HFV prior to administration of iNO. If oxygenation remains

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low in spite of ventilator and hemodynamic optimization oniNO, ECMO is considered as a therapeutic option. The Com-mittee on the Fetus and Newborn, American Academy ofPediatrics, has suggested that iNO use be limited to tertiarycare centers where ECMO is available.89 However, manycenters without ECMO capability have access to iNO. Careshould be taken to continue iNO therapy during transportfrom non-ECMO to ECMO centers.Inhaled NO is contraindicated in infants with congenital

heart disease known to be dependent on right-to-left shuntingof blood (such as hypoplastic left heart syndrome and inter-rupted aortic arch). There is also a high risk of pulmonaryedema in patients with pre-existing left ventricular dysfunc-tion who are placed on iNO (Fig. 5—right lower quadrant). Thisunderlines the importance of obtaining an echocardiogramprior to iNO therapy, not only to document PPHN and shunt-ing but also to rule out congenital heart disease.

Prostacyclin (PGI2)

It acts as a vasodilator in the intravenous as well as inhaledform by activating adenylate cyclase and increasing cAMP inpulmonary arterial smooth muscle cell (Fig. 1). Unlike inhaledvasodilators, intravenous vasodilators often cause systemichypotension. Inhaled PGI2 (Epoprostenol) use has been descri-bed in case reports.90,91 It may act synergistically with iNO tocause effective pulmonary vasodilation and also prevent therebound hypertension seen while weaning iNO. Use of theoral PGI2 analog Beraprost sodium as reported from Thailandhas caused significant improvement in the oxygenation indexin five neonates with PPHN who did not respond to alkalitherapy and HFOV.92 Another analog, Iloprost, has also beenused endotracheally and in the inhaled form along with iNOin intractable PPHN.93 As yet there are no randomized con-trolled trials evaluating the effect of these vasodilators andconsequently their use remains limited.

Inhaled PGE1

Aerosolized prostaglandin E1 (Alprostadil) has been used totreat pulmonary hypertension in adults as well as in exper-imental animal models. In a small pilot phase I–II study, Soodet al. suggested that inhaled PGE1 was a safe selectivepulmonary vasodilator in hypoxemic respiratory failure.94 Apilot trial evaluating the use of inhaled PGE1 (IPGE trial) iniNO-resistant PPHN was stopped due to poor enrollment.

Systemic vasodilators: Phosphodiesteraseinhibitors

The high rate of failure to obtain a sustained oxygenationresponse to iNO therapy has led to the search for othertargets to enhance pulmonary vasodilation (Fig. 1). Inhibitionof the cGMP-degrading phosphodiesterase (PDE5) by sildenafiland inhibition of the cAMP-degrading phosphodiesterase(PDE3) by milrinone are two of the most promising therapies.

Sildenafil

This drug is presently available both in oral and intravenousform in the United States and is FDA approved only for adultswith pulmonary hypertension. Studies have shown that oralsildenafil (dose range: 1–3 mg/kg every 6 h) improves oxygen-ation and reduces mortality in centers limited by non-availability of iNO.95,96 Intravenous sildenafil was shown tobe effective in improving oxygenation in patients with PPHNwith and without prior exposure to iNO.97 Being systemicallyadministered, the risk of side effects like hypotension due tosystemic vasodilation is high. This risk may be diminished byslowly administering a loading dose (0.4-mg load over 3 h),followed by a maintenance dose (0.07 mg/kg/h). Sildenafilmay reduce the rebound pulmonary hypertension notedduring iNO weaning. A randomized control pilot trial of IVsildenafil prior to the use of iNO was stopped due to poorenrollment.

Milrinone

This inotropic vasodilator is commonly used in the pediatricand adult intensive care units but is not currently licensed foruse in treating PPHN. It inhibits PDE3 and relaxes pulmonaryarteries in the fetal lamb model of PPHN.98 Infants with PPHNrefractory to iNO therapy have responded to IV milrinone in 3case series.99–101 A loading dose (50 mcg/kg) followed by amaintenance dose (0.33–1 mcg/kg/h) is commonly used. Aswith any systemic vasodilator, hypotension is a clinicalconcern and blood pressure needs to be closely monitored.Milrinone may be the pulmonary vasodilator of choice in thepresence of PPHN with left ventricular dysfunction.102

Bosentan

This non-specific endothelin-1 receptor blocker has beenused in the treatment of PPHN, mainly in adults. In the fetallamb model of pulmonary hypertension, Ivy et al.50 showedthat chronic intrauterine ET receptor blockade decreased PAPin utero, decreased RVH and distal muscularization of smallpulmonary arteries, and increased the fall in PVR at delivery.Use of bosentan in neonates was described by Goissenet al.103 in two infants with transposition of the great vesselsassociated with pulmonary hypertension and has beenshown to be effective in PPHN.104

Steroids in PPHN

Postnatal systemic steroids have been shown to decrease theduration of hospital length of stay and oxygen dependence inMAS.105 In the fetal lamb model of PPHN, hydrocortisonetreatment postnatally has been shown to improve oxygen-ation, increase cGMP levels, and reduce ROS levels.106 Thesedata suggest a potential role for hydrocortisone in PPHN. Careshould be taken to avoid using steroids in the presence ofbacterial or viral infection. Recent evidence that geneticabnormalities in the cortisol pathway are associated withPPHN provide further basis for exploring the role of steroidsin PPHN.30

Fig. 7 – Management of PPHN by “gentle ventilation”—see text for details. (Copyright Satyan Lakshminrusimha.)

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Extracorporeal membrane oxygenation

ECMO is a supportive measure that essentially gives time forthe neonatal heart and lung to recover from the underlyingpathology. With improved ventilation techniques (Fig. 7) andlimitation of oxygen toxicity and the use of therapies likeHFOV, surfactant, iNO, and other vasodilators, ECMO use forneonatal respiratory disorders has decreased. The technicaldetails as well as types of ECMO are discussed at length inother articles.

Newer therapies

Several newer therapies for PPHN are under investigation.These include free-radical scavengers like recombinanthuman superoxide dismutase (SOD), which has improvedoxygenation in lambs with PPHN.107,108 Apocynin, an NADPHoxidase inhibitor, has also been shown to attenuate ROS-mediated vasoconstriction and increase NOS activity in PPHNlambs.109 Use of antenatal betamethasone in animal studieshas been shown to improve pulmonary arterial relaxation toATP and NO donor in PPHN lambs.110 Increased endothelialNOS and reduced markers of oxidative stress were alsorevealed in the steroid-exposed group. Activators ofsGC111,112 may be more effective than iNO in inducing

pulmonary vasodilation especially in the presence of oxida-tive stress.

Neurodevelopmental outcomes

PPHN is a disease with significant long-term morbidity,irrespective of the treatment modality. These infants sufferfrom long-term sequelae, such as neurodevelopmental, cog-nitive, and hearing abnormalities.113–115 Thus, it is essentialto provide long-term multidisciplinary follow-up after dis-charge. Konduri et al. in their long-term follow-up of infantsrandomized to early iNO in PPHN noted neurodevelopmentalimpairment in about 25% of infants (early iNO, 27% andcontrol, 25%).113 The incidence of hearing impairment (earlyiNO, 23% and control, 24%) was also similar in their study.

Summary

PPHN is associated with high morbidity and mortality.Inhaled nitric oxide, along with HFV, surfactant, and suppor-tive measures including sedation and blood pressure support,remains the mainstay of PPHN management. ECMO is anoption when these measures fail. Oral/IV sildenafil, IV milri-none, and inhaled PGI2 may have a synergistic effect with iNOand are being used more frequently. However, larger clinical

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trials are necessary to establish their treatment effect. Free-radical scavengers like SOD, sGC activators, and antenatalsteroids are potential future therapies currently under inves-tigation. Clinical trials to evaluate these newer therapies aredifficult to perform because of a narrow window betweenfailure to respond to iNO and need for cannulation for ECMO.Long-term follow-up is essential for these infants due to thehigh risk of neurodevelopmental and hearing abnormalities.

r e f e r e n c e s

1. Dawes GS. Pulmonary circulation in the foetus and new-born.Br Med Bull. 1966;22(1):61–65.

2. Ardran G, Dawes GS, Prichard MM, Reynolds SR, Eyatt DG. Theeffect of ventilation of the foetal lungs upon the pulmonarycirculation. J Physiol. 1952;118(1):12–22.

3. Rasanen J, Wood DC, Weiner S, Ludomirski A, Huhta JC. Roleof the pulmonary circulation in the distribution of humanfetal cardiac output during the second half of pregnancy.Circulation. 1996;94(5):1068–1073.

4. Lakshminrusimha S, Steinhorn RH. Pulmonary vascular biol-ogy during neonatal transition. Clin Perinatol. 1999;26(3):601–619.

5. Delaney C, Gien J, Grover TR, Roe G, Abman SH. Pulmonaryvascular effects of serotonin and selective serotonin reuptakeinhibitors in the late-gestation ovine fetus. Am J Physiol LungCell Mol Physiol. 2011;301(6):L937–L944.

6. Delaney C, Gien J, Roe G, Isenberg N, Kailey J, Abman SH.Serotonin contributes to high pulmonary vascular tone in asheep model of persistent pulmonary hypertension of thenewborn. Am J Physiol Lung Cell Mol Physiol. 2013;304(12):L894–L901.

7. Chambers CD, Hernandez-Diaz S, Van Marter LJ, et al. Selec-tive serotonin-reuptake inhibitors and risk of persistentpulmonary hypertension of the newborn. N Engl J Med.2006;354(6):579–587.

8. Luscher TF. Endothelin: systemic arterial and pulmonaryeffects of a new peptide with potent biologic properties. AmRev Respir Dis. 1992;146(5 Pt 2):S56–S60.

9. Ivy DD, Kinsella JP, Abman SH. Physiologic characterization ofendothelin A and B receptor activity in the ovine fetalpulmonary circulation. J Clin Invest. 1994;93(5):2141–2148.

10. Mann J, Farrukh IS, Michael JR. Mechanisms by which endo-thelin 1 induces pulmonary vasoconstriction in the rabbit. JAppl Physiol. 1991;71(2):410–416.

11. Tod ML, Cassin S. Endothelin-1-induced pulmonary arterialdilation is reduced by N omega-nitro-L-arginine in fetallambs. J Appl Physiol. 1992;72(5):1730–1734.

12. Wong J, Reddy VM, Hendricks-Munoz K, Liddicoat JR, GerretsR, Fineman JR. Endothelin-1 vasoactive responses in lambswith pulmonary hypertension and increased pulmonaryblood flow. Am J Physiol. 1995;269(6 Pt 2):H1965–H1972.

13. Dawes GS, Mott JC. The vascular tone of the fetal lung. JPhysiol. 1962;164(3):465–477.

14. Heymann MA. Control of the pulmonary circulation in theperinatal period. J Dev Physiol. 1984;6(3):281–290.

15. Mensah E, Morin FC 3rd, Russell JA, Taggart TP, Gugino SF,Steinhorn RH. Soluble guanylate cyclase mRNA expressionchange during ovine lung development. Pediatr Res. 1998;43:290.

16. Bloch KD, Filippov G, Sanchez LS, Nakane M, de la Monte SM.Pulmonary soluble guanylate cyclase, a nitric oxide receptor,is increased during the perinatal period. Am J Physiol. 1997;272(3 Pt 1):L400–L406.

17. Kumar VH, Hutchison AA, Lakshminrusimha S, Morin FC 3rd,Wynn RJ, Ryan RM. Characteristics of pulmonary hyperten-sion in preterm neonates. J Perinatol. 2007;27(4):214–219.

18. Teitel DF, Iwamoto HS, Rudolph AM. Changes in the pulmo-nary circulation during birth-related events. Pediatr Res.1990;27(4 Pt 1):372–378.

19. Haworth SG, Hislop AA. Adaptation of the pulmonary circu-lation to extra-uterine life in the pig and its relevance to thehuman infant. Cardiovasc Res. 1981;15(2):108–119.

20. Konduri GG, Theodorou AA, Mukhopadhyay A, DeshmukhDR. Adenosine triphosphate and adenosine increase thepulmonary blood flow to postnatal levels in fetal lambs.Pediatr Res. 1992;31(5):451–457.

21. Steinhorn RH, Morin FC 3rd, Van Wylen DG, Gugino SF, GieseEC, Russell JA. Endothelium-dependent relaxations to adeno-sine in juvenile rabbit pulmonary arteries and veins. Am JPhysiol. 1994;266(5 Pt 2):H2001–H2006.

22. Shaul PW, Farrar MA, Zellers TM. Oxygen modulatesendothelium-derived relaxing factor production in fetal pul-monary arteries. Am J Physiol. 1992;262(2 Pt 2):H355–H364.

23. Hanson KA, Ziegler JW, Rybalkin SD, Miller JW, Abman SH,Clarke WR. Chronic pulmonary hypertension increases fetallung cGMP phosphodiesterase activity. Am J Physiol. 1998;275(5 Pt 1):L931–L941.

24. Sanchez LS, de la Monte SM, Filippov G, Jones RC, Zapol WM,Bloch KD. Cyclic-GMP-binding, cyclic-GMP-specific phospho-diesterase (PDE5) gene expression is regulated during ratpulmonary development. Pediatr Res. 1998;43(2):163–168.

25. Van Marter LJ, Hernandez-Diaz S, Werler MM, Louik C,Mitchell AA. Nonsteroidal antiinflammatory drugs in latepregnancy and persistent pulmonary hypertension of thenewborn. Pediatrics. 2013;131(1):79–87.

26. Lakshminrusimha S, D’Angelis CA, Russell JA, et al. C-typenatriuretic peptide system in fetal ovine pulmonary vascula-ture. Am J Physiol Lung Cell Mol Physiol. 2001;281(2):L361–L368.

27. Singh SA, Ibrahim T, Clark DJ, Taylor RS, George DH. Persis-tent pulmonary hypertension of newborn due to congenitalcapillary alveolar dysplasia. Pediatr Pulmonol. 2005;40(4):349–353.

28. Hamvas A, Cole FS, Nogee LM. Genetic disorders of surfactantproteins. Neonatology. 2007;91(4):311–317.

29. Shulenin S, Nogee LM, Annilo T, Wert SE, Whitsett JA, DeanM. ABCA3 gene mutations in newborns with fatal surfactantdeficiency. N Engl J Med. 2004;350(13):1296–1303.

30. Byers HM, Dagle JM, Klein JM, Ryckman KK, McDonald EL,Murray JC, et al. Variations in CRHR1 are associated withpersistent pulmonary hypertension of the newborn. PediatrRes. 2012;71(2):162–167.

31. Dargaville PA, South M, McDougall PN. Surfactant and sur-factant inhibitors in meconium aspiration syndrome. J Pediatr.2001;138(1):113–115.

32. Dobyns EL, Wescott JY, Kennaugh JM, Ross MN, Stenmark KR.Eicosanoids decrease with successful extracorporeal mem-brane oxygenation therapy in neonatal pulmonary hyper-tension. Am J Respir Crit Care Med. 1994;149(4 Pt 1):873–880.

33. Langleben D, DeMarchie M, Laporta D, Spanier AH, Schle-singer RD, Stewart DJ. Endothelin-1 in acute lung injury andthe adult respiratory distress syndrome. Am Rev Respir Dis.1993;148(6 Pt 1):1646–1650.

34. Lapointe A, Barrington KJ. Pulmonary hypertension and theasphyxiated newborn. J Pediatr. 2011;158(2 Suppl):e19–e24.

35. Shah PS, Ohlsson A, Perlman M. Hypothermia to treat neo-natal hypoxic ischemic encephalopathy: systematic review.Arch Pediatr Adolesc Med. 2007;161(10):951–958.

36. Van Meurs KP, Wright LL, Ehrenkranz RA, et al. Inhaled nitricoxide for premature infants with severe respiratory failure. NEngl J Med. 2005;353(1):13–22.

37. Check J, Gotteiner N, Liu X, et al. Fetal growth restriction andpulmonary hypertension in premature infants with broncho-pulmonary dysplasia. J Perinatol. 2013;33(7):553–557.

S E M I N A R S I N P E R I N A T O L O G Y 3 8 ( 2 0 1 4 ) 7 8 – 9 1 89

38. Mourani PM, Abman SH. Pulmonary vascular disease inbronchopulmonary dysplasia: pulmonary hypertension andbeyond. Curr Opin Pediatr. 2013;25(3):329–337.

39. Alano MA, Ngougmna E, Ostrea EM Jr., Konduri GG. Analysisof nonsteroidal antiinflammatory drugs in meconium and itsrelation to persistent pulmonary hypertension of the new-born. Pediatrics. 2001;107(3):519–523.

40. Wild LM, Nickerson PA, Morin FC 3rd. Ligating the ductusarteriosus before birth remodels the pulmonary vasculatureof the lamb. Pediatr Res. 1989;25(3):251–257.

41. Kallen B, Olausson PO. Maternal use of selective serotonin re-uptake inhibitors and persistent pulmonary hypertension ofthe newborn. Pharmacoepidemiol Drug Saf. 2008;17(8):801–806.

42. Fornaro E, Li D, Pan J, Belik J. Prenatal exposure to fluoxetineinduces fetal pulmonary hypertension in the rat. Am J RespirCrit Care Med. 2007;176(10):1035–1040.

43. Andrade SE, McPhillips H, Loren D, et al. Antidepressantmedication use and risk of persistent pulmonary hypertensionof the newborn. Pharmacoepidemiol Drug Saf. 2009;18(3):246–252.

44. Dukarm RC, Morin FC 3rd, Russell JA, Steinhorn RH. Pulmo-nary and systemic effects of the phosphodiesterase inhibitordipyridamole in newborn lambs with persistent pulmonaryhypertension. Pediatr Res. 1998;44(6):831–837.

45. Shaul PW, Yuhanna IS, German Z, Chen Z, Steinhorn RH,Morin FC 3rd. Pulmonary endothelial NO synthase geneexpression is decreased in fetal lambs with pulmonary hyper-tension. Am J Physiol. 1997;272(5 Pt 1):L1005–L1012.

46. Steinhorn RH, Russell JA, Morin FC 3rd. Disruption of cGMPproduction in pulmonary arteries isolated from fetal lambswith pulmonary hypertension. Am J Physiol. 1995;268(4 Pt 2):H1483–H1489.

47. Villanueva ME, Zaher FM, Svinarich DM, Konduri GG.Decreased gene expression of endothelial nitric oxide syn-thase in newborns with persistent pulmonary hypertension.Pediatr Res. 1998;44(3):338–343.

48. Pearson DL, Dawling S, Walsh WF, et al. Neonatal pulmonaryhypertension—urea-cycle intermediates, nitric oxide produc-tion, and carbamoyl-phosphate synthetase function. N Engl JMed. 2001;344(24):1832–1838.

49. Ivy DD, Le Cras TD, Horan MP, Abman SH. Chronic intra-uterine pulmonary hypertension increases preproendothelin-1 and decreases endothelin B receptor mRNA expression inthe ovine fetal lung. Chest. 1998;114(1 suppl):65S.

50. Ivy DD, Parker TA, Ziegler JW, et al. Prolonged endothelin Areceptor blockade attenuates chronic pulmonary hyperten-sion in the ovine fetus. J Clin Invest. 1997;99(6):1179–1186.

51. Wedgwood S, Steinhorn RH, Bunderson M, et al. Increasedhydrogen peroxide downregulates soluble guanylate cyclasein the lungs of lambs with persistent pulmonary hyper-tension of the newborn. Am J Physiol Lung Cell Mol Physiol.2005;289(4):L660–L666.

52. Brennan LA, Steinhorn RH, Wedgwood S, et al. Increasedsuperoxide generation is associated with pulmonary hyper-tension in fetal lambs: a role for NADPH oxidase. Circ Res.2003;92(6):683–691.

53. Konduri GG, Bakhutashvili I, Eis A, Pritchard K Jr.. Oxidantstress from uncoupled nitric oxide synthase impairs vaso-dilation in fetal lambs with persistent pulmonary hyper-tension. Am J Physiol Heart Circ Physiol. 2007;292(4):H1812–H1820.

54. Storme L, Aubry E, Rakza T, et al. Pathophysiology of persis-tent pulmonary hypertension of the newborn: impact of theperinatal environment. Arch Cardiovasc Dis. 2013;106(3):169–177.

55. Kattwinkel J. Textbook of Neonatal Resuscitation, 6th ed. ElkGrove Village, IL: American Academy of Pediatrics; 2011.

56. Kattwinkel J, Perlman JM, Aziz K, et al. Part 15: neonatalresuscitation: 2010 American Heart Association Guidelines for

cardiopulmonary resuscitation and emergency cardiovascu-lar care. Circulation. 2010;122(16 Suppl 2):S909–S919.

57. Perlman JM, Wyllie J, Kattwinkel J, et al. Part 11: neonatalresuscitation: 2010 International Consensus on cardiopulmo-nary resuscitation and emergency cardiovascular care sci-ence with treatment recommendations. Circulation. 2010;122:S516–S538.

58. Lakshminrusimha S, Swartz DD, Gugino SF, et al. Oxygenconcentration and pulmonary hemodynamics in newbornlambs with pulmonary hypertension. Pediatr Res. 2009;66(5):539–544.

59. Lakshminrusimha S, Russell JA, Steinhorn RH, et al. Pulmo-nary arterial contractility in neonatal lambs increases with100% oxygen resuscitation. Pediatr Res. 2006;59(1):137–141.

60. Lakshminrusimha S, Wynn RJ, Youssfi M, et al. Use of CTangiography in the diagnosis of total anomalous venousreturn. J Perinatol. 2009;29(6):458–461.

61. Reynolds EW, Ellington JG, Vranicar M, Bada HS. Brain-typenatriuretic peptide in the diagnosis and management ofpersistent pulmonary hypertension of the newborn. Pediatrics.2004;114(5):1297–1304.

62. Vijlbrief DC, Benders MJ, Kemperman H, van Bel F, de VriesWB. B-type natriuretic peptide and rebound during treatmentfor persistent pulmonary hypertension. J Pediatr. 2012;160(1):111–115.e1.

63. Walsh-Sukys MC, Tyson JE, Wright LL, et al. Persistentpulmonary hypertension of the newborn in the era beforenitric oxide: practice variation and outcomes. Pediatrics.2000;105(1 Pt 1):14–20.

64. Bifano EM, Pfannenstiel A. Duration of hyperventilation andoutcome in infants with persistent pulmonary hypertension.Pediatrics. 1988;81(5):657–661.

65. Hendricks-Munoz KD, Walton JP. Hearing loss in infants withpersistent fetal circulation. Pediatrics. 1988;81(5):650–656.

66. Dworetz AR, Moya FR, Sabo B, Gladstone I, Gross I. Survival ofinfants with persistent pulmonary hypertension withoutextracorporeal membrane oxygenation. Pediatrics. 1989;84(1):1–6 [comment].

67. Wung JT, James LS, Kilchevsky E, James E. Management ofinfants with severe respiratory failure and persistence of thefetal circulation, without hyperventilation. Pediatrics. 1985;76(4):488–494.

68. Rudolph AM, Yuan S. Response of the pulmonary vasculatureto hypoxia and Hþ ion concentration changes. J Clin Invest.1966;45(3):399–411.

69. Lakshminrusimha S, Russell JA, Steinhorn RH, et al.Pulmonary hemodynamics in neonatal lambs resuscita-ted with 21%, 50%, and 100% oxygen. Pediatr Res. 2007;62(3):313–318.

70. Pabalan MJ, Nayak SP, Ryan RM, Kumar VH, Lakshminru-simha S. Methemoglobin to cumulative nitric oxide ratio andresponse to inhaled nitric oxide in PPHN. J Perinatol. 2009;29(10):698–701.

71. Carlo WA, Beoglos A, Chatburn RL, Walsh MC, Martin RJ.High-frequency jet ventilation in neonatal pulmonary hyper-tension. Am J Dis Child. 1989;143(2):233–238.

72. Clark RH, Yoder BA, Sell MS. Prospective, randomized com-parison of high-frequency oscillation and conventional ven-tilation in candidates for extracorporeal membraneoxygenation. J Pediatr. 1994;124(3):447–454.

73. Engle WA, Yoder MC, Andreoli SP, Darragh RK, Langefeld CD,Hui SL. Controlled prospective randomized comparison ofhigh-frequency jet ventilation and conventional ventilationin neonates with respiratory failure and persistent pulmo-nary hypertension. J Perinatol. 1997;17(1):3–9.

74. Kinsella JP, Abman SH. Clinical approaches to the use of high-frequency oscillatory ventilation in neonatal respiratory fail-ure. J Perinatol. 1996;16(2 Pt 2 Su):S52–S55.

S E M I N A R S I N P E R I N A T O L O G Y 3 8 ( 2 0 1 4 ) 7 8 – 9 190

75. Kinsella JP, Abman SH. High-frequency oscillatory ventilationaugments the response to inhaled nitric oxide in persistentpulmonary hypertension of the newborn: nitric Oxide StudyGroup. Chest. 1998;114(1 Suppl):100S.

76. Kinsella JP, Truog WE, Walsh WF, et al. Randomized, multi-center trial of inhaled nitric oxide and high-frequency oscil-latory ventilation in severe, persistent pulmonary hypertensionof the newborn. J Pediatr. 1997;131(1 Pt 1):55–62.

77. Lotze A, Mitchell BR, Bulas DI, Zola EM, Shalwitz RA, GunkelJH. Multicenter study of surfactant (beractant) use in thetreatment of term infants with severe respiratory failure.Survanta in Term Infants Study Group. J Pediatr. 1998;132(1):40–47.

78. Lotze A, Mitchell BR, Bulas DI, Zola EM, Shalwitz RA, GunkelJH. Multicenter study of surfactant (beractant) use in thetreatment of term infants with severe respiratory failure.Survanta in Term Infants Study Group. J Pediatr. 1998;132(1):40–47 [comment].

79. The Neonatal Inhaled Nitric Oxide Study Group. Inhaled nitricoxide in full-term and nearly full-term infants with hypoxicrespiratory failure. N Engl J Med. 1997;336(9):597–604.

80. Clark RH, Kueser TJ, Walker MW, et al. Low-dose nitric oxidetherapy for persistent pulmonary hypertension of the new-born. Clinical Inhaled Nitric Oxide Research Group. N Engl JMed. 2000;342(7):469–474.

81. Roberts JD Jr, Fineman JR, Morin FC 3rd, et al. Inhaled nitricoxide and persistent pulmonary hypertension of the new-born. The Inhaled Nitric Oxide Study Group. N Engl J Med.1997;336(9):605–610.

82. Finer NN, Barrington KJ. Nitric oxide for respiratory failure ininfants born at or near term. Cochrane Database Syst Rev. 2000[CD000399].

83. Tworetzky W, Bristow J, Moore P, et al. Inhaled nitric oxide inneonates with persistent pulmonary hypertension. Lancet.2001;357(9250):118–120.

84. Davidson D, Barefield ES, Kattwinkel J, et al. Inhaled nitricoxide for the early treatment of persistent pulmonary hyper-tension of the term newborn: a randomized, double-masked,placebo-controlled, dose-response, multicenter study. TheI-NO/PPHN Study Group. Pediatrics. 1998;101(3 Pt 1):325–334.

85. Aschner JL, Fike CD. New developments in the pathogenesisand management of neonatal pulmonary hypertension. In:Bancalari E, ed, The Newborn Lung. Philadelphia: SaundersElsevier; 2008. 241–299.

86. Konduri GG, Kim UO. Advances in the diagnosis and manage-ment of persistent pulmonary hypertension of the newborn.Pediatr Clin North Am. 2009;56(3):579–600.

87. Aly H, Sahni R, Wung JT. Weaning strategy with inhaled nitricoxide treatment in persistent pulmonary hypertension of thenewborn. Arch Dis Child Fetal Neonatal Ed. 1997;76(2):F118–F122.

88. Sokol GM, Fineberg NS, Wright LL, Ehrenkranz RA. Changes inarterial oxygen tension when weaning neonates from inhalednitric oxide. Pediatr Pulmonol. 2001;32(1):14–19.

89. AAP. American Academy of Pediatrics. Committee on Fetusand Newborn. Use of inhaled nitric oxide. Pediatrics. 2000;106(2 Pt 1):344–345.

90. Kelly LK, Porta NF, Goodman DM, Carroll CL, Steinhorn RH.Inhaled prostacyclin for term infants with persistent pulmo-nary hypertension refractory to inhaled nitric oxide. J Pediatr.2002;141(6):830–832.

91. Soditt V, Aring C, Groneck P. Improvement of oxygenationinduced by aerosolized prostacyclin in a preterm infant withpersistent pulmonary hypertension of the newborn. IntensiveCare Med. 1997;23(12):1275–1278.

92. Nakwan N, Nakwan N, Wannaro J. Persistent pulmonaryhypertension of the newborn successfully treated with bera-prost sodium: a retrospective chart review. Neonatology.2011;99(1):32–37.

93. Ehlen M, Wiebe B. Iloprost in persistent pulmonary hyper-tension of the newborn. Cardiol Young. 2003;13(4):361–363.

94. Sood BG, Delaney-Black V, Aranda JV, Shankaran S. Aero-solized PGE1: a selective pulmonary vasodilator in neonatalhypoxemic respiratory failure results of a phase I/II openlabel clinical trial. Pediatr Res. 2004;56(4):579–585.

95. Baquero H, Soliz A, Neira F, Venegas ME, Sola A. Oralsildenafil in infants with persistent pulmonary hypertensionof the newborn: a pilot randomized blinded study. Pediatrics.2006;117(4):1077–1083.

96. Vargas-Origel A, Gomez-Rodriguez G, Aldana-Valenzuela C,Vela-Huerta MM, Alarcon-Santos SB, Amador-Licona N. Theuse of sildenafil in persistent pulmonary hypertension of thenewborn. Am J Perinatol. 2009;27(3):225–230.

97. Steinhorn RH, Kinsella JP, Butrous G, Dilleen M, Oakes M,Wessel DL. Intravenous sildenafil in the treatment of neo-nates with persistent pulmonary hypertension of the new-born. J Pediatr. 2009;155(6):841-847.

98. Lakshminrusimha S, Porta NF, Farrow KN, et al. Milrinoneenhances relaxation to prostacyclin and iloprost in pulmo-nary arteries isolated from lambs with persistent pulmonaryhypertension of the newborn. Pediatr Crit Care Med. 2009;10(1):106–112.

99. McNamara PJ, Shivananda SP, Sahni M, Freeman D, TaddioA. Pharmacology of milrinone in neonates with persistentpulmonary hypertension of the newborn and suboptimalresponse to inhaled nitric oxide. Pediatr Crit Care Med. 2013;14(1):74–84.

100. Bassler D, Choong K, McNamara P, Kirpalani H. Neonatalpersistent pulmonary hypertension treated with milrinone:four case reports. Biol Neonate. 2006;89(1):1–5.

101. McNamara PJ, Laique F, Muang-In S, Whyte HE. Milrinoneimproves oxygenation in neonates with severe persistentpulmonary hypertension of the newborn. J Crit Care. 2006;21(2):217–222.

102. Lakshminrusimha S, Steinhorn RH. Inodilators in nitricoxide resistant persistent pulmonary hypertension of thenewborn. Pediatr Crit Care Med. 2013;14(1):107–109.

103. Goissen C, Ghyselen L, Tourneux P, et al. Persistent pulmo-nary hypertension of the newborn with transposition of thegreat arteries: successful treatment with bosentan. Eur JPediatr. 2008;167(4):437–440.

104. Mohamed WA, Ismail M. A randomized, double-blind, pla-cebo-controlled, prospective study of bosentan for the treat-ment of persistent pulmonary hypertension of the newborn.J Perinatol. 2012;32(8):608–613.

105. Tripathi S, Saili A. The effect of steroids on the clinicalcourse and outcome of neonates with meconium aspirationsyndrome. J Trop Pediatr. 2007;53(1):8–12.

106. Perez M, Lakshminrusimha S, Wedgwood S, et al. Hydro-cortisone normalizes oxygenation and cGMP regulation inlambs with persistent pulmonary hypertension of the new-born. Am J Physiol Lung Cell Mol Physiol. 2011;302(6):L595–L603.

107. Lakshminrusimha S, Russell JA, Wedgwood S, et al. Super-oxide dismutase improves oxygenation and reduces oxida-tion in neonatal pulmonary hypertension. Am J Respir CritCare Med. 2006;174(12):1370–1377.

108. Steinhorn RH, Albert G, Swartz DD, Russell JA, Levine CR,Davis JM. Recombinant human superoxide dismutaseenhances the effect of inhaled nitric oxide in persistentpulmonary hypertension. Am J Respir Crit Care Med. 2001;164(5):834–839.

109. Wedgwood S, Lakshminrusimha S, Farrow KN, et al. Apoc-ynin improves oxygenation and increases eNOS in persis-tent pulmonary hypertension of the newborn. Am J PhysiolLung Cell Mol Physiol. 2012;302(6):L616–L626.

110. Chandrasekar I, Eis A, Konduri GG. Betamethasone attenu-ates oxidant stress in endothelial cells from fetal lambs with

S E M I N A R S I N P E R I N A T O L O G Y 3 8 ( 2 0 1 4 ) 7 8 – 9 1 91

persistent pulmonary hypertension. Pediatr Res. 2008;63(1):67–72.

111. Deruelle P, Balasubramaniam V, Kunig AM, Seedorf GJ,Markham NE, Abman SH. BAY 41-2272, a direct activator ofsoluble guanylate cyclase, reduces right ventricular hyper-trophy and prevents pulmonary vascular remodeling duringchronic hypoxia in neonatal rats. Biol Neonate. 2006;90(2):135–144.

112. Deruelle P, Grover TR, Storme L, Abman SH. Effects of BAY41-2272, a soluble guanylate cyclase activator, on pulmonaryvascular reactivity in the ovine fetus. Am J Physiol Lung CellMol Physiol. 2005;288(4):L727–L733.

113. Konduri GG, Vohr B, Robertson C, et al. Early inhaled nitricoxide therapy for term and near-term newborn infants withhypoxic respiratory failure: neurodevelopmental follow-up. JPediatr. 2007;150(3):235–240 [240 e1].

114. Robertson CM, Tyebkhan JM, Hagler ME, Cheung PY, Peliow-ski A, Etches PC. Late-onset progressive sensorineural hear-ing loss after severe neonatal respiratory failure. OtolNeurotol. 2002;23(3):353–356.

115. Lipkin PH, Davidson D, Spivak L, Straube R, Rhines J, ChangCT. Neurodevelopmental and medical outcomes of persis-tent pulmonary hypertension in term newborns treatedwith nitric oxide. J Pediatr. 2002;140(3):306–310 [comment].