Infant lung function after inhaled nitric oxide therapy for persistent pulmonary hypertension of the newborn

Persistent Pulmonary Hypertension: A Look Into the Future Therapy

Persistent pulmonary hypertension (PPHN) of the newborn is a lung parenchymal disorder that causes a wide range of hemodynamic changes in the newborn's systemic circulation. Arising from a multifactorial web of etiology, PPHN is one of the most common reasons for neonatal intensive care unit hospitalization and is associated with increased morbidity and mortality. Historically, multiple treatment modalities have been explored, ranging from oxygen and surfactant therapy to newer upcoming medications like magnesium sulfate and adenosine. This review article has discussed the pathogenesis of PPHN and its relationship with the clinical implications of PPHN, such as heart failure and so on. This article has also explored the diagnostic guidelines and analyzed the existing and the upcoming modalities for treating PPHN.

Persistent pulmonary hypertension of the newborn

Failure of the normal circulatory adaptation to extrauterine life results in persistent pulmonary hypertension of the newborn (PPHN). Although this condition is most often secondary to parenchymal lung disease or lung hypoplasia, it may also be idiopathic. PPHN is characterized by elevated pulmonary vascular resistance with resultant right-to-left shunting of blood and hypoxemia. Although the preliminary diagnosis of PPHN is often based on differential cyanosis and labile hypoxemia, the diagnosis is confirmed by echocardiography. Management strategies include optimal lung recruitment and use of surfactant in patients with parenchymal lung disease, maintaining optimal oxygenation and stable blood pressures, avoidance of respiratory and metabolic acidosis and alkalosis, and pulmonary vasodilator therapy. Extracorporeal membrane oxygenation is considered when medical management fails. Although mortality associated with PPHN has decreased significantly with improvements in medical care, there remains the potential risk for neurodevelopmental disability which warrants close follow-up of affected infants after discharge.

Nitric oxide for respiratory failure in infants born at or near term

BACKGROUND

Nitric oxide (NO) is a major endogenous regulator of vascular tone. Inhaled nitric oxide (iNO) gas has been investigated as treatment for persistent pulmonary hypertension of the newborn.

OBJECTIVES

To determine whether treatment of hypoxaemic term and near-term newborn infants with iNO improves oxygenation and reduces rate of death and use of extracorporeal membrane oxygenation (ECMO), or affects long-term neurodevelopmental outcomes.

SEARCH METHODS

We used the standard search strategy of the Cochrane Neonatal Review Group to search the Cochrane Central Register of Controlled Trials (CENTRAL; 2016, Issue 1), MEDLINE via PubMed (1966 to January 2016), Embase (1980 to January 2016) and the Cumulative Index to Nursing and Allied Health Literature (CINAHL; 1982 to January 2016). We searched clinical trials databases, conference proceedings and reference lists of retrieved articles for randomised controlled trials and quasi-randomised trials. We contacted the principal investigators of studies published as abstracts to ascertain the necessary information.

SELECTION CRITERIA

Randomised studies of iNO in term and near-term infants with hypoxic respiratory failure, with clinically relevant outcomes, including death, use of ECMO and oxygenation.

DATA COLLECTION AND ANALYSIS

We analysed trial reports to assess methodological quality using the criteria of the Cochrane Neonatal Review Group. We tabulated mortality, oxygenation, short-term clinical outcomes (particularly use of ECMO) and long-term developmental outcomes.

STATISTICS

For categorical outcomes, we calculated typical estimates for risk ratios and risk differences. For continuous variables, we calculated typical estimates for weighted mean differences. We used 95% confidence intervals and assumed a fixed-effect model for meta-analysis.

MAIN RESULTS

We found 17 eligible randomised controlled studies that included term and near-term infants with hypoxia.Ten trials compared iNO versus control (placebo or standard care without iNO) in infants with moderate or severe severity of illness scores (Ninos 1996; Roberts 1996; Wessel 1996; Davidson 1997; Ninos 1997; Mercier 1998; Christou 2000; Clark 2000; INNOVO 2007; Liu 2008). Mercier 1998 compared iNO versus control but allowed back-up treatment with iNO for infants who continued to satisfy the same criteria for severity of illness after two hours. This trial enrolled both preterm and term infants but reported most results separately for the two groups. Ninos 1997 studied only infants with congenital diaphragmatic hernia.One trial compared iNO versus high-frequency ventilation (Kinsella 1997).Six trials enrolled infants with moderate severity of illness scores (oxygenation index (OI) or alveolar-arterial oxygen difference (A-aDO2)) and randomised them to immediate iNO treatment or iNO treatment only after deterioration to more severe criteria (Barefield 1996; Day 1996; Sadiq 1998; Cornfield 1999; Konduri 2004; Gonzalez 2010).Inhaled nitric oxide appears to have improved outcomes in hypoxaemic term and near-term infants by reducing the incidence of the combined endpoint of death or use of ECMO (high-quality evidence). This reduction was due to a reduction in use of ECMO (with number needed to treat for an additional beneficial outcome (NNTB) of 5.3); mortality was not affected. Oxygenation was improved in approximately 50% of infants receiving iNO. The OI was decreased by a (weighted) mean of 15.1 within 30 to 60 minutes after the start of therapy, and partial pressure of arterial oxygen (PaO2) was increased by a mean of 53 mmHg. Whether infants had clear echocardiographic evidence of persistent pulmonary hypertension of the newborn (PPHN) did not appear to affect response to iNO. Outcomes of infants with diaphragmatic hernia were not improved; outcomes were slightly, but not significantly, worse with iNO (moderate-quality evidence).Infants who received iNO at less severe criteria did not have better clinical outcomes than those who were enrolled but received treatment only if their condition deteriorated. Fewer of the babies who received iNO early satisfied late treatment criteria, showing that earlier iNO reduced progression of the disease but did not further decrease mortality nor the need for ECMO (moderate-quality evidence). Incidence of disability, incidence of deafness and infant development scores were all similar between tested survivors who received iNO and those who did not.

AUTHORS' CONCLUSIONS

Inhaled nitric oxide is effective at an initial concentration of 20 ppm for term and near-term infants with hypoxic respiratory failure who do not have a diaphragmatic hernia.

Diagnosis and management of persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of new born (PPHN) is associated with mortality and morbidity; it may be idiopathic or secondary to a number of conditions. The mainstay of diagnosis and to exclude structural abnormalities is echocardiography. Brain type natriuretic peptide (BNP) levels are elevated in PPHN, but are insufficiently sensitive to contribute to routine diagnosis. Management includes improving oxygenation by optimising lung volume by ventilatory techniques and/or surfactant and administering pulmonary vasodilator agents. Inhaled nitric oxide (iNO), a selective pulmonary vasodilator, reduces the need for extracorporeal membrane oxygenation in term infants; it does not, however, improve mortality or have any long term positive effects in prematurely born infants or infants with congenital diaphragmatic hernia. Other pulmonary vasodilators have been reported in case series to be efficacious alone or in combination with iNO. Randomised trials with long term follow up are required to identify the optimum therapeutic strategies in PPHN.

The Respiratory System

This chapter addresses upper airway physiology for the pediatric intensivist, focusing on functions that affect ventilation, with an emphasis on laryngeal physiology and control in breathing. Effective control of breathing ensures that the airway is protected, maintains volume homeostasis, and provides ventilation. Upper airway structures are effectors for all of these functions that affect the entire airway. Nasal functions include air conditioning and protective reflexes that can be exaggerated and involve circulatory changes. Oral cavity and pharyngeal patency enable airflow and feeding, but during sleep pharyngeal closure can result in apnea. Coordination of breathing with sucking and nutritive swallowing alters during development, while nonnutritive swallowing at all ages limits aspiration. Laryngeal functions in breathing include protection of the subglottic airway, active maintenance of its absolute volume, and control of tidal flow patterns. These are vital functions for normal lung growth in fetal life and during rapid adaptations to breathing challenges from birth through adulthood. Active central control of breathing focuses on the coordination of laryngeal and diaphragmatic activities, which adapts according to the integration of central and peripheral inputs. For the intensivist, knowledge of upper airway physiology can be applied to improve respiratory support. In a second part the mechanical properties of the respiratory system as a critical component of the chain of events that result in translation of the output of the respiratory rhythm generator to ventilation are described. A comprehensive understanding of respiratory mechanics is essential to the delivery of optimized and individualized mechanical ventilation. The basic elements of respiratory mechanics will be described and developmental changes in the airways, lungs, and chest wall that impact on measurement of respiratory mechanics with advancing postnatal age are reviewed. This will be follwowed by two sections, the first on respiratory mechanics in various neonatal pathologies and the second in pediatric pathologies. The latter can be classified in three categories. First, restrictive diseases may be of pulmonary origin, such as chronic interstitial lung diseases or acute lung injury/acute respiratory distress syndrome, which are usually associated with reduced lung compliance. Restrictive diseases may also be due to chest wall abnormalities such as obesity or scoliosis (idiopathic or secondary to neuromuscular diseases), which are associated with a reduction in chest wall compliance. Second, obstructive diseases are represented by asthma and wheezing disorders, cystic fibrosis, long term sequelae of neonatal lung disease and bronchiolitis obliterans following hematopoietic stem cell transplantation. Obstructive diseases are defined by a reduced FEV1/VC ratio. Third, neuromuscular diseases, mainly represented by DMD and SMA, are associated with a decrease in vital capacity linked to respiratory muscle weakness that is better detected by PImax, PEmax and SNIP measurements.

Respiratory function during infancy in survivors of the INNOVO trial

RATIONALE

Despite encouraging reports suggesting that inhaled nitric oxide (iNO) appear to improve outcome in hypoxemic term and near term infants by improving oxygenation and reducing need for ECMO, the long-term benefits of iNO remain unclear. This study aimed to compare lung function at approximately 1 year in infants who were and were not randomly allocated to iNO as part of their neonatal management for severe respiratory failure at birth. Furthermore, results were compared to lung function of healthy infants.

METHODS

Maximal expiratory flow at functional residual capacity (V'maxFRC) was measured at approximately 1 year of age (corrected for any prematurity) in survivors of the INNOVO trial. Results were expressed as Z-scores, adjusted for sex and body size, based on data from healthy controls using identical techniques.

RESULTS

Technically satisfactory results were obtained in 30 infants (53% < 34 weeks gestation), 19 of whom were randomized to receive iNO V'maxFRC. Z-score was significantly reduced in infants with prior respiratory failure, whether or not they had been allocated to iNO (mean (SD) Z-score: -2.0 (1.2) and -2.6 (1.1), respectively, 95% CI difference; iNO vs. no iNO: -0.3; 1.6, P = 0.2). There was significant respiratory morbidity in both groups during the first year of life.

CONCLUSIONS

These results suggest that airway function remains reduced at 1 year of age following severe respiratory failure at birth, whether or not iNO is administered.

Airway function in infants treated with inhaled nitric oxide for persistent pulmonary hypertension

RATIONALE

Inhaled nitric oxide (iNO), used for treatment of persistent pulmonary hypertension of newborn (PPHN), is an oxygen free radical with potential for lung injury. Deferring ECMO with iNO in these neonates could potentially have long-term detrimental effects on lung function. We studied respiratory morbidity (defined as occurrence of respiratory infections requiring treatment, episodes of wheezing, and/or need for ongoing medications following discharge) and airway function at 1 year postnatal age in term neonates treated with iNO but not ECMO for PPHN, and compared data from similar infants recruited to the UK ECMO Trial randomized to receive ECMO or conventional management (CM).

METHODS

Maximal expiratory flow at FRC (V(') (maxFRC)) was measured in infants treated with iNO for PPHN (oxygenation index >or=25) at birth.

RESULTS

V(') (maxFRC) was measured in 23 infants and expressed as z-scores, to adjust for sex and body size and compared to data from 71 (46 ECMO, 25 CM) infants studied at a similar age in the ECMO Trial. Respiratory morbidity was low in iNO group. V(') (maxFRC) z-score was lower than predicted in all groups (P < 0.001), with no significant difference between those treated with iNO [mean (SD) z-score: -1.65 (1.2)] and those treated with ECMO [-1.59 (1.2)] or CM [-2.1(1.0)]. Within iNO, ECMO and CM groups; 26%, 37% and 56%, respectively, had V(') (maxFRC) z-scores below normal.

CONCLUSIONS

Respiratory outcome at 1 year in iNO treated neonates with moderately severe PPHN is encouraging, with no apparent increase in respiratory morbidity when compared to the general population. Sub-clinical reductions in airway function are evident at 1 year, suggesting that continuing efforts to minimize lung injury in the neonatal period are warranted to maximize lung health in later life.

Nitric oxide for respiratory failure in infants born at or near term

BACKGROUND

Nitric oxide is a major endogenous regulator of vascular tone. Inhaled nitric oxide gas has been investigated as a treatment for persistent pulmonary hypertension of the newborn.

OBJECTIVES

To determine whether treatment of hypoxaemic term and near-term newborn infants with inhaled nitric oxide (iNO) improves oxygenation and reduces the rates of death, the requirement for extracorporeal membrane oxygenation (ECMO), or affects long term neurodevelopmental outcomes.

SEARCH STRATEGY

Electronic and hand searching of pediatric/neonatal literature and personal data files. In addition we contacted the principal investigators of articles which have been published as abstracts to ascertain the necessary information.

SELECTION CRITERIA

Randomized and quasi-randomized studies of inhaled nitric oxide in term and near term infants with hypoxic respiratory failure. Clinically relevant outcomes, including death, requirement for ECMO, and oxygenation.

DATA COLLECTION AND ANALYSIS

Trial reports were analysed for methodologic quality using the criteria of the Cochrane Neonatal Review Group. Results of mortality, oxygenation, short term clinical outcomes (particularly need for ECMO), and long term developmental outcomes were tabulated.

STATISTICS

For categorical outcomes, typical estimates for relative risk and risk difference were calculated. For continuous variables, typical estimates for weighted mean difference were calculated. 95% confidence intervals were used. A fixed effect model was assumed for meta-analysis.

MAIN RESULTS

Fourteen eligible randomized controlled studies were found in term and near term infants with hypoxia. Seven of the trials compared iNO to control (placebo or standard care without iNO) in infants with moderate or severe severity of illness scores. Four of the trials compared iNO to control, but allowed back up treatment with iNO if the infants continued to satisfy the same criteria for severity of illness after a defined period of time. Two trials enrolled infants with moderate severity of illness score (OI or AaDO2) and randomized to immediate iNO treatment or iNO treatment only if they deteriorated to more severe criteria. One trial studied only infants with congenital diaphragmatic hernia (Ninos 1997), and one trial enrolled both preterm and term infants (Mercier 1998), but reported the majority of the results separately for the two groups. Inhaled nitric oxide appears to improve outcome in hypoxaemic term and near term infants by reducing the incidence of the combined endpoint of death or need for ECMO. The reduction seems to be entirely a reduction in need for ECMO; mortality is not reduced. Oxygenation improves in approximately 50% of infants receiving nitric oxide. The Oxygenation Index decreases by a (weighted) mean of 15.1 within 30 to 60 minutes after commencing therapy and PaO2 increases by a mean of 53 mmHg. Whether infants have clear echocardiographic evidence of persistent pulmonary hypertension of the newborn (PPHN) or not does not appear to affect outcome. The outcome of infants with diaphragmatic hernia was not improved; indeed there is a suggestion that outcome was slightly worsened. The incidence of disability, incidence of deafness and infant development scores are all similar between tested survivors who received nitric oxide or not.

AUTHORS' CONCLUSIONS

On the evidence presently available, it appears reasonable to use inhaled nitric oxide in an initial concentration of 20 ppm for term and near term infants with hypoxic respiratory failure who do not have a diaphragmatic hernia.

Lung function tests in neonates and infants with chronic lung disease: lung and chest-wall mechanics

This is the fifth paper in a review series that summarizes available data and critically discusses the potential role of lung function testing in infants and young children with acute neonatal respiratory disorders and chronic lung disease of infancy (CLDI). This review focuses on respiratory mechanics, including chest-wall and tissue mechanics, obtained in the intensive care setting and in infants during unassisted breathing. Following orientation of the reader to the subject area, we focused comments on areas of enquiry proposed in the introductory paper to this series. The quality of the published literature is reviewed critically with respect to relevant methods, equipment and study design, limitations and strengths of different techniques, and availability and appropriateness of reference data. Recommendations to guide future investigations in this field are provided. Numerous different methods have been used to assess respiratory mechanics with the aims of describing pulmonary status in preterm infants and assessing the effect of therapeutic interventions such as surfactant treatment, antenatal or postnatal steroids, or bronchodilator treatment. Interpretation of many of these studies is limited because lung volume was not measured simultaneously. In addition, populations are not comparable, and the number of infants studied has generally been small. Nevertheless, results appear to support the pathophysiological concept that immaturity of the lung leads to impaired lung function, which may improve with growth and development, irrespective of the diagnosis of chronic lung disease. To fully understand the impact of immaturity on the developing lung, it is unlikely that a single parameter such as respiratory compliance or resistance will accurately describe underlying changes. Assessment of respiratory mechanics will have to be supplemented by assessment of lung volume and airway function. New methods such as the low-frequency forced oscillation technique, which differentiate the tissue and airway components of respiratory mechanics, are likely to require further development before they can be of clinical significance.

The use of nitric oxide in neonatal care

Knowledge of NO and its role in the human body currently is limited. Further scientific research involving this unique molecule will expand its clinical usefulness. It is an exciting era in research,involving numerous body processes and systems. The initial work on pulmonary vascular response in newborns who have PPHN has opened the door to seemingly endless possibilities involving many aspects of health.

Inhaled nitric oxide increases endothelin-1 levels: a potential cause of rebound pulmonary hypertension

OBJECTIVE

Inhaled nitric oxide (iNO) is front-line therapy for pulmonary hypertension after repair of congenital heart disease. However, little clinical data exists regarding the effects of iNO on regulators of pulmonary vascular resistance. An imbalance between primary vasodilators, such as NO, and vasoconstrictors, such as endothelin-1 (ET-1), has been implicated in rebound pulmonary hypertension upon iNO withdrawal. The objective of this study was to determine whether iNO therapy alters plasma ET-1 levels.

DESIGN

This is a prospective study involving pediatric and adult patients at risk for pulmonary hypertension.

SETTING

Pediatric patients were in the cardiac intensive care unit and adult patients were in a tertiary-care hospital.

PATIENTS

Group 1 included children with congenital heart disease requiring iNO for treatment of pulmonary hypertension after cardiopulmonary bypass (n = 15), group 2 was adults receiving iNO (n = 10), and group 3 included children at risk for pulmonary hypertension after bypass that did not require iNO (n = 8).

INTERVENTIONS

Dosages of iNO were 2-60 ppm. The duration of therapy ranged from 23 to 188 hrs in group 1 and 29 to 108 hrs in group 2.

MEASUREMENTS AND MAIN RESULTS

Arterial blood was obtained for the measurement of ET-1 levels before and during iNO therapy and 24 hrs after iNO withdrawal. Group 1 mean ET-1 levels increased to 127% of baseline by 12 hrs of iNO, remained elevated at 48 hrs (p < .05), then decreased to 71% of iNO levels 24 hrs after withdrawal (p < .01). Group 2 ET-1 levels increased to 147%, and 137% of baseline at 12 and 24 hrs of iNO therapy, then fell to 68% of baseline within 24 hrs of discontinuing iNO. ET-1 levels in group 3 decreased after surgery (p < .05).

CONCLUSIONS

These data suggest that iNO increased plasma ET-1 levels, which subsequently decreased when iNO was discontinued. Increased circulating ET-1 levels might contribute to rebound pulmonary hypertension upon iNO withdrawal.

Inhaled nitric oxide in the neonatal period

Pulmonary hypertension of the newborn is an important cause of hypoxaemia, particularly in the at term or near term neonate. It can occur as a primary condition or secondary to a variety of other diseases. Endogenous nitric oxide is an important modulator of vascular tone in pulmonary circulation. Initial uncontrolled studies indicated that inhalation of nitric oxide resulted in a reduction in pulmonary hypertension, with improvement in oxygenation, but no change in the systemic vascular resistance. There have now been a number of randomised trials performed exploring the efficacy of inhaled nitric oxide. These trials have demonstrated that in at term or near term infants, inhaled nitric oxide reduces the combined end point of death or the need for extracorporeal membrane oxygenation. The significant effect seems due to the reduced extracorporeal membrane oxygenation requirement. No such beneficial effect has been consistently reported in infants with congenital diaphragmatic hernia. Randomised trials have failed to highlight long-term positive results in preterm infants. Inhaled nitric oxide has side effects, although those due to nitrogen dioxide and methaemoglobin formation can be minimised by appropriate nitric oxide delivery. It is important to use the smallest effective nitric oxide dose, continuous nitric oxide and nitrogen dioxide monitoring and frequent methaemoglobin analyses. Careful patient selection should be undertaken, avoiding those at high risk of haemorrhagic complications. Longer term follow-up studies are required to determine the real risk:benefit ratio of inhaled nitric oxide treatment.

Nitric oxide for respiratory failure in infants born at or near term

BACKGROUND

This section is under preparation and will be included in the next issue.

OBJECTIVES

To determine whether treatment of hypoxemic newborn infants with inhaled nitric oxide (INO) improves oxygenation and reduces the rates of death, or the requirement for ECMO.

SEARCH STRATEGY

Electronic and hand searching of pediatric/neonatal literature and personal data files. In addition we contacted the principal investigators of articles which have been published as abstracts to ascertain the necessary information.

SELECTION CRITERIA

Randomized and quasi randomized studies in term and near term infants. Administration of inhaled nitric oxide. Clinically relevant outcomes, including death, requirement for ECMO, and oxygenation.

DATA COLLECTION AND ANALYSIS

Eight randomized controlled studies were found in term and near term infants with hypoxia. Entry criteria were reasonably consistent except for the one trial that studied only infants with congenital diaphragmatic hernia (Ninos 1997).

MAIN RESULTS

Inhaled nitric oxide appears to improve outcome in hypoxemic term and near term infants by reducing the incidence of the combined endpoint of death or need for ECMO. The reduction seems to be entirely a reduction in need for ECMO; mortality is not reduced. Oxygenation improves in approximately 50% of infants receiving nitric oxide. The Oxygenation Index decreases by a (weighted) mean of 15.1 within 30 to 60 minutes after commencing therapy and PaO2 increases by a mean of 53 mmHg. It does not appear to affect outcome whether infants have clear echocardiographic evidence of PPHN or not. The outcome of infants with diaphragmatic hernia was not improved; indeed there is a suggestion that outcome was slightly worsened.

REVIEWER'S CONCLUSIONS

On the evidence presently available, it appears reasonable to use inhaled nitric oxide in a concentration of 20 ppm for term and near term infants with hypoxic respiratory failure who do not have a diaphragmatic hernia. Longterm neurodevelopmental and pulmonary followup of surviving infants enrolled in randomized trials of INO are required to establish more firmly the role of INO in the treatment of neonatal respiratory failure.