Nitric oxide for preemies--not so fast

Antenatal hypoxia and pulmonary vascular function and remodeling

This review provides evidence that antenatal hypoxia, which represents a significant and worldwide problem, causes prenatal programming of the lung. A general overview of lung development is provided along with some background regarding transcriptional and signaling systems of the lung. The review illustrates that antenatal hypoxic stress can induce a continuum of responses depending on the species examined. Fetuses and newborns of certain species and specific human populations are well acclimated to antenatal hypoxia. However, antenatal hypoxia causes pulmonary vascular disease in fetuses and newborns of most mammalian species and humans. Disease can range from mild pulmonary hypertension, to severe vascular remodeling and dangerous elevations in pressure. The timing, length, and magnitude of the intrauterine hypoxic stress are important to disease development, however there is also a genetic-environmental relationship that is not yet completely understood. Determining the origins of pulmonary vascular remodeling and pulmonary hypertension and their associated effects is a challenging task, but is necessary in order to develop targeted therapies for pulmonary hypertension in the newborn due to antenatal hypoxia that can both treat the symptoms and curtail or reverse disease progression.

Efficacy of inhaled nitric oxide in preterm neonates

Over the past 20 years, the recognition of nitric oxide (NO) as an endothelial-derived vasodilator has led to remarkable advances in vascular biology awareness. The signaling molecule NO, produced by NO synthase, is a molecule that is widespread in the body and important in multiple organ systems. Soon after its discovery, investigators found NO to be a potent pulmonary vasodilator in term neonates. Nitric oxide has come to perform a key function in neonatal therapy and management since its identification, especially in those with respiratory failure. It is conventionally used in the neonatal population for the treatment of persistent pulmonary hypertension, resulting in hypoxic respiratory failure of the term or near-term newborn. Inhaled NO has been successful in acutely improving oxygenation and in reducing the need for extracorporeal membrane oxygenation treatment. In recent years, the efficacy of inhaled NO for the prevention of pulmonary disability as well as its neuroprotective capabilities in preterm infants has been explored.

Inhaled nitric oxide for preterm neonates

The evidence for the benefits of inhaled nitric oxide (iNO) on gas exchange, cytokine-induced lung inflammation, and vascular dysfunction has been demonstrated by several animal and human studies. The use of iNO in extremely low birth weight neonates for the prevention of adverse outcomes like chronic lung disease and neurologic injury has been investigated, but the findings remain inconclusive. This review briefly outlines the biologic rationale for the use of iNO in preterm neonates and the results on the outcome measures of bronchopulmonary dysplasia and brain injury from the recent clinical trials. This article focuses on the potential toxicities, persistent controversies, and unanswered questions regarding the use of this treatment modality in this patient population at high risk for adverse outcomes.

Progress in Discovery and Evaluation of Treatments to Prevent Bronchopulmonary Dysplasia

BACKGROUND

Recent improvements in the survival of extremely preterm infants have been accompanied by evolution in the pathogenesis and histopathology of bronchopulmonary dysplasia (BPD). Although oxygen and barotrauma-induced injury remain important contributing factors, pulmonary developmental arrest appears to play an equally important causal role in prolonged respiratory illness, especially among the most immature surviving preterm newborns. To date, clinical trials have failed to demonstrate a substantial benefit of a single treatment or preventive strategy for BPD.

OBJECTIVES

To evaluate the current evidence in favor of treatments that might prevent BPD.

METHODS

Review of clinical studies of preventive treatment strategies for BPD.

RESULTS

High frequency oscillatory ventilation, permissive hypercapnea, and inhaled nitric oxide might offer benefit to infants at risk of BPD. These and other potential preventive therapies for BPD, such as superoxide dismutase, inositol, and alpha(1)-proteinase inhibitor, deserve further study.

CONCLUSIONS

Although some current treatments offer promise, no preventive therapy for BPD has proven safe and effective, except for intramuscular vitamin A. Additional studies of respiratory technologies, management strategies, and protective molecules are needed.

Neonatal Ventilation With Inhaled Nitric Oxide Versus Ventilatory Support Without Inhaled Nitric Oxide for Preterm Infants With Severe Respiratory Failure: the INNOVO multicentre randomised controlled trial (ISRCTN 17821339)

BACKGROUND

Although inhaled nitric oxide (iNO) may be a promising treatment for newborn infants with severe respiratory failure, the results from 3 previous small trials were inconclusive.

METHODS

Infants of <34 weeks' gestation, <28 days old, and with severe respiratory failure requiring ventilatory support were randomized to receive or not receive iNO. The study was not blinded.

FINDINGS

Recruited were 108 infants (55 allocated to receive iNO and 53 not allocated to receive iNO) from 15 neonatal units in the United Kingdom and Republic of Ireland. Fifty-nine percent (64 of 108) died, and 84% of the survivors (37 of 44) had signs of some impairment or disability, 9 (20%) of them classified as severely disabled. There was no evidence of an effect of iNO on the primary outcomes: death or severe disability at 1 year corrected age (relative risk [RR]: 0.99; 95% confidence interval [CI]: 0.76 to 1.29); death or supplemental oxygen on expected date of delivery (RR: 0.84; 95% CI: 0.68 to 1.02); or death or supplemental oxygen at 36 weeks' postmenstrual age (RR: 0.98; 95% CI: 0.87 to 1.12). There was a trend for infants allocated to the iNO group to spend more time on the ventilator (log rank: 3.6), on supplemental oxygen (log rank: 1.4), and in hospital (log rank: 3.5) than those allocated to receive no iNO. This pattern predominantly reflected the infants who died. Mean total costs at 1 year corrected age were significantly higher in the iNO group, partly because of the costs of the gas but mainly because of the difference in initial hospitalization costs.

INTERPRETATION

Evidence of prolongation of intensive care and increased costs of such care, without clear beneficial effects, implies that iNO cannot be recommended for preterm infants with severe hypoxic respiratory failure.

Strategies for preventing bronchopulmonary dysplasia

PURPOSE OF REVIEW

Neonatologists and pulmonary biologists have long sought preventive treatments for bronchopulmonary dysplasia (BPD). The purpose of this review is to highlight recent reports of a number of potential treatments intended to prevent BPD and to discuss the controversies surrounding preventive strategies.

RECENT FINDINGS

The evolution of BPD from a disorder of pulmonary injury affecting moderately preterm infants, to one characterized by a developmental pulmonary arrest among survivors of extreme prematurity has important implications for BPD prevention. Recent recognition that the pathogenesis of BPD might have prenatal origins raises new challenges and opportunities for studies of BPD prevention; however, most current preventive strategies for BPD focus on respiratory management. Neither past nor current clinical trials have shown a conclusive benefit of a single preventive treatment strategy. Promising but still largely unproven preventive respiratory treatments include: high frequency oscillatory ventilation, permissive hypercapnea, and inhaled nitric oxide. Observational and recent laboratory data support the need for randomized clinical trials of continuous positive airway pressure versus mechanical ventilation. Additionally, clinical trials are needed to address the deficit in our knowledge of the potential benefits and risks of postnatal low dose corticosteroid treatment. Further study of superoxide dismutase, inositol, and alpha-1 proteinase inhibitor also are warranted on the basis of recent clinical trials or meta-analyses.

SUMMARY

Only Vitamin A has proven a safe and effective preventive treatment for BPD. Additional studies of respiratory technologies, management strategies, and protective molecules are needed. Directed cytokine and genetic therapies are on the horizon.

The safety and efficacy of nitric oxide therapy in premature infants

OBJECTIVES

To assess the safety-efficacy balance of low-dose inhaled nitric oxide (iNO) in hypoxemic premature infants because no sustained beneficial effect has been demonstrated clearly and there are concerns about side effects.

STUDY DESIGN

Eight hundred and sixty infants <32 weeks were randomized at birth to receive 5 ppm iNO or placebo when they presented with hypoxemic respiratory failure (HRF) defined by a requirement for mechanical ventilation, fraction of inspired oxygen (FIO 2 ) >40%, and arterio-alveolar ratio in oxygen (aAO 2 ) <0.22. The primary end point was intact survival at 28 days of age.

RESULTS

Sixty-one of 415 infants presented with HRF and were compared with 84 of 445 controls who presented with HRF. There was no difference in the primary end point (61.4% in infants [23% with HRF who were treated with iNO] vs 61.1% in controls [21.4% in controls with HRF]; P = .943). For the infants with HRF who were treated with iNO, there was no significant difference from controls for intraventricular hemorrhage (IVH) (6% vs 7%), necrotizing enterocolitis (8% vs 6 %), or patent ductus arteriosus (PDA) (34% vs 37%). Compared with nonhypoxemic infants, the risk of bronchopulmonary displasia (BPD) increased significantly in HRF controls (OR = 3.264 [CI 1.461-7.292]) but not in infants with HRF who were treated with iNO (OR = 1.626 [CI 0.633-4.178]).

CONCLUSIONS

iNO appears to be safe in premature infants but did not lead to a significant improvement in intact survival on day 28.

Modeling and Remodeling of the Lung in Neonatal Chronic Lung Disease

Neonatal chronic lung disease (CLD) is the major long-term pulmonary complication of preterm birth affecting about 20% of infants who need mechanical ventilation. CLD is the result of abnormal repair processes following inflammatory lung injury that lead to remodeling of the lung. Inflammation may be initiated by a variety of stimuli including mechanical ventilation, oxygen toxicity and infection. The resultant neutrophil chemotaxis and degranulation leads to the release of enzymes such as matrix metalloproteinases that can cause proteolysis of the lung extracellular matrix. Abnormal healing with remodeling leads to poorly compliant lungs with reduced capacity for gas exchange. Drugs can influence the normal process of lung modeling or remodeling. Fetal lung development can be influenced by glucocorticosteroids and inflammation. Both can cause abnormal lung modeling with fewer, larger alveoli and accelerated lung maturation, which confers benefits in terms of reduced morbidity and mortality from respiratory distress syndrome but potentially increases the risk of subsequent lung injury. Antioxidants, such as retinol (vitamin A), administered post-natally may reduce the effects of oxidative stress leading to a modest reduction in CLD but they require repeated intramuscular injections. Postnatal glucocorticosteroid therapy can modify the lung inflammatory response and reduce CLD but it can also have detrimental effects on the developing brain and lung, thereby creating a clinical dilemma for neonatologists. Proteinase inhibitors may be a rational therapy but more research is needed before they can be accepted as a treatment for preterm neonates.'Modeling' is defined as planning or forming that follows a set pattern. The term is used to describe the normal process of lung growth and development that culminates in mature branching alveolar air spaces surrounded by a network of capillaries. Normal lung modeling occurs under a variety of genetic and hormonal influences that can be altered, leading to abnormal patterns of growth. 'Remodeling' is defined as altering the structure of or re-making and, in the case of the lung, is used to describe the abnormal patterns of lung growth that occur after lung injury. Modeling and remodeling of the lungs occur to an extent throughout life but never more rapidly than during the fetal and early neonatal periods, and factors that influence this process may lead to development of neonatal CLD. Some of the factors involved in normal and abnormal lung modeling and inflammation and glucocorticosteroid-induced remodeling in the perinatal period, in the context of neonatal CLD, are reviewed with considerations of how various drugs may influence these processes.

Inhaled nebulized nitrite is a hypoxia-sensitive NO-dependent selective pulmonary vasodilator

The blood anion nitrite contributes to hypoxic vasodilation through a heme-based, nitric oxide (NO)-generating reaction with deoxyhemoglobin and potentially other heme proteins. We hypothesized that this biochemical reaction could be harnessed for the treatment of neonatal pulmonary hypertension, an NO-deficient state characterized by pulmonary vasoconstriction, right-to-left shunt pathophysiology and systemic hypoxemia. To test this, we delivered inhaled sodium nitrite by aerosol to newborn lambs with hypoxic and normoxic pulmonary hypertension. Inhaled nitrite elicited a rapid and sustained reduction ( approximately 65%) in hypoxia-induced pulmonary hypertension, with a magnitude approaching that of the effects of 20 p.p.m. NO gas inhalation. This reduction was associated with the immediate appearance of NO in expiratory gas. Pulmonary vasodilation elicited by aerosolized nitrite was deoxyhemoglobin- and pH-dependent and was associated with increased blood levels of iron-nitrosyl-hemoglobin. Notably, from a therapeutic standpoint, short-term delivery of nitrite dissolved in saline through nebulization produced selective, sustained pulmonary vasodilation with no clinically significant increase in blood methemoglobin levels. These data support the concept that nitrite is a vasodilator acting through conversion to NO, a process coupled to hemoglobin deoxygenation and protonation, and evince a new, simple and inexpensive potential therapy for neonatal pulmonary hypertension.

The story of nitric oxide: from rascally radical to miracle molecule

ONCE CONSIDERED AN environmental nuisance at best and a dangerous poison at worst, nitric oxide (NO) has been discovered to be an important biologic messenger, sending a shock wave through the scientific community. This article briefly reviews the discovery of NO as a regulator of vascular muscle tone and describes important events on its road to fame. The application of inhaled NO (iNO) for the treatment of hypoxic respiratory failure in neonates is discussed.