Peter M. MacFarlane

Severity of neonatal hyperoxia determines structural and functional changes in developing mouse airway

Wheezing is a major long-term respiratory morbidity in preterm infants with and without bronchopulmonary dysplasia. We hypothesized that mild vs. severe hyperoxic exposure in neonatal mice differentially affects airway smooth muscle hypertrophy and resultant airway reactivity. Newborn mice were exposed to 7 days of mild (40% oxygen) or severe (70% oxygen) hyperoxia vs. room air controls. Respiratory system resistance (Rrs), compliance (Crs), and airway reactivity were measured 14 days after oxygen exposure ended under ketamine/xylazine anesthesia. Baseline Rrs increased and Crs decreased in both treatment groups. Methacholine challenge dose dependently increased Rrs and decreased Crs in 40% oxygen-exposed mice, whereas Rrs and Crs responses were similar between 70% oxygen-exposed and normoxic controls. Airway smooth muscle thickness was increased in 40%- but not 70%-exposed mice, whereas collagen increased and both alveolar number and radial alveolar counts decreased after 40% and 70% oxygen. These data indicate that severity of hyperoxia may differentially affect structural and functional changes in the developing mouse airway that contribute to longer-term hyperreactivity. These findings may be important to our understanding of the complex role of neonatal supplemental oxygen therapy in postnatal development of airway responsiveness.

Mechanisms of Injury to the Preterm Lung and Airway: Implications for Long-Term Pulmonary Outcome

Despite changes in the epidemiology of bronchopulmonary dysplasia (BPD), longer-term morbidity, particularly in the form of airway dysfunction, remains a substantial problem in former preterm infants. The stage for this respiratory morbidity may begin as early as the transition from fetal to neonatal life. Newer therapeutic approaches for BPD should be directed toward minimizing this longer-term respiratory morbidity. Neonatal animal models focused primarily on hyperoxic exposure may provide important insights into the pathogenesis of longer-term airway hyperreactivity in this population.

Physiologic Basis for Intermittent Hypoxic Episodes in Preterm Infants

Intermittent hypoxic episodes are typically a consequence of immature respiratory control and remain a troublesome challenge for the neonatologist. Furthermore, their frequency and magnitude are commonly underestimated by clinically employed pulse oximeter settings. In extremely low birth weight infants the incidence of intermittent hypoxia [IH] progressively increases over the first 4 weeks of postnatal life, with a subsequent plateau followed by a slow decline beginning at weeks six to eight. Over this period of unstable respiratory control, increased oxygen-sensitive peripheral chemoreceptor activity has been associated with a higher incidence of apnea of prematurity. In contrast, infants with bronchopulmonary dysplasia [chronic neonatal lung disease] exhibit decreased peripheral chemosensitivity, although the effect on respiratory stability in this population is unclear. Such episodic hypoxia/reoxygenation in early life has the potential to sustain a proinflammatory cascade with resultant multisystem, including respiratory, morbidity. Therapeutic approaches for intermittent hypoxic episodes comprise careful titration of baseline or supplemental inspired oxygen as well as xanthine therapy to prevent apnea of prematurity. Characterization of the pathophysiologic basis for such intermittent hypoxic episodes and their consequences during early life is necessary to provide an evidence-based approach to their management.

Anti-Inflammatory Effect of Caffeine Is Associated with Improved Lung Function after Lipopolysaccharide-Induced Amnionitis

Background: Although caffeine enhances respiratory control and decreases the need for mechanical ventilation and resultant bronchopulmonary dysplasia, it may also have anti-inflammatory properties in protecting lung function. Objective: We hypothesized that caffeine improves respiratory function via an anti-inflammatory effect in lungs of a lipopolysaccharide (LPS)-induced pro-inflammatory amnionitis rat pup model. Methods: Caffeine was given orally (10 mg/kg/day) from postnatal day (p)1 to p14 to pups exposed to intra-amniotic LPS or normal saline. Expression of IL-1β was assessed in lung homogenates at p8 and p14, and respiratory system resistance (Rrs) and compliance (Crs) as well as CD68 cell counts and radial alveolar counts were assessed at p8. Results: In LPS-exposed rats, IL-1β and CD68 cell counts both increased at p8 compared to normal saline controls. These increases in pro-inflammatory markers were no longer present in caffeine-treated LPS-exposed pups. Rrs was higher in LPS-exposed pups (4.7 ± 0.9 cm H2O/ml·s) at p8 versus controls (1.6 ± 0.3 cm H2O/ml·s, p < 0.01). LPS-exposed pups no longer exhibited a significant increase in Rrs (2.8 ± 0.5 cm H2O/ml·s) after caffeine. Crs did not differ significantly between groups, although radial alveolar counts were lower in both groups of LPS-exposed pups. Conclusions: Caffeine promotes anti-inflammatory effects in the immature lung of prenatal LPS-exposed rat pups associated with improvement of Rrs, suggesting a protective effect of caffeine on respiratory function via an anti-inflammatory mechanism.

The 'Effects of Transfusion Thresholds on Neurocognitive Outcome of Extremely Low Birth-Weight Infants (ETTNO)' Study: Background Aims, and Study Protocol

Background: Infants with extremely low birth weight uniformly develop anemia of prematurity and frequently require red blood cell transfusions (RBCTs). Although RBCT is widely practiced, the indications remain controversial in the absence of conclusive data on the long-term effects of RBCT. Objectives: To summarize the current equipoise and to outline the study protocol of the ‘Effects of Transfusion Thresholds on Neurocognitive Outcome of extremely low birth-weight infants (ETTNO)’ study. Methods: Review of the literature and design of a large pragmatic randomized controlled trial of restrictive versus liberal RBCT guidelines enrolling 920 infants with birth weights of 400–999 g with long-term neurodevelopmental follow-up. Results and Conclusions: The results of ETTNO will provide definite data about the efficacy and safety of restrictive versus liberal RBCT guidelines in very preterm infants.

Perinatal Oxygen in the Developing Lung

Lung diseases, such as bronchopulmonary dysplasia (BPD), wheezing, and asthma, remain significant causes of morbidity and mortality in the pediatric population, particularly in the setting of premature birth. Pulmonary outcomes in these infants are highly influenced by perinatal exposures including prenatal inflammation, postnatal intensive care unit interventions, and environmental agents. Here, there is strong evidence that perinatal supplemental oxygen administration has significant effects on pulmonary development and health. This is of particular importance in the preterm lung, where premature exposure to room air represents a hyperoxic insult that may cause harm to a lung primed to develop in a hypoxic environment. Preterm infants are also subject to increased episodes of hypoxia, which may also result in pulmonary damage and disease. Here, we summarize the current understanding of the effects of oxygen on the developing lung and how low vs. high oxygen may predispose to pulmonary disease that may extend even into adulthood. Better understanding of the underlying mechanisms will help lead to improved care and outcomes in this vulnerable population.

Cardiorespiratory events in preterm infants: interventions and consequences

Stabilization of respiration and oxygenation continues to be one of the main challenges in clinical care of the neonate. Despite aggressive respiratory support including mechanical ventilation, continuous positive airway pressure, oxygen and caffeine therapy to reduce apnea and accompanying intermittent hypoxemia, the incidence of intermittent hypoxemia events continues to increase during the first few months of life. Even with improvements in clinical care, standards for oxygen saturation targeting and modes of respiratory support have yet to be identified in this vulnerable infant cohort. In addition, we are only beginning to explore the association between the incidence and pattern of cardiorespiratory events during early postnatal life and both short- and long-term morbidity including retinopathy of prematurity, growth, sleep-disordered breathing and neurodevelopmental impairment. Part 1 of this review included a summary of lung development and diagnostic methods of cardiorespiratory monitoring. In Part 2 we focus on clinical interventions and the short- and long-term consequences of cardiorespiratory events in preterm infants.

Vulnerability of neonatal respiratory neural control to sustained hypoxia during a uniquely sensitive window of development

The first postnatal weeks represent a period of development in the rat during which the respiratory neural control system may be vulnerable to aberrant environmental stressors. In the present study, we investigated whether sustained hypoxia (SH; 11% O2) exposure starting at different postnatal ages differentially modifies the acute hypoxic (HVR) and hypercapnic ventilatory response (HCVR). Three different groups of rat pups were exposed to 5 days of SH, starting at either postnatal age 1 (SH1-5), 11 (SH11-15), or 21 (SH21-25) days. Whole body plethysmography was used to assess the HVR and HCVR the day after SH exposure ended. The primary results indicated that 1) the HVR and HCVR of SH11-15 rats were absent or attenuated (respectively) compared with age-matched rats raised in normoxia; 2) there was a profoundly high (∼84% of pups) incidence of unexplained mortality in the SH11-15 rats; and 3) these phenomena were unique to the SH11-15 group with no comparable effect of the SH exposure on the HVR, HCVR, or mortality in the younger (SH1-5) or older (SH21-25) rats. These results share several commonalities with the risk factors thought to underlie the etiology of sudden infant death syndrome, including 1) a vulnerable neonate; 2) a critical period of development; and 3) an environmental stressor.

Carotid chemoreceptor development and neonatal apnea.

The premature transition from fetal to neonatal life is accompanied by an immature respiratory neural control system. Most preterm infants exhibit recurrent apnea, resulting in repetitive oscillations in O(2) saturation (intermittent hypoxia, IH). Numerous factors are likely to play a role in the etiology of apnea including inputs from the carotid chemoreceptors. Despite major advances in our understanding of carotid chemoreceptor function in the early neonatal period, however, their contribution to the initiation of an apneic event and its eventual termination are still largely speculative. Recent findings have provided a detailed account of the postnatal changes in the incidence of hypoxemic events associated with apnea, and there is anecdotal evidence for a positive correlation with carotid chemoreceptor maturation. Furthermore, studies on non-human animal models have shown that chronic IH sensitizes the carotid chemoreceptors, which has been proposed to perpetuate the occurrence of apnea. An alternative hypothesis is that sensitization of the carotid chemoreceptors could represent an important protective mechanism to defend against severe hypoxemia. The purpose of this review, therefore, is to discuss how the carotid chemoreceptors may contribute to the initiation and termination of an apneic event in the neonate and the use of xanthine therapy in the prevention of apnea.

cAMP-mediated secretion of brain-derived neurotrophic factor in developing airway smooth muscle

Moderate hyperoxic exposure in preterm infants contributes to subsequent airway dysfunction and to risk of developing recurrent wheeze and asthma. The regulatory mechanisms that can contribute to hyperoxia-induced airway dysfunction are still under investigation. Recent studies in mice show that hyperoxia increases brain-derived neurotrophic factor (BDNF), a growth factor that increases airway smooth muscle (ASM) proliferation and contractility. We assessed the mechanisms underlying effects of moderate hyperoxia (50% O2) on BDNF expression and secretion in developing human ASM. Hyperoxia increased BDNF secretion, but did not alter endogenous BDNF mRNA or intracellular protein levels. Exposure to hyperoxia significantly increased [Ca2+]i responses to histamine, an effect blunted by the BDNF chelator TrkB-Fc. Hyperoxia also increased ASM cAMP levels, associated with reduced PDE4 activity, but did not alter protein kinase A (PKA) activity or adenylyl cyclase mRNA levels. However, 50% O2 increased expression of Epac2, which is activated by cAMP and can regulate protein secretion. Silencing RNA studies indicated that Epac2, but not Epac1, is important for hyperoxia-induced BDNF secretion, while PKA inhibition did not influence BDNF secretion. In turn, BDNF had autocrine effects of enhancing ASM cAMP levels, an effect inhibited by TrkB and BDNF siRNAs. Together, these novel studies suggest that hyperoxia can modulate BDNF secretion, via cAMP-mediated Epac2 activation in ASM, resulting in a positive feedback effect of BDNF-mediated elevation in cAMP levels. The potential functional role of this pathway is to sustain BDNF secretion following hyperoxic stimulus, leading to enhanced ASM contractility and proliferation.