Ventilation and oxygen: dose-related effects of oxygen on ventilation-induced lung injury

In utero ventilation induces lung parenchymal and vascular alterations in extremely preterm fetal sheep

Extremely preterm infants are often exposed to long durations of mechanical ventilation to facilitate gas exchange, resulting in ventilation-induced lung injury (VILI). New lung protective strategies utilizing noninvasive ventilation or low tidal volumes are now common but have not reduced rates of bronchopulmonary dysplasia. We aimed to determine the effect of 24 h of low tidal volume ventilation on the immature lung by ventilating preterm fetal sheep in utero. Preterm fetal sheep at 110 ± 1(SD) days' gestation underwent sterile surgery for instrumentation with a tracheal loop to enable in utero mechanical ventilation (IUV). At 112 ± 1 days' gestation, fetuses received either in utero mechanical ventilation (IUV, n = 10) targeting 3-5 mL/kg for 24 h, or no ventilation (CONT, n = 9). At necropsy, fetal lungs were collected to assess molecular and histological markers of lung inflammation and injury. IUV significantly increased lung mRNA expression of interleukin (IL)-1β, IL-6, IL-8, IL-10, and tumor necrosis factor (TNF) compared with CONT, and increased surfactant protein (SP)-A1, SP-B, and SP-C mRNA expression compared with CONT. IUV produced modest structural changes to the airways, including reduced parenchymal collagen and myofibroblast density. IUV increased pulmonary arteriole thickness compared with CONT but did not alter overall elastin or collagen content within the vasculature. In utero ventilation of an extremely preterm lung, even at low tidal volumes, induces lung inflammation and injury to the airways and vasculature. In utero ventilation may be an important model to isolate the confounding mechanisms of VILI to develop effective therapies for preterm infants requiring prolonged respiratory support.NEW & NOTEWORTHY Preterm infants often require prolonged respiratory support, but the relative contribution of ventilation to the development of lung injury is difficult to isolate. In utero mechanical ventilation allows for mechanistic investigations into ventilation-induced lung injury without confounding factors associated with sustaining extremely preterm lambs ex utero. Twenty-four hours of in utero ventilation, even at low tidal volumes, increased lung inflammation and surfactant protein expression and produced structural changes to the lung parenchyma and vasculature.

Moving Bronchopulmonary Dysplasia Research from the Bedside to the Bench

Although advances in the respiratory management of extremely preterm infants have led to improvements in survival, this progress has not yet extended to a reduction in the incidence of bronchopulmonary dysplasia (BPD). BPD is a complex multifactorial condition that primarily occurs due to disturbances in the regulation of normal pulmonary airspace and vascular development. Preterm birth and exposure to invasive mechanical ventilation also compromises large airway development, leading to significant morbidity and mortality. Although both predisposing and protective genetic and environmental factors have been frequently described in the clinical literature, these findings have had limited impact on the development of effective therapeutic strategies. This gap is likely because the molecular pathways that underlie these observations are yet not fully understood, limiting the ability of researchers to identify novel treatments that can preserve normal lung development and/or enhance cellular repair mechanisms. In this review article, we will outline various well-established clinical observations whilst identifying key knowledge gaps that need to be filled with carefully designed pre-clinical experiments. We will address these issues by discussing controversial topics in the pathophysiology, the pathology and the treatment of BPD, including an evaluation of existing animal models that have been used to answer important questions.

Ventilation-induced lung injury is not exacerbated by growth restriction in preterm lambs

Intrauterine growth restriction (IUGR) and preterm birth are frequent comorbidities and, combined, increase the risk of adverse respiratory outcomes compared with that in appropriately grown (AG) infants. Potential underlying reasons for this increased respiratory morbidity in IUGR infants compared with AG infants include altered fetal lung development, fetal lung inflammation, increased respiratory requirements, and/or increased ventilation-induced lung injury. IUGR was surgically induced in preterm fetal sheep (0.7 gestation) by ligation of a single umbilical artery. Four weeks later, preterm lambs were euthanized at delivery or delivered and ventilated for 2 h before euthanasia. Ventilator requirements, lung inflammation, early markers of lung injury, and morphological changes in lung parenchymal and vascular structure and surfactant composition were analyzed. IUGR preterm lambs weighed 30% less than AG preterm lambs, with increased brain-to-body weight ratio, indicating brain sparing. IUGR did not induce lung inflammation or injury or alter lung parenchymal and vascular structure compared with AG fetuses. IUGR and AG lambs had similar oxygenation and respiratory requirements after birth and had significant, but similar, increases in proinflammatory cytokine expression, lung injury markers, gene expression, and surfactant phosphatidylcholine species compared with unventilated controls. IUGR does not induce pulmonary structural changes in our model. Furthermore, IUGR and AG preterm lambs have similar ventilator requirements in the immediate postnatal period. This study suggests that increased morbidity and mortality in IUGR infants is not due to altered lung tissue or vascular structure, or to an altered response to early ventilation.

Transient vascular and long-term alveolar deficits following a hyperoxic injury to neonatal mouse lung

Background

The lungs of very preterm human babies display deficits in alveolarization and vascularization as a result of the clinical use of high oxygen treatment (leading to hyperoxia) required to decrease the risk of mortality. Detailed analyses of the persistence of the respiratory deficits following this treatment and means to restore a normal state have not been investigated in full detail. In this study, high oxygen administration to neonatal mouse lungs was established as a proxy to hyperoxia in human preterm infant lungs, to better characterize the associated deficits and thus provide a means to assist in the development of treatments in the future.

Methods

Ninety percent oxygen was administered to newborn mice for four consecutive days. The effects of this treatment upon alveolarization and vascularization were investigated by morphometric, histochemical, immunohistochemical and protein analyses at day five (D5), D28 and D56 postpartum.

Results

Relative to control untreated lungs, septation of hyperoxic lungs was significantly reduced and airspaces were significantly enlarged at all stages examined. Furthermore, compared to controls, the number of secondary septa per tissue area was significantly reduced at D5, significantly increased at D28 and then the same as controls at D56. Analysis of vascularization parameters indicated a reduction in mature blood vessel number and the amount of Pecam1 at D5. Both of these parameters returned to control levels by D28.

Conclusions

This study suggests that administration of high oxygen to underdeveloped lungs has a transient reductive effect on secondary septal number and pulmonary vascularization and a significant long-term reduction in alveolarization persisting into adulthood. This model can be used for future research of premature lung disease therapies in humans, addressing these short term septal and vascular and long term alveolar deficits, specifically relating to injury by hyperoxia.

Hyperoxia induces alveolar epithelial-to-mesenchymal cell transition

Myofibroblast accumulation is a pathological feature of lung diseases requiring oxygen therapy. One possible source for myofibroblasts is through the epithelial-to-mesenchymal transition (EMT) of alveolar epithelial cells (AEC). To study the effects of oxygen on alveolar EMT, we used RLE-6TN and ex vivo lung slices and found that hyperoxia (85% O2, H85) decreased epithelial proteins, presurfactant protein B (pre-SpB), pro-SpC, and lamellar protein by 50% and increased myofibroblast proteins, α-smooth muscle actin (α-SMA), and vimentin by over 200% (P < 0.05). In AEC freshly isolated from H85-treated rats, mRNA for pre-SpB and pro-SpC was diminished by ∼50% and α-SMA was increased by 100% (P < 0.05). Additionally, H85 increased H2O2 content, and H2O2 (25-50 μM) activated endogenous transforming growth factor-β1 (TGF-β1), as evident by H2DCFDA immunofluorescence and ELISA (P < 0.05). Both hyperoxia and H2O2 increased SMAD3 phosphorylation (260% of control, P < 0.05). Treating cultured cells with TGF-β1 inhibitors did not prevent H85-induced H2O2 production but did prevent H85-mediated α-SMA increases and E-cadherin downregulation. Finally, to determine the role of TGF-β1 in hyperoxia-induced EMT in vivo, we evaluated AEC from H85-treated rats and found that vimentin increased ∼10-fold (P < 0.05) and that this effect was prevented by intraperitoneal TGF-β1 inhibitor SB-431542. Additionally, SB-431542 treatment attenuated changes in alveolar histology caused by hyperoxia. Our studies indicate that hyperoxia promotes alveolar EMT through a mechanism that is dependent on activation of TGF-β1 signaling.

Mechanical ventilation-induced apoptosis in newborn rat lung is mediated via FasL/Fas pathway

Mechanical ventilation induces pulmonary apoptosis and inhibits alveolar development in preterm infants, but the molecular basis for the apoptotic injury is unknown. The objective was to determine the signaling mechanism(s) of ventilation (stretch)-induced apoptosis in newborn rat lung. Seven-day-old rats were ventilated with room air for 24 h using moderate tidal volumes (8.5 ml/kg). Isolated fetal rat lung epithelial and fibroblast cells were subjected to continuous cyclic stretch (5, 10, or 17% elongation) for up to 12 h. Prolonged ventilation significantly increased the number of apoptotic alveolar type II cells (i.e., terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling and anti-cleaved caspase-3 immunochemistry) and was associated with increased expression of the apoptotic mediator Fas ligand (FasL). Fetal lung epithelial cells, but not fibroblasts, subjected to maximal (i.e., 17%, but not lesser elongation) cyclic stretch exhibited increased apoptosis (i.e., nuclear fragmentation and DNA laddering), which appeared to be mediated via the extrinsic pathway (increased expression of FasL and cleaved caspase-3, -7, and -8). The intrinsic pathway appeared not to be involved [minimal mitochondrial membrane depolarization (JC-1 flow analysis) and no activation of caspase-9]. Universal caspases inhibition and neutralization of FasL abrogated the stretch-induced apoptosis. Prolonged mechanical ventilation induces apoptosis of alveolar type II cells in newborn rats and the mechanism appears to involve activation of the extrinsic death pathway via the FasL/Fas system.

Mechanical ventilation injury and repair in extremely and very preterm lungs

Background

Extremely preterm infants often receive mechanical ventilation (MV), which can contribute to bronchopulmonary dysplasia (BPD). However, the effects of MV alone on the extremely preterm lung and the lung’s capacity for repair are poorly understood.

Aim

To characterise lung injury induced by MV alone, and mechanisms of injury and repair, in extremely preterm lungs and to compare them with very preterm lungs.

Methods

Extremely preterm lambs (0.75 of term) were transiently exposed by hysterotomy and underwent 2 h of injurious MV. Lungs were collected 24 h and at 15 d after MV. Immunohistochemistry and morphometry were used to characterise injury and repair processes. qRT-PCR was performed on extremely and very preterm (0.85 of term) lungs 24 h after MV to assess molecular injury and repair responses.

Results

24 h after MV at 0.75 of term, lung parenchyma and bronchioles were severely injured; tissue space and myofibroblast density were increased, collagen and elastin fibres were deformed and secondary crest density was reduced. Bronchioles contained debris and their epithelium was injured and thickened. 24 h after MV at 0.75 and 0.85 of term, mRNA expression of potential mediators of lung repair were significantly increased. By 15 days after MV, most lung injury had resolved without treatment.

Conclusions

Extremely immature lungs, particularly bronchioles, are severely injured by 2 h of MV. In the absence of continued ventilation these injured lungs are capable of repair. At 24 h after MV, genes associated with injurious MV are unaltered, while potential repair genes are activated in both extremely and very preterm lungs.

The effects of intrauterine growth restriction and antenatal glucocorticoids on ovine fetal lung development

INTRODUCTION

Intrauterine growth restriction (IUGR) is associated with high rates of neonatal morbidity. IUGR babies are often born preterm and are, therefore, exposed to antenatal glucocorticoids. Antenatal glucocorticoids significantly improve overall survival rates of preterm infants, but there is a paucity of information about their effects on IUGR Infants.

METHODS

We induced IUGR in sheep by single umbilical artery ligation (SUAL), or sham in control fetuses. To half the ewes, we administered betamethasone (BM) on d 5 (BM1) and 6 (BM2) following surgery, and collected fetal lung tissue on d 7.

RESULTS

SUAL alone was associated with higher circulating fetal cortisol levels (2.8 ± 0.4 vs. 1.0 ± 0.4, P = 0.001) as compared with controls but not with changes in lung morphology or surfactant protein (SP) gene expression. BM was associated with a significant reduction in lung tissue density (P = 0.048). There were no significant differences between groups in lung DNA concentration or septal crest density. SP-A, SP-B, and SP-C gene expressions were significantly increased in control and SUAL fetuses that were administered BM.

DISCUSSION

These results show that in SUAL fetuses, maternal BM is associated with acceleration of fetal lung structure, as occurs in normally grown fetuses, and that BM induces SP production, an effect not observed in SUAL-induced IUGR fetuses alone.

Injury and repair in the very immature lung following brief mechanical ventilation

Mechanical ventilation (MV) of very premature infants contributes to lung injury and bronchopulmonary dysplasia (BPD), the effects of which can be long-lasting. Little is currently known about the ability of the very immature lung to recover from ventilator-induced lung injury. Our objective was to determine the ability of the injured very immature lung to repair in the absence of continued ventilation and to identify potential mechanisms. At 125 days gestational age (days GA, 0.85 of term), fetal sheep were partially exposed by hysterotomy under anesthesia and aseptic conditions; they were intubated and ventilated for 2 h with an injurious MV protocol and then returned to the uterus to continue development. Necropsy was performed at either 1 day (short-term group, 126 days GA, n = 6) or 15 days (long-term group, 140 days GA, n = 5) after MV; controls were unventilated (n = 7-8). At 1 day after MV, lungs displayed signs of injury, including hemorrhage, disorganized elastin and collagen deposition in the distal airspaces, altered morphology, significantly reduced secondary septal crest density, and decreased airspace. Bronchioles had thickened epithelium with evidence of injury and sloughing. Relative mRNA levels of early response genes (connective tissue growth factor, cysteine-rich 61, and early growth response-1) and proinflammatory cytokines [interleukins (IL)-1β, IL-6, IL-8, tumor necrosis factor-α, and transforming growth factor-β] were not different between groups 1 day after MV. At 15 days after MV, lung structure was normal with no evidence of injury. We conclude that 2 h of MV induces severe injury in the very immature lung and that these lungs have the capacity to repair spontaneously in the absence of further ventilation.

Tailoring oxygen needs of extremely low birth weight infants in the delivery room

Fetal to neonatal transition poses an extraordinary challenge for the extremely low birth weight (ELBW) neonate. Indeed a significant number of ELBW neonates will need proactive resuscitation to achieve postnatal stabilization. Positive pressure ventilation and oxygenation are the most relevant interventions in the delivery room (DR). Oxygen needs during resuscitation still represent a conundrum for neonatologists. While hyperoxemia favors oxidative stress and subsequent organ injury, hypoxemia is associated with long-term neurodevelopmental impairment. It has been shown that ELBW neonates can be successfully resuscitated with lower concentrations of oxygen as had been done traditionally. Moreover, reducing oxygen load has resulted in achievement of arterial partial pressures of oxygen at admission closer to the physiologic range, less oxidative stress and less inflammation. The availability of reference ranges for arterial oxygen saturation (SpO(2)) for ELBW neonates in the first 10 min after birth has been an extraordinary step forward in our ability to individually titrate oxygen needs thus avoiding the risks of both hypo- and hyperoxemia. The optimal fraction of inspired oxygen (FiO(2)) to initiate resuscitation and the safest SpO(2) percentiles for ELBW neonates during the first minutes of life are still unknown and will need further research in the future. Until then, optimal ventilation at birth and individually tailoring FiO(2) according to the nomogram seem to be the most reasonable and safe approach.