Effects of ventilation with different positive end-expiratory pressures on cytokine expression in the preterm lamb lung

High Tidal Volume Ventilation Causes Different Inflammatory Responses in Newborn versus Adult Lung

We investigated the effect of high VT ventilation on adult and newborn rats by examining pulmonary injury and cytokine messenger RNA (mRNA). On the basis of compliance, edema formation, and histology, ventilation with 25 ml.kg(-1) was more injurious to adult rats than newborns. Ventilation with 40 ml kg(-1) minimally affected compliance in newborns but caused death in adults. Ventilation of adults for 30 minutes at 25 ml kg(-1) upregulated the mRNA expression of interleukin (IL)-1beta, IL-6, tumor necrosis factor-alpha (TNF-alpha), macrophage inflammatory protein-2 (MIP-2), and IL-10, whereas in newborns such ventilation only increased mRNA expression of MIP-2 and IL-10. When VT was raised to 40 ml kg(-1) in newborns, IL-1beta mRNA levels were additionally increased at 30 minutes, whereas ventilation for 3 hours additionally increased IL-6 and TNF-alpha mRNA. In newborns, the addition of 100% oxygen (O2) to 30 minutes of ventilation blunted the high VT induction of IL-1beta, IL-10, and MIP-2 mRNA expressions, whereas at 3 hours, 100% O2 concentration synergistically increased the mRNAs for TNF-alpha and IL-6. Overall, adult rats are more susceptible to high VT-induced lung injury compared with newborns. In newborns, the inflammatory response is dependent on VT, duration, and supplemental O2. Thus, recommendations for VT limitation based on adult data may be inappropriate for newborns.

Standardized lung recruitment during high frequency and conventional ventilation: similar pathophysiologic and inflammatory responses in an animal model of respiratory distress syndrome

OBJECTIVE

To evaluate standardized lung recruitment strategy during both high frequency oscillation (HFO) and volume-targeted conventional ventilation (CV+V) in spontaneously breathing piglets with surfactant washout on pathophysiologic and inflammatory responses.

DESIGN

Prospective animal study.

SETTING

Research laboratory.

SUBJECTS

Twenty-four newborn piglets.

INTERVENTIONS

We compared pressure support and synchronized intermittent mandatory ventilation, both with targeted tidal volumes, (PSV+V, SIMV+V) to HFO. Animals underwent saline lavage to produce lung injury, received artificial surfactant and were randomized to one of the three treatment groups (each n=8). After injury and surfactant replacement, lung volumes were recruited in all groups using a standard protocol. Ventilation continued for 6 h.

MEASUREMENTS AND MAIN RESULTS

Arterial and central venous pressures, heart rates, blood pressure and arterial blood gases were continuously monitored. At baseline, post lung injury and 6 h we collected serum and bronchoalveolar lavage samples for proinflammatory cytokines: IL 6, IL 8 and TNF-alpha, and performed static pressure-volume (P/V) curves. Lungs were fixed for morphometrics and histopathologic analysis. No physiologic differences were found. Analysis of P/V curves showed higher opening pressures after lung injury in the HFO group compared to the SIMV+V group ( p<0.05); no differences persisted after treatment. We saw no differences in change in proinflammatory cytokine levels. Histopathology and morphometrics were similar. Mean airway pressure (P(aw)) was highest in the HFO group compared to SIMV+V ( p<0.002).

CONCLUSIONS

Using a standardized lung recruitment strategy in spontaneously breathing animals, CV+V produced equivalent pathophysiologic outcomes without an increase in proinflammatory cytokines when compared to HFO.

Initial responses to ventilation of premature lambs exposed to intra-amniotic endotoxin 4 days before delivery

Preterm delivery is frequently preceded by chorioamnionitis, resulting in exposure of the fetal lung to inflammation. We hypothesized that ventilation of the antenatally inflamed lung would result in amplification of the lung injury. Therefore, we induced fetal lung inflammation with intra-amniotic endotoxin (10 mg of Escherichia coli 055:B5) 4 days before premature delivery at 130 days of gestation. Lung function and lung inflammation after surfactant treatment and 4 h of mechanical ventilation were evaluated. Inflammatory cell numbers in amniotic fluid were increased >10-fold by antenatal endotoxin exposure. Antenatal endotoxin exposure had minimal effects on blood pressure, heart rate, lung compliance, and blood gas values. The endotoxin-exposed lungs required higher ventilation pressures. Ventilation did not increase the number of inflammatory cells or the protein in bronchoalveolar lavage fluid of the endotoxin-exposed animals above that measured in endotoxin-exposed fetuses that were not ventilated. IL-1β, IL-6, and IL-8 mRNA in cells from bronchoalveolar lavage fluid were increased by antenatal endotoxin exposure but not changed by ventilation. IL-1β and IL-8 protein was increased in lung tissue by 4 h of ventilation. Very little inflammation was induced by ventilation in this premature lamb model of surfactant treatment and gentle ventilation. After lung inflammation was induced by intra-amniotic endotoxin injection, ventilation did not increase lung injury.

Alveolar instability causes early ventilator-induced lung injury independent of neutrophils

Intratracheal instillation of Tween causes a heterogeneous surfactant deactivation in the lung, with areas of unstable alveoli directly adjacent to normal stable alveoli. We employed in vivo video microscopy to directly assess alveolar stability in normal and surfactant-deactivated lung and tested our hypothesis that alveolar instability causes a mechanical injury, initiating an inflammatory response that results in a secondary neutrophil-mediated proteolytic injury. Pigs were mechanically ventilated (VT 10 cc/kg, positive end-expiratory pressure [PEEP] 3 cm H2O), randomized to into three groups, and followed for 4 hours: Control group (n = 3) surgery only; Tween group (n = 4) subjected to intratracheal Tween (surfactant deactivator causing alveolar instability); and Tween + PEEP group (n = 4) subjected to Tween with increased PEEP (15 cm H2O) to stabilize alveoli. The magnitude of alveolar instability was quantified by computer image analysis. Surfactant-deactivated lungs developed significant histopathology only in lung areas with unstable alveoli without an increase in neutrophil-derived proteases. PEEP stabilized alveoli and significantly reduced histologic evidence of lung injury. Thus, in this model, alveolar instability can independently cause ventilator-induced lung injury. To our knowledge, this is the first study to directly confirm that unstable alveoli are subjected to ventilator-induced lung injury whereas stable alveoli are not.

Transfer of the active form of transforming growth factor-beta 1 gene to newborn rat lung induces changes consistent with bronchopulmonary dysplasia

Bronchopulmonary dysplasia is a chronic lung disease of premature human infancy that shows pathological features comprising varying sized areas of interstitial fibrosis in association with distorted large alveolar spaces. We have previously shown that transfer of active transforming growth factor (TGF)-beta 1 (AdTGF beta 1(223/225)) genes by adenovirus vector to embryonic lungs results in inhibition of branching morphogenesis and primitive peripheral lung development, whereas transfer to adult lungs results in progressive interstitial fibrosis. Herein we show that transfer of TGF-beta1 to newborn rat pups results in patchy areas of interstitial fibrosis developing throughout a period of 28 days after transfer. These areas of fibrosis appear alongside areas of enlarged alveolar spaces similar to the prealveoli seen at birth, suggesting that postnatal lung development and alveolarization has been inhibited. In rats treated with AdTGF beta 1(223/225), enlarged alveolar spaces were evident by day 21, and by 28 days, the mean alveolar cord length was nearly twice that in control vector or untreated rats. Hydroxyproline measurements confirmed the presence of fibrosis. These data suggest that overexpression of TGF-beta 1 during the critical period of postnatal rat lung alveolarization gives rise to pathological, biochemical, and morphological changes consistent with those seen in human bronchopulmonary dysplasia, thus inferring a pathogenic role for TGF-beta in this disorder.

Effects of protective and conventional mechanical ventilation on pulmonary function and systemic cytokine release after cardiopulmonary bypass

OBJECTIVE

To evaluate the effects of protective and conventional ventilation with or without positive end-expiratory pressure (PEEP), on systemic tumor necrosis factor-alpha, interleukin-6 levels and pulmonary function during open heart surgery.

DESIGN

Prospective, randomized clinical study.

SETTING

Single university hospital.

PATIENTS AND PARTICIPANTS

Forty-four patients undergoing elective coronary artery bypass grafting surgery with cardiopulmonary bypass.

INTERVENTIONS

Patients ventilated with (1) protective tidal volumes (6 ml/kg, respiratory rate: 15 breaths/min, PEEP 5 cmH(2)O, n=15) group PV; (2) conventional tidal volumes (10 ml/kg, respiratory rate: 9 breaths/min, PEEP 5 cmH(2)O, n=14) group CV+PEEP and (3) conventional tidal volumes (10 ml/kg, respiratory rate: 9 breaths/min, n=15) without PEEP, group CV+ZEEP. Various pulmonary parameters, systemic TNF-alpha and IL-6 levels were determined throughout the study.

MEASUREMENTS AND RESULTS

There were no differences among the groups regarding the systemic TNF- alpha and IL-6 levels. The plateau airway pressures of group PV were lower than those of groups CV+PEEP ( p=0.02) and CV+ZEEP ( p=0.001) after cardiopulmonary bypass. The shunt fraction of group PV was significantly lower than that of group CV+ZEEP 24 h after surgery ( p<0.05). Oxygenation and the alveolar-arterial oxygen difference were better in both PEEP groups than in group CV+ZEEP 24 h after the operation.

CONCLUSIONS

We could not find any evidence that protective mechanical ventilation prevents some of the adverse effects of cardiopulmonary bypass on the lung, nor systemic cytokine levels, postoperative pulmonary function or length of hospitalization.

Na,K-ATPase gene transfer increases liquid clearance during ventilation-induced lung injury

Mechanical ventilation with high tidal volumes (HVT) downregulates alveolar Na,K-ATPase function and impairs lung liquid clearance. We hypothesized that overexpression of Na,K-ATPase in the alveolar epithelium could counterbalance these changes and increase clearance in a rat model of mild ventilation-induced lung injury. We used a surfactant-based system to deliver 4 x 10(9) plaque-forming units of E1a-/E3- recombinant adenovirus containing either a rat beta1 Na,K-ATPase subunit cDNA (adbeta1) or no cDNA (adnull) to rat lungs 7 days before ventilation with a VT of approximately 40 ml/kg (peak airway pressure of less than 35 cm H2O) for 40 minutes. Lung liquid clearance and Na, K-ATPase activity and protein abundance were increased in HVT adbeta1-infected lungs as compared with sham and adnull-infected HVT lungs. These results suggest that Na,K-ATPase subunit gene overexpression in the alveolar epithelium increases Na,K-ATPase function and lung liquid clearance in a model of HVT. We provide here the first evidence that using a genetic approach improves active Na+ transport and thus liquid clearance in the setting of mild ventilation-induced lung injury.

Effects of high PCO2 on ventilated preterm lamb lungs

High PCO(2) levels attenuate reperfusion injury and ventilation-induced injury in isolated and perfused lungs. We asked whether premature lambs could tolerate 6 h of ventilation with a PCO(2) >80 mm Hg and whether the high PCO(2) modulated the ventilator-induced injury. Preterm surfactant-treated lambs were ventilated for 30 min with a high tidal volume (V(T)) to induce lung injury. The lambs then were ventilated for 5.5 h with a V(T) of 6-9 mL/kg to achieve a PCO(2) of 40-50 mm Hg in the control group. CO(2) was added to the ventilator circuit of a high PCO(2) group to maintain an average PCO(2) of 95 +/- 5 mm Hg. The high PCO(2) lambs had heart rates, blood pressures, plasma cortisol values, and oxygenation equivalent to the control lambs. The lungs of the high PCO(2) group had significantly higher gas volumes and had less lung injury by histopathology. Indicators of inflammation (white blood cells, hydrogen peroxide production, and IL-1beta and IL-8 cytokine mRNA expression in cells from the alveolar wash) qualitatively indicated less injury in the high PCO(2) group, although the differences were not significant. Preterm lambs tolerated a very high PCO(2) without physiologic compromise for 6 h. The high PCO(2) may attenuate ventilator-induced lung injury in the preterm.

Effect of surfactant on ventilation-induced mediator release in isolated perfused mouse lungs

The human acute respiratory distress syndrome (ARDS) is a severe pulmonary complication with high mortality rates. To support their vital functions, patients suffering from ARDS are mechanically ventilated. Recently it was shown that low tidal volume ventilation reduces mortality and pro-inflammatory mediator release in these patients, suggesting biotrauma as a side effect of mechanical ventilation. Because the application of exogenous surfactant has been proposed as a treatment for ARDS, we investigated the effect of surfactant on ventilation-induced release of tumor necrosis factor (TNF), interleukin-6 (IL-6) and 6-keto-PGF(1 alpha) (the stable metabolite of prostacyclin) in isolated perfused mouse lungs ventilated with high end-inspiratory pressures. Instillation of 100mg/kg surfactant into the lungs was well tolerated and improved tidal volume, pulmonary compliance and alveolar expansion. Exogenous surfactant increased the ventilation-induced liberation of TNF and IL-6 into the perfusate, but had no effect on the release of 6-keto-PGF(1 alpha). The surfactant preparation used reduced baseline TNF production by murine alveolar macrophages, indicating that the exaggeration of ventilation-induced TNF release cannot be explained by a direct effect of surfactant on these cells. We hypothesize that ventilation-induced mediator release is explained by stretching of lung cells, which is reinforced by surfactant. The findings that in this model of ventilation-induced lung injury exogenous surfactant at the same time improved lung functions and enhanced mediator release suggest that surfactant treatment may prevent barotrauma and augment biotrauma.

Bronchopulmonary dysplasia-oxidative stress and antioxidants

There is increasing evidence that oxidative stress is implicated in the development of bronchopulmonary dysplasia. Several important factors contribute to augmented oxidative stress in the newborn and especially the preterm infant: first, because of its immaturity, the lung of preterm infants is frequently exposed to oxygen therapy and hyperoxia. Second, the antioxidant defense and its ability to be induced during an hyperoxic challenge are impaired. Third, the preterm infant has an increased susceptibility to infection and inflammation, which increases oxidative stress. Fourth, free iron, which catalyzes the production of toxic reactive oxygen species, can be detected in preterm infants. The molecular and cellular mechanisms for free radical-induced injury are now understood in more detail, and it is clear that oxidative stress plays an important role in triggering apoptosis, in serving as second messenger and in signal transduction. This new insight might lead to novel and efficient therapies. So far, there has been no significant breakthrough regarding antioxidant therapies. Care should, however, be exercised in supplementing the preterm infant with antioxidants since this may affect growth and development.

Decreased indicators of lung injury with continuous positive expiratory pressure in preterm lambs

Continuous positive airway pressure (CPAP) is being used clinically to avoid mechanical ventilation of preterm infants as a strategy to minimize lung injury. There is little experimental information about how CPAP might minimize lung injury after preterm birth. We induced preterm labor in antenatal glucocorticoid-treated sheep carrying twins at 133 d gestation with an inhibitor of progesterone synthesis. The lambs delivered spontaneously approximately 2 d later and were randomized to three groups: no ventilation (n = 4), conventional mechanical ventilation to a target PCO(2) of 40 mm Hg (n = 5), or CPAP using a bubble CPAP device set to deliver 5 cm H(2)O pressure (n = 6). The CPAP lambs breathed without distress and maintained PCO(2) values of approximately 60 mm Hg. At 2 h of age, the lungs of the CPAP lambs held 74 +/- 4 mL/kg air at 40 cm H(2)O pressure, which was more than the 60 +/- 3 mL/kg for the ventilated lambs (p < 0.05). Lymphocyte and monocyte numbers in alveolar washes were equivalent in the unventilated, ventilated, and CPAP lambs. However, no neutrophils were found in the unventilated lambs, and the ventilated lambs had 6.6 times more neutrophils in alveolar washes than did the CPAP lambs (p < 0.05). The cells in alveolar wash from CPAP lambs contained less hydrogen peroxide than did the cells from ventilated lambs (p < 0.05). In this model in preterm lambs CPAP results in lower indicators of acute lung injury than does mechanical ventilation during the first 2 h of life.

Postnatal lung inflammation increased by ventilation of preterm lambs exposed antenatally to Escherichia coli endotoxin

Chorioamnionitis resulting in exposure of the fetal lung to inflammation is frequent before preterm delivery. The initiation of mechanical ventilation in the preterm recruits granulocytes to the lungs and increases proinflammatory cytokine expression in the lungs. We hypothesized that when the prematurely born newborn with chorioamnionitis was ventilated, inflammation would increase. Therefore, we asked whether inflammatory exposure to the fetal lung caused by intra-amniotic endotoxin (10 mg, Escherichia coli 055:beta 5) given at 100 d gestation would alter the inflammatory responses to the mechanical ventilation in surfactant-treated preterm lambs delivered at 130 d gestation. Cells in alveolar washes, proinflammatory cytokine expression, and surfactant protein mRNA expression were not different for saline and endotoxin exposed lambs that were not ventilated. The endotoxin- and saline-exposed control animals had similar lung function for 6 h of ventilation. Bronchoalveolar lavage fluid from the ventilated and antenatal endotoxin-exposed animals contained 5.7 times more monocytes, 12 times more lymphocytes, and a nonsignificant increase in neutrophils. Cells from the bronchoalveolar lavage fluid expressed 3-fold more IL-6 and IL-8 mRNA than did cells from the saline exposed comparison animals. An antenatal exposure of the fetal lung to endotoxin enhanced the subsequent inflammatory response of the ventilated preterm lung.

Conventional mechanical ventilation of healthy lungs induced pro-inflammatory cytokine gene transcription

We investigated the potential inflammatory reaction induced by mechanical ventilation (MV) using 10 ml/kg tidal volume and no positive end-expiratory pressure (PEEP) in control (C, n = 8), spontaneously breathing (SB, n = 12) and mechanically ventilated (MV, n = 12) rabbits with normal lungs. After 6 h (MV and SB groups) or immediately (C group), lungs were removed for measurement of wet-to-dry (W/D) weight ratio and for bronchoalveolar lavage (BAL). Pulmonary mechanics were also studied. MV animals developed a modest but significant (P < 0.01) impairment of arterial blood oxygenation and had higher W/D lung weight ratio than C ones. In MV group, BAL macrophage count was greater (P < 0.05) than in SB one. MV induced an upregulation of MCP-1, TNF-alpha, and IL-1beta gene transcription (mRNAs), without significant elevation of the corresponding protein cytokines in the BAL supernatant, except for MCP-1 (P < 0.05). These data suggest that MV, even using moderate tidal volume, elicits a pro-inflammatory stimulus to the lungs.

Injury responses to different surfactants in ventilated premature lamb lungs

The lung of the preterm infant is easily injured and an initial indication of the injury is an inflammatory response. Surfactant treatment and gentle ventilation will minimize the initiation and progression of injury. We asked if the initial lung injury response differed when preterm ventilated lambs were treated with complete natural sheep surfactant, a lipid extract of sheep surfactant, a surfactant used to treat RDS (Survanta), or a synthetic surfactant containing recombinant SP-C (Venticute). We used a gentle style of ventilation and a positive end expiratory pressure of 4 cmH(2)0 to minimize injury. The surfactants were not distinguishable based on gas exchange, compliance or lung gas volumes over the 6h ventilation period. When compared with unventilated controls the ventilated lambs had increased protein and inflammatory cells in alveolar lavages. The cells from the alveolar lavages produced more H(2)0(2), expressed more surface adhesion antigens and CD-14 receptors, and expressed more mRNA for the pro-inflammatory cytokines IL-1 beta and IL-8 than did cells from unventilated lungs. Lung tissue expressed primarily increased IL-6 mRNA relative to unventilated controls. However, there were no consistent differences in any of the inflammatory indicators between the different surfactant treated groups. Because endotoxin free natural surfactant containing SP-A was not superior to three other surfactants containing differing amounts of the surfactant proteins, additions of these proteins to clinical surfactants may not decrease the indicators of lung inflammation that accompany the initiation of ventilation of the preterm lung.

Neonatal respiratory failure

The classic entity of neonatal distress syndrome, as a lung disease expressing predominant surfactant deficiency, is currently changing to a more complex disease of the developing lung as a result of the number of extremely immature preterm infants. Prenatal factors, such as the fetal inflammatory response syndrome influence short- and long-term outcome in these premature infants presenting with respiratory distress syndrome at birth. Therefore, various previously dismissed treatment strategies, such as surfactant prophylaxis or newer anti-inflammatory approaches have to be reinvestigated in this emerging population. Despite the resurrection of a new picture of bronchopulmonary dysplasia, lung injury induced by mechanical ventilation remains a major issue in neonatal intensive care. With the advance in understanding of mechanical ventilation, it is becoming evident, that improvement in outcome can not be achieved by restoring normal lung physiology in the diseased lung using sophisticated ventilators and ventilation modes. A more disease specific ventilator strategy that will target as early as possible homogenous lung opening while at the same time avoiding overdistention of the lung, has the potential to affect outcome. The possible antiinflammatory properties of surfactant-proteins, nitric oxide and corticosteroids, despite some drawbacks, may show to have a synergistic effect. However, this needs further exploration.

Intratracheal endotoxin causes systemic inflammation in ventilated preterm lambs

Intratracheal endotoxin causes acute inflammation in the adult lung, and injurious styles of mechanical ventilation can result in systemic inflammation derived from the lungs. We asked how ventilated premature and near-term lungs responded to intratracheal endotoxin and if systemic inflammation occurred. Lambs delivered at 130 d gestational age (GA) were treated with surfactant or surfactant plus endotoxin (0.1 mg/kg or 10 mg/kg) (Escherichia coli, serotype O55:B5) and were ventilated for 6 h. Both endotoxin doses resulted in impaired gas exchange and systemic inflammation in the preterm lambs. Lambs at 141 d GA (term 146 d) were given either 10 mg/kg intratracheal endotoxin, 10 mg/kg endotoxin plus high tidal volume ventilation for the first 30 min of life, or 5 microg/kg endotoxin given intravenously. Endotoxin alone (10 mg/kg) caused lung inflammation but no systemic effects after 6 h of ventilation. Lambs given 10 mg/kg endotoxin plus high tidal volume ventilation or 5 microg/kg endotoxin intravenously had decreased gas exchange and systemic inflammation. Endotoxin was detected in the plasma of lambs at 130 d GA but not at 141 d GA. Inflammation in the lungs was more severe in preterm animals. Mechanical ventilation of the endotoxin-exposed preterm lung resulted in systemic effects at a low endotoxin dose and without high tidal volume ventilation.

Deficiency of SP-B reveals protective role of SP-C during oxygen lung injury

Although the surface properties of surfactant protein (SP)-B and SP-C are similar, the contributions that either protein may make to lung function have not been identified in vivo. Mutations in SP-B cause lethal respiratory failure at birth; however, SP-B null mice are deficient in both SP-B and SP-C. To identify potential contributions of SP-C to lung function in vivo, the following transgenic mice were generated and exposed to 95% O(2) for 3 days: (SP-B(+/+),SP-C(+/+)), (SP-B(+/+), SP-C(-/-)), (SP-B(+/-),SP-C(+/+)), (SP-B(+/-),SP-C(+/-)), and (SP-B(+/-),SP-C(-/-)). Hyperoxia altered pressure-volume curves in mice that were heterozygous for SP-B, and these values were further decreased in (SP-B(+/-),SP-C(-/-)) mice. Likewise, alveolar interleukin (IL)-6 and IL-1 beta were maximally increased by O(2) exposure of (SP-B(+/-),SP-C(-/-)) mice compared with the other genotypes. Lung hysteresivity was lower in the (SP-B(+/-),SP-C(-/-)) mice. Surfactant isolated from (SP-B(+/+),SP-C(-/-)) and (SP-B(+/-),SP-C(-/-)) mice failed to stabilize the surface tension of microbubbles, showing that SP-C plays a role in stabilization or recruitment of phospholipid films at low bubble radius. Genetically decreased levels of SP-B combined with superimposed O(2)-induced injury reveals the distinct contribution of SP-C to pulmonary function in vivo.

Glucocorticoids, inflammation and the perinatal lung

Many women delivering preterm infants at less than 30 weeks gestation have subclinical chorioamnionitis. Based on current guidelines, maternal glucocorticoid treatment is given to induce lung maturation. Fetal exposure to proinflammation can cause acute and chronic injury, but inflammation also can induce fetal lung maturation. Both antenatal glucocorticoids and inflammation modulate lung development, by inducing the surfactant system, inducing structural maturation, and inhibiting alveolarization. The opportunities for the future are to develop new safer strategies to mature the preterm foetus, and the risks are potential adverse interactions of repetitive glucocorticoid exposures and unrecognized fetal exposure to inflammation.