Injurious ventilatory strategies increase cytokines and c-fos m-RNA expression in an isolated rat lung model

Brief, large tidal volume ventilation initiates lung injury and a systemic response in fetal sheep

RATIONALE

Premature infants are exposed to potentially injurious ventilation in the delivery room. Assessments of lung injury are confounded by effects of subsequent ventilatory support.

OBJECTIVES

To evaluate the injury response to a brief period of large tidal volume (Vt) ventilation, simulating neonatal resuscitation in preterm neonates.

METHODS

Preterm lambs (129 d gestation; term is150 d) were ventilated (Vt = 15 ml/kg, no positive end-expiratory pressure) for 15 minutes to simulate delivery room resuscitation, either with the placental circulation intact (fetal resuscitation [ FR]) or after delivery (neonatal resuscitation [NR]). After the initial 15 minutes, lambs received surfactant and were maintained with either ventilatory support (FR-VS and NR-VS) or placental support (FR-PS) for 2 hours, 45 minutes. A control group received no resuscitation and was maintained with placental support. Samples of bronchoalveolar lavage fluid, lung, and liver were analyzed.

MEASUREMENTS AND MAIN RESULTS

Inflammatory cells and protein in bronchoalveolar lavage fluid, heat shock protein-70 immunostaining, IL-1beta, IL-6, IL-8, monocyte chemotactic protein-1, serum amyloid A (SAA)-3, Toll-like receptor (TLR)-2, and TLR4 mRNA in the lungs were increased in the FR-PS group compared with control animals. There were further elevations in neutrophils, IL-6, and IL-8 mRNA in the FR-VS and NR-VS groups compared with FR-PS. SAA3, TLR2, and TLR4 mRNA increased in the liver in all resuscitation groups relative to control animals.

CONCLUSIONS

Ventilation for 15 minutes with a Vt of 15 ml/kg initiates an injurious process in the preterm lung and a hepatic acute-phase response. Subsequent ventilatory support causes further increases in some injury indicators.

Differential roles of p55 and p75 tumor necrosis factor receptors on stretch-induced pulmonary edema in mice

Ventilator-induced lung injury plays a crucial role in the outcome of patients with acute lung injury. Previous studies have shown a role for the cytokine tumor necrosis factor-alpha (TNF) in stretch-induced alveolar neutrophil recruitment, but the involvement of TNF in stretch-induced pulmonary edema is unclear. We investigated the effects of TNF through its individual p55 and p75 receptors on early pulmonary edema formation during high stretch ventilation, before neutrophil infiltration. Anesthetized wild-type or TNF receptor single/double knockout mice were ventilated with high tidal volume ( approximately 38 ml/kg) for 2 h or until they developed arterial hypotension. Pulmonary edema was assessed by physiological parameters including respiratory mechanics and blood gases, and by lavage fluid protein, lung wet:dry weight ratio, and lung permeability measurements using fluorescence-labeled albumin. High stretch ventilation in wild-type and TNF receptor double knockout animals induced similar pulmonary edema, and only 25-30% of mice completed the protocol. In contrast, the p55 receptor knockout mice were strongly protected from edema formation, with all animals completing the protocol. Myeloperoxidase assay indicated that this protective effect was not associated with decreased pulmonary neutrophil sequestration. The p75 receptor knockout mice, however, displayed increased susceptibility to edema formation, and no animals survived the full 2 h. These results demonstrate a novel role for TNF signaling (independent from its effects on neutrophil recruitment) specifically through the p55 receptor, in promoting high stretch-induced pulmonary edema, whereas p75 signaling may play an opposing role.

Reduction in alveolar macrophages attenuates acute ventilator induced lung injury in rats

OBJECTIVE

Alveolar macrophages are the sentinel cell for activation of the inflammatory cascade when the lung is exposed to noxious stimuli. We investigated the role of macrophages in mechanical lung injury by comparing the effect of high-volume mechanical ventilation with or without prior depletion of macrophages.

DESIGN AND SETTING

Randomized sham-controlled animal study in anesthetized rats.

METHODS

Lung injury was induced by 15 min of mechanical ventilation (intermittent positive pressure ventilation) using high peak pressures and zero end-expiratory pressure. The mean tidal volume was 40+/-0.7 ml/kg. One group of animals was killed immediately after this period of volutrauma (HV), while in a second group normoventilation was continued for 2 h at a tidal volume less than 10 ml/kg (HV-LV). One-half of the animals were depleted of alveolar macrophages by pretreatment with intratracheal liposomal clodronate (CL2MDP).

MEASUREMENTS

Arterial blood gas, blood pressure. After kill: lung static pressure volume curves, bronchoalveolar fluid concentration for protein, macrophage inflammatory protein 2, tumor necrosis factor alpha, and wet/dry lung weight ratio (W/D).

RESULTS

During HV and HV+LV oxygenation, lung compliance, and alveolar stability were better preserved in animals pretreated with CL2MDP. In both groups W/D ratio was significantly greater in ventilated than in nonventilated animals (4.5+/-0.6), but the increase in W/D was significantly less in CL2MDP treated HV and HV-LV groups (6.1+/-0.4, 6.6+/-0.6) than in the similarly ventilated nontreated groups (8.7+/-0.2 and 9.2+/-0.5).

CONCLUSIONS

Alveolar macrophages participate in the early phase of ventilator-induced lung injury.

[Mechanical ventilation induced lung injury]

Mechanical ventilation is associated with important complications, among which production or perpetuation of acute lung injury and product of distant organ injuries of the lung basically through the release of inflammatory mediators to the systemic circulation. There is increasingly greater evidence in both in vitro and in vivo experimental models that show the reality of this lesional mechanism. The main lesional mechanisms are both stretching and rupture of the lung structures (volutrauma) and cyclical opening and closure of the closed alveolar zones (atelectrauma). Studies on the use of protective lung ventilation strategies have shown a beneficial effect in patients with ARDS of the use of open lung ventilation strategies, use of circulating volumes less than 10 ml/kg and of maintaining alveolar pressure under 30 cm of H2O. It should be investigated if these same strategies would be useful in preventing the appearance of ARDS in mechanically ventilated patients for another reason, basically in those with risk factors for the development of this condition.

Neutrophil elastase inhibitor (sivelestat) attenuates subsequent ventilator-induced lung injury in mice

Mechanical ventilation can paradoxically cause acute lung injury, which is termed ventilator-induced lung injury. Neutrophil recruitment and neutrophil elastase release play a central role in the pathogenesis of ventilator-induced lung injury including cell damage, extracellular matrix degradation and alveolar-capillary hyperpermeability. We therefore speculated that neutrophil elastase inhibition ameliorates ventilator-induced lung injury. Anesthetized C57/BL6 mice received mechanical ventilation with a high tidal volume (V(T); 20 ml/kg) for 4 h. The neutrophil elastase inhibitor (sivelestat, 100 mg/kg) or saline was given intraperitoneally (i.p.) 30 min before ventilation. Sivelestat completely inhibited both neutrophil elastase and myeloperoxidase activities that were increased by ventilation, and attenuated the histopathological degree of lung damage, neutrophil accumulation and lung water content, as well as the concentration of macrophage inflammatory protein (MIP)-2, interleukin (IL)-6 and tumor necrosis factor (TNF)-alpha in bronchoalveolar lavage fluid and serum. Moreover, mechanical ventilation increased the phosphorylation of c-Jun NH2-terminal kinase (JNK) and the expression of early growth response gene-1 (Egr-1) mRNA, and these increases were also recovered by sivelestat. The terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) staining revealed apoptotic cells mainly in alveolar epithelial cells and their numbers corresponded to histological damage. These data suggested that sivelestat could protect against ventilator-induced lung injury by suppressing apoptotic responses through mechanical stress-induced cell signaling in addition to inhibiting neutrophil chemotaxis.

Effects of overinflation on procollagen type III expression in experimental acute lung injury

Introduction

In acute lung injury (ALI), elevation of procollagen type III (PC III) occurs early and has an adverse impact on outcome. We examined whether different high-inflation strategies of mechanical ventilation (MV) in oleic acid (OA) ALI alter regional expression of PC III.

Methods

We designed an experimental, randomized, and controlled protocol in which rats were allocated to two control groups (no injury, recruited [alveolar recruitment maneuver after tracheotomy without MV; n = 4 rats] and control [n = 5 rats]) or four injured groups (one exposed to OA only [n = 10 rats] and three OA-injured and ventilated). The three OA-injured groups were ventilated for 1 hour according to the following strategies: LVHP-S (low volume-high positive end-expiratory pressure [PEEP], supine; n = 10 rats, tidal volume [V_T] = 8 ml/kg, PEEP = 12 cm H₂O), HVLP-S (high volume-low PEEP, supine; n = 10 rats, V_T = 20 ml/kg, PEEP = 5 cm H₂O), and HVLP-P (high volume-low PEEP, prone; n = 10 rats). Northern blot analysis for PC III and interleukin-1-beta (IL-1β) and polymorphonuclear infiltration index (PMI) counting were performed in nondependent and dependent regions. Regional differences between groups were assessed by two-way analysis of variance after logarithmic transformation and post hoc tests.

Results

A significant interaction for group and region effects was observed for PC III (p = 0.012) with higher expression in the nondependent region for HVLP-S and LVHP-S, intermediate for OA and HVLP-P, and lower for control (group effect, p < 0.00001, partial η² = 0.767; region effect, p = 0.0007, partial η² = 0.091). We found high expression of IL-1β (group effect, p < 0.00001, partial η² = 0.944) in the OA, HVLP-S, and HVLP-P groups without regional differences (p = 0.16). PMI behaved similarly (group effect, p < 0.00001, partial η² = 0.832).

Conclusion

PC III expression is higher in nondependent regions and in ventilatory strategies that caused overdistension. This response was partially attenuated by prone positioning.

Acute lung injury and the acute respiratory distress syndrome: a clinical review

Acute respiratory distress syndrome and acute lung injury are well defined and readily recognised clinical disorders caused by many clinical insults to the lung or because of predispositions to lung injury. That this process is common in intensive care is well established. The mainstay of treatment for this disorder is provision of excellent supportive care since these patients are critically ill and frequently have coexisting conditions including sepsis and multiple organ failure. Refinements in ventilator and fluid management supported by data from prospective randomised trials have increased the methods available to effectively manage this disorder.

The open lung concept of mechanical ventilation: the role of recruitment and stabilization

This article describes the pathophysiologic basis and clinical role for lung recruitment maneuvers. It reviews the literature and presents the authors' clinical experience of over 15 years in the collaboration between Erasmus MC and the University of Rochester. The authors are hopeful that these lung-protective strategies are presented in a useful format that may be useful to the practicing intensivist, thus bringing laboratory and clinical research to bedside practice.

Physiology of mechanical ventilation

Mechanical ventilation, although essential in taking care of acute lung injury and widely used during surgical procedures worldwide, remains a highly debated field. Clinical trials in the last decade have shown convincingly that mechanical ventilation can result in additional mortality in patients with acute lung injury. This understanding has resulted in a resurged interest in mechanical ventilation, and especially in techniques and strategies to further improve mechanical ventilation. This article discusses physiological principles to improve the understanding of mechanical ventilation.

Mechanical ventilation in ARDS: a state-of-the-art review

Mechanical ventilation is an essential component of the care of patients with ARDS, and a large number of randomized controlled clinical trials have now been conducted evaluating the efficacy and safety of various methods of mechanical ventilation for the treatment of ARDS. Low tidal volume ventilation (</= 6 mL/kg predicted body weight) should be utilized in all patients with ARDS as it is the only method of mechanical ventilation that, to date, has been shown to improve survival. High positive end-expiratory pressure, alveolar recruitment maneuvers, and prone positioning may each be useful as rescue therapy in a patient with severe hypoxemia, but these methods of ventilation do not improve survival for the wide population of patients with ARDS. Although not specific to the treatment of ARDS, protocol-driven weaning that utilizes a daily spontaneous breathing trial and ventilation in the semirecumbent position have proven benefits and should be used in the management of ARDS patients.

NF-kappaB in the lungs of premature rabbits during mechanical ventilation--comparison between conventional mechanical ventilation (CMV) and high-frequency oscillation (HFO)

The purpose of this study is to compare pulmonary nuclear factor-kappaB (NF-kappaB) activity of conventional mechanical ventilation (CMV) with that of high-frequency oscillation (HFO) in premature rabbit lungs. For surfactant-depleted model, we used premature rabbits in order to exclude the effect of lung lavage on the activation of NF-kappaB. The premature rabbits were delivered at a gestational age of 27 days by hysterotomy. Both modes of the ventilator were set at the same MAP and FiO(2). We used animals that had PCO(2) levels of approximately 50-mmHg. Animals were sacrificed after 1-hr ventilation with CMV or HFO. Then activity of pulmonary NF-kappaB was assessed. We observed that NF-kappaB activity was higher in the lungs of CMV compared with those of HFO, as measured by Western blot analysis. The activity level of NF-kappaB in the lungs measured by ELISA was significantly higher in CMV group than in HFO group. We conclude that a higher level of NF-kappaB activation was associated with CMV when compared to HFO.

High-frequency oscillation (HFO) prevents activation of NF-kappaB found with conventional mechanical ventilation (CMV) in surfactant-depleted rabbit lung

High-frequency oscillation (HFO) has been recognized as an effective ventilatory strategy to minimize lung injury during respiratory support. Conventional mechanical ventilation (CMV) compared with HFO was shown to result in an increased number of PMNs and inflammatory cytokines in the lung lavage fluid. However how mechanical forces can be sensed by cells and converted into biochemical signals for intracellular signal transduction is still unknown. In this current study, we sought to determine whether the activation of Nuclear factor-kappa B (NF-kappaB) might be involved in the lung injury caused by CMV. Surfactant-depleted Japanese white rabbits received 1- or 4-hr CMV or 1- or 4-hr HFO. Then, activation of NF-kappaB in the lungs was assessed by conducting electrophoretic mobility shift assays (EMSA). In the experiment with whole lungs, NF-kappaB activity was much higher in the 4-hr CMV lungs than in the 4-hr HFO lungs. To clarify the origin of the cells in which NF-kappaB was activated, we did a second lung lavage at the end of ventilation and washed out the cells that had infiltrated the alveoli. The levels of NF-kappaB activity were the similar in the lungs of 4-hr HFO rabbits and in those of 4-hr CMV ones. On the other hand, NF-kappaB activity was much higher in the 4-hr CMV lungs than in the 4-hr HFO lungs in the experiment with the lung lavage fluid cells. These results show that the increase in NF-kappaB activity in the lungs of 4-hr CMV rabbits was due mainly to the cells that had infiltrated the alveoli.

A cluster-randomized trial of benchmarking and multimodal quality improvement to improve rates of survival free of bronchopulmonary dysplasia for infants with birth weights of less than 1250 grams

OBJECTIVE

We tested whether NICU teams trained in benchmarking and quality improvement would change practices and improve rates of survival without bronchopulmonary dysplasia in inborn neonates with birth weights of <1250 g.

METHODS

A cluster-randomized trial enrolled 4093 inborn neonates with birth weights of <1250 g at 17 centers of the National Institute of Child Health and Human Development Neonatal Research Network. Three centers were selected as best performers, and the remaining 14 centers were randomized to intervention or control. Changes in rates of survival free of bronchopulmonary dysplasia were compared between study year 1 and year 3.

RESULTS

Intervention centers implemented potentially better practices successfully; changes included reduced oxygen saturation targets and reduced exposure to mechanical ventilation. Five of 7 intervention centers and 2 of 7 control centers implemented use of high-saturation alarms to reduce oxygen exposure. Lower oxygen saturation targets reduced oxygen levels in the first week of life. Despite these changes, rates of survival free of bronchopulmonary dysplasia were all similar between intervention and control groups and remained significantly less than the rate achieved in the best-performing centers (73.3%).

CONCLUSIONS

In this cluster-randomized trial, benchmarking and multimodal quality improvement changed practices but did not reduce bronchopulmonary dysplasia rates.

Lung protective ventilatory strategies in acute lung injury and acute respiratory distress syndrome: from experimental findings to clinical application

This review addresses the physiological background and the current status of evidence regarding ventilator-induced lung injury and lung protective strategies. Lung protective ventilatory strategies have been shown to reduce mortality from adult respiratory distress syndrome (ARDS). We review the latest knowledge on the progression of lung injury by mechanical ventilation and correlate the findings of experimental work with results from clinical studies. We describe the experimental and clinical evidence of the effect of lung protective ventilatory strategies and open lung strategies on the progression of lung injury and current controversies surrounding these subjects. We describe a rational strategy, the open lung strategy, to accomplish an open lung, which may further prevent injury caused by mechanical ventilation. Finally, the clinician is offered directions on lung protective ventilation in the early phase of ARDS which can be applied on the intensive care unit.

Closing volume: a reappraisal (1967-2007)

Measurement of closing volume (CV) allows detection of presence or absence of tidal airway closure, i.e. cyclic opening and closure of peripheral airways with concurrent (1) inhomogeneity of distribution of ventilation and impaired gas exchange; and (2) risk of peripheral airway injury. Tidal airway closure, which can occur when the CV exceeds the end-expiratory lung volume (EELV), is commonly observed in diseases characterised by increased CV (e.g. chronic obstructive pulmonary disease, asthma) and/or decreased EELV (e.g. obesity, chronic heart failure). Risk of tidal airway closure is enhanced by ageing. In patients with tidal airway closure (CV > EELV) there is not only impairment of pulmonary gas exchange, but also peripheral airway disease due to injury of the peripheral airways. In view of this, the causes and consequences of tidal airway closure are reviewed, and further studies are suggested. In addition, assessment of the "open volume", as opposed to the "closing volume", is proposed because it is easier to perform and it requires less equipment.

Neutrophil apoptosis: a marker of disease severity in sepsis and sepsis-induced acute respiratory distress syndrome

Introduction

Apoptosis of neutrophils (polymorphonuclear neutrophils [PMNs]) may limit inflammatory injury in sepsis and acute respiratory distress syndrome (ARDS), but the relationship between the severity of sepsis and extent of PMN apoptosis and the effect of superimposed ARDS is unknown. The objective of this study was to correlate neutrophil apoptosis with the severity of sepsis and sepsis-induced ARDS.

Methods

A prospective cohort study was conducted in intensive care units of three tertiary hospitals in Porto Alegre, southern Brazil. Fifty-seven patients with sepsis (uncomplicated sepsis, septic shock, and sepsis-induced ARDS) and 64 controls were enrolled. Venous peripheral blood was collected from patients with sepsis within 24 hours of diagnosis. All surgical groups, including controls, had their blood drawn 24 hours after surgery. Control patients on mechanical ventilation had blood collected within 24 hours of initiation of mechanical ventilation. Healthy controls were blood donors. Neutrophils were isolated, and incubated ex vivo, and apoptosis was determined by light microscopy on cytospun preparations. The differences among groups were assessed by analysis of variance with Tukeys.

Results

In medical patients, the mean percentage of neutrophil apoptosis (± standard error of the mean [SEM]) was lower in sepsis-induced ARDS (28% ± 3.3%; n = 9) when compared with uncomplicated sepsis (57% ± 3.2%; n = 8; p < 0.001), mechanical ventilation without infection, sepsis, or ARDS (53% ± 3.0%; n = 11; p < 0.001) and healthy controls (69% ± 1.1%; n = 33; p < 0.001) but did not differ from septic shock (38% ± 3.7%; n = 12; p = 0.13). In surgical patients with sepsis, the percentage of neutrophil apoptosis was lower for all groups when compared with surgical controls (52% ± 3.6%; n = 11; p < 0.001).

Conclusion

In medical patients with sepsis, neutrophil apoptosis is inversely proportional to the severity of sepsis and thus may be a marker of the severity of sepsis in this population.

Alpha glucocorticoid receptor expression in different experimental rat models of acute lung injury

BACKGROUND AND OBJECTIVES

Acute respiratory distress syndrome (ARDS) is a frequent form of hypoxiemic respiratory failure caused by the acute development of diffuse lung inflammation. Dysregulated systemic inflammation with persistent elevation of circulating inflammatory cytokines is the pathogenetic mechanism for pulmonary and extrapulmonary organ dysfunction in patients with ARDS. Glucocorticoids (GCs) have a broad range of inhibitory inflammatory effects, including inhibition of cytokines transcription, cellular activation and growth factor production. They inhibit the inflammatory pathways through two specific intracellular glucocorticoid receptors (GRs), named GR alpha and GR beta. The aim of our study was to evaluate the histologic evidence of inflammatory injury and the GR alpha uptake of resident and inflammatory cells in different experimental models of acute lung injury (ALI).

METHODS

We studied four groups of rats: three different experimental rat models of lung injury and a control group. The ALI was caused by barotrauma (due to an overventilation), oleic acid injection and mechanical ventilation. Results were compared to nonventilated rat control group. The duration of mechanical ventilation was of 2.5h. At the end of each experiment, rats were sacrificed. Lung biopsies were evaluated for morphologic changes. The immunohistochemistry was performed to study GR alpha expression.

RESULTS

Histologic evidence of lung injury (alveolar and interstitial edema, vascular congestion, alveolar haemorrhage, emphysema, number of interstitial cells and neutrophils, and destruction of alveolar attachments) were present in all ventilated groups. Barotrauma lead to an additional inflammatory response. GR alpha expression significantly increased in the three ventilated groups compared with nonventilated groups. GR alpha expression was highest in barotrauma group.

CONCLUSIONS

These data indicate that ALI is associated with diffuse alveolar damage, up-regulation of the inflammatory response and GR alpha overexpression. Barotrauma is the most effective mechanism inducing acute lung inflammation and GR alpha overexpression.

High tidal volume ventilation induces lung injury after hepatic ischemia-reperfusion

Ischemia-reperfusion not only damages the affected organ but also leads to remote organ injuries. Hepatic inflow interruption usually occurs during hepatic surgery. To investigate the influence of liver ischemia-reperfusion on lung injury and to determine the contribution of tidal volume settings on liver ischemia-reperfusion-induced lung injury, we studied anesthetized and mechanically ventilated rats in which the hepatic inflow was transiently interrupted twice for 15 min. Two tidal volumes, 6 ml/kg as a low tidal volume (IR-LT) and 24 ml/kg as a high tidal volume (IR-HT), were assessed after liver ischemia-reperfusion, as well as after a sham operation, 6 ml/kg (NC-LT) and 24 ml/kg (NC-HT). Both the IR-HT and IR-LT groups had a gradual decline in the systemic blood pressure and a significant increase in plasma TNF-α concentrations. Of the four groups, only the IR-HT group developed lung injury, as assessed by an increase in the lung wet-to-dry weight ratio, the presence of significant histopathological changes, such as perivascular edema and intravascular leukocyte aggregation, and an increase in the bronchoalveolar lavage fluid TNF-α concentration. Furthermore, only in the IR-HT group was airway pressure increased significantly during the 6-h reperfusion period. These findings suggest that liver ischemia-reperfusion caused systemic inflammation and that lung injury is triggered when high tidal volume ventilation follows liver ischemia-reperfusion.

Lung-protective ventilation strategies in neonatology: What do we know—What do we need to know?

OBJECTIVE

Randomized controlled trials (RCTs) investigating various lung-protective ventilation modes or strategies in newborn infants have failed to show clear differences in mortality or bronchopulmonary dysplasia. This review tries to identify possible reasons for this observation, applying modern concepts on ventilator-induced lung injury and lung-protective ventilation.

DATA SOURCE

Published RCTs and systematic reviews on mechanical ventilation in newborn infants were identified by searching PubMed and the Cochrane Library.

DATA SYNTHESIS

A total of 16 RCTs and four systematic reviews comparing high-frequency ventilation with conventional mechanical ventilation (CMV) failed to show consistent differences in mortality and bronchopulmonary dysplasia. Unfortunately, clear information or data on ventilation and oxygenation targets in the search for optimal lung volumes during high-frequency ventilation or CMV is lacking in many RCTs, questioning the validity of the results and the meta-analytic subgroup analysis. Based on improvement in oxygenation, only three RCTs successfully applied the optimal lung volume strategy during high-frequency ventilation. A total of 24 RCTs and three systematic reviews comparing various CMV modes and settings and two RCTs investigating permissive hypercapnia reported no differences in mortality or bronchopulmonary dysplasia. However, the intervention arms in these RCTs did not differ in tidal volume or positive end-expiratory pressures, variables that are considered important determinants in ventilator-induced lung injury. In fact, no RCT in newborn infants has substantiated so far the experimental finding that avoiding large tidal volumes and low positive end-expiratory pressure during CMV is lung protective in newborn infants.

CONCLUSION

RCTs investigating lung-protective ventilation in neonates have mainly focused on comparing high-frequency ventilation with CMV. Most of these RCTs show weaknesses in the design, which may explain the inconsistent effect of high-frequency ventilation on bronchopulmonary dysplasia. RCTs on CMV only focused on comparing various modes and settings, leaving the important question whether reducing tidal volume or increasing positive end-expiratory pressure is also lung protective in newborn infants unanswered.

Basic ventilator management: lung protective strategies

Acute respiratory failure is manifested clinically as a patient with variable degrees of respiratory distress, but characteristically an abnormal arterial blood partial pressure of oxygen or carbon dioxide. The application of mechanical ventilation in this setting can be life-saving. An emerging body of clinical and basic research, however, has highlighted the potential adverse effects of positive pressure ventilation. Clinicians involved with the care of critically ill patients must recognize and seek to prevent these complications using lung-protective ventilation strategies. This article discusses the basic concepts of mechanical ventilation, reviews the categories of ventilator-associated lung injury, and discusses current strategies for the recognition and prevention of these adverse effects in the application of mechanical ventilation.

Atelectasis in the perioperative patient

PURPOSE OF REVIEW

To report the impact of atelectasis on perioperative outcomes. Atelectasis occurs in the dependent parts of the lungs of most patients who are anesthetized. Development of atelectasis is associated with decreased lung compliance, impairment of oxygenation, increased pulmonary vascular resistance and development of lung injury. Here, we examine the etiology, contributing factors, consequences, diagnosis and treatment of atelectasis.

RECENT FINDINGS

Atelectasis describes the state of absent air in alveoli attributable to collapse, but recent findings suggest that alveoli are filled with foam and fluid. It is now known that atelectasis plays an important role beyond abnormal gas exchange and that prevention or reversal of atelectasis in some populations of postoperative patients may improve outcome.

SUMMARY

Atelectasis in the presence of preexisting lung disease or limited cardiopulmonary reserve may have significant consequences. Increasing understanding of the underlying nature of atelectasis and its contribution to acute lung injury will improve our approach to the prevention and management of atelectasis.

Lipopolysaccharide exposure modifies high tidal volume ventilation-induced proinflammatory mediator expression in newborn rat lungs

Infection/inflammation and mechanical ventilation have both independently been shown to increase cytokine/chemokine levels in lung tissue and blood samples of premature patients. Little is known about the combined effect of systemic inflammation and mechanical ventilation on cytokine expression in the lung. We tested whether pre-existing inflammation induced by lipopolysaccharide (LPS) exposure would modify cytokine/chemokine response in newborn rat lungs to high tidal volume ventilation (HTVV). Newborn rats were randomly assigned to four groups: groups I and II (saline); groups III and IV: 3 mg/kg LPS. Groups II and IV were 24h later subjected to 3h of ventilation with a tidal volume of 25 mL/kg. HTVV alone increased IL-1beta, IL-6 and the chemokine (C-X-C motif) ligand 2 (CXCL2) mRNA expression. Although the cytokine response to LPS alone had disappeared after 24 h, the combination of LPS pretreatment and HTVV significantly increased the expression of IL-6 and IL-1beta mRNA when compared with HTVV alone. TNF-alpha expression was increased neither by HTVV alone nor in combination with LPS. IL-6 protein content in bronchoalveolar lavage increased due to the combined treatment. Thus, a subtle pre-existing inflammation combined with HTVV amplifies the proinflammatory cytokine/chemokine expression in the newborn rat lung compared with HTVV alone.

Effect of positive end-expiratory pressure on inflammatory response in oleic acid-induced lung injury and whole-lung lavage-induced lung injury

PURPOSE

The present study investigated the effects of positive end-expiratory pressure (PEEP) on the inflammatory response in two different lung injury models: edematous lung induced by oleic acid (OA); and atelectatic lung induced by whole-lung lavage (LAV).

METHODS

Japanese white rabbits (n = 28) were allocated to one of the two lung injury (OA or LAV) groups, and each group was treated with intermittent positive pressure ventilation, using zero end-expiratory pressure (ZEEP) or PEEP (1 cm H(2)O above the lower inflection point [LIP]). Thus, the animals were divided into LAV-ZEEP, LAV-PEEP, OA-ZEEP, and OA-PEEP groups. Blood and bronchoalveolar lavage fluid (BALF) were sampled 3 h after ventilatory treatment to analyze interleukin (IL)-8 levels.

RESULTS

Pa(O) (2) was significantly decreased after the induction of lung injury, but was significantly higher in the PEEP groups compared to the ZEEP groups for each lung injury. Serum IL-8 levels were elevated in both experimental models. Serum IL-8 levels were significantly lower in LAV-PEEP than in LAV-ZEEP, whereas no difference was noted between OA-PEEP and OA-ZEEP. BALF IL-8 levels were lower in LAV-PEEP than in LAV-ZEEP. PEEP above LIP attenuated the elevation of IL-8 in BALF and serum in atelectatic lungs, but did not attenuate these increases in the edematous lungs.

CONCLUSION

These results suggest that the protective effects of PEEP on injured lungs may depend on the underlying lung pathology.

Role of the renin-angiotensin system in ventilator-induced lung injury: an in vivo study in a rat model

BACKGROUND

Injurious mechanical ventilation can cause a pro-inflammatory reaction in the lungs. Recent evidence suggests an association of the renin-angiotensin system (RAS) with lung inflammation. A study was undertaken to investigate the pathogenic role of the RAS in ventilator-induced lung injury (VILI) and to determine whether VILI can be attenuated by angiotensin converting enzyme (ACE) inhibition.

METHODS

Male Sprague-Dawley rats were mechanically ventilated for 4 h with low (7 ml/kg) or high (40 ml/kg) tidal volumes; non-ventilated rats were used as controls. Lung injury and inflammation were measured by the lung injury score, protein leakage, myeloperoxidase activity, pro-inflammatory cytokine levels and nuclear factor (NF)-kappaB activity. Expression of the RAS components was also assessed. Some rats were pretreated with the ACE inhibitor captopril (10 mg/kg) for 3 days or received a concomitant infusion with losartan or PD123319 (type 1 or type 2 angiotensin II receptor antagonist) during mechanical ventilation to assess possible protective effects on VILI.

RESULTS

In the high-volume group (n=6) the lung injury score, bronchoalveolar lavage fluid protein concentration, pro-inflammatory cytokines and NF-kappaB activities were significantly increased compared with controls (n=6). Lung tissue angiotensin II levels and mRNA levels of angiotensinogen and type 1 and type 2 angiotensin II receptors were also significantly increased in the high-volume group. Pretreatment with captopril or concomitant infusion with losartan or PD123319 in the high-volume group attenuated the lung injury and inflammation (n=6 for each group).

CONCLUSIONS

The RAS is involved in the pathogenesis of ventilator-induced lung injury. ACE inhibitor or angiotensin receptor antagonists can attenuate VILI in this rat model.

Biological markers of lung injury before and after the institution of positive pressure ventilation in patients with acute lung injury

BACKGROUND

Several biological markers of lung injury are predictors of morbidity and mortality in patients with acute lung injury (ALI). The low tidal volume lung-protective ventilation strategy is associated with a significant decrease in plasma biomarker levels compared to the high tidal volume ventilation strategy. The primary objective of this study was to test whether the institution of lung-protective positive pressure ventilation in spontaneously ventilating patients with ALI exacerbates pre-existing lung injury by using measurements of biomarkers of lung injury before and after intubation.

MATERIALS AND METHODS

A prospective observational cohort study was conducted in the intensive care unit of a tertiary care university hospital. Twenty-five intubated, mechanically ventilated patients with ALI were enrolled. Physiologic data and serum samples were collected within 6 hours before intubation and at two different time points within the first 24 hours after intubation to measure the concentration of interleukin (IL)-6, IL-8, intercellular adhesion molecule 1 (ICAM-1), and von Willebrand factor (vWF). The differences in biomarker levels before and after intubation were analysed using repeated measures analysis of variance and a paired t test with correction for multiple comparisons.

RESULTS

Before endotracheal intubation, all of the biological markers (IL-8, IL-6, ICAM-1, and vWF) were elevated in the spontaneously breathing patients with ALI. After intubation and the institution of positive pressure ventilation (tidal volume 7 to 8 ml/kg per ideal body weight), none of the biological markers was significantly increased at either an early (3 +/- 2 hours) or later (21 +/- 5 hours) time point. However, the levels of IL-8 were significantly decreased at the later time point (21 +/- 5 hours) after intubation. During the 24-hour period after intubation, the PaO2/FiO2 (partial pressure of arterial oxygen/fraction of the inspired oxygen) ratio significantly increased and the plateau airway pressure significantly decreased.

CONCLUSION

Levels of IL-8, IL-6, vWF, and ICAM-1 are elevated in spontaneously ventilating patients with ALI prior to endotracheal intubation. The institution of a lung-protective ventilation strategy with positive pressure ventilation does not further increase the levels of biological markers of lung injury. The results suggest that the institution of a lung-protective positive pressure ventilation strategy does not worsen the pre-existing lung injury in most patients with ALI.

Pathogenetic significance of biological markers of ventilator-associated lung injury in experimental and clinical studies

For patients with acute lung injury, positive pressure mechanical ventilation is life saving. However, considerable experimental and clinical data have demonstrated that how clinicians set the tidal volume, positive end-expiratory pressure, and plateau airway pressure influences lung injury severity and patient outcomes including mortality. In order to better identify ventilator-associated lung injury (VALI), clinical investigators have sought to measure blood-borne and airspace biological markers of VALI. At the same time, several laboratory-based studies have focused on biological markers of inflammation and organ injury in experimental models in order to clarify the mechanisms of ventilator-induced lung injury (VILI) and VALI. This review summarizes data on biological markers of VALI and VILI from both clinical and experimental studies with an emphasis on markers identified in patients and in the experimental setting. This analysis suggests that measurement of some of these biological markers may be of value in diagnosing VALI and in understanding its pathogenesis.

The Role of CT-scan Studies for the Diagnosis and Therapy of Acute Respiratory Distress Syndrome

CT has provided new insights on the pathophysiology of acute respiratory distress syndrome (ARDS), demonstrating that ARDS does not affect the lung parenchyma homogeneously. These findings suggest that lung edema, as assessed by CT scan, should be included in the definition. Lung CT findings may provide a firm rationale for tailoring tidal volume during mechanical ventilation. Ideally, tidal volume should be proportional to the portion of the lung open to ventilation, as assessed by CT scan, rather than to the body weight. CT assessment of lung recruitability seems to be a prerequisite for a rational setting of positive end-expiratory pressure.

Neuromuscular blocking agents decrease inflammatory response in patients presenting with acute respiratory distress syndrome*

OBJECTIVE

To evaluate the effects of neuromuscular blocking agents (NMBAs) on pulmonary and systemic inflammation in patients with acute respiratory distress syndrome ventilated with a lung-protective strategy.

DESIGN

Multiple-center, prospective, controlled, and randomized trial.

SETTING

One medical and two medical-surgical intensive care units.

PATIENTS

A total of 36 patients with acute respiratory distress syndrome (Pao2/Fio2 ratio of < or =200 at a positive end-expiratory pressure of > or =5 cm H2O) were included within 48 hrs of acute respiratory distress syndrome onset.

INTERVENTIONS

Patients were randomized to receive conventional therapy plus placebo (n = 18) or conventional therapy plus NMBAs (n = 18) for 48 hrs. Both groups were ventilated with a lung-protective strategy (tidal volume between 4 and 8 mL/kg ideal body weight, plateau pressure of < or =30 cm H2O).

MEASUREMENTS AND MAIN RESULTS

Bronchoalveolar lavages and blood samples were performed, before randomization and at 48 hrs, to determine the concentrations of tumor necrosis factor-alpha, interleukin (IL)-1beta, IL-6, and IL-8. Pao2/Fio2 ratio was evaluated before randomization and at 24, 48, 72, 96, and 120 hrs. A decrease over time in IL-8 concentrations (p = .034) was observed in the pulmonary compartment of the NMBA group. At 48 hrs after randomization, pulmonary concentrations of IL-1beta (p = .005), IL-6 (p = .038), and IL-8 (p = .017) were lower in the NMBA group as compared with the control group. A decrease over time in IL-6 (p = .05) and IL-8 (p = .003) serum concentrations was observed in the NMBA group. At 48 hrs after randomization, serum concentrations of IL-1beta (p = .037) and IL-6 (p = .041) were lower in the NMBA group as compared with the control group. A sustained improvement in Pao2/Fio2 ratio was observed and was reinforced in the NMBA group (p < .001).

CONCLUSION

Early use of NMBAs decrease the proinflammatory response associated with acute respiratory distress syndrome and mechanical ventilation.

Assisted spontaneous breathing during early acute lung injury

In the early phase of their disease process, patients with acute lung injury are often ventilated with strategies that control the tidal volume or airway pressure, while modes employing spontaneous breathing are applied later to wean the patient from the ventilator. Spontaneous breathing modes may integrate intrinsic feedback mechanisms that should help prevent ventilator-induced lung injury, and should improve synchrony between the ventilator and the patient's demand. Airway pressure release ventilation with spontaneous breathing was shown to decrease cyclic collapse/recruitment of dependent, juxtadiaphragmatic lung areas compared with airway pressure release ventilation without spontaneous breathing. Combined with previous data demonstrating improved cardiorespiratory variables, airway pressure release ventilation with spontaneous breathing may turn out to be a less injurious ventilatory strategy.

Choosing the frequency of deep inflation in mice: balancing recruitment against ventilator-induced lung injury

Low tidal volume (Vt) ventilation is protective against ventilator-induced lung injury but can promote development of atelectasis. Periodic deep inflation (DI) can open the lung, but if delivered too frequently may cause damage via repeated overdistention. We therefore examined the effects of varying DI frequency on lung mechanics, gas exchange, and biomarkers of injury in mice. C57BL/6 males were mechanically ventilated with positive end-expiratory pressure (PEEP) of 2 cmH2O for 2 h. One high Vt group received a DI with each breath (HV). Low Vt groups received 2 DIs after each hour of ventilation (LV) or 2 DIs every minute (LVDI). Control groups included a nonventilated surgical sham and a group receiving high Vt with zero PEEP (HVZP). Respiratory impedance was measured every 4 min, from which tissue elastance (H) and damping (G) were derived. G and H rose progressively during LV and HVZP, but returned to baseline after hourly DI during LV. During LVDI and HV, G and H remained low and gas exchange was superior to that of LV. Bronchoalveolar lavage fluid protein was elevated in HV and HVZP but was not different between LV and LVDI. Lung tissue IL-6 and IL-1beta levels were elevated in HVZP and lower in LVDI compared with LV. We conclude that frequent DI can safely improve gas exchange and lung mechanics and may confer protection from biotrauma. Differences between LVDI and HV suggest that an optimal frequency range of DI exists, within which the benefits of maintaining an open lung outweigh injury incurred from overdistention.

Inflammation and bronchopulmonary dysplasia: a continuing story

Increasing evidence indicates that bronchopulmonary dysplasia (BPD) results, at least in part, from an imbalance between pro-inflammatory and anti-inflammatory mechanisms, with a persistent imbalance that favours pro-inflammatory mechanisms. The inflammatory response is characterised by an accumulation of neutrophils and macrophages in the airways and pulmonary tissue of preterm infants and, moreover, by an arsenal of pro-inflammatory mediators which affect the alveolar capillary unit and tissue integrity. As well as pro-inflammatory cytokines and toxic oxygen radicals, various lipid mediators as well as potent proteases may be responsible for acute lung injury. During the last decade it has become evident that multiple pre- and postnatal events contribute to the development of BPD in preterm infants. Chorioamnionitis and cytokine exposure in utero, plus sequential lung injury caused by postnatal resuscitation, oxygen toxicity, volu-, barotrauma and infection all lead to a pulmonary inflammatory response which is most probably associated with aberrant wound healing and an inhibition of alveolarisation as well as vascular development in the immature lungs of very preterm infants, causing the 'new BPD'.

Maintenance of end-expiratory recruitment with increased respiratory rate after saline-lavage lung injury

Cyclical recruitment of atelectasis with each breath is thought to contribute to ventilator-associated lung injury. Extrinsic positive end-expiratory pressure (PEEPe) can maintain alveolar recruitment at end exhalation, but PEEPe depresses cardiac output and increases overdistension. Short exhalation times can also maintain end-expiratory recruitment, but if the mechanism of this recruitment is generation of intrinsic PEEP (PEEPi), there would be little advantage compared with PEEPe. In seven New Zealand White rabbits, we compared recruitment from increased respiratory rate (RR) to recruitment from increased PEEPe after saline lavage. Rabbits were ventilated in pressure control mode with a fraction of inspired O(2) (Fi(O(2))) of 1.0, inspiratory-to-expiratory ratio of 2:1, and plateau pressure of 28 cmH(2)O, and either 1) high RR (24) and low PEEPe (3.5) or 2) low RR (7) and high PEEPe (14). We assessed cyclical lung recruitment with a fast arterial Po(2) probe, and we assessed average recruitment with blood gas data. We measured PEEPi, cardiac output, and mixed venous saturation at each ventilator setting. Recruitment achieved by increased RR and short exhalation time was nearly equivalent to recruitment achieved by increased PEEPe. The short exhalation time at increased RR, however, did not generate PEEPi. Cardiac output was increased on average 13% in the high RR group compared with the high PEEPe group (P < 0.001), and mixed venous saturation was consistently greater in the high RR group (P < 0.001). Prevention of end-expiratory derecruitment without increased end-expiratory pressure suggests that another mechanism, distinct from intrinsic PEEP, plays a role in the dynamic behavior of atelectasis.

Respuesta inflamatoria y apoptosis en la lesión pulmonar aguda

One of the principal mechanisms of pulmonary injury in acute respiratory distress is due to the effects of the precipitated inflammatory response. The damage produced to the alveolar epithelium and underlying endothelium depends on the sequestration and activation of inflammatory cells, which in turn exert their actions through mediators. On the other hand, apoptosis is a mechanism responsible for epithelial damage and regulation of inflammation. Response of the lung tissue subjected to mechanical ventilation stimulus is added to the previous mechanisms. All these processes flow into a series of common pathways of cellular activation. Knowledge of these mechanisms could serve to identify which patients would benefit from a specific treatment before applying therapies that act indiscriminately in the inflammatory response.

Mechanical stretch has contrasting effects on mediator release from bronchial epithelial cells, with a rho-kinase-dependent component to the mechanotransduction pathway

INTRODUCTION

In vivo, the airway epithelium stretches and relaxes with each respiratory cycle, but little is known about the effect this pattern of elongation and relaxation has on bronchial epithelial cells. We have used a model of cell deformation to measure the effect of stretch on inflammatory cytokine release by the BEAS 2B cell line, and to examine the method of mechanotransduction in these cells.

METHODS

BEAS 2B cells were cyclically stretched using the Flexercell system. IL-8 and RANTES protein and RNA levels were measured after different elongations, rates and duration of stretch. An inhibitor of Rho (Ras Homologous)-associated kinases was used, to assess the effect of blocking downstream of integrin signalling. Immunofluorescent staining of paxillin was used to study the effect of stretch on the distribution of focal contacts and the organisation of the actin cytoskeleton.

RESULTS

IL-8 release by BEAS 2B cells was increased by cytokine stimulation and stretch, whereas RANTES levels in the cell supernatant decreased after stretch in a dose-, time- and rate-dependent manner. Thirty percent elongation at 20 cycles/min for 24h increased IL-8 levels by over 100% (P < 0.01). Blocking rho kinase using Y-27632 inhibited the effect of stretch on IL-8 release by the BEAS 2B cells. Immunofluorescent staining demonstrated that stretch caused dramatic disassembly of focal adhesions and resulted in the redistribution of paxillin to the peri-nuclear region.

CONCLUSION

This study demonstrates a marked effect of stretch on bronchial epithelial cell function. We propose that stretch modulates epithelial cell function via the activation of rho kinases. The observation that stretch promotes focal adhesion disassembly suggests a mechanism whereby focal adhesion turnover (coordination of assembly and disassembly) is essential for mechanotransduction in bronchial epithelial cells.

Short-term mechanical ventilation increases airway reactivity in rat pups

We used a rat pup model to delineate whether mechanical ventilation of <or=4 h duration in the absence of supplemental oxygen contributes to the development of airway hyperreactivity. Eight-day-old rat pups were assigned to unventilated normoxic controls, ventilated under normoxic conditions, ventilated under hyperoxic conditions (100% O2), or unventilated hyperoxic groups (>95% O2). After each intervention, they were returned to their mothers. On d 10 of life, all animals were anesthetized, paralyzed, and ventilated to measure pulmonary function. Total lung resistance (RL) and dynamic lung compliance (Cdyn) were measured in response to increasing intravenous doses of methacholine (0.03-1 microg/g) by head-out body plethysmography. Injection of methacholine caused a dose-dependent increase in RL and decrease in Cdyn. The response of both RL and Cdyn to methacholine was significantly potentiated by prior exposure to mechanical ventilation when compared with unventilated normoxic controls. The addition of hyperoxia to mechanical ventilation did not further potentiate responses to methacholine. Mechanical ventilation did not alter lung myosin or the number of inflammatory cells in airways of room air ventilated versus unventilated control animals. We conclude that a brief period of mechanical ventilation in rat pups increases airway reactivity 48 h after such exposure in the presence as well as absence of hyperoxic exposure. This represents a potentially important model to investigate the mechanisms involved in airway hyperreactivity induced by neonatal lung injury.

Ischemia and Reperfusion Increases Susceptibility to Ventilator-induced Lung Injury in Rats

OBJECTIVES

Hemorrhagic shock followed by resuscitation (HSR) commonly triggers an inflammatory response that leads to acute respiratory distress syndrome.

HYPOTHESIS

HSR exacerbates mechanical stress-induced lung injury by rendering the lung more susceptible to ventilator-induced lung injury.

METHODS

Rats were subjected to HSR, and were randomized into an HSR + high tidal volume and zero positive end-expiratory pressure (PEEP) or a HSR + low tidal volume with 5 cm H(2)O PEEP. A sham-operated rat + high tidal volume and zero PEEP served as a control.

RESULTS

HSR increased susceptibility to ventilator-induced lung injury as evidenced by an increase in lung elastance and the wet/dry ratio and a reduction in Pa(O(2)) as compared with the other groups. The lung injury observed in the HSR + high tidal volume group was associated with a higher level of interleukin 6 in the lung and blood, increased epithelial cell apoptosis in the kidney and small intestine villi, and a tendency toward high levels of alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, and creatinine in plasma.

CONCLUSIONS

HSR priming renders the lung and kidney more susceptible to mechanical ventilation-induced organ injury.

Microarray analysis of regional cellular responses to local mechanical stress in acute lung injury

Human acute lung injury is characterized by heterogeneous tissue involvement, leading to the potential for extremes of mechanical stress and tissue injury when mechanical ventilation, required to support critically ill patients, is employed. Our goal was to establish whether regional cellular responses to these disparate local mechanical conditions could be determined as a novel approach toward understanding the mechanism of development of ventilator-associated lung injury. We utilized cross-species genomic microarrays in a unilateral model of ventilator-associated lung injury in anesthetized dogs to assess regional cellular responses to local mechanical conditions that potentially contribute pathogenic mechanisms of injury. Highly significant regional differences in gene expression were observed between lung apex/base regions as well as between gravitationally dependent/nondependent regions of the base, with 367 and 1,544 genes differentially regulated between these regions, respectively. Major functional groupings of differentially regulated genes included inflammation and immune responses, cell proliferation, adhesion, signaling, and apoptosis. Expression of genes encoding both acute lung injury-associated inflammatory cytokines and protective acute response genes were markedly different in the nondependent compared with the dependent regions of the lung base. We conclude that there are significant differences in the local responses to stress within the lung, and consequently, insights into the cellular responses that contribute to ventilator-associated lung injury development must be sought in the context of the mechanical heterogeneity that characterizes this syndrome.

Sepsis-induced lung injury in rats increases alveolar epithelial vulnerability to stretch

OBJECTIVE

Previous in vitro models have shown that cellular deformation causes dose-dependent injury and death in healthy rat alveolar epithelial cells (AECs). We compared the viability of AECs from septic rats with those from nonseptic rats after 1 hr of cyclic equibiaxial stretch. We hypothesized that sepsis would increase stretch-induced cell death.

DESIGN

Laboratory investigation.

SETTING

University research laboratory.

SUBJECTS

Thirty-seven male Sprague-Dawley rats weighing 240-260 g.

INTERVENTIONS

Anesthetized rats were subjected to cecal ligation and double puncture (2CLP) or sham laparotomy without cecal ligation or puncture (sham). After 24 or 48 hrs, AECs were isolated, seeded in custom wells, and maintained in culture for 48 hrs before study. AECs were stretched cyclically (15/min) to a 0%, 12%, 25%, or 37% change in surface area (DeltaSA) for 1 hr. Cell viability, phenotypic markers, and nuclear factor-kappaB intracellular localization were assessed using fluorescent immunocytochemistry.

MEASUREMENTS AND MAIN RESULTS

Phase and fluorescent images were evaluated for all studies. Response to stretch was the same at 24 and 48 hrs after 2CLP. Relative to sham, 2CLP significantly increased cell death at 25 and 37% DeltaSA (p<.003, analysis of variance). Relative to sham, 2CLP did not alter expression of type I or type II phenotypic markers. Nuclear factor-kappaB within the nuclear compartment was observed after 2CLP in unstretched cells and after 1 hr of cyclic stretch at 37% DeltaSA. In sham, nuclear factor-kappaB within the nuclear compartment was seen only after stretch.

CONCLUSIONS

AECs isolated from septic rats are more vulnerable to mechanical deformation injury than AECs from nonseptic animals.

The contribution of biophysical lung injury to the development of biotrauma

Patients with severe acute respiratory distress syndrome who die usually succumb to multiorgan failure as opposed to hypoxia. Despite appropriate resuscitation, some patients' symptoms persist on a downward spiral, apparently propagated by an uncontained systemic inflammatory response. This phenomenon is not well understood. However, a novel hypothesis to explain this observation proposes that it is related to the life-saving ventilatory support used to treat the respiratory failure. According to this hypothesis, mechanical ventilation per se, by altering both the magnitude and the pattern of lung stretch, can cause changes in gene expression and/or cellular metabolism that ultimately can lead to the development of an overwhelming inflammatory response-even in the absence of overt structural damage. This mechanism of injury has been termed biotrauma. In this review we explore the biotrauma hypothesis, the causal relationship between biophysical injury and organ failure, and its implications for the future therapy and management of critically ill patients.

Various adhesion molecules impair microvascular leukocyte kinetics in ventilator-induced lung injury

Although the endothelial expression of various adhesion molecules substantially differs between pulmonary microvessels, their importance for neutrophil and lymphocyte sequestration in ventilator-induced lung injury (VILI) has not been systematically analyzed. We investigated the kinetics of polymorphonuclear cells (PMN) and mononuclear cells (MN) in the acinar microcirculation of the isolated rat lung with VILI by real-time confocal laser fluorescence microscopy, with or without inhibition of ICAM-1, VCAM-1, or P-selectin by monoclonal antibodies (MAb). Adhesion molecules in each microvessel were estimated by intravital fluorescence microscopy or immunohistochemical staining. In high tidal volume-ventilated lungs, 1) ICAM-1, VCAM-1, and P-selectin were differently upregulated in venules, arterioles, and capillaries; 2) venular PMN rolling was improved by inhibition of ICAM-1, VCAM-1, or P-selectin, whereas arteriolar PMN rolling was improved by ICAM-1 or VCAM-1 inhibition; 3) capillary PMN entrapment was ameliorated only by anti-ICAM-1 MAb; and 4) MN rolling in venules and arterioles and MN entrapment in capillaries were improved by ICAM-1 and VCAM-1 inhibition. In conclusion, the contribution of endothelial adhesion molecules to abnormal leukocyte behavior in VILI-injured microcirculation is microvessel and leukocyte specific. ICAM-1- and VCAM-1-dependent, but P-selectin-independent, arteriolar PMN rolling, which is expected to reflect the initial stage of tissue injury, should be taken as a phenomenon unique to ventilator-associated lung injury.

Contribution of high-mobility group box-1 to the development of ventilator-induced lung injury

RATIONALE

Proinflammatory cytokines play an important role in ventilator-induced lung injury (VILI). High-mobility group box-1 (HMGB1) is a macrophage-derived proinflammatory cytokine that can cause lung injury.

OBJECTIVES

This study tested the hypothesis that HMGB1 is released in intact lungs ventilated with large Vt. A second objective was to identify the source of HMGB1. A third objective was to examine the effects of blocking HMGB1 on the subsequent development of VILI.

METHODS

Bronchoalveolar lavage fluid (BALF) and lung tissues were obtained from rabbits mechanically ventilated for 4 h with a small (8 ml/kg) versus a large (30 ml/kg) Vt. BALF was also obtained from rabbits with intratracheal instillation of anti-HMGB1 antibody before the initiation of large Vt ventilation.

MEASUREMENTS AND MAIN RESULTS

The concentrations of HMGB1 in BALF were fivefold higher in the large than in the small Vt group. Immunohistochemistry and immunofluorescence studies revealed expression of HMGB1 in the cytoplasm of macrophages and neutrophils in lungs ventilated with large Vt. Blocking HMGB1 improved oxygenation, limited microvascular permeability and neutrophil influx into the alveolar lumen, and decreased concentrations of tumor necrosis factor-alpha in BALF.

CONCLUSIONS

These observations suggest that HMGB1 could be one of the deteriorating factors in the development of VILI.

Early ventilation and outcome in patients with moderate to severe traumatic brain injury

OBJECTIVES

An increase in mortality has been reported with early intubation in severe traumatic brain injury, possibly due to suboptimal ventilation. This analysis explores the impact of early ventilation on outcome in moderate to severe traumatic brain injury.

DESIGN

Retrospective, registry-based analysis.

SETTING

This study was conducted in a large county trauma system that includes urban, suburban, and rural jurisdictions.

PATIENTS

Nonarrest trauma victims with a Head Abbreviated Injury Score of > or =3 were identified from our county trauma registry.

INTERVENTIONS

Intubated patients were stratified into 5 mm Hg arrival PCO(2) increments. Logistic regression was used to calculate odds ratios for each increment, adjusting for age, gender, mechanism of injury, year of injury, preadmission Glasgow Coma Scale score, hypotension, Head Abbreviated Injury Score, Injury Severity Score, PO(2), and base deficit. Increments with the highest relative survival were used to define the optimal PCO(2) range. Outcomes for patients with arrival PCO(2) values inside and outside this optimal range were then explored for both intubated and nonintubated patients, adjusting for the same factors as defined previously. In addition, the independent outcome effect of hyperventilation and hypoventilation was assessed.

MEASUREMENTS AND MAIN RESULTS

A total of 890 intubated and 2,914 nonintubated patients were included. Improved survival was observed for the arrival PCO(2) range 30-49 mm Hg. Patients with arrival PCO(2) values inside this optimal range had improved survival and a higher incidence of good outcomes. Conversely, there was no improvement in outcomes for patients within this optimal PCO(2) range for nonintubated patients after adjusting for all of the factors defined previously. Both hyperventilation and hypoventilation were associated with worse outcomes in intubated but not nonintubated patients. The proportion of arrival PCO(2) values within the optimal range was lower for intubated vs. nonintubated patients.

CONCLUSIONS

Arrival hypercapnia and hypocapnia are common and associated with worse outcomes in intubated but not spontaneously breathing patients with traumatic brain injury.

A high positive end-expiratory pressure, low tidal volume ventilatory strategy improves outcome in persistent acute respiratory distress syndrome: a randomized, controlled trial

OBJECTIVE

It has been shown in a two-center study that high positive end-expiratory pressure (PEEP) and low tidal volume (LTV) improved outcome in ARDS. However, that study involved patients with underlying diseases unique to the study area, was conducted at only two centers, and enrolled a small number of patients. We similarly hypothesized that a ventilatory strategy based on PEEP above the lower inflection point of the pressure volume curve of the respiratory system (Pflex) set on day 1 with a low tidal volume would result in improved outcome in patients with severe and persistent acute respiratory distress syndrome (ARDS).

DESIGN

Randomized, controlled clinical trial.

SETTING

Network of eight Spanish multidisciplinary intensive care units (ICUs) under the acronym of ARIES (Acute Respiratory Insufficiency: España Study).

PATIENTS

All consecutive patients admitted into participating Spanish ICUs from March 1999 to March 2001 with a diagnosis of ARDS were considered for the study. If 24 hrs after meeting ARDS criteria, the Pao2/Fio2 remained < or =200 mm Hg on standard ventilator settings, patients were randomized into two groups: control and Pflex/LTV.

INTERVENTIONS

In the control group, tidal volume was 9-11 mL/kg of predicted body weight (PBW) and PEEP > or =5 cm H2O. In the Pflex/LTV group, tidal volume was 5-8 mL/kg PBW and PEEP was set on day 1 at Pflex + 2 cm H2O. In both groups, Fio2 was set to maintain arterial oxygen saturation >90% and Pao2 70-100 mm Hg, and respiratory rate was adjusted to maintain Paco2 between 35 and 50 mm Hg.

MEASUREMENTS AND MAIN RESULTS

The study was stopped early based on an efficacy stopping rule as described in the methods. Of 103 patients who were enrolled (50 control and 53 Pflex), eight patients (five in control, three in Pflex) were excluded from the final evaluation because the random group assignment was not performed in one center according to protocol. Main outcome measures were ICU and hospital mortality, ventilator-free days, and nonpulmonary organ dysfunction. ICU mortality (24 of 45 [53.3%] vs. 16 of 50 [32%], p = .040), hospital mortality (25 of 45 [55.5%] vs. 17 of 50 [34%], p = .041), and ventilator-free days at day 28 (6.02 +/- 7.95 in control and 10.90 +/- 9.45 in Pflex/LTV, p = .008) all favored Pflex/LTV. The mean difference in the number of additional organ failures postrandomization was higher in the control group (p < .001).

CONCLUSIONS

A mechanical ventilation strategy with a PEEP level set on day 1 above Pflex and a low tidal volume compared with a strategy with a higher tidal volume and relatively low PEEP has a beneficial impact on outcome in patients with severe and persistent ARDS.

Inhibition of matrix metalloproteinase-9 prevents neutrophilic inflammation in ventilator-induced lung injury

Neutrophils are considered to play a central role in ventilator-induced lung injury (VILI). However, the pulmonary consequences of neutrophil accumulation have not been fully elucidated. Matrix metalloproteinase-9 (MMP-9) had been postulated to participate in neutrophil transmigration. The purpose of this study was to investigate the role of MMP-9 in the neutrophilic inflammation of VILI. Male Sprague-Dawley rats were divided into three groups: 1) low tidal volume (LVT), 7 ml/kg of tidal volume (VT); 2) high tidal volume (HVT), 30 ml/kg of VT; and 3) HVT with MMP inhibitor (HVT+MMPI). As a MMPI, CMT-3 was administered daily from 3 days before mechanical ventilation. Degree of VILI was assessed by wet-to-dry weight ratio and acute lung injury (ALI) scores. Neutrophilic inflammation was determined from the neutrophil count in the lung tissue and myeloperoxidase (MPO) activity in the bronchoalveolar lavage fluid (BALF). MMP-9 expression and activity were examined by immunohistochemical staining and gelatinase zymography, respectively. The wet-to-dry weight ratio, ALI score, neutrophil infiltration, and MPO activity were increased significantly in the HVT group. However, in the HVT+MMPI group, pretreatment with MMPI decreased significantly the degree of VILI, as well as neutrophil infiltration and MPO activity. These changes correlated significantly with MMP-9 immunoreactivity and MMP-9 activity. Most outcomes were significantly worse in the HVT+MMPI group compared with the LVT group. In conclusion, VILI mediated by neutrophilic inflammation is closely related to MMP-9 expression and activity. The inhibition of MMP-9 protects against the development of VILI through the downregulation of neutrophil-mediated inflammation.

Reversibility of lung collapse and hypoxemia in early acute respiratory distress syndrome

RATIONALE

The hypothesis that lung collapse is detrimental during the acute respiratory distress syndrome is still debatable. One of the difficulties is the lack of an efficient maneuver to minimize it.

OBJECTIVES

To test if a bedside recruitment strategy, capable of reversing hypoxemia and collapse in > 95% of lung units, is clinically applicable in early acute respiratory distress syndrome.

METHODS

Prospective assessment of a stepwise maximum-recruitment strategy using multislice computed tomography and continuous blood-gas hemodynamic monitoring.

MEASUREMENTS AND MAIN RESULTS

Twenty-six patients received sequential increments in inspiratory airway pressures, in 5 cm H(2)O steps, until the detection of Pa(O(2)) + Pa(CO(2)) >or= 400 mm Hg. Whenever this primary target was not met, despite inspiratory pressures reaching 60 cm H(2)O, the maneuver was considered incomplete. If there was hemodynamic deterioration or barotrauma, the maneuver was to be interrupted. Late assessment of recruitment efficacy was performed by computed tomography (9 patients) or by online continuous monitoring in the intensive care unit (15 patients) up to 6 h. It was possible to open the lung and to keep the lung open in the majority (24/26) of patients, at the expense of transient hemodynamic effects and hypercapnia but without major clinical consequences. No barotrauma directly associated with the maneuver was detected. There was a strong and inverse relationship between arterial oxygenation and percentage of collapsed lung mass (R = - 0.91; p < 0.0001).

CONCLUSIONS

It is often possible to reverse hypoxemia and fully recruit the lung in early acute respiratory distress syndrome. Due to transient side effects, the required maneuver still awaits further evaluation before routine clinical application.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.