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

Permissive hypercapnia--role in protective lung ventilatory strategies

"Permissive hypercapnia" is an inherent element of accepted protective lung ventilation. However, there are no clinical data evaluating the efficacy of hypercapnia per se, independent of ventilator strategy. In the absence of such data, it is necessary to determine whether the potential exists for an active role for hypercapnia, distinct from the demonstrated benefits of reduced lung stretch. In this review, we consider four key issues. First, we consider the evidence that protective lung ventilatory strategies improve survival and we explore current paradigms regarding the mechanisms underlying these effects. Second, we examine whether hypercapnic acidosis may have effects that are additive to the effects of protective ventilation. Third, we consider whether direct elevation of CO(2), in the absence of protective ventilation, is beneficial or deleterious. Fourth, we address the current evidence regarding the buffering of hypercapnic acidosis in ARDS. These perspectives reveal that the potential exists for hypercapnia to exert beneficial effects in the clinical context. Direct administration of CO(2) is protective in multiple models of acute lung and systemic injury. Nevertheless, several specific concerns remain regarding the safety of hypercapnia. At present, protective ventilatory strategies that involve hypercapnia are clinically acceptable, provided the clinician is primarily targeting reduced tidal stretch. There are insufficient clinical data to suggest that hypercapnia per se should be independently induced, nor do outcome data exist to support the practice of buffering hypercapnic acidosis. Rapidly advancing basic scientific investigations should better delineate the advantages, disadvantages, and optimal use of hypercapnia in ARDS.

The Effects of Different Ventilatory Settings on Pulmonary and Systemic Inflammatory Responses During Major Surgery

Mechanical ventilation with high tidal volumes (V(T)) and zero or low positive end-expiratory pressure increased mediator release to inflammatory stimuli or acute lung injury. We studied whether mechanical ventilation modifies the inflammatory responses during major thoracic or abdominal surgery. Sixty-four patients undergoing elective thoracotomy (n = 34) or laparotomy (n = 30) were randomized to receive either mechanical ventilation with V(T) = 12 or 15 mL/kg ideal body weight, respectively, and zero end-expiratory pressure, or V(T) = 6 mL/kg ideal body weight with positive end-expiratory pressure of 10 cm H(2)O. In 62 patients who completed the study, arterial oxygenation was not different between groups. Tumor necrosis factor, interleukin (IL)-1, IL-6, IL-8, IL-10, and IL-12 were determined by cytometric bead array in plasma after 0, 1, 2, and 3 h and in tracheal aspirates after 3 h of mechanical ventilation. Data were log-transformed and analyzed using parametric or nonparametric tests, as indicated. All plasma mediators increased more during abdominal than during thoracic surgery, although the differences were small. However, neither time course nor concentrations of pulmonary or systemic mediators differed between the two ventilatory settings. Our data suggest that the ventilatory settings we studied do not affect inflammatory reactions during major surgery within 3 h.

IMPLICATIONS

In 62 patients undergoing elective major thoracic or abdominal surgery, mechanical ventilation with low tidal volumes and positive end-expiratory pressure or high tidal volumes and zero end-expiratory pressure did not result in different pulmonary or systemic levels of measured inflammatory markers.

Ventilation-induced Neutrophil Infiltration Depends on c-Jun N-Terminal Kinase

Positive pressure ventilation with large VTs has been shown to cause release of cytokines, including macrophage inflammatory protein-2 (MIP-2), a functional equivalent of human interleukin-8. The mechanisms regulating ventilation-induced cytokine production are unclear. Based on our previous in vitro model of lung cell stretch, we hypothesized that high VT ventilation-induced MIP-2 production is dependent on the activation of the c-Jun N-terminal kinase (JNK). We exposed C57BL/6 mice to high VT (30 ml/kg) or low VT (6 ml/kg) mechanical ventilation for 5 hours. High VT ventilation-induced neutrophil migration into the lung, MIP-2 protein production, MIP-2 messenger RNA expression, and JNK activation. Large VT ventilation of JNK knockout mice and pharmacologic JNK inhibition with SP600125 attenuated neutrophil sequestration and blocked MIP-2 messenger RNA expression and MIP-2 production. We conclude that lung cell stretch in vivo results in increased lung neutrophil sequestration and increased MIP-2 production, which was, at least in part, dependent upon the JNK pathway.

Effects of cyclic opening and closing at low- and high-volume ventilation on bronchoalveolar lavage cytokines

OBJECTIVE

To examine the mechanisms of ventilator-induced lung injury at low and high lung volumes.

DESIGN

Prospective, randomized, laboratory study.

SETTING

University research laboratory.

SUBJECTS

Eighty-eight adult male Sprague-Dawley rats.

INTERVENTIONS

Mechanical ventilation using low and high lung volumes.

MEASUREMENTS AND MAIN RESULTS

An ex vivo rat lung model was used. In study I (ventilation at low lung volumes), rat lungs (n = 40) were randomly assigned to various modes of ventilation: a) opening and closing with positive end-expiratory pressure (PEEP; control): tidal volume 7 mL/kg and PEEP 5 cm H2O; b) opening and closing from zero end-expiratory pressure (ZEEP): tidal volume 7 mL/kg and PEEP 0; or c) atelectasis. Peak inspiratory pressure was monitored at the beginning and end of 3 hrs of ventilation. At the end of 3 hrs of ventilation, the lungs were lavaged, and the concentrations of tumor necrosis factor-alpha, macrophage inflammatory protein-2, and interleukin-6 cytokines were measured in the lavage. In study II (ventilation at high volumes), rat lungs (n = 45) were randomly assigned to a) cyclic lung stretch: pressure-controlled ventilation, peak inspiratory pressure 50 cm H2O, and PEEP 8 cm H2O; b) continuous positive airway pressure at 50 cm H2O (CPAP50); or c) CPAP at the mean airway pressure of the cyclic stretch group (CPAP 31 cm H2O). Bronchoalveolar lavage cytokine concentrations (tumor necrosis factor-alpha, macrophage inflammatory protein-2, and interleukin-6) were measured at the end of 3 hrs of ventilation. In the low volume study, there was no difference in bronchoalveolar lavage cytokine concentrations between the PEEP group and the atelectatic group. All cytokines were significantly higher in the ZEEP group compared with the atelectasis group. Macrophage inflammatory protein-2 was significantly higher in the ZEEP group compared with the PEEP group. Lung compliance, as reflected by change in peak inspiratory pressure, was also significantly worse in the ZEEP compared with the PEEP group. In the high-volume study, tumor necrosis factor-alpha and interleukin-6 were significantly higher in the cyclic stretch group compared with the CPAP 31 group. There was no significant difference between the cytokine concentrations in the cyclic stretch group compared with the CPAP 50 group.

CONCLUSION

We conclude that at low lung volumes, cyclic opening and closing from ZEEP leads to greater increases in bronchoalveolar lavage cytokines than atelectasis. With high-volume ventilation, over time, the degree of overdistension is more associated with increases in bronchoalveolar lavage cytokines than cyclic opening and closing alone.

Protective ventilation of patients with acute respiratory distress syndrome

The majority of patients with acute respiratory distress syndrome (ARDS) require mechanical ventilation. This support provides time for the lungs to heal, but the adverse effects of mechanical ventilation significantly influence patient outcome. Traditionally, these were ascribed to mechanical effects, such as haemodynamic compromise from decreased venous return or gross air leaks induced by large transpulmonary pressures. More recently, however, the ARDS Network study has established the clinical importance of lowering the tidal volume to limit overdistension of the lung when ventilating patients with ARDS. This study suggests that ventilator-associated lung injury (VALI) caused by overdistension of the lung contributes to the mortality of patients with ARDS. Moreover, the results from clinical and basic research have revealed more subtle types of VALI, including upregulation of the inflammatory response in the injured and overdistended lung. This not only damages the lung, but the overflow of inflammatory mediators into the systemic circulation may explain why most patients who die with ARDS succumb to multi-organ failure rather than respiratory failure. The results of these studies, the present understanding of the pathophysiology of VALI, and protective ventilatory strategies are reviewed.

High frequency oscillatory ventilation suppresses inflammatory response in lung tissue and microdissected alveolar macrophages in surfactant depleted piglets

The impact of high frequency oscillatory ventilation (HFOV) compared with intermittent mandatory ventilation (IMV) on oxygenation and pulmonary inflammatory response was studied in a surfactant depleted piglet model. After establishment of lung injury by bronchoalveolar lavage, piglets either received HFOV (n =5) or IMV (control; n = 5) for eight hours. PaO(2) was higher and mean pulmonary arterial pressure (MPAP) was lower with HFOV (HFOV versus control, mean +/- SEM; endpoint PaO(2): 252 +/- 73 versus 68 +/- 8.4 mm Hg; p < 0.001; MPAP: 22 +/- 2.3 versus 34 +/- 2.5 mm Hg; p < 0.01). mRNA expression of interleukin (IL)-1 beta, IL-6, IL-8, IL-10, TGF-beta 1, Endothelin-1, and adhesion molecules (E-selectin, P-selectin, ICAM-1) in lung tissue was quantified by real time PCR normalized to beta-actin and hypoxanthine-guanine-phosphoribosyl-transferase (HPRT). mRNA expression of all cytokines and adhesion molecules/HPRT was higher in controls (e.g.: HFOV versus control, mean +/- SEM; IL-1 beta/HPRT: 1.6 +/- 0.3 versus 23.1 +/- 8.6 relative units (RU), p < 0.001; IL-8/HPRT: 8.5 +/- 2.0 versus 63.5 +/- 15.2 RU, p < 0.001). IL-8/HPRT gene expression was quantified in microdissected single cells. With HFOV, IL-8 gene expression was highly reduced in alveolar macrophages: 10 +/- 3.4 copies IL-8 mRNA/copy HPRT mRNA versus 356 +/- 142; p < 0.05 (bronchiolar epithelial cells: 33 +/- 16 versus 208 +/- 108; alveolar septum: 2.1 +/- 1.3 versus 26 +/- 11; bronchiolar smooth muscle cells: 1.3 +/- 0.3 versus 2.8 +/- 1.0; vascular smooth muscle cells: 0.7 +/- 0.3 versus 1.1 +/- 0.4). In conclusion, HFOV improved oxygenation, reduced pulmonary arterial pressure and attenuated pulmonary inflammatory response.

Neutrophil depletion attenuates interleukin-8 production in mild-overstretch ventilated normal rabbit lung

OBJECTIVE

Acute lung injury induced by lung overstretch is associated with neutrophil influx, but the pathogenic role of neutrophils in overstretch-induced lung injury remains unclear.

DESIGN

To assess the contribution of neutrophils, we compared the effects of noninjurious large tidal volume (Vt) ventilation on lungs in normal and neutrophil-depleted animals.

SETTING

Research animal laboratory.

SUBJECTS

Twenty-six male Japanese white rabbits.

INTERVENTIONS

Animals were mechanically ventilated for 4 hrs with one of the three following protocols: large Vt (20 mL/kg), small Vt (8 mL/kg), and large Vt (20 mL/kg) with neutrophil depletion achieved by a single dose of vinblastine injection (0.75 mg/kg) intravenously 4 days before the experiment.

MEASUREMENTS AND MAIN RESULTS

Large Vt ventilation produced alveolar neutrophil influx compared with low Vt (p =.002) without evidence of edema or increased epithelial permeability. The neutrophil influx was accompanied by increases in interleukin-8 in bronchoalveolar lavage fluid (p =.04). Immunohistochemistry of large Vt lungs showed increased interleukin-8 staining in bronchial epithelial cells, alveolar epithelium, alveolar macrophages, and smooth muscles of pulmonary vessels. Neutrophil depletion attenuated the interleukin-8 increase in the lung. Large Vt did not increase plasma interleukin-8 or tumor necrosis factor-alpha in plasma and bronchoalveolar lavage fluid. No expression of p-selectin or intercellular adhesion molecule-1 was observed.

CONCLUSIONS

Cyclic overstretching of normal rabbit lungs with noninjurious large Vt produced neutrophil influx and interleukin-8 increase in bronchoalveolar lavage fluid. Production of pulmonary interleukin-8 by lung overstretch might require the interaction between resident lung cells and migrated neutrophils. This study suggests that large Vt ventilation potentiates the predisposed, subclinical lung injury, such as nosocomial pneumonia or aspiration of gastric contents.

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.

Effect of core body temperature on ventilator-induced lung injury

OBJECTIVE

Ventilator-induced lung injury is a risk in patients requiring elevated ventilatory support pressures. We hypothesized that thermal stress modulates the development of ventilator-induced lung injury.

DESIGN

Experimental study.

SETTING

University laboratory.

SUBJECTS

Anesthetized rabbits.

INTERVENTIONS

Two experimental studies were designed to determine the role of temperature as a cofactor in ventilator-induced lung injury. In the first study, three groups of anesthetized rabbits were randomized to be ventilated for 2 hrs at core body temperatures of 33, 37, or 41 degrees C while ventilated with pressure control ventilation of 15/3 cm H2O (noninjurious settings-control) or 35/3 cm H2O (potentially injurious settings-experimental). To exclude effects arising from cardiac output fluctuations or from extrapulmonary organs, an isolated lung model was used for the second study, perfused at a fixed rate and studied at either 33 degrees C or 41 degrees C.

MEASUREMENTS AND MAIN RESULTS

In the first study, the hyperthermic group compared with the hypothermic animals had significantly reduced mean PaO2 (-114 vs. + 14 mm Hg, p <.05), increased lung edema formation (mean wet weight/dry weight ratio of 8.1 vs. 5.7), and altered pressure-volume curves. The hyperthermic isolated, perfused lungs had an increased ultrafiltration coefficient, formed more edema, and experienced greater alveolar hemorrhage than hypothermic lungs.

CONCLUSIONS

In two studies of ventilator-induced lung injury in rabbits, maintaining hyperthermia compared with hypothermia augmented the development of lung injury. Similar results from both the in vivo and isolated, perfused lung studies suggest that the observed effects were not due to cardiovascular factors or consequences of heating nonpulmonary organs.

Effect of neuromuscular blocking agents on gas exchange in patients presenting with acute respiratory distress syndrome*

OBJECTIVE

To evaluate the effects of a 48-hr neuromuscular blocking agents (NMBA) infusion on gas exchange over a 120-hr time period in patients with acute respiratory distress syndrome.

DESIGN

Multiple center, prospective, controlled, and randomized trial.

SETTING

Four adult medical or mixed medical-surgical intensive care units.

PATIENTS

A total of 56 patients with acute respiratory distress syndrome with a PaO2/FiO2 ratio of <150 at a positive end-expiratory pressure of > or =5 cm H2O.

INTERVENTIONS

After randomization, patients received either conventional therapy without NMBA (control group) or conventional therapy plus NMBA for the next 48 hrs. The initial ventilator mode was volume-assist/control. The ventilator remained on assist-control mode throughout the initial 48-hr period in both groups. Tidal volume was 6-8 mL/kg ideal body weight.

MEASUREMENTS AND MAIN RESULTS

When analyzed for the entire 120 hrs, there was a significant effect of the NMBA on the course of PaO2/FiO2 ratio (p =.021). Separate comparisons at each time point indicated that patients randomized to the NMBA group had a higher PaO2/FiO2 at 48, 96, and 120 hrs after randomization. Moreover, a decrease of positive end-expiratory pressure (p =.036) was only found in the NMBA group. Two-way repeated-measures analysis of variance exhibited a decrease in positive end-expiratory pressure over time (p =.036). Concerning short-term effects, there was no modification of PaO2/FiO2 ratio 1 hr after randomization in either group. Only one patient (from the control group) developed pneumothorax.

CONCLUSIONS

Use of NMBA during a 48-hr period in patients with acute respiratory distress syndrome is associated with a sustained improvement in oxygenation.

Current concepts in the diagnosis and management of trauma-related sepsis

Traumatic injury is common, and accounts for a large health care burden. Trauma and in particular haemorrhagic shock are closely related to the onset of multiple organ failure, the systemic inflammatory response and sepsis. Despite overall improvements in the care of septic critically ill patients there has been little impact on morbidity and mortality. In recent years our understanding of sepsis both as an illness and at a molecular level has led to the development of a number of therapeutic interventions. This article outlines the current evidence for such interventions and points to possible future research that is required in the diagnosis and management of trauma-related sepsis.

Liquid ventilation: an adjunct for respiratory management

Although significant advances in respiratory care have reduced mortality of patients with respiratory failure, morbidity persists, often resulting from iatrogenic mechanisms. Mechanical ventilation with gas has been shown to initiate as well as exacerbate underlying lung injury, resulting in progressive structural damage and release of inflammatory mediators within the lung. Alternative means to support pulmonary gas exchange while preserving lung structure and function are therefore required. Perfluorochemical (PFC) liquids are currently used clinically in a number of ways, such as intravascular PFC emulsions for volume expansion/oxygen carrying/angiography and intracavitary neat PFC liquid for image contrast enhancement or vitreous fluid replacement. As a novel approach to replace gas as the respiratory medium, liquid assisted ventilation (LAV) with PFC liquids has been investigated as an alternative respiratory modality for over 30 years. Currently, there are several theoretical and practical applications of LAV in the immature or mature lung at risk for acute respiratory distress and injury associated with mechanical ventilation.

Therapeutic hypercapnia is not protective in the in vivo surfactant-depleted rabbit lung

Permissive hypercapnia because of reduced tidal volume is associated with improved survival in lung injury, whereas therapeutic hypercapnia-deliberate elevation of arterial Pco2-protects against in vivo reperfusion injury and injury produced by severe lung stretch. No published studies to date have examined the effects of CO2 on in vivo models of neonatal lung injury. We used an established in vivo rabbit model of surfactant depletion to investigate whether therapeutic hypercapnia would improve oxygenation and protect against ventilator-induced lung injury. Animals were randomized to injurious (tidal volume, 12 mL/kg; positive end-expiratory pressure, 0 cm H2O) or protective ventilatory strategy (tidal volume, 5 mL/kg; positive end-expiratory pressure, 12.5 cm H2O), and to receive either control conditions or therapeutic hypercapnia (fraction of inspired CO2, 0.12). Oxygenation (alveolar-arterial O2 difference, arterial Po2), lung injury (alveolar-capillary protein leak, impairment of static compliance), and selected bronchoalveolar lavage and plasma cytokines (IL-8, growth-related oncogene, monocyte chemoattractant protein-1, and tumor necrosis factor-alpha) were measured. Injurious ventilation resulted in a large alveolar-arterial O2 gradient, elevated peak airway pressure, increased protein leak, and impaired lung compliance. Therapeutic hypercapnia did not affect any of these outcomes. Tumor necrosis factor-alpha was not increased by mechanical stretch in any of the groups. Therapeutic hypercapnia abolished the stretch-induced increase in bronchoalveolar lavage monocyte chemoattractant protein-1, but did not affect any of the other mediators studied. Therapeutic hypercapnia may attenuate the impairment in oxygenation and inhibit certain cytokines. Because hypercapnia inhibits certain cytokines but does not alter lung injury, the pathogenic role of these cytokines in lung injury is questionable.

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 recruitment maneuvers in patients with acute lung injury and acute respiratory distress syndrome ventilated with high positive end-expiratory pressure

OBJECTIVE

Positive end-expiratory pressure (PEEP) and recruitment maneuvers (RMs) may partially reverse atelectasis and reduce ventilation-associated lung injury. The purposes of this study were to assess a) magnitude and duration of RM effects on arterial oxygenation and on requirements for oxygenation support (Fio2/PEEP) in patients with acute lung injury and acute respiratory distress syndrome (ALI/ARDS) receiving ventilation with low tidal volumes and high levels of PEEP; and b) frequency of adverse respiratory and circulatory events attributable to RMs.

DESIGN

Prospective, randomized, crossover study.

SETTING

Thirty-four intensive care units at 19 hospitals.

PATIENTS

Seventy-two patients with early ALI/ARDS. Baseline PEEP and Fio2 were 13.8 +/- 3.0 cm H2O and 0.39 +/- 0.10, respectively (mean +/- sd).

INTERVENTIONS

We conducted RMs by applying continuous positive airway pressure of 35-40 cm H2O for 30 secs. We conducted sham RMs on alternate days. We monitored oxyhemoglobin saturation by pulse oximetry (SpO2), Fio2/PEEP, blood pressure, and heart rate for 8 hrs after RMs and sham RMs. We examined chest radiographs for barotrauma.

MEASUREMENTS AND MAIN RESULTS

Responses to RMs were variable. Greatest increments from baseline SpO2 within 10 mins after RMs were larger than after sham RMs (1.7 +/- 0.2 vs. 0.6 +/- 0.3 %, mean +/- SEM, p < .01). Systolic blood pressure decreased more +/- 1.1 mm Hg, p < .01). Changes in Fio2/PEEP requirements were not significantly different at any time after RMs vs. sham RMs. Barotrauma was apparent on first radiographs after one RM and one sham RM.

CONCLUSIONS

In ALI/ARDS patients receiving mechanical ventilation with low tidal volumes and high PEEP, short-term effects of RMs as conducted in this study are variable. Beneficial effects on gas exchange in responders appear to be of brief duration. More information is needed to determine the role of recruitment maneuvers in the management of ALI/ARDS.

Enhancement of the endotoxin recognition pathway by ventilation with a large tidal volume in rabbits

Ventilation with a small tidal volume (V(t)) is associated with better clinical outcomes than with a large V(t), particularly in critical settings, including acute lung injury. To determine whether V(t) influences the lipopolysaccaharide (LPS) recognition pathway, we studied CD14 expression in rabbit lungs and the release of TNF-alpha by cultured alveolar macrophages after 240 min of ventilation with a large (20 ml/kg) vs. a small (5 ml/kg) V(t). We also applied small or large V(t) to lungs instilled with 50 microg/kg of LPS. The alveolar macrophages collected after large V(t) ventilation revealed a 20-fold increase in LPS-induced TNF-alpha release compared with those collected after small V(t) ventilation, whereas TNF-alpha was undetectable without LPS stimulation. In animals ventilated with a large V(t), the expression of CD14 mRNA in whole lung homogenates and the expression of CD14 protein on alveolar macrophages, assessed by immunohistochemistry, were both significantly increased in the absence of LPS stimulation. A large V(t) applied to LPS-instilled lungs increased the pulmonary albumin permeability and TNF-alpha release into the plasma. These results suggest that mechanical stress caused by a large V(t) sensitizes the lungs to endotoxin, a phenomenon that may occur partially via the upregulation of CD14.

Early changes in lung gene expression due to high tidal volume

The purpose of this study was to use gene expression profiling to understand how adult rat lung responds to high tidal volume (HV) ventilation in vivo. HV ventilation for 30 minutes did not cause discernable lung injury (in terms of altered mechanics or histology) but caused obvious injury when continued for 90 minutes. However, at 30-minute ventilation, HV caused significant upregulation of 10 genes and suppression of 12 genes. Among the upregulated genes were transcription factors, stress proteins, and inflammatory mediators; the downregulated genes were exemplified by metabolic regulatory genes. On the basis of cluster analysis, we studied Egr-1, c-Jun, heat shock protein 70, and interleukin (IL)-1beta in further detail. Temporal studies demonstrated that Egr-1 and c-Jun were increased early and before heat shock protein 70 and IL-1beta. Spatial studies using in situ hybridization and laser capture microscopy revealed that all four genes were upregulated primarily in the bronchiolar airway epithelium. Furthermore, at 90 minutes of HV ventilation, a significant increase in intracellular IL-1beta protein was observed. Although there are limitations to gene array methodology, the current data suggest a global hypothesis that (1). the effects of HV are cumulative; (2). specific patterns of gene activation and suppression precede lung injury; and (3). alteration of gene expression after mechanical stretch is pathogenic.

Phosphoinositide 3-OH kinase inhibition prevents ventilation-induced lung cell activation

In acute respiratory distress syndrome patients, protective ventilation strategies reduce mortality and proinflammatory mediator levels. It has been suggested that some of the side effects of mechanical ventilation are caused by the excessive release of mediators capable of causing pulmonary inflammation and tissue destruction (biotrauma). Selective inhibition of this process might be used to minimize the side effects of artificial mechanical ventilation. This study was designed to identify the cell types and specific signaling mechanisms that are activated by ventilation with increased pressure/volume (overventilation). In isolated perfused mouse lungs, overventilation caused nuclear translocation of nuclear factor-kappaB (NF-kappaB) and enhanced expression of interleukin-6 mRNA in alveolar macrophages and alveolar epithelial type II cells. The phosphoinositide 3-OH kinase inhibitor Ly294002 prevented nuclear translocation of NF-kappaB and the subsequent release of interleukin-6 and macrophage inflammatory protein-2alpha in overventilated but not in endotoxic lungs. Similar results were obtained in rats in vivo, where Ly294002 prevented NF-kappaB activation by overventilation but not by endotoxin. These findings show that alveolar macrophages and alveolar epithelial type II cells contribute to the ventilation-induced release of proinflammatory mediators and that selective inhibition of this process is possible without inhibiting the activation of NF-kappaB by endotoxin.

High tidal volume upregulates intrapulmonary cytokines in an in vivo mouse model of ventilator-induced lung injury

Mechanical ventilation has been demonstrated to exacerbate lung injury, and a sufficiently high tidal volume can induce injury in otherwise healthy lungs. However, it remains controversial whether injurious ventilation per se, without preceding lung injury, can initiate cytokine-mediated pulmonary inflammation. To address this, we developed an in vivo mouse model of acute lung injury produced by high tidal volume (Vt) ventilation. Anesthetized C57BL6 mice were ventilated at high Vt (34.5 +/- 2.9 ml/kg, mean +/- SD) for a duration of 156 +/- 17 min until mean blood pressure fell below 45 mmHg (series 1); high Vt for 120 min (series 2); or low Vt (8.8 +/- 0.5 ml/kg) for 120 or 180 min (series 3). High Vt produced progressive lung injury with a decrease in respiratory system compliance, increase in protein concentration in lung lavage fluid, and lung pathology showing hyaline membrane formation. High-Vt ventilation was associated with increased TNF-alpha in lung lavage fluid at the early stage of injury (series 2) but not the later stage (series 1). In contrast, lavage fluid macrophage inflammatory protein-2 (MIP-2) was increased in all high-Vt animals. Lavage fluid from high-Vt animals contained bioactive TNF-alpha by WEHI bioassay. Low-Vt ventilation induced minimal changes in physiology and pathology with negligible TNF-alpha and MIP-2 proteins and TNF-alpha bioactivity. These results demonstrate that high-Vt ventilation in the absence of underlying injury induces intrapulmonary TNF-alpha and MIP-2 expression in mice. The apparently transient nature of TNF-alpha upregulation may help explain previous controversy regarding the involvement of cytokines in ventilator-induced lung injury.

Low tidal volume ventilation induces proinflammatory and profibrogenic response in lungs of rats

OBJECTIVE

We examined whether mechanical ventilation with low tidal volume induces polymorphonuclear infiltration and proinflammatory and profibrogenic responses in rat lungs compared dependent and nondependent lung region to expression of interleukin-1beta (IL-1beta) and alpha-1 procollagen III (PC III) mRNA.

DESIGN

An experimental, randomized and controlled protocol with previously normal rats.

INTERVENTIONS

Three groups of ten animals were studied. Two groups were ventilated (FIO2=0.3) in supine position for 1 h without positive end expiratory pressure, one group with a low tidal volume (6 ml/kg), and the other with a high tidal volume (24 ml/kg). In the third group animals were kept in spontaneous ventilation for 1 h.

MEASUREMENTS AND RESULTS

After ventilation the right lung was used to quantify polymorphonuclear infiltration. The left lung was divided into dependent and nondependent regions, and expression of IL-1beta and PC III mRNA was quantified by northern blot analysis. The group ventilated with low tidal volume had greater polymorphonuclear infiltration IL-1beta and PC III mRNA expression than the nonventilated group. Similar results were observed with high tidal volumes. There was no difference between low and high tidal volume ventilation. Expression levels of IL-1beta and PC III mRNA were higher in the nondependent region of ventilated groups and equal in the nonventilated group.

CONCLUSIONS

Even a low tidal volume mode of mechanical ventilation induces proinflammatory and profibrogenic response, with a nondependent predominance for IL-1beta and PC III mRNA expression in supine, ventilated, previously normal rats.

Pentoxifylline decreases tumor necrosis factor and interleukin-1 during high tidal volume

Tumor necrosis factor-alpha (TNF-alpha) is one of the most important proinflammatory cytokines which plays a central role in host defense and in the acute inflammatory response related to tissue injury. The major source of TNF-alpha are immune cells such as neutrophils and macrophages. We tested the hypothesis that pentoxifylline, a methylxanthine derivative, down-regulates proinflammatory cytokine expression during acute lung injury in rats. Male Wistar rats weighing 250 to 450 g were anesthetized ip with 50 mg/kg sodium thiopental and randomly divided into three groups: group 1 (N = 7): tidal volume (V T) = 7 ml/kg, respiratory rate (RR) = 50 breaths/min and normal saline infusion; group 2 (N = 7): V T = 42 ml/kg, RR = 9 breaths/min and normal saline infusion; group 3 (N = 7): V T = 42 ml/kg, RR = 9 breaths/min and pentoxifylline infusion. The animals were ventilated with an inspired oxygen fraction of 1.0, a positive end-expiratory pressure of 3 cmH2O, and normal saline or pentoxifylline injected into the left femoral vein. The mRNA of TNF-alpha rapidly increased in the lung tissue within 180 min of ventilation with a higher V T with normal saline infusion. The concentrations of inflammatory mediators were decreased in plasma and bronchoalveolar lavage (BAL) in the presence of higher V T with pentoxifylline infusion (TNF-alpha: plasma, 102.2+/-90.9 and BAL, 118.2+/-82.1; IL-1 : plasma, 45.2+/-42.7 and BAL, 50.2+/-34.9, P < 0.05). We conclude that TNF-alpha produced by neutrophil influx may function as an alert signal in host defense to induce production of other inflammatory mediators.

Acute lung injury and the acute respiratory distress syndrome

Although ALI/ARDS mortality rates have improved over the last several decades, they remain high, particularly in the geriatric patient population. Although considerable progress has been made in understanding the pathogenesis of the disease, a large number of promising treatments have proven unsuccessful. One exception has been in the area of ventilator management, where a strategy of protective ventilation with low tidal volumes has demonstrated a significant mortality benefit. Basic research continues to help advance our understanding of this complex syndrome and identify interesting new directions of investigation. The results of several large, randomized trials of new ventilatory and pharmacologic strategies currently underway may help identify successful methods of treating this important disease.

Influence of tidal volume on pulmonary NO release, tissue lipid peroxidation and surfactant phospholipids

Mechanical stress during ventilation may cause or aggravate acute lung injury. This study investigates the influence of low vs. high tidal volume (V(t)) on factors known to play key roles in acute lung injury: nitric oxide release, eNOS and iNOS gene expression, lipid peroxidation (LPO), and surfactant phospholipids (PL). Isolated rabbit lungs were subjected to one of three ventilation patterns for 135 min (V(t)-PEEP): 6 ml/kg-0 cm H(2)O. 12 ml/kg-0 cm H(2)O 6 ml/kg-5 cm H(2)O, 12 ml/kg-0 cm H(2)O, and 6 ml/kg-5 cm H(2)O resulted in comparable peak inspiratory pressure (PIP). This allowed comparing low and high V(t) without dependence on PIP. Ventilatory patterns did not induce changes in pulmonary artery pressure, vascular permeability (K(f,c)), PIP or pulmonary compliance. High V(t) in comparison with both of the low V(t) groups caused an increase in BALF-nitrite (30.6+/-3.0* vs. 21.4+/-2.2 and 16.2+/-3.3 microM), BALF-PL (1110+/-19* vs. 750+/-68 and 634+/-82 microg/ml), and tissue LPO product accumulation (0.62+/-0.051* vs. 0.48+/-0.052 and 0.43+/-0.031 nmol/mg), *P<0.05 each. Perfusate nitrite and BALF-PL composition (assessed by use of 31P-NMR spectroscopy and MALDI-TOF mass spectrometry) did not differ among the groups. High V(t) ventilation reduced eNOS gene expression but did not affect iNOS expression. The increased release of NO and the accumulation of LPO products may represent early lung injury while elevated BALF-PL may reflect distension-induced surfactant secretion.

Differential effects of mechanical ventilatory strategy on lung injury and systemic organ inflammation in mice

Patients with acute respiratory distress syndrome are at increased risk for developing multiorgan system dysfunction. The goal of this study was to establish an in vivo murine model to assess the differential effects of ventilation-protective strategies on the development of acute lung injury and systemic organ inflammation. C57B/6 mice were randomized to mechanical ventilation (MV) with conventional, high (17 ml/kg) or protective, low (6 ml/kg) tidal volume (VT) after intratracheal hydrochloric acid or no intervention. Mean arterial pressure was continuously monitored during MV and did not differ between groups. After 4 h, lung injury was assessed by measurement of wet/dry lung weight, lung lavage protein concentration and cell count, and histology. Concentration of IL-6, TNF-alpha, VEGF, and VEGF receptor-2 (VEGFR2) was measured in lung, liver, kidney, and heart. Results were compared with control, spontaneously breathing mice. Lung injury and altered pulmonary cytokine expression were not detected after MV of healthy mice with low or high VT. Although MV did not significantly alter IL-6 or TNF-alpha in systemic organs, VEGF concentration significantly increased in liver and kidney. After acid aspiration, mice ventilated with high VT manifested lung injury and increased IL-6 and VEGFR2 in lung, liver, and kidney, whereas VEGF increased only in liver and kidney. MV with low VT after acid aspiration attenuated lung injury, both IL-6 and VEGFR2 expression in lung and systemic organs, and hepatic, but not renal, increased VEGF. Our data suggest that MV strategy has differential effects on systemic inflammatory changes and thus may selectively predispose to systemic organ dysfunction.

Positive end-expiratory pressure modulates local and systemic inflammatory responses in a sepsis-induced lung injury model

OBJECTIVE

Previous animal studies have shown that certain modes of mechanical ventilation (MV) can injure the lungs. Most of those studies were performed with models that differ from clinical causes of respiratory failure. We examined the effects of positive end-expiratory pressure (PEEP) in the setting of a clinically relevant, in vivo animal model of sepsis-induced acute lung injury ventilated with low or injurious tidal volume.

METHODS

Septic male Sprague-Dawley rats were anesthetized and randomized to spontaneous breathing or four different strategies of MV for 3 h at low (6 ml/kg) or high (20 ml/kg) tidal volume (V(T)) with zero PEEP or PEEP above inflection point in the pressure-volume curve. Sepsis was induced by cecal ligation and perforation. Mortality rates, pathological evaluation, lung tissue cytokine gene expression, and plasma cytokine concentrations were analyzed in all experimental groups.

RESULTS

Lung damage, cytokine synthesis and release, and mortality rates were significantly affected by the method of MV in the presence of sepsis. PEEP above the inflection point significantly attenuated lung damage and decreased mortality during 3 h of ventilation with low V(T) (25% vs. 0%) and increased lung damage and mortality in the high V(T) group (19% vs. 50%). PEEP attenuated lung cytokine gene expression and plasma concentrations during mechanical ventilation with low V(T).

CONCLUSIONS

The use of a PEEP level above the inflection point in a sepsis-induced acute lung injury animal model modulates the pulmonary and systemic inflammatory responses associated with sepsis and decreases mortality during 3 h of MV.

Stretch-induced IL-8 depends on c-Jun NH2-terminal and nuclear factor-kappaB-inducing kinases

Positive pressure ventilation with large tidal volumes has been shown to cause release of cytokines, including interleukin (IL)-8. The mechanisms regulating lung stretch-induced cytokine production are unclear. We hypothesized that stretch-induced IL-8 production is dependent on the activation of the mitogen-activated protein kinases, c-Jun NH2-terminal kinases (JNK), p38, and/or extracellular signal-regulated kinase (ERK) 1/2. We exposed A549 cells, a type II-like alveolar epithelial cell line, to cyclic stretch at 20 cycles/min for 5 min-2 h. Cyclic stretch induced IL-8 protein production, IL-8 mRNA expression, and JNK activation, but only transient activation of p38 and ERK1/2. Inhibition of stretch-induced JNK activation by adenovirus-mediated gene transfer of stress-activated protein kinase (SEK-1), a dominant-negative mutant of SEK-1, the immediate upstream activator of the JNKs, and pharmacological JNK inhibitor II SP-600125 blocked IL-8 mRNA expression and attenuated IL-8 production. Inhibition of p38 and ERK1/2 did not affect stretch-induced IL-8 production. Stretch-induced activation NF-kappaB and activator protein (AP)-1 was blocked by NF-kappaB inhibitor and JNK inhibitor, respectively. An NF-IL-6 site was not essential for cyclic stretch-induced IL-8 promoter activity. Stretch also induced NF-kappaB-inducing kinase (NIK) activation, and inhibition of NF-kappaB attenuated IL-8 mRNA expression and IL-8 production. We conclude that stretch-induced transcriptional regulation of IL-8 mRNA and IL-8 production was via activation of AP-1 and NF-kappaB and was dependent on JNK and NIK activation, respectively.

Gene expression analysis in interstitial lung edema induced by saline infusion

To investigate the molecular events taking place during the development of hydraulic interstitial edema, we analyzed by microarray and conventional molecular techniques the variation of gene expression in lung from rabbits treated with slow-rate saline infusions. This analysis indicates that even a condition characterized by a small increase in extravascular water can have a significant influence on the inflammatory milieu. In this regard, cytokines, in particular TNFalpha, can be considered early mediators capable of inducing secondary effects on the injured tissue. Moreover, two MT1 genes were strongly up-regulated, data consistent with their role as protective molecules.

Positive end-expiratory pressure delays the progression of lung injury during ventilator strategies involving high airway pressure and lung overdistention

OBJECTIVE

Many studies have investigated the protective role of positive end-expiratory pressure (PEEP) on ventilator-induced lung injury. Most assessed lung injury in protocols involving different ventilation strategies applied for the same length of time. This study, however, set out to investigate the protective role of PEEP with respect to the time needed to reach similar levels of lung injury.

DESIGN

Prospective, randomized laboratory animal investigation.

SETTING

The University Laboratory of Ospedale Maggiore, Milano, IRCCS.

SUBJECTS

Anesthetized, paralyzed, and mechanically ventilated Sprague-Dawley rats.

INTERVENTIONS

Three groups of five Sprague-Dawley rats were ventilated using zero end-expiratory pressure ZEEP (PEEP of 0 cm H(2)O) and PEEP of 3 and 6 cm H(2)O and a similar index of lung overdistension (Paw(p)/P(100) congruent with 1.1; where Paw(p) is peak airway pressure and P(100) is the pressure corresponding to total lung capacity). To obtain this, tidal volume was reduced depending on the PEEP. To reach similar levels of lung injury, we measured respiratory system elastance while ventilating the animals and killed them when respiratory system elastance was 150% of baseline. Once target respiratory system elastance was reached, the lung wet-to-dry ratio was obtained.

RESULTS

Rats were ventilated with comparable high airway pressure (Paw(p) of 42.8 +/- 3.1, 43.5 +/- 2.6, and 46.2 +/- 4.4, respectively, for PEEP 0, 3, and 6) obtaining similar overdistension (Paw(p)/P(100) - index of overdistension: 1.17 +/- 0.2, 1.06 +/- 0.1, and 1.19 +/- 0.2). The respiratory system elastance target was reached and wet-to-dry ratio was not different in the three groups, suggesting a similar degree of lung damage. The time taken to achieve the target respiratory system elastance was three times longer with PEEP 3 and 6 (55 +/- 14 mins and 60 +/- 17) as compared with zero end-expiratory pressure (18 +/- 3 mins, p <.001).

CONCLUSION

These findings confirm that PEEP is protective against ventilator-induced lung injury and may enable the clinician to "buy time" in the progression of lung injury.

Effect of ventilatory rate on airway cytokine levels and lung injury

BACKGROUND

Controversy exists regarding the effect of large-volume mechanical ventilation (MV), as a sole stimulus, on the pulmonary cytokine milieu. We used a well described experimental model of ventilator-induced lung injury (VILI) to examine the impact of large volume ventilation on pulmonary cytokines in vivo and to study the effect of respiratory rate (RR) variation on these levels.

MATERIALS AND METHODS

Sixty rats (410 +/- 47 g) were randomized to: 1) non ventilated control; 2) V(t) = 40 ml/kg, RR = 40 bpm; 3) V(t) = 40 ml/kg, RR = 20 bpm; 4) V(t) = 7 ml/kg, RR = 40 bpm; or 5) V(t) = 7 ml/kg, RR = 20 bpm. After 1 h of MV, bronchoalveolar lavage (BAL) and serum were collected. BAL was analyzed for urea, protein, lactate dehydrogenase (LDH), tumor necrosis factor (TNF)alpha and interleukin (IL)-6. Epithelial lining fluid volume (ELF) was calculated.

RESULTS

Regardless of RR, animals ventilated at 7 ml/kg did not differ from control in any outcome. In contrast, MV at 40 ml/kg V(t) with 40 bpm produced lung injury characterized by significant elevations of BAL TNFalpha, IL-6, protein, ELF, and LDH. At 40 ml/kg V(t), RR reduction (20 bpm) significantly reduced all injury measures.

CONCLUSION

This study confirms that large-volume MV, as a sole stimulus, produces lung injury and cytokine release. Whereas increasing RR at low V(t) has little impact on injury parameters, RR reduction under VILI-promoting conditions significantly limits lung injury.

Atelectasis causes vascular leak and lethal right ventricular failure in uninjured rat lungs

During mechanical ventilation, lung recruitment attenuates injury caused by high VT, improves oxygenation, and may optimize pulmonary vascular resistance (PVR). We hypothesized that ventilation without recruitment would induce injury in otherwise healthy lungs. Anesthetized rats were ventilated with conventional mechanical ventilation (VT 8 ml/kg; respiratory frequency 40 per minute) and 21% inspired oxygen, with or without a recruitment strategy consisting of recruitment maneuvers plus positive end-expiratory pressure, in the presence or absence of a laparotomy. Additional experiments examined the impact of atelectasis on right ventricular function using echocardiography, as well as functional residual capacity and PVR. Lack of recruitment resulted in reduced overall survival (59% nonrecruited vs. 100% recruited, p < 0.05), increased microvascular leak, greater impairment of oxygenation and lung compliance, increased PVR, and elevated plasma lactate. Echocardiography demonstrated that right ventricular dysfunction occurred in the absence of recruitment. Finally, samples from nonrecruited lungs demonstrated ultrastructural evidence of microvascular endothelial disruption. Although such effects clearly do not occur with comparable magnitude in the clinical context, the current data suggest novel mechanisms (microvascular leak, right ventricular dysfunction) whereby derecruitment may contribute to development of lung injury and adverse systemic outcome.

Ventilation-induced lung injury is associated with an increase in gut permeability

Mechanical ventilation is associated with several harmful effects mainly related to high tidal volumes (Vt). Ventilator-induced lung injury can be responsible for an increased production of inflammatory mediators. We evaluated remote consequences on the gut of lung triggered inflammatory response, neutralizing anti-tumor necrosis factor (TNF) antibody was administered to assess the role of TNF in lung and gut permeability changes. Rats were anesthetized and ventilated for 2 h. A control group (Con: Vt = 10 mL/kg) was compared with a high Vt group (HV: Vt = 30 ml/kg). One microCi of I125-labeled human serum albumin was injected to measure extravascular albumin space. Gut permeability was evaluated by plasma-to-lumen ratio leakage of I125 human serum albumin. Extravascular albumin space increased in the HV group from 446 +/- 50 microL to 2783 +/- 887 microL. Gut index of permeability increased from 5.1 +/- 1.2 to 14.2 +/- 4.9. Anti-TNF antibody prevented both lung and gut increase in permeability. High tidal volume ventilation resulted in an increase in lung edema and gut permeability, antagonism of TNF with neutralizing antibodies abrogated the increase in gut permeability as well as lung edema.

Ventilator-induced heat shock protein 70 and cytokine mRNA expression in a model of lipopolysaccharide-induced lung inflammation

OBJECTIVE

To investigate the effect of mechanical ventilation with no PEEP (ZEEP) and 4 cmH(2)O PEEP on heat shock protein 70 (HSP70) and pulmonary inflammatory cytokine expression in a model of lipopolysaccharide (LPS) induced lung inflammation.

DESIGN AND SETTING

Prospective, randomized, experimental animal study.

SUBJECTS AND INTERVENTIONS

We challenged 42 male Sprague-Dawley rats intratracheally with LPS. After 24 h the rats were randomly assigned to one of the ventilation strategies. Rats received either 4 h of mechanical ventilation with ZEEP or mechanical ventilation with 4 cmH(2)O PEEP. A nonventilated control group received LPS only. Lung pathology after LPS challenge was evaluated by histology to assess baseline lung injury. HSP70 and cytokine mRNA levels were measured in total lung homogenates.

RESULTS

PaO(2) levels and lung histology revealed no deterioration after PEEP ventilation and severe deterioration after ZEEP ventilation. There was a significant higher expression of HSP70 and IL-1beta mRNA in the lungs of the ZEEP group than in the PEEP group and nonventilated controls. In the ZEEP group high HSP70 levels were correlated inversely with low IL-1beta mRNA and low IL-6 mRNA.

CONCLUSIONS

We propose that HSP70 expression protects the lung against ventilator-induced lung injury by decreasing cytokine transcription in the lung.

Effects of albumin and Ringer's lactate on production of lung cytokines and hydrogen peroxide after resuscitated hemorrhage and endotoxemia in rats

RATIONALE AND HYPOTHESIS

Acute lung injury is a frequent complication of severe sepsis or blood loss and is often associated with an excessive inflammatory response requiring mechanical ventilation. We tested the hypothesis that the types of fluids used during early resuscitation have an important effect on the evolution of lung injury.

METHODS

Rats were subjected to either hemorrhage or endotoxemia for 1 hr, followed by resuscitation to a controlled mean blood pressure with Ringer's lactate, 5% albumin, or 25% albumin for 1 hr. After resuscitation, blood cytokine levels were measured. The lung was then excised and ventilated with a tidal volume of 30 mL/kg for 2 hrs.

RESULTS

The volume of fluids required was significantly smaller in the albumin-treated groups than in the Ringer's lactate groups. In the hemorrhagic shock model, plasma concentrations of tumor necrosis factor-alpha, interleukin-6, and macrophage inflammatory protein-2 were significantly lower and interleukin-10 was significantly higher in the albumin-treated groups compared with the Ringer's lactate-treated group. The levels of tumor necrosis factor-alpha and macrophage inflammatory protein-2 in bronchoalveolar lavage fluid were lower and interleukin-10 was higher in the albumin-treated groups than in the Ringer's lactate group. The decreased cytokine production was associated with a reduction of hydrogen peroxide formation with albumin resuscitation. The lung wet/dry ratio was lower in the 5% albumin (0.54 +/- 0.01) and 25% albumin (0.55 +/- 0.02) groups than in the Ringer's lactate group (0.62 +/- 0.02; both p <.05). These effects of albumin seen in the hemorrhagic shock model were not observed in the endotoxic shock model.

CONCLUSIONS

We conclude that resuscitation with albumin may have utility in reducing ventilator-induced lung injury after hemorrhagic shock, but not after endotoxic shock. These findings suggest that the mechanisms leading to ventilator-induced lung injury after hemorrhage differ from those after endotoxemia.

Pulmonary lymphatics and edema accumulation after brief lung injury

In a past study of hyperoxia-induced lung injury, the extensive lymphatic filling could have resulted from lymphatic proliferation or simple lymphatic recruitment. This study sought to determine whether brief lung injury could produce similar changes, to show which lymphatic compartments fill with edema, and to compare their three-dimensional structure. Tracheostomized rats were ventilated at high tidal volume (12-16 ml) or low tidal volume (3-5 ml) or allowed to breathe spontaneously for 25 min. Light microscopy showed more perivascular, interlobular septal, and alveolar edema in the animals ventilated at high tidal volume (P < 0.0001). Scanning electron microscopy of lymphatic casts showed extensive filling of the perivascular lymphatics in the group ventilated at high tidal volume (P < 0.01), but lymphatic filling was greater in the nonventilated group than in the group that was ventilated at low tidal volume (P < 0.01). The three-dimensional structures of the cast interlobular and perivascular lymphatics were similar. There was little filling and no difference in pleural lymphatic casts among the three groups. More edema accumulated in the surrounding lymphatics of larger blood vessels than smaller blood vessels. Brief high-tidal-volume lung injury caused pulmonary edema similar to that caused by chronic hyperoxic lung injury, except it was largely restricted to perivascular and septal lymphatics and prelymphatic spaces.

Mechanical ventilation may increase susceptibility to the development of bacteremia

OBJECTIVE

We examined the hypothesis that mechanical ventilation with a potentially injurious strategy would predispose animals to the detrimental effects of subsequent instillation of bacteria.

DESIGN

Interventional animal study.

SETTING

A university hospital research laboratory.

SUBJECTS

Fifty Sprague-Dawley male rats.

INTERVENTIONS

Rats were anesthetized and randomized to receive a protective (tidal volume 7 mL/kg, positive end-expiratory pressure 5 cm H(2)O, n = 25) or an injurious ventilatory strategy (tidal volume 21 mL/kg, zero positive end-expiratory pressure, n = 25). Hemodynamics were similar during the 1-hr ventilation period in the two groups. Animals were then disconnected from the ventilator and Pseudomonas aeruginosa was instilled intratracheally before extubation. Thereafter, animals breathed spontaneously; mortality rate was assessed up to 48 hrs, at which time the animals were killed.

MEASUREMENTS AND MAIN RESULTS

The 48-hr mortality rate was 28% in the protective group and 40% in the injurious group (p = not significant). A positive bacterial culture from the lung was obtained in 56% of the surviving rats in the low tidal volume group and 67% in the high tidal volume group (p =.059). A positive blood bacterial culture was found in 11% of the low tidal volume group and 33% in the high tidal volume group (p <.05). The absolute bacterial count in the blood was lower in the low tidal volume group compared with the high tidal volume group (p <.05). Concentrations of blood tumor necrosis factor-alpha and macrophage inflammatory protein-2, and lung macrophage inflammatory protein-2 at 48 hrs were significantly higher in the low tidal volume group than in the high tidal volume group.

CONCLUSIONS

An injurious ventilatory strategy predisposes animals to subsequent bacteremia associated with an impaired host defense reflected by cytokine response.

Early surfactant administration protects against lung dysfunction in a mouse model of ARDS

Sepsis can predispose the lung to insults such as mechanical ventilation (MV). It was hypothesized that treating the lung with exogenous surfactant early in the development of sepsis will reduce the lung dysfunction associated with MV 18 h later. Mice underwent sham or cecal ligation and perforation (CLP) surgery. Immediately after surgery, mice were either untreated or given 100 mg/kg of bovine lipid extract surfactant intratracheally. Eighteen hours later, the lungs were removed and analyzed either immediately or following ventilation ex vivo for 2 h by an "injurious" mode of ventilation (20 ml/kg, 0 cm positive end-expiratory pressure). In nonventilated lungs, exogenous surfactant had no impact on compliance or IL-6 concentrations in the lungs. In the ventilated groups, the administered surfactant had a significant protective effect on the lung dysfunction induced by MV, but only in the CLP lungs. We conclude that administration of exogenous surfactant at the time of a systemic insult can protect the lung from the damaging effects of MV 18 h later.

Septic shock, multiple organ failure, and acute respiratory distress syndrome

In 1914, Schottmueller wrote "Septicemia is a state of microbial invasion from a portal of entry into the blood stream which causes signs of illness." In the last few decades, the evidence that sepsis results from an exaggerated systemic inflammatory host response induced by infecting organisms is compelling; inflammatory mediators are the key players in the pathogenesis of septic shock and multiorgan failure. Sepsis and its sequelae represent a continuum of clinical syndrome encompassing systemic inflammation, coagulopathy, and hemodynamic abnormalities. Severe sepsis and septic shock continue to be the major causes of morbidity and mortality in the United States; sepsis deaths currently match mortality from myocardial infarction. Despite significant advances in our understanding of the pathophysiology and technological innovations in the supportive management, mortality from septic shock remains excessive. After many disappointments with strategies to manipulate the inflammatory response, modulation of coagulation cascade to decrease sepsis mortality has become a clinical reality. This review will highlight and discuss recent advances in the pathophysiology and management of sepsis.

Ventilator-induced cell wounding and repair in the intact lung

We tested the hypothesis that cells of ventilator-injured lungs are subject to reversible plasma membrane stress failure. Rat lungs were perfused with the membrane impermeable fluorescent marker propidium iodide and randomized to one of four ventilation strategies. Subpleural lung regions were imaged with confocal microscopy, and cell injury was quantified as the number of propidium iodide-positive cells per alveolus. The number of injured cells was significantly greater in lungs ventilated with large tidal volumes and zero end-expiratory pressure than in lungs ventilated with small tidal volumes and positive end-expiratory pressure (p < 0.01). Cell injury correlated with lung weight gain, change in dynamic compliance, and histologic injury scores. In a second set of experiments, lungs were mechanically ventilated for 30 minutes at high tidal volume settings, whereas propidium iodide was perfused either during or after injurious ventilation. Labeling after removal of injurious stress revealed significantly fewer injured cells (0.25 +/- 0.09 to 0.08 +/- 0.08, p < 0.01). We conclude that cells of ventilator-injured lungs are subject to reversible plasma membrane stress failure.

Molecular mechanisms of lung cell activation induced by cyclic stretch

OBJECTIVE

To review molecular mechanisms of lung cell activation by stretch.

DATA SOURCES

Published original and review articles.

DATA SUMMARY

Positive-pressure mechanical ventilation is associated with both beneficial and harmful effects. Data indicate that mechanical ventilation can induce, or increase, lung inflammation. This effect is clearly linked to the degree of lung cell stretching. By modeling cyclic stretch in cultured cells, it has been possible to investigate the cellular pathways activated by this mechanical strain. Integrin receptors, proteins of the focal adhesion plaque, and the cytoskeleton itself participate in the multiple molecular complex that senses cyclic stretch, transforming a mechanical signal into a biological response. Several intracellular signaling pathways then are activated and eventually result in increased transcription of genes harboring "stretch-response elements" in their promoters. Among these pathways, the mitogen-activated protein kinase signaling cascade appears to be central in mediating the effects of cell stretching. Other posttranscriptional mechanisms, such as messenger RNA stabilization and the secretion of preformed mediators, also may account for the secretion of inflammatory mediators after cyclic stretch.

CONCLUSION

Identification of the relevant molecular mechanisms will help in the development of novel ventilatory and pharmacologic therapeutic strategies aimed at preventing the deleterious effects of mechanical ventilation.

Future research directions in acute lung injury: summary of a National Heart, Lung, and Blood Institute working group

Acute lung injury (ALI) and its more severe form, the acute respiratory distress syndrome (ARDS), are syndromes of acute respiratory failure that result from acute pulmonary edema and inflammation. The development of ALI/ARDS is associated with several clinical disorders including direct pulmonary injury from pneumonia and aspiration as well as indirect pulmonary injury from trauma, sepsis, and other disorders such as acute pancreatitis and drug overdose. Although mortality from ALI/ARDS has decreased in the last decade, it remains high. Despite two major advances in treatment, low VT ventilation for ALI/ARDS and activated protein C for severe sepsis (the leading cause of ALI/ARDS), additional research is needed to develop specific treatments and improve understanding of the pathogenesis of these syndromes. The NHLBI convened a working group to develop specific recommendations for future ALI/ARDS research. Improved understanding of disease heterogeneity through use of evolving biologic, genomic, and genetic approaches should provide major new insights into pathogenesis of ALI. Cellular and molecular methods combined with animal and clinical studies should lead to further progress in the detection and treatment of this complex disease.

The effects of cyclic stretch on gene transfer in alveolar epithelial cells

Cyclic stretch has been shown to alter cell physiology, cytoskeletal structure, signal transduction, and gene expression in a variety of cell types. To determine the effects of stretch on the gene transfer process, we compared the transfection efficiencies of human A549 cells grown either statically or exposed to 10% cyclic stretch (Delta surface area) at 60 cycles/min (1 Hz) for 24 hours prior to, and/or after transfection with pEGFP-N1 and pCMV-lux-DTS using lipoplex or electroporation. Stretching the cells prior to transfection had no effect on gene transfer. By contrast, cyclic, but not continuous, stretch applied immediately after transfection for as little as 30 minutes resulted in a 10-fold increase in gene transfer and expression by either transfection technique. These stretch conditions did not result in rupture of the plasma membrane based on the fact that DNA was unable to enter stretched cells unless either an electric field was applied or the DNA was complexed with liposomes. Taken together with the timing of the stretch response and the known effects of stretch on transcription, these findings suggest that cyclic stretch may be altering the intracellular transport of plasmids to increase gene expression.

Bone marrow transplantation reveals an essential synergy between neuronal and hemopoietic cell neurokinin production in pulmonary inflammation

Neurogenic inflammation is believed to originate with the antidromic release of substance P, and of other neurokinins encoded by the preprotachykinin A (PPT-A) gene, from unmyelinated nerve fibers (C-fibers) following noxious stimuli. Consistent with this concept, we show here that selective sensory-fiber denervation with capsaicin and targeted deletion of the PPT-A gene protect murine lungs against both immune complex-mediated and stretch-mediated injuries. Reconstitution of PPT-A gene-deleted mice with WT bone marrow does not abrogate this protection, demonstrating a critical role for PPT-A gene expression by sensory neurons in pulmonary inflammation. Surprisingly, reconstitution of WT mice with PPT-A gene-deficient bone marrow also confers protection against pulmonary injury, revealing that PPT-A gene expression in hemopoietic cells has a previously unanticipated essential role in tissue injury. Taken together, these findings demonstrate a critical synergy between capsaicin-sensitive sensory fibers and hemopoietic cells in neurokinin-mediated inflammation and suggest that such synergy may be the basis for a stereotypical mechanism of response to injury in the respiratory tract.

The role of apoptosis in acute lung injury

OBJECTIVE

To discuss current aspects of our understanding of the role of apoptosis in lung injury.

DATA SOURCES

Review of English language literature.

DATA SUMMARY

Apoptosis is a process that produces timely death in senescent cells. Apoptosis is important in developmental biology and in remodeling of tissues during repair. Many apoptosis pathways converge in intracellular protease cascades that lead to DNA cleavage and cell death. Apoptosis pathways can be triggered by surface receptors, which interact with soluble proteins or membrane-bound proteins, such as Fas ligand. Fas ligand accumulates in soluble form at sites of tissue inflammation and has the potential to initiate apoptosis of leukocytes, epithelial cells, and other parenchymal cells. Dysregulation of apoptosis pathways could contribute to the epithelial injury that is characteristic of acute lung injury in humans. The effects of Fas ligand are modulated by factors in lung fluids, such as cytokines (e.g., transforming growth factor-beta, surfactant protein A, and angiotensin II) and a specific Fas ligand decoy receptor (DcR3).

CONCLUSION

Strategies to block apoptosis pathways could be useful in limiting some forms of acute lung injury in humans.

Mechanisms of pulmonary edema clearance during acute hypoxemic respiratory failure: role of the Na,K-ATPase

Pulmonary edema is the hallmark of acute respiratory distress syndrome. It occurs when the permeability of the alveolar-capillary barrier is increased, causing alveolar flooding and impaired gas exchange. The mechanisms of alveolar fluid resorption are different from those of alveolar edema formation. Alveolar fluid resorption into the vessels is brought about mainly by active transport of sodium ions (Na+) out of the alveolar spaces with water following the osmotic gradient. Na+ transport across the alveolar epithelium, and thus alveolar fluid resorption, is regulated by apical Na+ channels, the basolateral sodium potassium-adenosine triphosphatase (Na,K-ATPase), and possibly chloride channels. The Na,K-ATPase has been localized to the alveolar epithelium and the importance of its role in contributing to lung edema clearance has been demonstrated. In models of lung injury, several reports have shown that catecholamines such as isoproterenol and dopamine up-regulate Na+ channels and the Na,K-ATPase giving rise to increased alveolar fluid resorption. Although recombinant gene technology is not yet a therapeutic option for the treatment of pulmonary edema, several experimental studies have reported that overexpression of Na,K-ATPase genes causes increased fluid resorption during hyperoxic lung injury. There is significant evidence that fluid clearance is impaired in patients with lung injury. Therapeutic strategies aimed at increasing the ability of alveolar epithelium to resorb the edema should lead to benefits for patients with acute respiratory distress syndrome.

Epidemiology and outcome of acute respiratory failure in intensive care unit patients

OBJECTIVES

To summarize the prevalence of various forms of acute respiratory failure in acutely ill patients and review the major factors involved in the outcome of these patients.

DATA SOURCES AND SELECTION

MEDLINE search for published studies reporting the prevalence or outcome for patients with acute respiratory failure and cited reference studies and abstracts from a recent international meeting in the intensive care medicine field.

DATA SYNTHESIS AND EXTRACTION

From the selected articles, information was obtained regarding the prevalence of acute respiratory failure, including acute respiratory distress syndrome and acute lung injury as defined by the North American-European Consensus Conference, the outcome, and the factors influencing mortality rates in this population of patients.

CONCLUSIONS

The prevalence of acute respiratory failure varies according to the definition used and the population studied. Nonsurvivors of acute respiratory distress syndrome die predominantly of respiratory failure in <20% of cases. The relatively high mortality rates of acute lung injury/acute respiratory distress syndrome are primarily related to the underlying disease, the severity of the acute illness, and the degree of organ dysfunction.