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

Clara cell secretory protein and phospholipase A2activity modulate acute ventilator-induced lung injury in mice

Lung vascular permeability is acutely increased by high-pressure and high-volume ventilation. To determine the roles of mechanically activated cytosolic PLA2(cPLA2) and Clara cell secretory protein (CCSP), a modulator of cPLA2activity, we compared lung injury with and without a PLA2inhibitor in wild-type mice and CCSP-null mice (CCSP−/−) ventilated with high and low peak inflation pressures (PIP) for 2- or 4-h periods. After ventilation with high PIP, we observed significant increases in the bronchoalveolar lavage albumin concentrations, lung wet-to-dry weight ratios, and lung myeloperoxidase in both genotypes compared with unventilated controls and low-PIP ventilated mice. All injury variables except myeloperoxidase were significantly greater in the CCSP−/−mice relative to wild-type mice. Inhibition of cPLA2in wild-type and CCSP−/−mice ventilated at high PIP for 4 h significantly reduced bronchoalveolar lavage albumin and total protein and lung wet-to-dry weight ratios compared with vehicle-treated mice of the same genotype. Membrane phospho-cPLA2and cPLA2activities were significantly elevated in lung homogenates of high-PIP ventilated mice of both genotypes but were significantly higher in the CCSP−/−mice relative to the wild-type mice. Inhibition of cPLA2significantly attenuated both the phospho-cPLA2increase and increased cPLA2activity due to high-PIP ventilation. We propose that mechanical activation of the cPLA2pathway contributes to acute high PIP-induced lung injury and that CCSP may reduce this injury through inhibition of the cPLA2pathway and reduction of proinflammatory products produced by this pathway.

Lung Development and Susceptibility to Ventilator-induced Lung Injury

RATIONALE

Ventilator-induced lung injury has been predominantly studied in adults.

OBJECTIVES

To explore the effects of age and lung development on susceptibility to such injury.

METHODS

Ex vivo isolated nonperfused rat lungs (infant, juvenile, and adult) were mechanically ventilated where VT was based on milliliters per kilogram of body weight or as a percentage of the measured total lung capacity (TLC). In vivo anesthetized rats (infant, adult) were mechanically ventilated with pressure-limited VTs. Allocation to ventilation strategy was randomized.

MEASUREMENTS

Ex vivo injury was assessed by pressure-volume analysis, reduction in TLC, and histology, and in vivo injury by lung compliance, cytokine production, and wet- to dry-weight ratio.

MAIN RESULTS

Ex vivo ventilation (VT 30 ml.kg(-1)) resulted in a significant reduction (36.0 +/- 10.1%, p < 0.05) in TLC in adult but not in infant lungs. Ex vivo ventilation (VT 50% TLC) resulted in a significant reduction in TLC in both adult (27.8 +/- 2.8%) and infant (10.6 +/- 7.0%) lungs, but more so in the adult lungs (p < 0.05); these changes were paralleled by histology and pressure-volume characteristics. After high stretch in vivo ventilation, adult but not infant rats developed lung injury (total lung compliance, wet/dry ratio, tumor necrosis factor alpha). Surface video microscopy demonstrated greater heterogeneity of alveolar distension in ex vivo adult versus infant lungs.

CONCLUSION

These data provide ex vivo and in vivo evidence that comparable ventilator settings are significantly more injurious in the adult than infant rat lung, probably reflecting differences in intrinsic susceptibility or inflation pattern.

Protective effects of low respiratory frequency in experimental ventilator-associated lung injury*

OBJECTIVE

To determine whether ventilator-associated lung hyperinflation injury can be attenuated by a reduction in respiratory frequency.

DESIGN

Prospective comparative laboratory investigation.

SETTING

University medical center research laboratory.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

Eight groups of isolated, perfused rat lungs were exposed to cyclic ventilation at different respiratory frequencies and tidal volumes. Each group of six to eight lung preparations was assigned to one of four respiratory frequencies (10, 20, 40, or 80 breaths/min) and one of two tidal volumes (5 or 20 mL.kg). Measurement of capillary filtration coefficient (Kf,c), a sensitive index of lung microvascular permeability and injury, was made at baseline and at 30, 60, and 90 mins of the experimental conditions.

MEASUREMENTS AND MAIN RESULTS

Lungs exposed to 5 mL.kg tidal volume had no elevation in Kf,c at any time point regardless of respiratory frequency. Lungs exposed to 20 mL. kg tidal volume and a respiratory frequency of 80 had significant elevations in Kf,c at all times after baseline compared with lungs exposed to respiratory frequencies of 10, 20, or 40 (0.14 +/- 0.03, 0.16 +/- 0.02, 0.31 +/- 0.05 vs. 0.76 +/- 0.16). Furthermore, the Kf,c at 90 mins was significantly higher than permeability at baseline in this group (1.53 +/- 0.45 vs. 0.12 +/- 0.02 mL.min.cm H2O.100 g of lung tissue).

CONCLUSIONS

Reduction in respiratory frequency to values much lower than normal ameliorated experimental ventilator-induced hyperinflation lung injury as determined by pulmonary capillary filtration coefficient.

Comparison of the effect of aminophylline and low PEEP vs. high PEEP on EGF concentration in critically ill patients with ALI/ARDS

BACKGROUND

Acute respiratory distress syndrome (ARDS) remains a serious, often fatal, condition, despite progress in modern critical care treatment. Cytokines play important roles in the pathogenesis of the syndrome, although their roles in the evaluation and outcome have not been clearly elucidated yet.

OBJECTIVES

We tested whether serum concentration of epidermal growth factor (EGF), as one of the important inflammatory mediators, changes with time and administration of mechanical ventilation and aminophylline.

PATIENTS AND METHODS

Thirty patients [mean (SD): age = 56.6 (17.4) years] with ARDS were enrolled. After diagnosis based on inclusion and exclusion criteria, the patients were intubated and mechanically ventilated. Two hours after ventilation with definite positive end-expiratory pressure (PEEP), aminophylline with a specific dose was started. Serum samples were obtained at five time points of 0, 2, 2.5, 4 and 8 h post-starting PEEP.

RESULTS

Serum EGF concentration decreased after mechanical ventilation with PEEP (P < 0.05). The serum EGF concentrations 8 h after intervention was statistically lower in the low PEEP group than in the high PEEP group. The Acute Physiology and Chronic Health Evaluation (APACHE) Pi score and PaO2/FiO2 improved significantly after 8 h (P < 0.05).

CONCLUSION

Beneficial effects of mechanical ventilation and aminophylline on APACHE Pi score and PaO2/FiO2 influence serum EGF levels. These findings may have relevance to the development of multisystem organ failure.

Reduced activation of immunomodulatory transcription factors during positive end-expiratory pressure adjustment based on volume-dependent compliance in isolated perfused rabbit lungs

BACKGROUND

Repeated alveolar collapse and cyclic alveolar overdistension with associated activation of inflammatory signalling cascades contribute to ventilator-induced lung injury (VILI). The appropriate positive end-expiratory pressure (PEEP) which prevents or ameliorates VILI is unknown. In the isolated perfused lung, repeated adjustments of PEEP based on the continuously analysed intratidal compliance-volume curve have previously been shown to result in full end-expiratory alveolar recruitment and low risk of cyclic alveolar overdistension. Accordingly, we tested the hypothesis that such ventilatory management reduces intrapulmonary activation of the immunomodulatory transcription factors nuclear factor kappaB (NF-kappaB), activator protein 1 (AP-1) and cAMP-responsive element binding protein (CREB) which induce the expression of various chemokines and cytokines.

METHODS

Isolated perfused rabbit lungs were randomly allocated to one of three groups: zero end-expiratory pressure (ZEEP) to induce repeated alveolar collapse (n=6), high PEEP to induce cyclic alveolar overdistension (n=6) and repeated PEEP adjustments based on intratidal compliance-volume curve analysis by the slice method to minimize repeated alveolar collapse and overdistension (n=9). All lungs were ventilated with a tidal volume of 6 ml kg(-1) bodyweight for 120 min. Thereafter, activation of transcription factors NF-kappaB, AP-1 and CREB in lung tissue was analysed by electrophoretic mobility shift assay.

RESULTS

High PEEP was associated with the highest activation of NF-kappaB and AP-1 and repeated PEEP adjustments with the lowest activation when compared with the other two study groups (P<0.001). In contrast, activation of CREB did not differ between groups. Activated NF-kappaB and AP-1 protein complexes consisted mainly of the transactivators p50/p65 and c-Fos/Jun, respectively.

CONCLUSIONS

In isolated perfused rabbit lungs, repeated adjustments of PEEP based on the continuously analysed intratidal compliance-volume curve were associated with less activation of early steps of inflammatory signalling cascades than ventilation with ZEEP or high PEEP.

The role of hyaluronan synthase 3 in ventilator-induced lung injury

We recently found that low-molecular-weight hyaluronan was induced by cyclic stretch in lung fibroblasts and accumulated in lungs from animals with ventilator-induced lung injury. The low-molecular-weight hyaluronan produced by stretch increased interleukin-8 production in epithelial cells, and was accompanied by an upregulation of hyaluronan synthase-3 mRNA. We hypothesized that low-molecular-weight hyaluronan induced by high VT was dependent on hyaluronan synthase 3, and was associated with ventilator-induced lung injury. Effects of high VT ventilation in C57BL/6 wild-type and hyaluronan synthase-3 knockout mice were compared. Significantly increased neutrophil infiltration, macrophage inflammatory protein-2 production, and lung microvascular leak were found in wild-type animals ventilated with high VT. These reactions were significantly reduced in hyaluronan synthase-3 knockout mice, except the capillary leak. Wild-type mice ventilated with high VT were found to have increased low-molecular-weight hyaluronan in lung tissues and concomitant increased expression of hyaluronan synthase-3 mRNA, neither of which was found in hyaluronan synthase-3 knockout mice. We conclude that high VT induced low-molecular-weight hyaluronan production is dependent on de novo synthesis through hyaluronan synthase 3, and plays a role in the inflammatory response of ventilator-induced lung injury.

Effects of mechanical ventilation at low lung volume on respiratory mechanics and nitric oxide exhalation in normal rabbits

Lung mechanics, exhaled NO (NOe), and TNF-alpha in serum and bronchoalveolar lavage fluid were assessed in eight closed and eight open chest, normal anesthetized rabbits undergoing prolonged (3-4 h) mechanical ventilation (MV) at low volume with physiological tidal volumes (10 ml/kg). Relative to initial MV on positive end-expiratory pressure (PEEP), MV at low volume increased lung quasi-static elastance (+267 and +281%), airway (+471 and +382%) and viscolelastic resistance (+480 and +294%), and decreased NOe (-42 and -25%) in closed and open chest rabbits, respectively. After restoration of PEEP, viscoelastic resistance returned to control, whereas airway resistance remained elevated (+120 and +31%) and NOe low (-25 and -20%) in both groups of rabbits. Elastance remained elevated (+23%) only in closed-chest animals, being associated with interstitial pulmonary edema, as reflected by increased lung wet-to-dry weight ratio with normal albumin concentration in bronchoalveolar lavage fluid. In contrast, in 16 additional closed- and open-chest rabbits, there were no changes of lung mechanics or NOe after prolonged MV on PEEP only. At the end of prolonged MV, TNF-alpha was practically undetectable in serum, whereas its concentration in bronchoalveolar lavage fluid was low and similar in animals subjected or not subjected to ventilation at low volume (62 vs. 43 pg/ml). These results indicate that mechanical injury of peripheral airways due to their cyclic opening and closing during ventilation at low volume results in changes in lung mechanics and reduction in NOe and that these alterations are not mediated by a proinflammatory process, since this is expressed by TNF-alpha levels.

Dynamic alveolar mechanics and ventilator-induced lung injury

OBJECTIVES

To review the mechanism of dynamic alveolar mechanics (i.e., the dynamic change in alveolar size and shape during ventilation) in both the normal and acutely injured lung; to investigate the alteration in alveolar mechanics secondary to acute lung injury as a mechanism of ventilator-induced lung injury (VILI); and to examine the hypothesis that the reduced morbidity and mortality associated with protective strategies of mechanical ventilation is related to the normalization of alveolar mechanics.

DATA EXTRACTION AND SYNTHESIS

This review is based on original published articles and review papers dealing with the mechanism of lung volume change at the alveolar level and the role of altered alveolar mechanics as a mechanism of VILI. In addition, data from our laboratory directly visualizing dynamic alveolar mechanics is reviewed and related to the literature.

CONCLUSIONS

The mechanism of alveolar inflation in normal lungs is unclear. Nonetheless, normal alveoli are very stable and change size very little with ventilation. Acute lung injury causes marked destabilization of individual alveoli. Alveolar instability causes pulmonary damage and is believed to be a major component in the mechanism of VILI. Ventilator strategies that reduce alveolar instability may potentially reduce the morbidity and mortality associated with VILI.

Noninvasive evaluation of acute capillary permeability changes during high-volume ventilation in rats with and without hypercapnic acidosis

OBJECTIVE

To evaluate whether hypercapnic acidosis attenuates acute alterations of pulmonary capillary permeability due to high lung stretch in rats using a simple, noninvasive, scintigraphic method.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

University research laboratory.

SUBJECTS

Male adult Wistar rats weighing 291 +/- 7.5 g.

INTERVENTIONS

Three groups of rats were studied: controls ventilated with a low (6 mL/kg body weight) tidal volume and rats ventilated with a high (38 mL/kg body weight) tidal volume under normocapnic (Paco(2) = 35.2 +/- 1.65 mm Hg) or hypercapnic (Paco(2) = 102.5 +/- 5.63 mm Hg) conditions.

MEASUREMENTS AND MAIN RESULTS

Pulmonary capillary permeability alterations were assessed by monitoring the rate of (111)In-transferrin accumulation in lung tissue. Respiratory system pressure-volume curves were registered and analyzed. High tidal volume ventilation increased In-transferrin plasma to lung flux in such a way that I(111)In-transferrin behaved like a marker of water. The rate of initial (first 30 mins of high tidal volume ventilation) lung transferrin accumulation measured by scintigraphy (standardized lung/heart ratio) was steady, correlated with the percent decrease in respiratory system compliance (a marker of edema progression), and did not differ between normocapnic and hypercapnic groups (18.9 +/- 3.97 vs. 14.2 +/- 2.89%/hr, not significant). However, lung In-tranferrin accumulation rate was highly scattered due to variable interindividual mechanical properties of the respiratory system. This rate was correlated with initial values of volume of the upper inflection point of the pressure-volume curve (r = -.53, p < .001) and end-inspiratory pressure (r = .54, p < .001). Mechanical properties were similar in normocapnic and hypercapnic rats. There was no difference between In-transferrin accumulation rates in these rats when a stringent selection was made based on end-inspiratory pressure (28-32 cm H(2)O) or body weight (330-360 g).

CONCLUSIONS

Hypercapnic acidosis does not influence in vivo the acute increase in pulmonary capillary permeability due to high-volume ventilation.

Acute lung injury and acute respiratory distress syndrome after pulmonary resection

The occurrence of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) after thoracic surgery are perplexing and persistent problems. Variously described as postpneumonectomy pulmonary edema, noncardiogenic pulmonary edema, and postlung resection pulmonary edema, ALI and ARDS may be considered a single entity, with ALI being the less severe form of ARDS. It is characterized by the acute onset of hypoxemia with radiographic infiltrates consistent with pulmonary edema, without elevations in the pulmonary capillary wedge pressure. Although this syndrome does not occur frequently and is usually without identifiable cause, the mortality is high. However, the phenomenon has not been rigorously studied owing to the low incidence, with primarily retrospective case series reported. Thus, the nomenclature, risks, and pathogenesis are not well defined. Interest in this syndrome has recently been renewed as the rate of other perioperative complications has declined. ALI/ARDS is reviewed with a focus on potential etiologies and the spectrum of available interventions.

Prone position delays the progression of ventilator-induced lung injury in rats: Does lung strain distribution play a role?*

OBJECTIVE

To investigate if prone position delays the progression of experimental ventilator-induced lung injury, possibly due to a more homogeneous distribution of strain within lung parenchyma.

DESIGN

Prospective, randomized, controlled trial.

SETTING

Animal laboratory of a university hospital.

SUBJECTS

Thirty-five Sprague Dawley male rats (weight 257 +/- 45 g).

INTERVENTIONS

Mechanical ventilation in either supine or prone position and computed tomography scan analysis.

MEASUREMENTS

: Animals were ventilated in supine (n = 15) or prone (n = 15) position until a similar ventilator-induced lung injury was reached. To do so, experiments were interrupted when respiratory system elastance was 150% of baseline. Ventilator-induced lung injury was assessed as lung wet-to-dry ratio and histology. Time to reach lung injury was considered as a main outcome measure. In five additional animals, computed tomography scans (GE Light Speed QX/I, thickness 1.25 mm, interval 0.6 mm, 100 MA, 100 Kv) were randomly taken at end-expiration and end-inspiration in both positions, and quantitative analysis was performed. Data are shown as mean +/- sd.

MEASUREMENTS AND MAIN RESULTS

Similar ventilator-induced lung injury was reached (respiratory system elastance, wet-to-dry ratio, and histology). The time taken to achieve the target ventilator-induced lung injury was longer with prone position (73 +/- 37 mins vs. 112 +/- 42, supine vs. prone, p = .011). Computed tomography scan analysis performed before lung injury revealed that at end-expiration, the lung was wider in prone position (p = .004) and somewhat shorter (p = .09), despite similar lung volumes (p = .455). Lung density along the vertical axis increased significantly only in supine position (p = .002). Lung strain was greater in supine as opposed to prone position (width strain, 7.8 +/- 1.8% vs. 5.6 +/- 0.9, supine vs. prone, p = .029).

CONCLUSIONS

Prone position delays the progression of ventilator-induced lung injury. Computed tomography scan analysis suggests that a more homogeneous distribution of strain may be implicated in the protective role of prone position against ventilator-induced lung injury.

Bench-to-bedside review: Biotrauma and modulation of the innate immune response

The innate immune network is responsible for coordinating the initial defense against potentially noxious stimuli. This complex system includes anatomical, physical and chemical barriers, effector cells and circulating molecules that direct component and system interactions. Besides the direct effects of breaching pulmonary protective barriers, cyclic stretch generated during mechanical ventilation (MV) has been implicated in the modulation of the innate immunity. Evidence from recent human trials suggests that controlling MV-forces may significantly impact outcome in acute respiratory distress syndrome. In this paper, we explore the pertinent evidence implicating biotrauma caused by cyclic MV and its effect on innate immune responses.

Modeling and Remodeling of the Lung in Neonatal Chronic Lung Disease

Neonatal chronic lung disease (CLD) is the major long-term pulmonary complication of preterm birth affecting about 20% of infants who need mechanical ventilation. CLD is the result of abnormal repair processes following inflammatory lung injury that lead to remodeling of the lung. Inflammation may be initiated by a variety of stimuli including mechanical ventilation, oxygen toxicity and infection. The resultant neutrophil chemotaxis and degranulation leads to the release of enzymes such as matrix metalloproteinases that can cause proteolysis of the lung extracellular matrix. Abnormal healing with remodeling leads to poorly compliant lungs with reduced capacity for gas exchange. Drugs can influence the normal process of lung modeling or remodeling. Fetal lung development can be influenced by glucocorticosteroids and inflammation. Both can cause abnormal lung modeling with fewer, larger alveoli and accelerated lung maturation, which confers benefits in terms of reduced morbidity and mortality from respiratory distress syndrome but potentially increases the risk of subsequent lung injury. Antioxidants, such as retinol (vitamin A), administered post-natally may reduce the effects of oxidative stress leading to a modest reduction in CLD but they require repeated intramuscular injections. Postnatal glucocorticosteroid therapy can modify the lung inflammatory response and reduce CLD but it can also have detrimental effects on the developing brain and lung, thereby creating a clinical dilemma for neonatologists. Proteinase inhibitors may be a rational therapy but more research is needed before they can be accepted as a treatment for preterm neonates.'Modeling' is defined as planning or forming that follows a set pattern. The term is used to describe the normal process of lung growth and development that culminates in mature branching alveolar air spaces surrounded by a network of capillaries. Normal lung modeling occurs under a variety of genetic and hormonal influences that can be altered, leading to abnormal patterns of growth. 'Remodeling' is defined as altering the structure of or re-making and, in the case of the lung, is used to describe the abnormal patterns of lung growth that occur after lung injury. Modeling and remodeling of the lungs occur to an extent throughout life but never more rapidly than during the fetal and early neonatal periods, and factors that influence this process may lead to development of neonatal CLD. Some of the factors involved in normal and abnormal lung modeling and inflammation and glucocorticosteroid-induced remodeling in the perinatal period, in the context of neonatal CLD, are reviewed with considerations of how various drugs may influence these processes.

Lower tidal volume ventilation and plasma cytokine markers of inflammation in patients with acute lung injury*

OBJECTIVES

To evaluate the association between interleukin-6, interleukin-8, and interleukin-10 and clinical outcomes including mortality in patients with acute lung injury and to determine whether lower tidal volume ventilation was associated with a decrease in plasma cytokines in patients with acute lung injury.

DESIGN

Multiple-center, randomized trial.

SETTING

Intensive care units in ten university centers.

PATIENTS

The study included 861 patients enrolled in the National Heart, Lung and Blood Institute Acute Respiratory Distress Syndrome Clinical Network trial of lower tidal volumes compared with traditional tidal volumes for acute lung injury.

INTERVENTIONS

Patients were randomized to a 6 mL/kg or a 12 mL/kg tidal volume strategy that has been previously described.

MEASUREMENTS AND MAIN RESULTS

Baseline plasma levels of interleukin-6, interleukin-8, and interleukin-10 were each associated with an increased risk of death in both logistic regression analyses controlling for ventilator group (odds ratio 1.63 per log-10 increment, 95% confidence interval 1.33-1.98; odds ratio 2.33 per log-10 increment, 95% confidence interval 1.79-3.03; odds ratio 2.02 per log-10 increment, 95% confidence interval 1.47-2.76, respectively) and multivariate analyses controlling for ventilation strategy, Acute Physiology and Chronic Health Evaluation III score, Pao2/Fio2 ratio, creatinine, platelet count, and vasopressor use (odds ratio 1.63 per log-10 increment, 95% confidence interval 0.93-1.49; odds ratio 1.73 per log-10 increment, 95% confidence interval 1.29-2.34; odds ratio 1.23 per log-10 increment, 95% confidence interval 0.86-1.76, respectively). Interleukin-6 and interleukin-8 levels were also associated with a significant decrease in ventilator free and organ failure free days. Patients with sepsis had the highest cytokine levels and the greatest risk of death per cytokine elevation. By day 3, the 6 mL/kg strategy was associated with a greater decrease in interleukin-6 and interleukin-8 levels. There was a 26% reduction in interleukin-6 (95% confidence interval, 12-37%) and a 12% reduction in interleukin-8 (95% confidence interval, 1-23%) in the 6 mL/kg group compared with the 12 mL/kg group.

CONCLUSIONS

In patients with acute lung injury, plasma interleukin-6 and interleukin-8 levels are associated with morbidity and mortality. The severity of inflammation varies with clinical risk factor, suggesting that clinical risk factor should be considered when both developing and testing therapeutic interventions. Low tidal volume ventilation is associated with a more rapid attenuation of the inflammatory response.

Host response to intratracheally instilled bacteria in ventilated and nonventilated rats

OBJECTIVE

Pneumonia occurs in approximately 7% of hospitalized patients. Susceptibility to certain bacteria such as Pseudomonas aeruginosa increases in critically ill patients, particularly those requiring mechanical ventilation. Previous studies investigating this susceptibility have used injurious modes of ventilation. The objective of this study was to evaluate the host's response to intratracheal instillation of P. aeruginosa in the setting of noninjurious mechanical ventilation and compare this with normal, spontaneously breathing animals receiving bacteria.

DESIGN

Randomized, controlled in vivo animal study.

SETTING

Research laboratory at a university-affiliated institution.

SUBJECTS

Adult male Sprague-Dawley rats.

INTERVENTIONS

Rats were randomized into four groups: spontaneously breathing given saline, spontaneously breathing given bacteria, mechanically ventilated given saline, and mechanically ventilated given bacteria. The ventilation strategy used involved low stretch (tidal volume of 8 mL/kg) with a positive end-expiratory pressure of 5 cm H2O.

MEASUREMENTS AND MAIN RESULTS

Lung compliance, bacterial recovery, surfactant, total cells, and cytokine concentrations in the lung lavage were analyzed after 4 hrs. Results showed that neither ventilation nor bacteria alone altered lung function, although the combination of ventilation and Pseudomonas significantly decreased arterial oxygenation and lung compliance. Increases in lavage cell counts, cytokines, and surfactant were observed in both groups administered bacteria compared with animals given saline. However, there were no significant differences in bacterial recovery, cell counts, cytokines, and surfactant measurements in the groups given bacteria.

CONCLUSIONS

These data suggest that bacterial instillation with low-stretch ventilation had a significant effect on lung function but did not alter the inflammatory response to a bacterial challenge over this time course compared with spontaneously breathing animals.

Influence of positive end-expiratory pressure (PEEP) on histopathological and bacteriological aspects of pneumonia during low tidal volume mechanical ventilation

OBJECTIVE

Ventilatory strategies combining low tidal volume (V(T)) with positive end-expiratory pressure (PEEP) are considered to be lung protective. The influence of the PEEP level was investigated on bacteriology and histology in a model of ventilator-associated pneumonia.

SUBJECTS

Nineteen New Zealand rabbits.

INTERVENTIONS

The animals were mechanically ventilated with a positive inspiratory pressure of 15 cmH(2)O and received either a zero end-expiratory pressure (ZEEP, n=6), a 5 cmH(2)O PEEP (n=5) or a 10 cmH(2)O PEEP (n=4). An inoculum of Enterobacter aerogenes was then instilled intrabronchially. The non-ventilated pneumonia group (n=4) was composed of spontaneously breathing animals which received the same inoculum. Pneumonia was assessed 24 h later.

MAIN RESULTS

The lung bacterial burden was higher in mechanically ventilated animals compared with spontaneously breathing animals. All animals from the latter group had negative spleen cultures. The spleen bacterial concentration was found to be lower in the 5 cmH(2)O PEEP group when compared to the ZEEP and 10 cmH(2)O PEEP groups (3.1+/-1.5 vs 4.9+/-1.1 and 5.0+/-1.3 log(10) cfu/g, respectively; p<0.05). Lung weight and histological score values were lower in the spontaneously breathing animals as well as in the 5 cmH(2)O PEEP group compared with the ZEEP and 10 cmH(2)O groups.

CONCLUSIONS

Mechanical ventilation substantially increased the lung bacterial burden and worsened the histological aspects of pneumonia in this rabbit model. Variations in terms of lung injury and systemic spreading of infection were noted with respect to the ventilatory strategy.

Augmented lung injury due to interaction between hyperoxia and mechanical ventilation*

OBJECTIVE

Mechanical overdistension and hyperoxia can independently cause lung injury, yet little is known about their combined effects. We hypothesized that hyperoxia exacerbates lung injury caused by large tidal volume ventilation.

DESIGN

Experimental study.

SETTING

University laboratory.

SUBJECTS

Anesthetized, paralyzed rabbits.

INTERVENTIONS

In experiment 1, 12 rabbits were ventilated with 25 mL/kg tidal volumes at positive end-expiratory pressure of 0 cm H2O for 4 hrs with either hyperoxia (HO; FiO2 = 0.5) or normoxia (NO; FiO2 = 0.21). In experiment 2, a separate group of animals were randomized to one of four groups to assess the interaction of tidal volume and inspired oxygen concentration on potential mediators of injury after 2 hrs of ventilation, before significant injury occurs: a) NO+normal tidal volume (NV; VT = 10 mL/kg); b) HO+NV; c) NO+high tidal volume (HV; VT = 25 mL/kg); d) HO+HV (n = 3 per group).

MEASUREMENTS AND MAIN RESULTS

: In the first study, HO compared with the NO group had significantly reduced PaO2/FiO2 ratio (320 +/- 110 vs. 498 +/- 98, p = .014) and increased lung injury scores at 4 hrs. Hyperoxia also significantly increased polymorphonuclear leukocytes, growth-related oncogene-alpha (2073 +/- 535 vs. 463 +/- 236 pg/mL, p = .02), and monocyte chemotactic protein-1 (7517 +/- 1612 vs. 2983 +/- 1289 pg/mL, p = .05) concentrations in bronchoalveolar lavage fluid. The second study showed increased alveolar-capillary permeability to a 70-kD fluorescent-labeled dextran only in response to the combination of both HO and HV. Chemokines and bronchoalveolar lavage fluid neutrophils were elevated in both HV groups; however, hyperoxia did not further increase chemokine or neutrophil counts over normoxia. No difference in lipid peroxidation was seen between groups.

CONCLUSIONS

Moderate hyperoxia exacerbates lung injury in a large tidal volume model of ventilator-induced lung injury. The mechanism by which this occurs is not mediated by increased production of CXC chemokines or lipid peroxidation.

Acute ventilator-induced vascular permeability and cytokine responses in isolated and in situ mouse lungs

To determine the influence of experimental model and strain differences on the relationship of vascular permeability to inflammatory cytokine production after high peak inflation pressure (PIP) ventilation, we used isolated perfused mouse lung and intact mouse preparations of Balb/c and B6/129 mice ventilated at high and low PIP. Filtration coefficients in isolated lungs and bronchoalveolar lavage (BAL) albumin in intact mice increased within 20–30 min after initiation of high PIP in isolated Balb/c lungs and intact Balb/c, B6/129 wild-type, and p55 and p75 tumor necrosis factor (TNF) dual-receptor null mice. In contrast, the cytokine response was delayed and variable compared with the permeability response. In isolated Balb/c lungs ventilated with 25–27 cmH2O PIP, TNF-α, interleukin (IL)-1β, IL-1α, macrophage inflammatory protein (MIP)-2, and IL-6 concentrations in perfusate were markedly increased in perfusate at 2 and 4 h, but only MIP-2 was detectable in intact Balb/c mice using the same PIP. In intact wild-type and TNF dual-receptor null mice with ventilation at 45 cmH2O PIP, the MIP-2 and IL-6 levels in BAL were significantly increased after 2 h in both groups, but there were no differences between groups in the BAL albumin and cytokine concentrations or in lung wet-to-dry weight ratios. TNF-α was not be detected in BAL fluids in any group of intact mice. These results suggest that the alveolar hyperpermeability induced by high PIP ventilation occurs very rapidly and is initially independent of TNF-α participation and unlikely to depend on MIP-2 or IL-6.

Mechanisms of early pulmonary neutrophil sequestration in ventilator-induced lung injury in mice

Polymorphonuclear leukocytes (PMN) play an important role in ventilator-induced lung injury (VILI), but the mechanisms of pulmonary PMN recruitment, particularly early intravascular PMN sequestration during VILI, have not been elucidated. We investigated the physiological and molecular mechanisms of pulmonary PMN sequestration in an in vivo mouse model of VILI. Anesthetized C57/BL6 mice were ventilated for 1 h with high tidal volume (injurious ventilation), low tidal volume and high positive end-expiratory pressure (protective ventilation), or normal tidal volume (control ventilation). Pulmonary PMN sequestration analyzed by flow cytometry of lung cell suspensions was substantially enhanced in injurious ventilation compared with protective and control ventilation, preceding development of physiological signs of lung injury. Anesthetized, spontaneously breathing mice with continuous positive airway pressure demonstrated that raised alveolar pressure alone does not induce PMN entrapment. In vitro leukocyte deformability assay indicated stiffening of circulating leukocytes in injurious ventilation compared with control ventilation. PMN sequestration in injurious ventilation was markedly inhibited by administration of anti-L-selectin antibody, but not by anti-CD18 antibody. These results suggest that mechanical ventilatory stress initiates pulmonary PMN sequestration early in the course of VILI, and this phenomenon is associated with stretch-induced inflammatory events leading to PMN stiffening and mediated by L-selectin-dependent but CD18-independent mechanisms.

What increases type III procollagen mRNA levels in lung tissue: stress induced by changes in force or amplitude?

We hypothesized that stress determined by force could induce higher type III procollagen (PCIII) mRNA expression than the stress determined by amplitude. To that end, rat lung tissue strips were oscillated for 1h under different amplitudes [1, 5 and 10% of resting length (L(B)), at 0.5 x 10(-2) N] and forces (0.25 x 10(-2), 0.5 x 10(-2) and 10(-2)N, at 5% L(B)). Resistance (R), elastance (E) and hysteresivity (eta) were analysed during sinusoidal oscillations at 1Hz. After 1h of oscillation, PCIII mRNA expression was determined by Northern-blot and semiquantitative RT-PCR. Control value of PCIII mRNA was obtained from unstressed strips. E and R increased with augmenting force and decreased with increasing amplitude, while eta remained unaltered. PCIII mRNA expression increased significantly after 1h of oscillation at 10(-2)N and 5% L(B) and remained unchanged for 6h. In conclusion, the stress induced by force but not by amplitude led to the increment in PCIII mRNA expression.

Sphingosine 1-Phosphate Reduces Vascular Leak in Murine and Canine Models of Acute Lung Injury

Excessive mechanical stress is a key component of ventilator-associated lung injury, resulting in profound vascular leak and an intense inflammatory response. To extend our in vitro observations concerning the barrier-protective effects of the lipid growth factor sphingosine 1-phosphate (Sph 1-P), we assessed the ability of Sph 1-P to prevent regional pulmonary edema accumulation in clinically relevant rodent and canine models of acute lung injury induced by combined intrabronchial endotoxin administration and high tidal volume mechanical ventilation. Intravenously delivered Sph 1-P significantly attenuated both alveolar and vascular barrier dysfunction while significantly reducing shunt formation associated with lung injury. Whole lung computed tomographic image analysis demonstrated the capability of Sph 1-P to abrogate significantly the accumulation of extravascular lung water evoked by 6-hour exposure to endotoxin. Axial density profiles and vertical density gradients localized the Sph 1-P response to transitional zones between aerated and consolidated lung regions. Together, these results indicate that Sph 1-P represents a novel therapeutic intervention for the prevention of pulmonary edema related to inflammatory injury and increased vascular permeability.

Extracorporeal life support for severe acute respiratory distress syndrome in adults

OBJECTIVE

Severe acute respiratory distress syndrome (ARDS) is associated with a high level of mortality. Extracorporeal life support (ECLS) during severe ARDS maintains oxygen and carbon dioxide gas exchange while providing an optimal environment for recovery of pulmonary function. Since 1989, we have used a protocol-driven algorithm for treatment of severe ARDS, which includes the use of ECLS when standard therapy fails. The objective of this study was to evaluate our experience with ECLS in adult patients with severe ARDS with respect to mortality and morbidity.

METHODS

We reviewed our complete experience with ELCS in adults from January 1, 1989, through December 31, 2003. Severe ARDS was defined as acute onset pulmonary failure, with bilateral infiltrates on chest x-ray, and PaO2/fraction of inspired oxygen (FiO2) ratio < or =100 or A-aDO2 >600 mm Hg despite maximal ventilator settings. The indication for ECLS was acute severe ARDS unresponsive to optimal conventional treatment. The technique of ECLS included veno-venous or veno-arterial vascular access, lung "rest" at low FiO2 and inspiratory pressure, minimal anticoagulation, and optimization of systemic oxygen delivery.

RESULTS

During the study period, ECLS was used for 405 adult patients age 17 or older. Of these 405 patients, 255 were placed on ECLS for severe ARDS refractory to all other treatment. Sixty-seven percent were weaned off ECLS, and 52% survived to hospital discharge. Multivariate logistic regression analysis identified the following pre-ELCS variables as significant independent predictors of survival: (1) age (P = 0.01); (2) gender (P = 0.048); (3) pH < or =7.10 (P = 0.01); (4) PaO2/FiO2 ratio (P = 0.03); and (5) days of mechanical ventilation (P < 0.001). None of the patients who survived required permanent mechanical ventilation or supplemental oxygen therapy.

CONCLUSION

Extracorporeal life support for severe ARDS in adults is a successful therapeutic option in those patients who do not respond to conventional mechanical ventilator strategies.

Pulmonary inflammation induced by high-stretch ventilation is mediated by tumor necrosis factor signaling in mice

Although high-stretch mechanical ventilation has been demonstrated to induce lung inflammation, the roles of soluble mediators, in particular TNF, remain controversial. We have previously shown in mice that high-stretch ventilation, in the absence of preceding lung injury, induces expression of bioactive TNF in lung lavage fluid early in the course of injury, but the biological significance of this, if any, has yet to be determined. We therefore investigated the pulmonary inflammatory response to a transient period of high-stretch ventilation in anesthetized mice lacking TNF receptors and mice treated with anti-TNF antibodies. A standardized stretch-induced lung injury (assessed by lung mechanics, blood gases, and lavage protein content), followed by noninjurious low-stretch ventilation for 3 h, produced significant alveolar neutrophil infiltration in wild-type mice. However, neutrophil recruitment was substantially attenuated in TNF receptor double knockout mice and in wild-type mice treated with intratracheal anti-TNF antibody. This attenuation was not associated with decreased concentrations of neutrophil attractant CXC chemokines (macrophage inflammatory protein-2 and keratinocyte-derived chemokine) in lavage fluid. In contrast to intratracheal antibody, intravenous anti-TNF antibody did not reduce neutrophil infiltration, suggesting that the role of TNF signaling is localized within the alveolar space and does not require decompartmentalization of TNF into the circulation. These findings provide the first direct evidence that pulmonary inflammation induced by high-stretch ventilation without underlying lung injury possesses a significant TNF-dependent component. The results suggest a potential for regional anti-TNF treatment in attenuating stretch-induced pulmonary inflammation.

Hypercapnia via reduced rate and tidal volume contributes to lipopolysaccharide-induced lung injury

Appreciating that CO2 modifies the chemical reactivity of nitric oxide (NO)-derived inflammatory oxidants, we investigated whether hypercapnia would modulate pulmonary inflammatory responses. Rabbits (n = 72) were ventilated with approximately 7-ml/kg tidal volume for 6 hours. Animals were randomized to one of the following conditions: eucapnia (Pa(CO2) at approximately 35-40 mm Hg), eucapnia + lipopolysaccharide (LPS), eucapnia + LPS + inhaled NO (iNO delivered at approximately 20 ppm), hypercapnia (Pa(CO2) at approximately 60 mm Hg), hypercapnia + LPS, and hypercapnia + LPS + iNO. The hypercapnia + LPS groups compared with groups exposed to eucapnia + LPS displayed significantly increased bronchoalveolar lavage fluid protein concentrations (p < 0.05), lung wet-to-dry ratios (p < 0.05), bronchoalveolar lavage fluid cell counts (p < 0.05), and lung histologic alterations consistent with greater injury. Furthermore, expression of inducible nitric oxide synthase (p < 0.05), tissue myeloperoxidase content (p < 0.05), and formation of lung protein 3-nitrotyrosine derivatives (p < 0.05) was greatest under conditions of hypercapnia + LPS. Groups exposed to hypercapnic conditions without LPS did not manifest these changes. The inhalation of iNO attenuated selected indices of lung injury. We conclude that hypercapnia induced by means of reduced rate and tidal volume amplifies pulmonary inflammatory responses.

The Lung in Sepsis: Fueling the Fire

Advances in our understanding of the molecular mechanisms underlying the pathophysiology of sepsis have generated considerable efforts in manipulating the host response during this frequently lethal condition. While existing trials of immune modulation have been largely unsuccessful, an appreciation for the roles of individual organ systems in sepsis is important to enable clinicians to discern how each functions as both a target for injury and a contributor to the derangement in homeostasis seen in sepsis. Such awareness will encourage treatment decisions aimed at optimizing conventional therapy while minimizing the adverse effects of supportive care, and it may also guide the incorporation of newer immunomodulatory therapeutics into our existing modalities. This article discusses the lung's response to sepsis, from the standpoint of organ dysfunction related to sepsis as well as its participation in the generation and maintenance of the systemic inflammatory state.

Significance of Von Willebrand Factor in Septic and Nonseptic Patients with Acute Lung Injury

Systemic endothelial activation and injury are important causes of multiorgan system failure. We hypothesized that plasma levels of von Willebrand factor (VWF), a marker of endothelial activation and injury, would be associated with clinical outcomes in acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). In 559 patients with ALI/ARDS enrolled in the National Heart, Lung, and Blood Institute ARDS Network trial of two VT strategies, plasma VWF levels were measured at randomization (mean 350 +/- 265% of normal control plasma) and Day 3 (344 +/- 207%). Baseline VWF levels were similar in patients with and without sepsis, and were significantly higher in nonsurvivors (435 +/- 333%) versus survivors (306 +/- 209%) even when controlling for severity of illness, sepsis, and ventilator strategy (increased odds ratio of death of 1.6 per SD size increase in VWF; 95% confidence interval, 1.4-2.1). Higher VWF levels were also significantly associated with fewer organ failure-free days. Ventilator strategy had no effect on VWF levels. In conclusion, the degree of endothelial activation and injury is strongly associated with outcomes in ALI/ARDS, regardless of the presence or absence of sepsis, and is not modulated by a protective ventilatory strategy. To improve outcomes further, new treatment strategies targeted at the endothelium should be investigated.

Ventilator-induced lung injury and multiple system organ failure: a critical review of facts and hypotheses

OBJECTIVE

To review how biotrauma leads to the development of multiple system organ failure (MSOF).

DESIGN AND SETTING

Published articles on experimental and clinical studies and review articles in the English language were collected and analyzed.

RESULTS

The concept that ventilation strategies using "large" tidal volumes and zero PEEP of injured lungs can enhance injury by the release of inflammatory mediators into the lungs and circulation, a mechanism that has been called biotrauma, is supported by evidence from experimental models ranging from mechanically stressed cell systems, to isolated lungs, intact animals, and humans. Biotrauma may lead to MSOF via spillover of lung-borne inflammatory mediators into the systemic circulation. However, spillover of other agents such as bacteria and soluble proapoptotic factors may also contribute to the onset of MSOF. Other less well studied mechanisms such as peripheral immunosuppression and translocation of bacteria and/or products from the gut may play an important role. Finally, genetic variability is a crucial factor.

CONCLUSIONS

The development of MSOF is a multifactorial process. Our proposed mechanisms linking mechanical ventilation and MSOF suggest several novel therapeutic approaches. However, it will first be necessary to study the mechanisms described above to delineate more precisely the contribution of each proposed factor, their interrelationships, and their time course. We suggest that scientific advances in immunology may offer novel approaches for prevention of MSOF secondary to ventilator-induced lung injury.

DNA microarray analysis of gene expression in alveolar epithelial cells in response to TNFalpha, LPS, and cyclic stretch

Recent evidence suggests that alveolar epithelial cells (AECs) may contribute to the development, propagation, and resolution of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). Proinflammatory cytokines, pathogen products, and injurious mechanical ventilation are important contributors of excessive inflammatory responses in the lung. In the present study, we used cDNA microarrays to define the gene expression patterns of A549 cells (an AEC line) in the early stages of three models of pulmonary parenchymal cell activation: cells treated with tumor necrosis factor-alpha (TNFalpha) (20 ng/ml), lipopolysaccharide (LPS, 1 microg/ml), or cyclic stretch (20% elongation) for either 1 h or 4 h. Differential gene expression profiles were determined by gene array analysis. TNFalpha induced an inflammatory response pattern, including induction of genes for chemokines, inflammatory mediators, and cell surface membrane proteins. TNFalpha also increased genes related to pro- and anti-apoptotic proteins, signal transduction proteins, and transcriptional factors. TNFalpha further induced a group of genes that may form a negative feedback loop to silence the NFkappaB pathway. Stimulation of AECs with mechanical stretch changed cell morphology and activated Src protein tyrosine kinase. The combination of TNFalpha plus stretch enhanced or attenuated expression of multiple genes. LPS decreased microfilament polymerization but had less impact on NFkappaB translocation and gene expression. Results from this study indicate that AECs can tailor their response to different stimuli or/and combination of stimuli and subsequently play an important role in acute inflammatory responses in the lung.

Inflammation and the acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a clinical syndrome of non-cardiogenic pulmonary oedema associated with bilateral pulmonary infiltrates, stiff lungs and refractory hypoxaemia. ARDS is characterized by an explosive acute inflammatory response in the lung parenchyma, leading to alveolar oedema, decreased lung compliance and, ultimately, hypoxaemia. Although our understanding of the causes and pathophysiology of ARDS has increased, the mortality rate remains in the range of 30-50%. No major advances in pharmacological therapy have been achieved. Mechanical ventilation is the main therapeutic intervention in the management of ARDS. The only approach that has been shown to reduce the inflammatory response and mortality is the use of lung-protective ventilatory strategy with a low tidal volume and high positive-end expiratory pressure. This chapter will review the current state of the literature on the pathogenesis of ARDS and ventilatory and pharmacotherapy approaches to its management.

Mechanical ventilation with moderate tidal volumes synergistically increases lung cytokine response to systemic endotoxin

Previous animal studies have identified a role for activation of innate immunity in the pathogenesis of ventilator-associated lung injury. These studies have used large tidal volume ventilation to study the effect of alveolar overdistension on induction of inflammatory pathways. We hypothesized an alternative mechanism for the pathogenesis of lung injury in which moderate tidal volume ventilation does not independently cause clinical inflammation but rather interacts with innate immune activation by bacterial products, resulting in an enhanced inflammatory response. We measured cytokine expression and lung injury in normal and lipopolysaccharide (LPS)-treated anesthetized rabbits randomized to either spontaneous respiration or mechanical ventilation. Outcome parameters were analyzed by two-way factorial analysis of variance to identify synergism between ventilation and systemic LPS. Mechanical ventilation alone resulted in minimal cytokine expression in the lung but did enhance LPS-induced expression of tumor necrosis factor-α, the CXC chemokines interleukin-8 and growth-related protein-α, and the CC chemokine monocyte chemoattractant protein-1. Increased mRNA expression and activation of the transcription factors nuclear factor-κB and activator protein-1 accompanied the cytokine responses. We conclude that moderate volume ventilation strategies augment the innate immune response to bacterial products in the lung and may play a role in the development of acute lung injury in patients with sepsis.

Persistence of diaphragmatic contraction influences the pulmonary inflammatory response to mechanical ventilation

Because we already showed (Brégeon, F., Roch, A., Delpierre, S., Ghigo, E., Autillo-Touati, A., Kajikawa, O., Martin, T., Pugin, J., Portugal, H., Auffray, J., Jammes, Y., 2002. Conventional mechanical ventilation of healthy lungs induced pro-inflammatory cytokine gene transcription, Respir. Physiol. Neurobiol. 132, 191-203) that non-injurious mechanical ventilation (MV) elicited inflammatory signal in paralyzed rabbits having normal lungs, we examined the role of neuromuscular blockade in the pulmonary inflammatory response. In the bronchoalveolar lavage fluid (BALF), leukocyte count, MCP-1 and IL-8 cytokine concentrations (ELISA) and mRNAs (reverse transcription polymerase chain reaction, RT-PCR) were measured in paralyzed (P) or non-paralyzed (NP) rabbits ventilated for a 6-h period. Compared to the P group and despite the tidal volume was the same, we measured in the NP one a lower compliance of the respiratory system (Crs,stat), a longer inspiratory time (Ti), a negative inspiratory tracheal pressure (Ptr) wave preceding the pump-induced positive pressure wave, and a higher peak tracheal pressure. Moreover, in NP animals, gross autopsy showed negligible lung abnormalities, and marked reduction of leukocyte count and lung cytokines (P < 0.05). Thus, the absence of neuromuscular blockade decreased the pulmonary chemotactic response to MV suggesting that the total suppression of negative pressure waves elicited by the diaphragmatic (di) contractions could be involved in this lung response to positive pressure MV.

Is high PEEP low volume ventilation in burn patients beneficial? A retrospective study of 61 patients

In burn care lung damage by inhalation injury is a major cause of mortality in burn patients. In the field of intensive care medicine ventilation strategies to reduce lung injury caused by ventilation are under investigation. The promising results of the application of pressure controlled high PEEP low volume (HPLV) ventilation prompted us to use this ventilation strategy in our burn ICU. To establish whether this ventilation regime is beneficial the charts of 61 consecutive patients needing artificial respiration were reviewed. A scoring system for PEEP level and tidal volume was developed and treatment groups with high PEEP and low volume and low PEEP high volume regimes were compared. No statistically significant differences could be found when comparing treatment versus mortality, the number of pulmonary complications or incidence of pneumothoraces. However, the trend showed a benefit in mortality for patients with an ABSI scoring up to 9, but at the same time pulmonary complications increase. The Horovitz oxygenation index showed no advantage for the HPLV Group. In a separate analysis we found a significant correlation between absolute PEEP and mortality. The correlation between PEEP level and mortality is a dynamic factor predicting outcome, This not been described yet and can be an addition to the static ABSI score. Differences between ventilation strategies were not as evident as expected in this retrospective study, so prospective randomized studies are needed.

Bench-to-bedside review: Permissive hypercapnia

Current protective lung ventilation strategies commonly involve hypercapnia. This approach has resulted in an increase in the clinical acceptability of elevated carbon dioxide tension, with hypoventilation and hypercapnia 'permitted' in order to avoid the deleterious effects of high lung stretch. Advances in our understanding of the biology of hypercapnia have prompted consideration of the potential for hypercapnia to play an active role in the pathogenesis of inflammation and tissue injury. In fact, hypercapnia may protect against lung and systemic organ injury independently of ventilator strategy. However, there are no clinical data evaluating the direct effects of hypercapnia per se in acute lung injury. This article reviews the current clinical status of permissive hypercapnia, discusses insights gained to date from basic scientific studies of hypercapnia and acidosis, identifies key unresolved concerns regarding hypercapnia, and considers the potential clinical implications for the management of patients with acute lung injury.

Acute respiratory distress syndrome, the critical care paradigm: what we learned and what we forgot

In the last several years, we definitely learned that the acute respiratory distress syndrome lung is small, nonhomogeneous, and that mechanical ventilation in this baby lung may cause physical damage as well as inflammatory reaction. The clinical benefit of the gentle lung treatment, based on a decrease of global/regional stress and strain into the lung, has been finally proved. However, we forgot the importance of lung perfusion and its distribution in this syndrome and, besides a low tidal volume, we still do not know how to handle the other variables of mechanical ventilation. Measurements of variables as transpulmonary pressure and end expiratory lung volume, for a rational setting of mechanical ventilation, should be introduced in routine clinical practice.

Pathophysiology of acute lung injury in combined burn and smoke inhalation injury

In the U.S.A., more than 1 million burn injuries occur every year. Although the survival from burn injury has increased in recent years with the development of effective fluid resuscitation management and early surgical excision of burned tissue, the mortality of burn injury is still high. In these fire victims, progressive pulmonary failure and cardiovascular dysfunction are important determinants of morbidity and mortality. The morbidity and mortality increases when burn injury is associated with smoke inhalation. In the present review, we will describe the pathophysiological aspects of acute lung injury induced by combined burn and smoke inhalation and examine various therapeutic approaches.

Injurious ventilation strategies cause systemic release of IL-6 and MIP-2 in rats in vivo

In vivo experiments showed no increased production of tumour necrosis factor (TNF) in response to injurious ventilation strategies in otherwise untreated animals. Because interleukin-6 (IL-6) and macrophage inflammatory protein-2 (MIP-2) are more sensitive markers of ventilation-induced cytokine release, serum and bronchoalveolar lavage (BAL) samples were examined for these mediators. Eighty-five adult rats were randomized to three different ventilation strategies. Rats were ventilated with low pressures and low tidal volumes [13/3; peak inspiratory pressure (PIP)/positive end-expiratory pressure (PEEP) in cmH2O], the second group of rats was ventilated with high pressures and low PEEP resulting in high tidal volumes (32/6), and the third group was ventilated with the same high pressures but without PEEP (32/0). Animals were ventilated either for 90 or 240 min, subsequently serum and BAL were collected for analyses on IL-6 and MIP-2 content. Non-ventilated animals served as healthy controls. Ventilation with 32/0 for 90 or 240 min, led to increased serum IL-6 levels. Serum MIP-2 levels were increased by ventilation with 32/6 (90 min) and 32/0 (240 min). Ventilation under any condition, even at 13/3, resulted in elevated MIP-2 levels in the BAL fluid. Even at normal pressures pulmonary MIP-2 levels were increased, suggesting that ventilation may promote pro-inflammatory responses in healthy subjects.

Ventilation strategy affects cytokine release after mesenteric ischemia-reperfusion in rats

OBJECTIVE

To evaluate the impact of different ventilation modalities on lung and plasma concentrations of cytokines in a model of secondary lung inflammation, mesenteric ischemia-reperfusion, in rats.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

Research laboratory of a university.

SUBJECTS

Sixty-four male adult Wistar rats weighing 320-380 g.

INTERVENTIONS

Eight groups were studied. Two groups underwent no surgical procedure: They were either not ventilated or ventilated with an injurious modality consisting of 30 mL/kg tidal volume (Vt) without positive-end expiratory pressure (PEEP). Animals of the other groups underwent laparotomy with or without 2-hr mesenteric ischemia followed by 4 hrs of reperfusion during which the rats were mechanically ventilated. Ventilation modalities were conventional (tidal volume 10 mL/kg, PEEP 3 cm H2O), protective (6 mL/kg, 6 cm H(2)O), or injurious (tidal volume 30 mL/kg and no PEEP). Rats were killed by exsanguination, and their lungs were excised and homogenized in buffer. Supernatants of lung homogenates and plasmas were stored at -80 degrees C for subsequent measurements.

MEASUREMENTS AND MAIN RESULTS

Tumor necrosis factor-alpha, interleukin-1 beta, interleukin-6, macrophage inhibitory protein 2, and interleukin-10 were determined in lung supernatants and plasmas with a rat-specific enzyme-linked immunosorbent assay. Lung and plasma cytokine concentrations were not significantly different between rats ventilated with the injurious modality only and nonventilated rats. Lung and plasma cytokine concentrations were higher in rats that had undergone mesenteric ischemia-reperfusion than in rats with laparotomy only, whatever ventilation modality. Lung and plasma cytokine concentrations were higher in these rats after the injurious ventilation modality than after the other modalities.

CONCLUSION

This study shows that an injurious ventilation does not produce significant in vivo release of cytokines in intact animals but promotes the release of pro- and anti-inflammatory cytokines in an inflammatory context.

A consideration of neonatal resuscitation

There are currently two major areas of resuscitation of the newborn which have come into question: the use of intermittent positive pressure ventilation and the use of oxygen. There is evolving evidence that volutrauma associated with IPPV, especially in the premature infant, may induce changes in the lung which can lead to chronic lung disease. There is reason to believe that the use of continuous positive airway pressure in premature infants who are making respiratory efforts may be less harmful than the use of IPPV. With regard to the use of oxygen, it is clear that most infants can be successfully resuscitated with room air. Although we can identify markers for oxidative stress in newborns when resuscitated with 100% oxygen, the clinical importance of these markers remain an open issue. If the presence of these markers after resuscitation is shown to relate to clinical problems, then the use of oxygen may need to be considered.

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

OBJECTIVE

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

DESIGN

Prospective animal study.

SETTING

Research laboratory.

SUBJECTS

Twenty-four newborn piglets.

INTERVENTIONS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSIONS

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

Smoke/burn injury-induced respiratory failure elicits apoptosis in ovine lungs and cultured lung cells, ameliorated with arteriovenous CO2 removal

STUDY OBJECTIVE

s: The purpose of this study was to examine the effects of two supportive therapies, conventional mechanical ventilation (CMV) and arteriovenous CO(2) removal (AVCO(2)R), during treatment of severe smoke/burn injury-induced ARDS.

DESIGN

Sheep were exposed to a smoke/burn injury (lethal dose causing death in 40% of animals); lung tissue and blood was collected prior to injury (control), when an ARDS criterion was met (PaO(2)/fraction of inspired oxygen ratio < 200), then after 72 h of either CMV (group 1) or AVCO(2)R (group 2). Lung tissue was studied by standard histopathologic techniques; cultured lung cells were studied in media supplemented with serum from all four groups.

MEASUREMENTS AND RESULTS

In vivo assays demonstrate less apoptotic cell death, and in vitro assays show significantly greater (p < 0.05) cell survival in group 2 (AVCO(2)R) than in group 1 (CMV) or baseline. Differential gene expression demonstrates significantly higher messenger RNA levels of proapoptotic and tumor necrosis factor (TNF)-alpha in cells incubated in baseline media. After exposure of cultured lung cells to conditioned media, protein expression assay of the culture medium revealed no TNF-alpha, TNF receptor (TNFR)-1, or TNFR-2, however, cultured cell lysate reveals elevated levels of TNF-alpha, TNFR-1 and caspase-3 in all groups; most occurred in cells incubated in baseline media (p < 0.05). HOECHST stain, DNA fragmentation, and caspase-3 cleavage show that AVCO(2)R ameliorates apoptosis in this model.

CONCLUSIONS

This in vitro work specifically examines cell death in lung cells as a result of smoke/burn injury and effects of therapeutic interventions. Our in vivo studies temporally correlate the clinical pathology to that studied in these lung cells and show that both in vivo and in vitro cell death is predominantly apoptotic and is significantly reduced by AVCO(2)R.

Inhaled carbon monoxide confers antiinflammatory effects against ventilator-induced lung injury

Ventilator-induced lung injury (VILI) is a major cause of morbidity and mortality in intensive care units. The stress-inducible gene product, heme oxygenase-1, and carbon monoxide (CO), a major by-product of heme oxygenase catalysis of heme, have been shown to confer potent antiinflammatory effects in models of tissue and cellular injury. In this study, we observed increased expression of heme oxygenase-1 mRNA and protein in a rat model of VILI. To assess the physiologic function of heme oxygenase-1 induction in VILI, we determined whether low concentration of inhaled CO could serve to protect the lung against VILI. Low concentration of inhaled CO significantly reduced tumor necrosis factor-alpha levels and total cell count in lavage fluid, while simultaneously elevating levels of antiinflammatory interleukin-10 levels. To better characterize the mechanism of CO-mediated antiinflammatory effects, we examined key signaling pathways, which may mediate CO-induced antiinflammatory effects. We demonstrate that inhaled CO exerts antiinflammatory effects in VILI via the p38 mitogen-activated protein kinase pathway but independent of activator protein-1 and nuclear factor-kappaB pathways. Our data lead to a tempting speculation that inhaled CO might be useful in minimizing VILI.

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

OBJECTIVE

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

DESIGN

Prospective, randomized clinical study.

SETTING

Single university hospital.

PATIENTS AND PARTICIPANTS

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

INTERVENTIONS

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

MEASUREMENTS AND RESULTS

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

CONCLUSIONS

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

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

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

Management of post traumatic respiratory failure

Acute respiratory distress syndrome (ARDS) is a severe and common complication of major trauma. The most important early management principle is to identify the inciting event and remove the ongoing insult aggressively. It is important to immediately resuscitate the patients and prepare them for a complex and difficult hospitalization. Avoiding secondary insults is the cornerstone of supportive care, and this is based primarily on aggressive immune surveillance, full nutrition, and unrelenting oxygen delivery. The use of aggressive immune surveillance, nutritional support, and fluid management is critical to support ventilator management for oxygenation and ventilation. In general, although essential, the ventilator has great potential for harm in patients who are compromised seriously with ARDS. Physicians must establish reasonable therapeutic goals based on oxygen delivery rather than arbitrary normal values of blood gas measurement. The impact of the ventilator should be limited with regard to aspiratory pressure, tidal volume, inspired oxygen, and levels of expiratory end expiratory pressure. Use of pulmonary toilet, including therapeutic bronchoscopy; patient positioning, including intermittent prone positioning, and recruitment maneuvers are useful therapeutic complements for maintaining functional residual capacity and decreasing shunt. Overall, ARDS represents a clear indication that the patient is failing to meet the demands of their stress and without prompt attention likely will die. It is a challenge and an opportunity to identify the underlying situation and to manage the patient while not causing additional harm as the patient's intrinsic resources can bring about the healing necessary to recover from the situation of extremis.