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

Lung recruitment--a guide for clinicians

Recruitment manoeuvres play an important role in minimising ventilator associated lung injury (VALI) particularly when lung protective ventilation strategies are employed and as such clinicians should consider their application. This paper provides evidence-based recommendations for clinical practice with regard to alveolar recruitment. It includes recommendations for timing of recruitment, strategies of recruitment and methods of measuring the efficacy of recruitment manoeuvres and contributes to knowledge about the risks associated with recruitment manoeuvres. There are a range of methods for recruiting alveoli, most notably by manipulating positive end expiratory pressure (PEEP) and peak inspiratory pressure (PIP) with consensus as to the most effective not yet determined. A number of studies have demonstrated that improvement in oxygenation is rarely sustained following a recruitment manoeuvre and it is questionable whether improved oxygenation should be the clinician's goal. Transient haemodynamic compromise has been noted in a number of studies with a few studies reporting persistent, harmful sequelae to recruitment manoeuvres. No studies have been located that assess the impact of recruitment manoeuvres on length of ventilation, length of stay, morbidity or mortality. Recruitment manoeuvres restore end expiratory lung volume by overcoming threshold opening pressures and are most effective when applied after circuit disconnection and airway suction. Whether this ultimately improves outcomes in adult or paediatric populations is unknown.

PROTEIN INHIBITION OF SURFACTANT DURING MECHANICAL VENTILATION OF ISOLATED RAT LUNGS

This study tested the hypothesis that material leaking into the airspace from the vasculature during ventilation interferes with surfactant function and contributes to decreases in lung compliance. Rats were euthanized and the lungs were isolated either with or without flushing of the vasculature, followed by mechanical ventilation and analysis of lung compliance and lung lavage analysis. Flushed lungs had higher lung compliance compared to the non-flushed lungs. This was associated with lower protein concentrations and improved surfactant activity. It is concluded that during mechanical ventilation, leakage of proteins results in surfactant inhibition and thereby contribute to decreased lung compliance.

Liquid plug propagation in flexible microchannels: A small airway model

In the present study, we investigate the effect of wall flexibility on the plug propagation and the resulting wall stresses in small airway models with experimental measurements and numerical simulations. Experimentally, a flexible microchannel was fabricated to mimic the flexible small airways using soft lithography. Liquid plugs were generated and propagated through the microchannels. The local wall deformation is observed instantaneously during plug propagation with the maximum increasing with plug speed. The pressure drop across the plug is measured and observed to increase with plug speed, and is slightly smaller in a flexible channel compared to that in a rigid channel. A computational model is then presented to model the steady plug propagation through a flexible channel corresponding to the middle plane in the experimental device. The results show qualitative agreements with experiments on wall shapes and pressure drops and the discrepancies bring up interesting questions on current field of modeling. The flexible wall deforms inward near the plug core region, the deformation and pressure drop across the plug increase with the plug speed. The wall deformation and resulting stresses vary with different longitudinal tensions, i.e., for large wall longitudinal tension, the wall deforms slightly, which causes decreased fluid stress and stress gradients on the flexible wall comparing to that on rigid walls; however, the wall stress gradients are found to be much larger on highly deformable walls with small longitudinal tensions. Therefore, in diseases such as emphysema, with more deformable airways, there is a high possibility of induced injuries on lining cells along the airways because of larger wall stresses and stress gradients.

Protective effects of adenosine A2A receptor agonist in ventilator-induced lung injury in rats

OBJECTIVES

Mechanical ventilation is associated with overwhelming inflammatory responses that are associated with ventilator-induced lung injury (VILI) in patients with acute respiratory distress syndrome. The activation of adenosine A2A receptors has been reported to attenuate inflammatory cascades.

HYPOTHESIS

The administration of A2A receptors agonist ameliorates VILI.

METHODS

Rats were subjected to hemorrhagic shock and resuscitation as a first hit to induce systemic inflammation. The animals randomly received the selective A2A receptor agonist CGS-21680 or a vehicle control in a blinded fashion at the onset of resuscitation phase. They were then randomized to receive mechanical ventilation as a second hit with a high tidal volume of 20 mL/kg and zero positive end-expiratory pressure, or a low tidal volume of 6 mL/kg with positive end-expiratory pressure of 5 cm H2O.

RESULTS

The administration of CGS-21680 attenuated lung injury as evidenced by a decrease in respiratory elastance, lung edema, lung injury scores, neutrophil recruitment in the lung, and production of inflammatory cytokines, compared with the vehicle-treated animals.

CONCLUSIONS

The selective A2A receptor agonist may have a place as a novel therapeutic approach in reducing VILI.

Acute lung injury in patients with severe brain injury: a double hit model

The presence of pulmonary dysfunction after brain injury is well recognized. Acute lung injury (ALI) occurs in 20% of patients with isolated brain injury and is associated with a poor outcome. The "blast injury" theory, which proposes combined "hydrostatic" and "high permeability" mechanisms for the formation of neurogenic pulmonary edema, has been challenged recently by the observation that a systemic inflammatory response may play an integral role in the development of pulmonary dysfunction associated with brain injury. As a result of the primary cerebral injury, a systemic inflammatory reaction occurs, which induces an alteration in blood-brain barrier permeability and infiltration of activated neutrophils into the lung. This preclinical injury makes the lungs more susceptible to the mechanical stress of an injurious ventilatory strategy. Tight CO2 control is a therapeutic priority in patients with acute brain injury, but the use of high tidal volume ventilation may contribute to the development of ALI. Establishment of a therapeutic regimen that allows the combination of protective ventilation with the prevention of hypercapnia is, therefore, required. Moreover, in patients with brain injury, hypoxemia represents a secondary insult associated with a poor outcome. Optimal oxygenation may be achieved by using an adequate FiO2 and by application of positive end-expiratory pressure (PEEP). PEEP may, however, affect the cerebral circulation by hemodynamic and CO2-mediated mechanisms and the effects of PEEP on cerebral hemodynamics should be monitored in these patients and used to titrate its application.

Role of free radicals in vascular dysfunction induced by high tidal volume ventilation

OBJECTIVE

To demonstrate that increased formation of reactive oxygen (ROS) and nitrogen species (RNS) is involved in VILI-induced vascular dysfunction.

METHODS

Male Sprague-Dawley anesthetized rats were ventilated for 60 min using low V(T) ventilation [V(T) 9 ml/kg, positive end-expiratory pressure (PEEP) 5 cmH(2)O, n = 18], and high V(T) ventilation (V(T) 35 ml/kg, zero PEEP, n = 18). Arterial pressure and respiratory system mechanics were monitored. Blood samples for the determination of arterial blood gases and lactate concentration were drawn. Vascular rings from the thoracic aortae were mounted in organ baths for isometric tension recording. We studied endothelium-dependent relaxation in norepinephrine-precontracted rings (acetylcholine, 10 nM-10 microM) and contraction induced by norepinephrine (1 nM-10 microM) in resting vessels. Vascular rings were preincubated for 30 min with Zn-Mn-SOD (100 u/ml) or tempol (10(-4) M) (extracellular and intracellular superoxide scavengers, respectively) or MnTMPyP (10(-5) M) (a superoxide and peroxynitrite scavenger). The presence of superoxide and nitrotyrosine in aortic rings was evaluated by immunofluorescence.

RESULTS

High V(T) ventilation induced hypotension, systemic acidosis, hypoxemia and hyperlactatemia, as well as impairment in acetylcholine and norepinephrine-induced responses in vitro. Responses to acetylcholine were improved by tempol (P = 0.004) and completely corrected (P < 0.001) by MnTMPyP. Responses to norepinephrine were also improved by treatment with tempol (P < 0.001) and MnTMPyP (P < 0.001). However, Zn-Mn-SOD did not improve acetylcholine- or norepinephrine-induced responses. Immunostaining for both superoxide and nitrotyrosine was increased in aortic rings from the high V(T) group.

CONCLUSIONS

Our data support a role for intracellular ROS and peroxynitrite in the high V(T) ventilation-induced vascular dysfunction.

Positive end-expiratory pressure alters the severity and spatial heterogeneity of ventilator-induced lung injury: an argument for cyclical airway collapse

PURPOSE

Ventilator-induced lung injury (VILI) is a recognized complication of mechanical ventilation. Although the specific mechanism by which mechanical ventilation causes lung injury remains an active area of study, both alveolar overdistension and cyclical airway collapse and recruitment have been suggested as contributing causes. We hypothesized that mechanical ventilation in the absence of positive end-expiratory pressure (PEEP) causes VILI to be more severe and regionally variable as compared with PEEP = 8 cm H(2)O.

MATERIALS AND METHODS

To test this hypothesis, anesthetized, supine rabbits were mechanically ventilated with an end-inspiratory pressure of 28 cm H(2)O and either 0 or 8 cm H(2)O PEEP for 4 hours. Regional lung injury was determined by histologic scoring.

RESULTS

In the absence of PEEP, lung injury was regionally variable and greatest in the dorsal-caudal lung. This regional injury heterogeneity was abolished by the addition of PEEP = 8 cm H(2)O.

CONCLUSIONS

These results suggest that VILI is regionally heterogeneous and spatially correlates with regions in which cyclical airway collapse and recruitment is most likely to occur.

Paralytics in critical care: not always the bad guy

PURPOSE OF REVIEW

The use of neuromuscular blocking agents (NMBAs) in patients with acute respiratory distress syndrome (ARDS) and acute lung injury remains controversial, although frequent. This review analyzes the effects of NMBAs on thoracopulmonary mechanics, gas exchange, patient outcome and their potential adverse effects.

RECENT FINDINGS

NMBAs are used in 25-45% of acute lung injury/ARDS patients for a mean period of 12 days, especially in severe ARDS. Hypoxemia and facilitation of mechanical ventilation are the main indications of NMBAs. Two randomized controlled trials showed that the systematic early use of NMBAs is associated with a sustained improvement in oxygenation in ARDS patients. The most recent suggests a beneficial effect on proinflammatory response associated with ARDS and mechanical ventilation.

SUMMARY

The use of NMBAs in acute lung injury/ARDS patients is not marginal. Recent studies suggest a beneficial effect of early use of NMBAs on oxygenation and inflammation. The role of NMBAs in the occurrence of ICU-acquired neuromyopathies and lung atelectasis in ARDS patients remains largely questioned. The use of NMBAs in the early phase of ARDS could reinforce the beneficial effects of a lung-protective ventilation. In this context, the effect of NMBAs on the outcome of ARDS patients must be evaluated.

[Survival of rats subjected to different levels of pulmonary injury]

OBJECTIVE

To determine a pulmonary injury model in rats that is associated with moderate mortality after extubation.

DESIGN AND SETTING

An experimental study in an animal model of ventilator-induced lung injury in the animal research laboratory in Virgen de las Nieves University Hospital.

SUBJECTS AND METHOD

A total of 45 male Wistar-Kyoto rats weighing 250-300g received food and water ad libitum. The rats were anesthetized and a tracheotomy was performed by insertion of endotracheal tube by tracheotomy.

INTERVENTIONS

Pulmonary injury due to mechanical ventilation was maintained for 60 min with high tidal volume (25 ml/kg) combined with intratracheal instillation of different doses of 0,9% saline solution. Rats were randomly distributed into 3 groups (15 animals in each group) with different amounts of instilled saline solution: group I, 0.5 ml/250 g body weight; group II, 1 ml/250 g body weight, and group III, 1.5 ml/250 g body weight.

MAIN MEASUREMENTS

Survival of animals after extubation was recorded every 5 min for the first 40 min and then at 3 h, 24 h, 72 h, and 7 days.

RESULTS

Survival in rats that received 0.5, 1 and 1.5 ml/250 g of intratracheal saline solution was 60%, 43% and 0% respectively, with statistically significant differences between groups receiving 0.5 and 1.5 ml/250 g (p = 0.003).

CONCLUSIONS

Survival in rats mechanically ventilated with high moderate volume is influenced by increased doses of intratracheal saline solution and this is important to design studies that analyze the effect the interventions on mortality.

Pulmonary atelectasis during low stretch ventilation: "open lung" versus "lung rest" strategy

OBJECTIVE

Limiting tidal volume (VT) may minimize ventilator-induced lung injury (VILI). However, atelectasis induced by low VT ventilation may cause ultrastructural evidence of cell disruption. Apoptosis seems to be involved as protective mechanisms from VILI through the involvement of mitogen-activated protein kinases (MAPKs). We examined the hypothesis that atelectasis may influence the response to protective ventilation through MAPKs.

DESIGN

Prospective randomized study.

SETTING

University animal laboratory.

SUBJECTS

Adult male 129/Sv mice.

INTERVENTIONS

Isolated, nonperfused lungs were randomized to VILI: VT of 20 mL/kg and positive end-expiratory pressure (PEEP) zero; low stretch/lung rest: VT of 6 mL/kg and 8-10 cm H2O of PEEP; low stretch/open lung: VT of 6 mL/kg, two recruitment maneuvers and 14-16 cm H2O of PEEP. Ventilator settings were adjusted using the stress index.

MEASUREMENT AND MAIN RESULT

Both low stretch strategies equally blunted the VILI-induced derangement of respiratory mechanics (static volume-pressure curve), lung histology (hematoxylin and eosin), and inflammatory mediators (interleukin-6, macrophage inflammatory protein-2 [enzyme-linked immunosorbent assay], and inhibitor of nuclear factor-kB[Western blot]). VILI caused nuclear swelling and membrane disruption of pulmonary cells (electron microscopy). Few pulmonary cells with chromatin condensation and fragmentation were seen during both low stretch strategies. However, although cell thickness during low stretch/open lung was uniform, low stretch/lung rest demonstrated thickening of epithelial cells and plasma membrane bleb formation. Compared with the low stretch/open lung, low stretch/lung rest caused a significant decrease in apoptotic cells (terminal deoxynucleotidyl transferase mediated deoxyuridine-triphosphatase nick end-labeling) and tissue expression of caspase-3 (Western blot). Both low stretch strategies attenuated the activation of MAPKs. Such reduction was larger during low stretch/open lung than during low stretch/lung rest (p < 0.001).

CONCLUSION

Low stretch strategies provide similar attenuation of VILI. However, low stretch/lung rest strategy is associated to less apoptosis and more ultrastructural evidence of cell damage possibly through MAPKs-mediated pathway.

Noninjurious mechanical ventilation activates a proinflammatory transcriptional program in the lung

Mechanical ventilation is a life-saving intervention in patients with respiratory failure. However, human and animal studies have demonstrated that mechanical ventilation using large tidal volumes (>or=12 ml/kg) induces a potent inflammatory response and can cause acute lung injury. We hypothesized that mechanical ventilation with a "noninjurious" tidal volume of 10 ml/kg would still activate a transcriptional program that places the lung at risk for severe injury. To identify key regulators of this transcriptional response, we integrated gene expression data obtained from whole lungs of spontaneously breathing mice and mechanically ventilated mice with computational network analysis. Topological analysis of the gene product interaction network identified Jun and Fos families of proteins as potential regulatory hubs. Electrophoretic mobility gel shift assay confirmed protein binding to activator protein-1 (AP-1) consensus sequences, and supershift experiments identified JunD and FosB as components of ventilation-induced AP-1 binding. Specific recruitment of JunD to the regulatory region of the F3 gene by mechanical ventilation was confirmed by chromatin immunoprecipitation assay. In conclusion, we demonstrate a novel computational framework to systematically dissect transcriptional programs activated by mechanical ventilation in the lung, and show that noninjurious mechanical ventilation initiates a response that can prime the lung for injury from a subsequent insult.

Role of lung-marginated monocytes in an in vivo mouse model of ventilator-induced lung injury

RATIONALE

Recruited leukocytes play an important role in ventilator-induced lung injury, although studies have focused predominantly on neutrophils. Inflammatory subset Gr-1(high) monocytes are recruited to sites of inflammation and have been implicated in acute lung injury induced by systemic endotoxin.

OBJECTIVES

To investigate the recruitment and role of Gr-1(high) monocytes in an in vivo mouse model of ventilator-induced lung injury.

METHODS

Anesthetized mice were ventilated with low or high stretch. Flow cytometry was used to quantify monocyte subset margination to the lungs, and to assess their in situ cellular activation in response to mechanical stretch. To investigate monocyte involvement in lung injury progression, a two-hit model was used, with a subclinical dose of lipopolysaccharide (intraperitoneal) given 2 hours prior to high-stretch ventilation. In some animals, monocytes were depleted using intravenous clodronate liposomes. Development of lung injury was assessed in ventilated animals by peak inspiratory pressure and respiratory system mechanics.

MEASUREMENTS AND MAIN RESULTS

High-stretch ventilation induced significant pulmonary margination of Gr-1(high) but not Gr-1(low) monocytes compared with nonventilated mice. These monocytes displayed increased activation status, with higher CD11b (vs. nonventilated mice) and lower L-selectin expression (vs. low-stretch ventilation). Lipopolysaccharide challenge led to enhanced lung margination of Gr-1(high) monocytes and neutrophils, and sensitized the lungs to high stretch-induced pulmonary edema. Clodronate-liposome pretreatment depleted lung monocytes (but not neutrophils) and significantly attenuated lung injury.

CONCLUSIONS

High-stretch mechanical ventilation promotes pulmonary margination of activated Gr-1(high) monocytes, which play a role in the progression of ventilator-induced lung injury.

Permissive hypercapnia

Mechanical ventilation using high tidal volume (VT) and transpulmonary pressure can damage the lung, causing ventilator-induced lung injury. Permissive hypercapnia, a ventilatory strategy for acute respiratory failure in which the lungs are ventilated with a low inspiratory volume and pressure, has been accepted progressively in critical care for adult, pediatric, and neonatal patients requiring mechanical ventilation and is one of the central components of current protective ventilatory strategies.

High-frequency oscillatory ventilation in adults: clinical considerations and management priorities

Recently, there has been renewed interest in high-frequency oscillatory ventilation (HFOV) as a lung-protective strategy in adults. It limits overdistension and prevents cyclic collapse by maintaining end-expiratory lung volume. Studies have shown that HFOV is safely tolerated in the adult population and may offer more benefit if applied early in the course of disease. These findings have implications for clinicians as the use of HFOV may increase in the coming decade. Gas transport mechanisms, ventilator settings, patient monitoring, and clinical considerations for HFOV are substantially different from conventional mechanical ventilation. This article reviews management strategies and monitoring priorities currently recommended for management of adults receiving HFOV.

Critical illness-related corticosteroid insufficiency

The diagnosis of adrenal failure and the indications for corticosteroid therapy in critically ill patients are controversial. This controversy is fueled by the complexity of the issues and the paucity of data from high quality clinical trials. Nevertheless, while the use of high-dose corticosteroids in patients with severe sepsis and ARDS failed to improve outcome and was associated with increased complications, an extended course of stress-dose corticosteroids has been reported to increase the occurrence of ventilator-free days and survival in select groups of ICU patients. These patients typically have an exaggerated proinflammatory response. Until recently the exaggerated proinflammatory response that characterizes critically ill patients with systemic inflammation has focused on suppression of the hypothalamic-pituitary-adrenal axis and adrenal failure. However, experimental and clinical data suggest that glucocorticoid tissue resistance may also play an important role. This complex syndrome is referred to as critical illness-related corticosteroid insufficiency (CIRCI) and is defined as inadequate corticosteroid activity for the severity of the illness of a patient. The paper reviews cortisol physiology, CIRCI, and the role of corticosteroid therapy in critically ill patients.

Role of the pulmonary epithelium and inflammatory signals in acute lung injury

Acute lung injury (ALI) is a clinical disease marked by respiratory failure due to disruption of the epithelial and endothelial barrier, flooding of the alveolar compartment with protein-rich fluid and recruitment of neutrophils into the alveolar space. ALI affects approximately 200,000 patients annually in the USA and results in approximately 75,000 deaths. It is associated with prolonged mechanical ventilation, intensive medical care, high morbidity and mortality, and rising healthcare costs. Owing to its impact on public health, great strides have been made towards understanding the pathobiology of ALI to affect outcome. This review will focus on the role of the epithelial cell in the pathogenesis and resolution of ALI and the role of various inflammatory mediators in ALI.

Lung and systemic inflammation in preterm lambs on continuous positive airway pressure or conventional ventilation

Intratracheal lipopolysaccharide (LPS) causes acute inflammation and injurious mechanical ventilation results in pulmonary and systemic inflammation. We aimed to determine in preterm lungs if continuous positive airway pressure (CPAP) protects against pulmonary and systemic inflammation, compared with conventional mechanical ventilation (CMV) after intratracheal LPS. Preterm fetuses were exposed to maternal betamethasone and Epostane 36 h before delivery at 133 d gestational age (term = 150 d). Lambs were intubated and randomized to receive gentle CMV (tidal volume 8 mL/kg) or CPAP with 8 cm H2O pressure. Surfactant (10 mg/kg) mixed with 1 mg LPS or saline was instilled into the trachea at 15 min. Blood gas status, ventilation variables, and arterial pressures were recorded for 3 h. Static pressure-volume curves and lung and systemic inflammation were assessed postmortem. CPAP lambs had elevated Paco2 and minute ventilation compared with the CMV lambs. Cytokine mRNA was increased in the lungs and liver of CPAP and CMV lambs relative to unventilated controls. Intratracheal LPS amplified the cytokine mRNA responses of IL-1beta, IL-6, and IL-8 in the lung and liver. Blood neutrophils decreased similarly after LPS in CPAP and CMV groups. Cytokine markers of lung injury or the systemic response to intratracheal LPS were not decreased by CPAP relative to CMV, in preterm lambs

Acute lung inflammation and ventilator-induced lung injury caused by ATP via the P2Y receptors: an experimental study

BACKGROUND

Extracellular adenosine 5'-triphosphate (ATP) is an endogenous signaling molecule involved in multiple biological phenomena, including inflammation. The effects of extracellular ATP in the lung have not been fully clarified. This study examined 1) the biological roles of extracellular ATP in the pathogenesis of lung inflammation and 2) the possibility of involvement of extracellular ATP in mechanical ventilation-induced lung injury.

METHODS

The effects of intratracheal ATP on lung permeability, edema or lung inflammation were assessed by measurements of the lung wet-to-dry weight ratio and lung permeability index, immunohistochemistry and expression of key cytokines by real-time polymerase chain reaction. The ATP concentration in broncho-alveolar lavage (BAL) fluid from mice mechanically ventilated was measured by luciferin-luciferase assay. The suppressive effects of a P2 receptor antagonist on ventilator-induced lung inflammation were also examined.

RESULTS

ATP induced inflammatory reactions in the lung mainly via the ATP-P2Y receptor system. These reactions were alleviated by the co-administration of a specific P2 receptor antagonist. Mechanical ventilation with a large tidal volume caused lung inflammation and increased the ATP concentration in BAL fluid. P2 receptor antagonism partially mitigated the inflammatory effects of large tidal volume ventilation.

CONCLUSION

Our observations suggest that the ATP-P2Y receptor system is partially involved in the pathogenesis of ventilator-induced lung injury.

Nitrogen washout/washin, helium dilution and computed tomography in the assessment of end expiratory lung volume

INTRODUCTION

End expiratory lung volume (EELV) measurement in the clinical setting is routinely performed using the helium dilution technique. A ventilator that implements a simplified version of the nitrogen washout/washin technique is now available. We compared the EELV measured by spiral computed tomography (CT) taken as gold standard with the lung volume measured with the modified nitrogen washout/washin and with the helium dilution technique.

METHODS

Patients admitted to the general intensive care unit of Ospedale Maggiore Policlinico Mangiagalli Regina Elena requiring ventilatory support and, for clinical reasons, thoracic CT scanning were enrolled in this study. We performed two EELV measurements with the modified nitrogen washout/washin technique (increasing and decreasing inspired oxygen fraction (FiO2) by 10%), one EELV measurement with the helium dilution technique and a CT scan. All measurements were taken at 5 cmH2O airway pressure. Each CT scan slice was manually delineated and gas volume was computed with custom-made software.

RESULTS

Thirty patients were enrolled (age = 66 +/- 10 years, body mass index = 26 +/- 18 Kg/m2, male/female ratio = 21/9, partial arterial pressure of carbon dioxide (PaO2)/FiO2 = 190 +/- 71). The EELV measured with the modified nitrogen washout/washin technique showed a very good correlation (r2 = 0.89) with the data computed from the CT with a bias of 94 +/- 143 ml (15 +/- 18%, p = 0.001), within the limits of accuracy declared by the manufacturer (20%). The bias was shown to be highly reproducible, either decreasing or increasing the FiO2 being 117+/-170 and 70+/-160 ml (p = 0.27), respectively. The EELV measured with the helium dilution method showed a good correlation with the CT scan data (r2 = 0.91) with a negative bias of 136 +/- 133 ml, and appeared to be more correct at low lung volumes.

CONCLUSIONS

The EELV measurement with the helium dilution technique (at low volumes) and modified nitrogen washout/washin technique (at all lung volumes) correlates well with CT scanning and may be easily used in clinical practice.

TRIAL REGISTRATION

Current Controlled Trials NCT00405002.

Cell wounding and repair in ventilator injured lungs

Acute lung injury (ALI) is a common, frequently hospital-acquired condition with a high morbidity and mortality. The stress associated with invasive mechanical ventilation represents a potentially harmful exposure, and attempts to minimize deforming stress through low tidal ventilation have proven efficacious. Lung cells are both sensors and transducers of deforming stress, and are frequently wounded in the setting of mechanical ventilation. Cell wounding may be one of the drivers of the innate immunologic and systemic inflammatory response associated with mechanical ventilation. These downstream effects of mechanotransduction have been referred to collectively as "Biotrauma". Our review will focus on cellular stress failure, that is cell wounding, and the mechanisms mediating subsequent plasma membrane repair, we hold that a better mechanistic understanding of cell plasticity, deformation associated remodeling and repair will reveal candidate approaches for lung protective interventions in mechanically ventilated patients. We will detail one such intervention, lung conditioning with hypertonic solutions as an example of ongoing research in this arena.

Mechanical ventilation guided by esophageal pressure in acute lung injury

BACKGROUND

Survival of patients with acute lung injury or the acute respiratory distress syndrome (ARDS) has been improved by ventilation with small tidal volumes and the use of positive end-expiratory pressure (PEEP); however, the optimal level of PEEP has been difficult to determine. In this pilot study, we estimated transpulmonary pressure with the use of esophageal balloon catheters. We reasoned that the use of pleural-pressure measurements, despite the technical limitations to the accuracy of such measurements, would enable us to find a PEEP value that could maintain oxygenation while preventing lung injury due to repeated alveolar collapse or overdistention.

METHODS

We randomly assigned patients with acute lung injury or ARDS to undergo mechanical ventilation with PEEP adjusted according to measurements of esophageal pressure (the esophageal-pressure-guided group) or according to the Acute Respiratory Distress Syndrome Network standard-of-care recommendations (the control group). The primary end point was improvement in oxygenation. The secondary end points included respiratory-system compliance and patient outcomes.

RESULTS

The study reached its stopping criterion and was terminated after 61 patients had been enrolled. The ratio of the partial pressure of arterial oxygen to the fraction of inspired oxygen at 72 hours was 88 mm Hg higher in the esophageal-pressure-guided group than in the control group (95% confidence interval, 78.1 to 98.3; P=0.002). This effect was persistent over the entire follow-up time (at 24, 48, and 72 hours; P=0.001 by repeated-measures analysis of variance). Respiratory-system compliance was also significantly better at 24, 48, and 72 hours in the esophageal-pressure-guided group (P=0.01 by repeated-measures analysis of variance).

CONCLUSIONS

As compared with the current standard of care, a ventilator strategy using esophageal pressures to estimate the transpulmonary pressure significantly improves oxygenation and compliance. Multicenter clinical trials are needed to determine whether this approach should be widely adopted. (ClinicalTrials.gov number, NCT00127491.)

Inhaled IL-10 reduces biotrauma and mortality in a model of ventilator-induced lung injury

BACKGROUND

High-pressure ventilation induces barotrauma and pulmonary inflammation, thus leading to ventilator-induced lung injury (VILI). By limiting the pulmonal inflammation cascade the anti-inflammatory cytokine interleukin (IL)-10 may have protective effects. Via inhalation, IL-10 reaches the pulmonary system directly and in high concentrations.

METHODS

Thirty six male, anesthetized and mechanically ventilated Sprague-Dawley rats were randomly assigned to the following groups (n=9, each): SHAM: pressure controlled ventilation with p(max)=20cmH(2)O, PEEP=4; VILI: ventilator settings were changed for 20min to p(max)=45cmH(2)O, PEEP=0; IL-10(high): inhalation of 10microg/kg IL-10 prior to induction of VILI; and IL-10(low): inhalation of 1microg/kg IL-10 prior to induction of VILI. All groups were ventilated and observed for 4h.

RESULTS

High-pressure ventilation increased the concentrations of macrophage inflammatory protein (MIP)-2 and IL-1beta in bronchoalveolar lavage fluid (BALF) and plasma. This effect was reduced by the inhalation of IL-10 (10microg/kg). Additionally, IL-10 increased the animal survival time (78% vs. 22% 4-h mortality rate) and reduced NO-release from ex vivo cultured alveolar macrophages. Moreover, VILI-induced pulmonary heat shock protein-70 expression was reduced by IL-10 aerosol in a dose-dependent manner. Similarly, the activation of matrix metalloproteinase (MMP)-9 in BALF was reduced dose-dependently by IL-10. IL-10-treated animals showed a lower macroscopic lung injury score and less impairment of lung integrity and gas exchange.

CONCLUSIONS

Prophylactic inhalation of IL-10 improved survival and reduced lung injury in experimental VILI. Results indicate that this effect may be mediated by the inhibition of stress-induced inflammation and pulmonary biotrauma.

Mechanical ventilation enhances lung inflammation and caspase activity in a model of mouse pneumovirus infection

Severe infection with respiratory syncytial virus (RSV) in children can progress to respiratory distress and acute lung injury (ALI). Accumulating evidence suggests that mechanical ventilation (MV) is an important cofactor in the development of ALI by modulating the host immune responses to bacteria. This study investigates whether MV enhances the host response to pneumonia virus of mice (PVM), a mouse pneumovirus that has been used as a model for RSV infection in humans. BALB/c mice were inoculated intranasally with diluted clarified lung homogenates from mice infected with PVM strain J3666 or uninfected controls. Four days after inoculation, the mice were subjected to 4 h of MV (tidal volume, 10 ml/kg) or allowed to breathe spontaneously. When compared with that of mice inoculated with PVM only, the administration of MV to PVM-infected mice resulted in increased bronchoalveolar lavage fluid concentrations of the cytokines macrophage inflammatory protein (MIP)-2, MIP-1alpha (CCL3), and IL-6; increased alveolar-capillary permeability to high molecular weight proteins; and increased caspase-3 activity in lung homogenates. We conclude that MV enhances the activation of inflammatory and caspase cell death pathways in response to pneumovirus infection. We speculate that MV potentially contributes to the development of lung injury in patients with RSV infection.

Modulatory effects of hypercapnia on in vitro and in vivo pulmonary endothelial-neutrophil adhesive responses during inflammation

Reducing tidal volume as a part of a protective ventilation strategy may result in hypercapnia. In this study, we focused on the influence of hypercapnia on endothelial-neutrophil responses in models of inflammatory-stimulated human pulmonary microvascular endothelial cells (HMVEC) and in an animal model of lipopolysaccharide (LPS)-induced acute lung injury. Neutrophil adhesion and adhesion molecules expression and nuclear factor-kappaB (NF-kappaB) were analyzed in TNF-alpha and LPS-treated HMVEC exposed to either eucapnia or hypercapnia. In the in vivo limb, bronchoalveolar lavage fluid cell counts and differentials, adhesion molecule and chemokine expression were assessed in LPS-treated rabbits ventilated with either low tidal volume ventilation and eucapnia or hypercapnia. In both the in vitro and in vivo models, hypercapnia significantly increased neutrophil adhesion and adhesion molecule expression compared to eucapnia. Activity of NF-kappaB was significantly enhanced by hypercapnia in the in vitro experiments. IL-8 expression was greatest both in vitro and in vivo under conditions of hypercapnia and concomitant inflammation. CD11a expression was greatest in isolated human neutrophils exposed to hypercapnia+LPS. Our results demonstrate that endothelial-neutrophil responses per measurement of fundamental molecules of adhesion are significantly increased during hypercapnia and that hypercapnia mimics conditions of eucapnia+inflammation.

Interactions between mechanical and biological processes in acute lung injury

Human studies and animal models suggest that mechanical as well as biological processes contribute to acute lung injury. While mechanical stresses and bacterial products can directly alter the endothelial and epithelial barriers in the lungs, a growing body of evidence suggests that synergistic interactions between low levels of mechanical stress and bacterial products in the lungs can cause or exacerbate acute lung injury. New approaches to disrupting these synergistic interactions between mechanical stress and innate immunity have the potential to reduce the incidence or improve the outcome of acute lung injury in humans.

Essential role of pre-B-cell colony enhancing factor in ventilator-induced lung injury

RATIONALE

We previously demonstrated pre-B-cell colony enhancing factor (PBEF) as a biomarker in sepsis and sepsis-induced acute lung injury (ALI) with genetic variants conferring ALI susceptibility.

OBJECTIVES

To explore mechanistic participation of PBEF in ALI and ventilator-induced lung injury (VILI).

METHODS

Two models of VILI were utilized to explore the role of PBEF using either recombinant PBEF or PBEF(+/-) mice.

MEASUREMENTS AND MAIN RESULTS

Initial in vitro studies demonstrated recombinant human PBEF (rhPBEF) as a direct rat neutrophil chemotactic factor with in vivo studies demonstrating marked increases in bronchoalveolar lavage (BAL) leukocytes (PMNs) after intratracheal injection in C57BL/6J mice. These changes were accompanied by increased BAL levels of PMN chemoattractants (KC and MIP-2) and modest increases in lung vascular and alveolar permeability. We next explored the potential synergism between rhPBEF challenge (intratracheal) and a model of limited VILI (4 h, 30 ml/kg tidal volume) and observed dramatic increases in BAL PMNs, BAL protein, and cytokine levels (IL-6, TNF-alpha, KC) compared with either challenge alone. Gene expression profiling identified induction of ALI- and VILI-associated gene modules (nuclear factor-kappaB, leukocyte extravasation, apoptosis, Toll receptor pathways). Heterozygous PBEF(+/-) mice were significantly protected (reduced BAL protein, BAL IL-6 levels, peak inspiratory pressures) when exposed to a model of severe VILI (4 h, 40 ml/kg tidal volume) and exhibited significantly reduced expression of VILI-associated gene expression modules. Finally, strategies to reduce PBEF availability (neutralizing antibody) resulted in significant protection from VILI.

CONCLUSIONS

These studies implicate PBEF as a key inflammatory mediator intimately involved in both the development and severity of ventilator-induced ALI.

The cytokine storm and factors determining the sequence and severity of organ dysfunction in multiple organ dysfunction syndrome

Multiple organ dysfunction syndrome (MODS) is a major cause of morbidity and mortality in intensive care units. It is being encountered frequently in critically ill patients owing to advancements in organ-specific supportive technologies to survive the acute phase of severe sepsis and shock. It is now believed that MODS is the result of an inappropriate generalized inflammatory response of the host to a variety of acute insults. The pathologic mechanisms of MODS were reviewed, and factors determining the sequence and severity of organ dysfunction were discussed in depth. In the early phase of MODS, circulating cytokines cause universal endothelium injury in organs. In the later phase of MODS, overexpression of inflammatory mediators in the interstitial space of various organs is considered a main mechanism of parenchyma injury. The difference in constitutive expression and the upregulation of adhesion molecules in vascular beds and the density and potency of intrinsic inflammatory cells in different organs are the key factors determining the sequence and severity of organ dysfunction. By activating the intrinsic inflammatory cell in a distant organ, organ dysfunctions are linked in a positive feedback loop through circulating inflammatory mediators. Antagonists targeted at adhesion molecules may alleviate the severity of endothelial damage. And nonsteroidial anti-inflammatory drugs or steroids administered judiciously in the early phase of MODS may retard the progress of multiple organ failure.

Magnitude-dependent effects of cyclic stretch on HGF- and VEGF-induced pulmonary endothelial remodeling and barrier regulation

Mechanical ventilation at high tidal volumes compromises the blood-gas barrier and increases lung vascular permeability, which may lead to ventilator-induced lung injury and pulmonary edema. Using pulmonary endothelial cell (ECs) exposed to physiologically [5% cyclic stretch (CS)] and pathologically (18% CS) relevant magnitudes of CS, we evaluated the potential protective effects of hepatocyte growth factor (HGF) on EC barrier dysfunction induced by CS and vascular endothelial growth factor (VEGF). In static culture, HGF enhanced EC barrier function in a Rac-dependent manner and attenuated VEGF-induced EC permeability and paracellular gap formation. The protective effects of HGF were associated with the suppression of Rho-dependent signaling triggered by VEGF. Five percent CS promoted HGF-induced enhancement of the cortical F-actin rim and activation of Rac-dependent signaling, suggesting synergistic barrier-protective effects of physiological CS and HGF. In contrast, 18% CS further enhanced VEGF-induced EC permeability, activation of Rho signaling, and formation of actin stress fibers and paracellular gaps. These effects were attenuated by HGF pretreatment. EC preconditioning at 5% CS before HGF and VEGF further promoted EC barrier maintenance. Our data suggest synergistic effects of HGF and physiological CS in the Rac-mediated mechanisms of EC barrier protection. In turn, HGF reduced the barrier-disruptive effects of VEGF and pathological CS via downregulation of the Rho pathway. These results support the importance of HGF-VEGF balance in control of acute lung injury/acute respiratory distress syndrome severity via small GTPase-dependent regulation of lung endothelial permeability.

Inflammatory mediators in the immunobiology of bronchopulmonary dysplasia

Inflammation is important in the development of bronchopulmonary dysplasia (BPD). Polymorphonuclear cells and macrophages and proinflammatory cytokines/chemokines denote early inflammation in clinical scenarios such as in utero inflammation with chorioamnionitis or initial lung injury associated with respiratory distress syndrome or ventilator-induced lung injury. The persistence and non-resolution of lung inflammation contributes greatly to BPD, including altering the lung's ability to repair, contributing to fibrosis, and inhibiting secondary septation, alveolarization, and normal vascular development. Further understanding of the role of inflammation in the pathogenesis of BPD, in particular, during the chronic inflammatory period, offers us the opportunity to develop inflammation-related prevention and treatment strategies of this disease that has long-standing consequences for very premature infants.

Prone positioning in acute respiratory distress syndrome: a multicenter randomized clinical trial

OBJECTIVE

We examined the effect on survival of prone positioning as an early and continuous treatment in ARDS patients already treated with protective ventilation.

DESIGN AND SETTING

Open randomized controlled trial in 17 medical-surgical ICUs.

PATIENTS

Forty mechanically ventilated patients with early and refractory ARDS despite protective ventilation in the supine position.

INTERVENTIONS

Patients were randomized to remain supine or be moved to early (within 48[Symbol: see text]h) and continuous (> or = 20 h/day) prone position until recovery or death. The trial was prematurely stopped due to a low patient recruitment rate.

MEASUREMENTS AND RESULTS

Clinical characteristics, oxygenation, lung pressures, and hemodynamics were monitored. Need for sedation, complications, length of MV, ICU, and hospital stays, and outcome were recorded. PaO(2)/FIO(2) tended to be higher in prone than in supine patients after 6[Symbol: see text]h (202 +/-78 vs. 165+/-70 mmHg); this difference reached statistical significance on day 3 (234+/-85 vs. 159+/-78). Prone-related side effects were minimal and reversible. Sixty-day survival reached the targeted 15% absolute increase in prone patients (62% vs. 47%) but failed to reach significance due to the small sample.

CONCLUSIONS

Our study adds data that reinforce the suggestion of a beneficial effect of early continuous prone positioning on survival in ARDS patients.

Effect of Toll-like receptor 4 blockade on pulmonary inflammation caused by mechanical ventilation and bacterial endotoxin

Mechanical ventilation (MV) and lipopolysaccharide (LPS) synergistically increase inflammation and lung injury. The goal of this study was to determine whether blockade of CD14 or Toll-like receptor 4 (TLR4) would reduce inflammation caused by LPS and MV. Rabbits were pretreated with anti-TLR4 or anti-CD14 monoclonal antibodies, followed by endobronchial LPS and MV. Blockade of TLR4 reduced the number of neutrophils and the amount of CXCL8 in bronchoalveolar lavage fluid. In contrast, blockade of CD14 did not significantly decrease the number of neutrophils or the amount of CXCL8. These data show that TLR4 blockade reduces pulmonary inflammation caused by the combination of LPS and Mechanical ventilation.

Is there a best way to set tidal volume for mechanical ventilatory support?

Tidal breaths are an important component of mechanical ventilation. However, an inappropriate tidal volume setting can overstretch and injure the lung. Maximal stretch, tidal stretch, frequency of stretch, and rate of stretch are all implicated in such injury. Clinical trials have shown that limiting maximal and tidal stretch improves outcomes, even if gas exchange is partially compromised. Thus, current strategies should focus on limiting tidal and maximal stretch as much as possible.

Is there a best way to set positive expiratory-end pressure for mechanical ventilatory support in acute lung injury?

Airspace collapse is a hallmark of parenchymal lung injury. Strategies to reopen and maintain patency of these regions offer three advantages: improved gas exchange, less lung injury, and improved lung compliance. Elevations in intrathoracic pressure to achieve these goals, however, may overdistend healthier lung regions and compromise cardiac function. Positive expiratory-end pressure is a widely used technique to maintain alveolar patency, but its beneficial effects must be balanced against its harmful effects.

Bedside evaluation of pressure-volume curves in patients with acute respiratory distress syndrome

PURPOSE OF REVIEW

To describe the physiologic and diagnostic utility of static pressure-volume curves of the respiratory system at the bedside in patients with acute lung injury or acute respiratory distress syndrome.

RECENT FINDINGS

The pressure-volume curve of the respiratory system is a useful tool for the measurement of respiratory system mechanics in patients with acute lung injury or acute respiratory distress syndrome. The pressure-volume curve has a sigmoid shape, with lower and upper points on the inspiratory limb and a point of maximum curvature on the expiratory limb. Visual and mathematical pressure-volume curve analysis may be useful for understanding individual lung mechanics and for selecting ventilator settings. Among the different techniques for acquiring pressure-volume curves at the bedside, the constant slow flow method is the simplest to perform, the most clinically reliable and has the fewest limitations.

SUMMARY

Measurement of pressure-volume curves at the bedside in critically ill patients with acute lung injury or acute respiratory distress syndrome should be considered a useful respiratory monitoring tool to assess physiologic lung status and to adjust ventilator settings, when appropriate, to minimize superimposed lung injury associated with mechanical ventilators.

Tubulin acetylation and histone deacetylase 6 activity in the lung under cyclic load

Previous studies from our lab have demonstrated that upon exposure to physiologic levels of cyclic stretch, alveolar epithelial cells demonstrate a significant decrease in the amount of polymerized tubulin (Geiger et al., Gene Therapy 2006;13:725-731). However, not all microtubules are disassembled, although the mechanisms or implications of this were unknown. Using immunofluorescence microscopy, Western blotting, and immunohistochemistry approaches, we have compared the levels of acetylated tubulin in stretched and unstretched A549 cells and in murine lungs. In cultured cells exposed to cyclic stretch (10% change in basement membrane surface area at 0.25 Hz), nearly all of the remaining microtubules were acetylated, as demonstrated using immunofluorescence microscopy. In murine lungs ventilated for 20 minutes at 12 to 20 ml/kg followed by 48 hours of spontaneous breathing or for 3 hours at 16 to 40 ml/kg, levels of acetylated tubulin were increased in the peripheral lung. In both our in vitro and in vivo studies, we have found that mild to moderate levels of cyclic stretch significantly increases tubulin acetylation in a magnitude- and duration-dependent manner. This appears to be due to a decrease in histone deacetylase 6 activity (HDAC6), the major tubulin deacetylase. Since it has been previously shown that acetylated microtubules are positively correlated to a more stable population of microtubules, this result suggests that microtubule stability may be increased by cyclic stretch, and that tubulin acetylation is one way in which cells respond to changes in exogenous mechanical forces.

Animal models of acute lung injury

Acute lung injury in humans is characterized histopathologically by neutrophilic alveolitis, injury of the alveolar epithelium and endothelium, hyaline membrane formation, and microvascular thrombi. Different animal models of experimental lung injury have been used to investigate mechanisms of lung injury. Most are based on reproducing in animals known risk factors for ARDS, such as sepsis, lipid embolism secondary to bone fracture, acid aspiration, ischemia-reperfusion of pulmonary or distal vascular beds, and other clinical risks. However, none of these models fully reproduces the features of human lung injury. The goal of this review is to summarize the strengths and weaknesses of existing models of lung injury. We review the specific features of human ARDS that should be modeled in experimental lung injury and then discuss specific characteristics of animal species that may affect the pulmonary host response to noxious stimuli. We emphasize those models of lung injury that are based on reproducing risk factors for human ARDS in animals and discuss the advantages and disadvantages of each model and the extent to which each model reproduces human ARDS. The present review will help guide investigators in the design and interpretation of animal studies of acute lung injury.

[Sedation an acute respiratory distress syndrome]

AIM

To assess the role of sedation and myorelaxant agents in acute respiratory distress syndrome (ARDS) and to propose an updated management according to recent literature. EXTRACTION OF DATA: From Medline and Cochrane database of English and French language articles. Keywords were: acute respiratory distress syndrome, acute lung injury, general anaesthetics, inhalation, intravenous anaesthetics and intensive care. Selection of original articles, reviews and expert reports. Case reports have been included.

TOPIC

ARDS is a clinical picture in which respiratory constraints are major because of hypoxemia. To insure correct haematosis, mechanical ventilation has to be considered. It constitutes, then, the most frequent indication of sedation in the intensive care unit. The objectives are to help the ventilation of lungs and to improve gas exchange, by controlling agitation, fight against ventilation and to reduce mechanical ventilation associated injuries. In this situation, use of myorelaxant agents is aimed at facilitating synchronization of the patient with his/her ventilator and serves to improve oxygenation during the early inflammatory phase of ARDS. Several mechanisms may enflame this improvement of oxygenation. One of the most probable effect on optimization is the possibility of optimize protective ventilation at the cares phase of ARDS and to reduce mechanical ventilation-associated injuries.

CONCLUSION

With regard to benefits and inconvenient, sedation is considered as a treatment of ARDS. Its goals are the well being of patient and his/her adaptation to ventilator, but also the prevention on mechanical ventilation associated injuries. Hence, most authors suggest using a deep sedation at the early phase of ARDS. In this contact, use of myorelaxant agent is an intersecting adjuvant if sedation is not enough. The benefit is terms of survival and outcome remains to show.

[High PEEP vs. conventional PEEP in the acute respiratory distress syndrome: a systematic review and meta-analysis]

OBJECTIVE

To perform a systematic review and meta-analysis of the literature to evaluate the effects of high PEEP versus conventional PEEP on mortality and on the risk of barotrauma in patients with the acute respiratory distress syndrome (ARDS).

SOURCE OF DATA

Computer search of Medline, Embase, CINAHL, CANCERLIT, Pascal-Biomed, ACP Journal Club, Cochrane library (CDSR, DARE, CCTR), ISI Proceedings, Current Contents, and Web of Science, as well as manual search of selected references.

SELECTION OF STUDIES

Controlled random clinical trials published after NAECC (1994) that evaluated the effect of two levels of PEEP and that reported the mortality and incidence of barotrauma in the series.

DATA EXTRACTION

By two investigators working independently, with discrepancies resolved by group consensus. Contingency tables were elaborated and the RRs with corresponding confidence intervals were obtained for each study.

RESULTS

Four articles were selected for the meta-analysis of mortality and three for the meta-analysis of barotrauma. No effects of PEEP level on mortality were found (RR 0.73, 95% CI: 0.49 to 1.10) or on the incidence of barotrauma (RR 0.50, 95% CI: 0.14 to 1.73). However, an analysis of the studies in which PEEP was individualized in function of Pflex showed a significant decrease in mortality (RR 0.59, 95% CI: 0.43 to 0.82) (p=0.001)

CONCLUSIONS

The use of high or conventional PEEP in function of oxygenation does not affect mortality or the incidence of barotrauma in patients with ARDS. However, there might be a decrease in mortality associated to high PEEP individualized in function of the pulmonary mechanics of each patient.