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

Open-lung ventilation versus no ventilation during cardiopulmonary bypass in an innovative animal model of heart transplantation

Open-lung ventilation during cardiopulmonary bypass (CPB) in patients undergoing heart transplantation (HTx) is a potential strategy to mitigate postoperative acute respiratory distress syndrome (ARDS). We utilized an ovine HTx model to investigate whether open-lung ventilation during CPB reduces postoperative lung damage and complications. Eighteen sheep from an ovine HTx model were included, with ventilatory interventions randomly assigned during CPB: the OPENVENT group received low tidal volume (VT) of 3 mL/kg and positive end-expiratory pressure (PEEP) of 8 cm H20, while no ventilation was provided in the NOVENT group as per standard of care. The recipient sheep were monitored for 6 h post-surgery. The primary outcome was histological lung damage, scored at the end of the study. Secondary outcomes included pulmonary shunt, driving pressure, hemodynamics and inflammatory lung infiltration. All animals completed the study. The OPENVENT group showed significantly lower histological lung damage versus the NOVENT group (0.22 vs 0.27, p = 0.042) and lower pulmonary shunt (19.2 vs 32.1%, p = 0.001). In addition, the OPENVENT group exhibited a reduced driving pressure (9.6 cm H2O vs. 12.8 cm H2O, p = 0.039), lower neutrophil (5.25% vs 7.97%, p ≤ 0.001) and macrophage infiltrations (11.1% vs 19.6%, p < 0.001). No significant differences were observed in hemodynamic parameters. In an ovine model of HTx, open-lung ventilation during CPB significantly reduced lung histological injury and inflammatory infiltration. This highlights the value of an open-lung approach during CPB and emphasizes the need for further clinical evidence to decrease risks of lung injury in HTx patients.

Effect of mechanical ventilation under intubation on respiratory tract change of bacterial count and alteration of bacterial flora

Background: The most common 'second strike' in mechanically ventilated patients is a pulmonary infection caused by the ease with which bacteria can invade and colonize the lungs due to mechanical ventilation. At the same time, metastasis of lower airway microbiota may have significant implications in developing intubation mechanical ventilation lung inflammation. Thus, we establish a rat model of tracheal intubation with mechanical ventilation and explore the effects of mechanical ventilation on lung injury and microbiological changes in rats. To provide a reference for preventing and treating bacterial flora imbalance and pulmonary infection injury caused by mechanical ventilation of tracheal intubation. Methods: Sprague-Dawley rats were randomly divided into Control, Mechanical ventilation under intubation (1, 3, 6 h) groups, and Spontaneously breathing under intubation (1, 3, 6 h). Lung histopathological injury scores were evaluated. 16SrDNA sequencing was performed to explore respiratory microbiota changes, especially, changes of bacterial count and alteration of bacterial flora. Results: Compared to groups C and SV, critical pathological changes in pulmonary lesions occurred in the MV group after 6 h (p < 0.05). The Alpha diversity and Beta diversity of lower respiratory tract microbiota in MV6, SV6, and C groups were statistically significant (p < 0.05). The main dominant bacterial phyla in the respiratory tract of rats were Proteobacteria, Firmicutes, Bacteroidetes, and Cyanobacteria. Acinetobacter radioresistens in group C was significant, Megaonas in group MV6 was significantly increased, and Parvibacter in group SV6 was significantly increased. Anaerobic, biofilm formation, and Gram-negative bacteria-related functional genes were altered during mechanical ventilation with endotracheal intubation. Conclusion: Mechanical ventilation under intubation may cause dysregulation of lower respiratory microbiota in rats.

Molecular Dynamics of Lipopolysaccharide-Induced Lung Injury in Rodents

Acute respiratory distress syndrome (ARDS) is a common disease entity in critical care medicine and is still associated with a high mortality. Because of the heterogeneous character of ARDS, animal models are an insturment to study pathology in relatively standardized conditions. Rodent models can bridge the gap from in vitro investigations to large animal and clinical trials by facilitating large sample sizes under physiological conditions at comparatively low costs. One of the most commonly used rodent models of acute lung inflammation and ARDS is administration of lipopolysaccharide (LPS), either into the airways (direct, pulmonary insult) or systemically (indirect, extra-pulmonary insult). This narrative review discusses the dynamics of important pathophysiological pathways contributing to the physiological response to LPS-induced injury. Pathophysiological pathways of LPS-induced lung injury are not only influenced by the type of the primary insult (e.g., pulmonary or extra-pulmonary) and presence of additional stimuli (e.g., mechanical ventilation), but also by time. As such, findings in animal models of LPS-induced lung injury may depend on the time point at which samples are obtained and physiological data are captured. This review summarizes the current evidence and highlights uncertainties on the molecular dynamics of LPS-induced lung injury in rodent models, encouraging researchers to take accurate timing of LPS-induced injury into account when designing experimental trials.

Mitochondrial alarmins are tissue mediators of ventilator-induced lung injury and ARDS

Rationale

Endogenous tissue mediators inducing lung inflammation in the context of ventilator-induced lung injury (VILI) and acute respiratory distress syndrome (ARDS) are ill-defined.

Objectives

To test whether mitochondrial alarmins are released during VILI, and are associated with lung inflammation.

Methods

Release of mitochondrial DNA, adenosine triphosphate (ATP), and formyl-Met-Leu-Phe (fMLP) peptide-dependent neutrophil chemotaxis were measured in conditioned supernatants from human alveolar type II-like (A549) epithelial cells submitted to cyclic stretch in vitro. Similar measurements were performed in bronchoalveolar lavage fluids from rabbits submitted to an injurious ventilatory regimen, and from patients with ARDS.

Measurements and main results

Mitochondrial DNA was released by A549 cells during cell stretching, and was found elevated in BAL fluids from rabbits during VILI, and from ARDS patients. Cyclic stretch-induced interleukin-8 (IL-8) of A549 cells could be inhibited by Toll-like receptor 9 (TLR9) blockade. ATP concentrations were increased in conditioned supernatants from A549 cells, and in rabbit BAL fluids during VILI. Neutrophil chemotaxis induced by A549 cells conditioned supernatants was essentially dependent on fMLP rather than IL-8. A synergy between cyclic stretch-induced alarmins and lipopolysaccharide (LPS) was found in monocyte-derived macrophages in the production of IL-1ß.

Conclusions

Mitochondrial alarmins are released during cyclic stretch of human epithelial cells, as well as in BAL fluids from rabbits ventilated with an injurious ventilatory regimen, and found in BAL fluids from ARDS patients, particularly in those with high alveolar inflammation. These alarmins are likely to represent the proximal endogenous mediators of VILI and ARDS, released by injured pulmonary cells.

Lung-Protective Ventilation Strategies for Relief from Ventilator-Associated Lung Injury in Patients Undergoing Craniotomy: A Bicenter Randomized, Parallel, and Controlled Trial

Current evidence indicates that conventional mechanical ventilation often leads to lung inflammatory response and oxidative stress, while lung-protective ventilation (LPV) minimizes the risk of ventilator-associated lung injury (VALI). This study evaluated the effects of LPV on relief of pulmonary injury, inflammatory response, and oxidative stress among patients undergoing craniotomy. Sixty patients undergoing craniotomy received either conventional mechanical (12 mL/kg tidal volume [V_T] and 0 cm H₂O positive end-expiratory pressure [PEEP]; CV group) or protective lung (6 mL/kg V_T and 10 cm H₂O PEEP; PV group) ventilation. Hemodynamic variables, lung function indexes, and inflammatory and oxidative stress markers were assessed. The PV group exhibited greater dynamic lung compliance and lower respiratory index than the CV group during surgery (P < 0.05). The PV group exhibited higher plasma interleukin- (IL-) 10 levels and lower plasma malondialdehyde and nitric oxide and bronchoalveolar lavage fluid, IL-6, IL-8, tumor necrosis factor-α, IL-10, malondialdehyde, nitric oxide, and superoxide dismutase levels (P < 0.05) than the CV group. There were no significant differences in hemodynamic variables, blood loss, liquid input, urine output, or duration of mechanical ventilation between the two groups (P > 0.05). Patients receiving LPV during craniotomy exhibited low perioperative inflammatory response, oxidative stress, and VALI.

Mechanisms of ventilator-induced lung injury in healthy lungs

Mechanical ventilation is an essential method of patient support, but it may induce lung damage, leading to ventilator-induced lung injury (VILI). VILI is the result of a complex interplay among various mechanical forces that act on lung structures, such as type I and II epithelial cells, endothelial cells, macrophages, peripheral airways, and the extracellular matrix (ECM), during mechanical ventilation. This article discusses ongoing research focusing on mechanisms of VILI in previously healthy lungs, such as in the perioperative period, and the development of new ventilator strategies for surgical patients. Several experimental and clinical studies have been conducted to evaluate the mechanisms of mechanotransduction in each cell type and in the ECM, as well as the role of different ventilator parameters in inducing or preventing VILI. VILI may be attenuated by reducing the tidal volume; however, the use of higher or lower levels of positive end-expiratory pressure (PEEP) and recruitment maneuvers during the perioperative period is a matter of debate. Many questions concerning the mechanisms of VILI in surgical patients remain unanswered. The optimal threshold value of each ventilator parameter to reduce VILI is also unclear. Further experimental and clinical studies are necessary to better evaluate ventilator settings during the perioperative period in different types of surgery.

Submarine rescue decompression procedure from hyperbaric exposures up to 6 bar of absolute pressure in man: effects on bubble formation and pulmonary function

Recent advances in submarine rescue systems have allowed a transfer under pressure of crew members being rescued from a disabled submarine. The choice of a safe decompression procedure for pressurised rescuees has been previously discussed, but no schedule has been validated when the internal submarine pressure is significantly increased i.e. exceeding 2.8 bar absolute pressure. This study tested a saturation decompression procedure from hyperbaric exposures up to 6 bar, the maximum operating pressure of the NATO submarine rescue system. The objective was to investigate the incidence of decompression sickness (DCS) and clinical and spirometric indices of pulmonary oxygen toxicity. Two groups were exposed to a Nitrogen-Oxygen atmosphere (pO2 = 0.5 bar) at either 5 bar (N = 14) or 6 bar (N = 12) for 12 h followed by 56 h 40 min resp. 60 h of decompression. When chamber pressure reached 2.5 bar, the subjects breathed oxygen intermittently, otherwise compressed air. Repeated clinical examinations, ultrasound monitoring of venous gas embolism and spirometry were performed during decompression. During exposures to 5 bar, 3 subjects had minor subjective symptoms i.e. sensation of joint discomfort, regressing spontaneously, and after surfacing 2 subjects also experienced joint discomfort disappearing without treatment. Only 3 subjects had detectable intravascular bubbles during decompression (low grades). No bubbles were detected after surfacing. About 40% of subjects felt chest tightness when inspiring deeply during the initial phase of decompression. Precordial burning sensations were reported during oxygen periods. During decompression, vital capacity decreased by about 8% and forced expiratory flow rates decreased significantly. After surfacing, changes in the peripheral airways were still noticed; Lung Diffusion for carbon monoxide was slightly reduced by 1% while vital capacity was normalized. The procedure did not result in serious symptoms of DCS or pulmonary oxygen toxicity and may be considered for use when the internal submarine pressure is significantly increased.

Impact of the Prone Position in an Animal Model of Unilateral Bacterial Pneumonia Undergoing Mechanical Ventilation

Background:

The prone position (PP) has proven beneficial in patients with severe lung injury subjected to mechanical ventilation (MV), especially in those with lobar involvement. We assessed the impact of PP on unilateral pneumonia in rabbits subjected to MV.

Methods:

After endobronchial challenge with Enterobacter aerogenes, adult rabbits were subjected to either “adverse” (peak inspiratory pressure = 30 cm H2O, zero end-expiratory pressure; n = 10) or “protective” (tidal volume = 8 ml/kg, 5 cm H2O positive end-expiratory pressure; n = 10) MV and then randomly kept supine or turned to the PP. Pneumonia was assessed 8 h later. Data are presented as median (interquartile range).

Results:

Compared with the supine position, PP was associated with significantly lower bacterial concentrations within the infected lung, even if a “protective” MV was applied (5.93 [0.34] vs. 6.66 [0.86] log10 cfu/g, respectively; P = 0.008). Bacterial concentrations in the spleen were also decreased by the PP if the “adverse” MV was used (3.62 [1.74] vs. 6.55 [3.67] log10 cfu/g, respectively; P = 0.038). In addition, the noninfected lung was less severely injured in the PP group. Finally, lung and systemic inflammation as assessed through interleukin-8 and tumor necrosis factor-α measurement was attenuated by the PP.

Conclusions:

The PP could be protective if the host is subjected to MV and unilateral bacterial pneumonia. It improves lung injury even if it is utilized after lung injury has occurred and nonprotective ventilation has been administered.

Low tidal volume mechanical ventilation and oxidative stress in healthy mouse lungs

OBJECTIVE

Mechanical ventilation (MV) itself can directly contribute to lung injury. Therefore, the aim of the present study was to investigate early biomarkers concerning oxidant/antioxidant balance, oxidative stress, and inflammation caused by short-term MV in healthy mouse lungs.

METHODS

Twenty male C57BL/6 mice were randomly divided into two groups: MV, submitted to low tidal volume (V T, 6 mL/kg) MV for 30 min; and spontaneous respiration (SR), used as controls. Lung homogenate samples were tested regarding the activity of various antioxidant enzymes, lipid peroxidation, and TNF-α expression.

RESULTS

In comparison with the SR group, the MV group showed a significant decrease in the activity of superoxide dismutase (≈35%; p < 0.05), together with an increase in the activity of catalase (40%; p < 0.01), glutathione peroxidase (500%; p < 0.001), and myeloperoxidase (260%; p < 0.001), as well as a reduction in the glutathione/oxidized glutathione ratio (≈50%; p < 0.05) and an increase in TNF-α expression in the MV group. Oxidative damage, assessed by lipid peroxidation, was also greater in the MV group (45%; p < 0.05).

CONCLUSIONS

Our results show that short-term low V T MV can directly contribute to lung injury, generating oxidative stress and inflammation in healthy mouse lungs.

Intraoperative ventilation: incidence and risk factors for receiving large tidal volumes during general anesthesia

Background

There is a growing concern of the potential injurious role of ventilatory over-distention in patients without lung injury. No formal guidelines exist for intraoperative ventilation settings, but the use of tidal volumes (V_T) under 10 mL/kg predicted body weight (PBW) has been recommended in healthy patients. We explored the incidence and risk factors for receiving large tidal volumes (V_T > 10 mL/kg PBW).

Methods

We performed a cross-sectional analysis of our prospectively collected perioperative electronic database for current intraoperative ventilation practices and risk factors for receiving large tidal volumes (V_T > 10 mL/kg PBW). We included all adults undergoing prolonged (≥ 4 h) elective abdominal surgery and collected demographic, preoperative (comorbidities), intraoperative (i.e. ventilatory settings, fluid administration) and postoperative (outcomes) information. We compared patients receiving exhaled tidal volumes > 10 mL/kg PBW with those that received 8-10 or < 8 mL/kg PBW with univariate and logistic regression analyses.

Results

Ventilatory settings were non-uniform in the 429 adults included in the analysis. 17.5% of all patients received V_T > 10 mL/kg PBW. 34.0% of all obese patients (body mass index, BMI, ≥ 30), 51% of all patients with a height < 165 cm, and 34.6% of all female patients received V_T > 10 mL/kg PBW.

Conclusions

Ventilation with V_T > 10 mL/kg PBW is still common, although poor correlation with PBW suggests it may be unintentional. BMI ≥ 30, female gender and height < 165 cm may predispose to receive large tidal volumes during general anesthesia. Further awareness of patients' height and PBW is needed to improve intraoperative ventilation practices. The impact on clinical outcome needs confirmation.

Linking lung function and inflammatory responses in ventilator-induced lung injury

Despite decades of research, the mechanisms of ventilator-induced lung injury are poorly understood. We used strain-dependent responses to mechanical ventilation in mice to identify associations between mechanical and inflammatory responses in the lung. BALB/c, C57BL/6, and 129/Sv mice were ventilated using a protective [low tidal volume and moderate positive end-expiratory pressure (PEEP) and recruitment maneuvers] or injurious (high tidal volume and zero PEEP) ventilation strategy. Lung mechanics and lung volume were monitored using the forced oscillation technique and plethysmography, respectively. Inflammation was assessed by measuring numbers of inflammatory cells, cytokine (IL-6, IL-1β, and TNF-α) levels, and protein content of the BAL. Principal components factor analysis was used to identify independent associations between lung function and inflammation. Mechanical and inflammatory responses in the lung were dependent on ventilation strategy and mouse strain. Three factors were identified linking 1) pulmonary edema, protein leak, and macrophages, 2) atelectasis, IL-6, and TNF-α, and 3) IL-1β and neutrophils, which were independent of responses in lung mechanics. This approach has allowed us to identify specific inflammatory responses that are independently associated with overstretch of the lung parenchyma and loss of lung volume. These data provide critical insight into the mechanical responses in the lung that drive local inflammation in ventilator-induced lung injury and the basis for future mechanistic studies in this field.

Management of mechanical ventilation during laparoscopic surgery

Laparoscopy is widely used in the surgical treatment of a number of diseases. Its advantages are generally believed to lie on its minimal invasiveness, better cosmetic outcome and shorter length of hospital stay based on surgical expertise and state-of-the-art equipment. Thousands of laparoscopic surgical procedures performed safely prove that mechanical ventilation during anaesthesia for laparoscopy is well tolerated by a vast majority of patients. However, the effects of pneumoperitoneum are particularly relevant to patients with underlying lung disease as well as to the increasing number of patients with higher-than-normal body mass index. Moreover, many surgical procedures are significantly longer in duration when performed with laparoscopic techniques. Taken together, these factors impose special care for the management of mechanical ventilation during laparoscopic surgery. The purpose of the review is to summarise the consequences of pneumoperitoneum on the standard monitoring of mechanical ventilation during anaesthesia and to discuss the rationale of using a protective ventilation strategy during laparoscopic surgery. The consequences of chest wall derangement occurring during pneumoperitoneum on airway pressure and central venous pressure, together with the role of end-tidal-CO2 monitoring are emphasised. Ventilatory and non-ventilatory strategies to protect the lung are discussed.

Substance P receptor blockade decreases stretch-induced lung cytokines and lung injury in rats

Overdistension of lung tissue during mechanical ventilation causes cytokine release, which may be facilitated by the autonomic nervous system. We used mechanical ventilation to cause lung injury in rats, and studied how cervical section of the vagus nerve, or substance P (SP) antagonism, affected the injury. The effects of 40 or 25 cmH(2)O high airway pressure injurious ventilation (HV(40) and HV(25)) were studied and compared with low airway pressure ventilation (LV) and spontaneous breathing (controls). Lung mechanics, lung weight, gas exchange, lung myeloperoxidase activity, lung concentrations of interleukin (IL)-1 beta and IL-6, and amounts of lung SP were measured. Control rats were intact, others were bivagotomized, and in some animals we administered the neurokinin-1 (NK-1) receptor blocking agent SR140333. We first determined the durations of HV(40) and HV(25) that induced the same levels of lung injury and increased lung contents of IL-1 beta and IL-6. They were 90 min and 120 min, respectively. Both HV(40) and HV(25) increased lung SP, IL-1 beta and IL-6 levels, these effects being markedly reduced by NK-1 receptor blockade. Bivagotomy reduced to a lesser extent the HV(40)- and HV(25)-induced increases in SP but significantly reduced cytokine production. Neither vagotomy nor NK-1 receptor blockade prevented HV(40)-induced lung injury but, in the HV(25) group, they made it possible to maintain lung injury indices close to those measured in the LV group. This study suggests that both neuronal and extra-neuronal SP might be involved in ventilator-induced lung inflammation and injury. NK-1 receptor blockade could be a pharmacological tool to minimize some adverse effects of mechanical ventilation.

Mechanical ventilation with high tidal volume induces inflammation in patients without lung disease

INTRODUCTION

Mechanical ventilation (MV) with high tidal volumes may induce or aggravate lung injury in critical ill patients. We compared the effects of a protective versus a conventional ventilatory strategy, on systemic and lung production of tumor necrosis factor-alpha (TNF-alpha) and interleukin-8 (IL-8) in patients without lung disease.

METHODS

Patients without lung disease and submitted to mechanical ventilation admitted to one trauma and one general adult intensive care unit of two different university hospitals were enrolled in a prospective randomized-control study. Patients were randomized to receive MV either with tidal volume (VT) of 10 to 12 ml/kg predicted body weight (high VT group) (n = 10) or with VT of 5 to 7 ml/kg predicted body weight (low VT group) (n = 10) with an oxygen inspiratory fraction (FIO2) enough to keep arterial oxygen saturation >90% with positive end-expiratory pressure (PEEP) of 5 cmH2O during 12 hours after admission to the study. TNF-alpha and IL-8 concentrations were measured in the serum and in the bronchoalveolar lavage fluid (BALF) at admission and after 12 hours of study observation time.

RESULTS

Twenty patients were enrolled and analyzed. At admission or after 12 hours there were no differences in serum TNF-alpha and IL-8 between the two groups. While initial analysis did not reveal significant differences, standardization against urea of logarithmic transformed data revealed that TNF-alpha and IL-8 levels in bronchoalveolar lavage (BAL) fluid were stable in the low VT group but increased in the high VT group (P = 0.04 and P = 0.03). After 12 hours, BALF TNF-alpha (P = 0.03) and BALF IL-8 concentrations (P = 0.03) were higher in the high VT group than in the low VT group.

CONCLUSIONS

The use of lower tidal volumes may limit pulmonary inflammation in mechanically ventilated patients even without lung injury.

CLINICAL TRIAL REGISTRATION

NCT00935896.

Protective mechanical ventilation does not exacerbate lung function impairment or lung inflammation following influenza A infection

The degree to which mechanical ventilation induces ventilator-associated lung injury is dependent on the initial acute lung injury (ALI). Viral-induced ALI is poorly studied, and this study aimed to determine whether ALI induced by a clinically relevant infection is exacerbated by protective mechanical ventilation. Adult female BALB/c mice were inoculated with 104.5 plaque-forming units of influenza A/Mem/1/71 in 50 μl of medium or medium alone. This study used a protective ventilation strategy, whereby mice were anesthetized, tracheostomized, and mechanically ventilated for 2 h. Lung mechanics were measured periodically throughout the ventilation period using a modification of the forced oscillation technique to obtain measures of airway resistance and coefficients of tissue damping and tissue elastance. Thoracic gas volume was measured and used to obtain specific airway resistance, tissue damping, and tissue elastance. At the end of the ventilation period, a bronchoalveolar lavage sample was collected to measure inflammatory cells, macrophage inflammatory protein-2, IL-6, TNF-α, and protein leak. Influenza infection caused significant increases in inflammatory cells, protein leak, and deterioration in lung mechanics that were not exacerbated by mechanical ventilation, in contrast to previous studies using bacterial and mouse-specific viral infection. This study highlighted the importance of type and severity of lung injury in determining outcome following mechanical ventilation.

Isoflurane attenuates pulmonary interleukin-1beta and systemic tumor necrosis factor-alpha following mechanical ventilation in healthy mice

BACKGROUND

Mechanical ventilation (MV) induces an inflammatory response in healthy lungs. The resulting pro-inflammatory state is a risk factor for ventilator-induced lung injury and peripheral organ dysfunction. Isoflurane is known to have protective immunological effects on different organ systems. We tested the hypothesis that the MV-induced inflammatory response in healthy lungs is reduced by isoflurane.

METHODS

Healthy C57BL6 mice (n=34) were mechanically ventilated (tidal volume, 8 ml/kg; positive end-expiratory pressure, 4 cmH(2)O; and fraction of inspired oxygen, 0.4) for 4 h under general anesthesia using a mix of ketamine, medetomidine and atropine (KMA). Animals were divided into four groups: (1) Unventilated control group; (2) MV group using KMA anesthesia; (3) MV group using KMA with 0.25 MAC isoflurane; (4) MV group using KMA with 0.75 MAC isoflurane. Cytokine levels were measured in lung homogenate and plasma. Leukocytes were counted in lung tissue.

RESULTS

Lung homogenates: MV increased pro-inflammatory cytokines. In mice receiving KMA+ isoflurane 0.75 MAC, no significant increase in interleukin (IL)-1beta was found compared with non-ventilated control mice. PLASMA: MV induced a systemic pro-inflammatory response. In mice anesthetized with KMA+ isoflurane (both 0.25 and 0.75 MAC), no significant increase in tumor necrosis factor (TNF)-alpha was found compared with non-ventilated control mice.

CONCLUSIONS

The present study is the first to show that isoflurane attenuates the pulmonary IL-1beta and systemic TNF-alpha response following MV in healthy mice.

History of mechanical ventilation may affect respiratory mechanics evolution in acute respiratory distress syndrome

PURPOSE

The aim of this study was to investigate the effect of mechanical ventilation (MV) before acute respiratory distress syndrome (ARDS) on subsequent evolution of respiratory mechanics and blood gases in protectively ventilated patients with ARDS.

METHODS

Nineteen patients with ARDS were stratified into 2 groups according to ARDS onset relative to the onset of MV: In group A (n = 11), MV was applied at the onset of ARDS; in group B (n = 8), MV had been initiated before ARDS. Respiratory mechanics and arterial blood gas were assessed in early (<or=3 days) and late (8-11 days) ARDS, on zero positive end-expiratory pressure and positive end-expiratory pressure of 10 cm H(2)O.

RESULTS

In group A, Pao(2)/fractional inspired oxygen concentration increased (121 +/- 43 vs 161 +/- 60 mm Hg) and minimal resistance of respiratory system decreased (8.3 +/- 1.8 vs 6.0 +/- 2.1 cm H(2)O L(-1) s(-1)) from early to late ARDS. In group B, static elastance of respiratory system increased in the late stage (30.4 +/- 7.8 vs 36.4 +/- 9.9 cm H(2)O/L). In both groups, positive end-expiratory pressure application resulted in Pao(2)/fractional inspired oxygen concentration improvement and minimal resistance of respiratory system decreases in both stages.

CONCLUSION

In protectively ventilated patients with ARDS, late alteration of respiratory mechanics occurs more commonly in patients who have been ventilated before ARDS onset, suggesting that the history of MV affects the subsequent progress of ARDS even when using protective ventilation.

Extracellular matrix and mechanical ventilation in healthy lungs: back to baro/volotrauma?

PURPOSE OF REVIEW

The extracellular matrix plays an important role in the biomechanical behaviour of the lung parenchyma. The matrix is composed of a three-dimensional fibre mesh filled with different macromolecules, including proteoglycans which have important functions in many lung pathophysiological processes, as they regulate tissue hydration, macromolecular structure and function, response to inflammatory agents, and tissue repair and remodelling. The aim of this review is to describe the role of mechanical ventilation on pulmonary extracellular matrix structure and function.

RECENT FINDINGS

Recent experimental and clinical data suggest that in healthy lungs, mechanical ventilation with tidal volume ranging between 7 and 12 ml/kg in the absence of positive end-expiratory pressure may lead to endothelial, extracellular matrix and peripheral airways damage without major inflammatory response. Several mechanisms may explain damage to the lung structure induced by mechanical ventilation: regional overdistension, 'low lung volume' associated with tidal airway closure, and inactivation of surfactant.

SUMMARY

Tidal volume reduction to 6 ml/kg may be useful during mechanical ventilation of healthy lungs. The study of the extracellular matrix may be useful to better understand the pathophysiology of ventilator-induced lung injury in healthy and diseased lungs.

Mitogen-activated protein kinases regulate susceptibility to ventilator-induced lung injury

Background

Mechanical ventilation causes ventilator-induced lung injury in animals and humans. Mitogen-activated protein kinases have been implicated in ventilator-induced lung injury though their functional significance remains incomplete. We characterize the role of p38 mitogen-activated protein kinase/mitogen activated protein kinase kinase-3 and c-Jun-NH₂-terminal kinase-1 in ventilator-induced lung injury and investigate novel independent mechanisms contributing to lung injury during mechanical ventilation.

Methodology and Principle Findings

C57/BL6 wild-type mice and mice genetically deleted for mitogen-activated protein kinase kinase-3 (mkk-3^−/−) or c-Jun-NH₂-terminal kinase-1 (jnk1^−/−) were ventilated, and lung injury parameters were assessed. We demonstrate that mkk3^−/− or jnk1^−/− mice displayed significantly reduced inflammatory lung injury and apoptosis relative to wild-type mice. Since jnk1^−/− mice were highly resistant to ventilator-induced lung injury, we performed comprehensive gene expression profiling of ventilated wild-type or jnk1^−/− mice to identify novel candidate genes which may play critical roles in the pathogenesis of ventilator-induced lung injury. Microarray analysis revealed many novel genes differentially expressed by ventilation including matrix metalloproteinase-8 (MMP8) and GADD45α. Functional characterization of MMP8 revealed that mmp8^−/− mice were sensitized to ventilator-induced lung injury with increased lung vascular permeability.

Conclusions

We demonstrate that mitogen-activated protein kinase pathways mediate inflammatory lung injury during ventilator-induced lung injury. C-Jun-NH₂-terminal kinase was also involved in alveolo-capillary leakage and edema formation, whereas MMP8 inhibited alveolo-capillary protein leakage.

Airway closure: the silent killer of peripheral airways

Tidal airway closure occurs when the closing volume exceeds the end-expiratory lung volume, and it is commonly observed in general anaesthesia, particularly in obese patients. Animal studies suggest that tidal airway closure causes injury to peripheral airways, characterized histologically by rupture of alveolar-airway attachments, denuded epithelium, disruption of airway smooth muscle and increased numbers of polymorphonuclear leucocytes in the alveolar walls. Functionally, this injury is characterized by increased airway resistance. Peripheral airway injury may be a common yet unrecognized complication and may be avoided by application of low levels of positive end-expiratory pressure. Measurement of exhaled nitric oxide is a simple method that may permit early detection of unsuspected peripheral airway injury during mechanical ventilation, both in healthy and diseased lungs.

Delayed extubation to nasal continuous positive airway pressure in the immature baboon model of bronchopulmonary dysplasia: lung clinical and pathological findings

OBJECTIVE

Using the 125-day baboon model of bronchopulmonary dysplasia treated with prenatal steroid and exogenous surfactant, we hypothesized that a delay of extubation from low tidal volume positive pressure ventilation to nasal continuous positive airway pressure at 5 days (delayed nasal continuous positive airway pressure group) would not induce more lung injury when compared with baboons aggressively weaned to nasal continuous positive airway pressure at 24 hours (early nasal continuous positive airway pressure group), because both received positive pressure ventilation.

METHODS AND RESULTS

After delivery by cesarean section at 125 days (term: 185 days), infants received 2 doses of Curosurf (Chiesi Farmaceutica S.p.A., Parma, Italy) and daily caffeine citrate. The delay in extubation to 5 days resulted in baboons in the delayed nasal continuous positive airway pressure group having a lower arterial to alveolar oxygen ratio, high PaCO2, and worse respiratory function. The animals in the delayed nasal continuous positive airway pressure group exhibited a poor respiratory drive that contributed to more reintubations and time on mechanical ventilation. A few animals in both groups developed necrotizing enterocolitis and/or sepsis, but infectious pneumonias were not documented. Cellular bronchiolitis and peribronchiolar alveolar wall thickening were more frequently seen in the delayed nasal continuous positive airway pressure group. Bronchoalveolar lavage levels of interleukin-6, interleukin-8, monocyte chemotactic protein-1, macrophage inflammatory protein-1 alpha, and growth-regulated oncogene-alpha were significantly increased in the delayed nasal continuous positive airway pressure group. Standard and digital morphometric analyses showed no significant differences in internal surface area and nodal measurements between the groups. Platelet endothelial cell adhesion molecule vascular staining was not significantly different between the 2 nasal continuous positive airway pressure groups.

CONCLUSIONS

Volutrauma and/or low-grade colonization of airways secondary to increased reintubations and ventilation times are speculated to play causative roles in the delayed nasal continuous positive airway pressure group findings.

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

OBJECTIVES

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

HYPOTHESIS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The contribution of biophysical lung injury to the development of biotrauma

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

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

RATIONALE

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

OBJECTIVES

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

METHODS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSIONS

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

The impact of mechanical ventilation on the moxifloxacin treatment of experimental pneumonia caused by Streptococcus pneumoniae

OBJECTIVE

Streptococcus pneumoniae is a leading cause of community-acquired pneumonia and is responsible for early-onset ventilator-associated pneumonia as well. In intensive care units, community-acquired pneumonia is still associated with a mortality rate of up to 30%, especially when mechanical ventilation is required. Our objective was to study to what extent MV could influence the efficacy of moxifloxacin in a rabbit model of pneumonia.

DESIGN

Prospective experimental study.

SETTING

University hospital laboratory.

SUBJECTS

Male New Zealand White rabbits (n = 75).

INTERVENTIONS

S. pneumoniae (16089 strain; minimal inhibitory concentration for moxifloxacin = 0.125 mg/L) was instilled intrabronchially. Four hours later, a human-like moxifloxacin treatment was initiated in spontaneously breathing (SB) and mechanically ventilated (MV) animals. Untreated rabbits were used as controls. Survivors were killed 48 hrs later. Pneumonia was assessed and moxifloxacin pharmacokinetics were analyzed.

MEASUREMENTS AND MAIN RESULTS

Moxifloxacin treatment was associated with an improvement in survival in the SB animals (13 of 13 [100%] vs. eight of 37 [21.6%] controls). The survival rate was less influenced by treatment in MV rabbits (seven of 15 [46.1%] vs. one of eight [12.5%] controls). The lung bacterial burden was greater in MV compared with SB rabbits (5.1 +/- 2.4 vs. 1.6 +/- 1.4 log10 colony-forming units/g, respectively). Nearly all the untreated animals presented bacteremia as reflected by a positive spleen culture. No bacteremia was found in SB animals treated with moxifloxacin. In contrast, three of 13 (23.1%) moxifloxacin-treated and MV animals had positive spleen cultures. The apparent volume of distribution of moxifloxacin was lower in MV compared with SB rabbits.

CONCLUSIONS

In our model of moxifloxacin-treated S. pneumoniae pneumonia, mechanical ventilation was associated with a higher mortality rate and seemed to promote bacterial growth as well as systemic spread of the infection. In addition, the volume of distribution of moxifloxacin was reduced in the presence of mechanical ventilation. Although the roles of factors such as anesthesia, paralysis, and endotracheal tube insertion could not be established, these results suggest that mechanical ventilation may impair host lung defense, rendering antibiotic therapy less effective.

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.

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.

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.

The vaccinia virus K1L gene product inhibits host NF-kappaB activation by preventing IkappaBalpha degradation

Vaccinia virus wild-type strains such as Ankara and WR synthesize proteins capable of inhibiting the activation of host NF-kappaB, a family of transcription factors that regulate the expression of inflammatory genes. In contrast, an infection by the attenuated MVA strain, whose genome lacks many immunoregulatory genes present in the DNA of its Ankara parent, induces NF-kappaB activation. Insertion of NF-kappaB inhibitory genes into the MVA DNA, then, would alter the MVA phenotype. By this method, a 5.2-kb region of Ankara DNA containing the K1L gene and two other genes that are absent in the MVA genome that was identified as NF-kappaB was inhibited in cells infected with the MVA/5.2kb virus. To determine if K1L was responsible, the relevant biological properties of both a recombinant MVA containing a copy of the WR strain's K1L (MVA/K1L) and a WR deletion mutant lacking the K1L gene (DeltaK1L) were examined. Indeed, unlike its progenitor, the altered MVA halted degradation of the host regulatory protein IkappaBalpha-a key event in the pathway of transcriptional activation by NF-kappaB factors. Moreover, MVA/K1L gained the ability to repress artificially contrived and natural NF-kappaB-regulated expression of a transfected luciferase and the cellular tumor necrosis factor gene, respectively. In contrast, although these functions could also be performed by WR, the DeltaK1L virus lost these abilities. Thus, one apparent molecular function of K1L is to prevent IkappaBalpha degradation. This impediment to NF-kappaB-induced host proinflammatory gene expression, in turn, might enhance virus survival.

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

OBJECTIVE

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

DESIGN

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

SETTING

Research animal laboratory.

SUBJECTS

Twenty-six male Japanese white rabbits.

INTERVENTIONS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSIONS

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

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

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

Molecular mechanisms of lung cell activation induced by cyclic stretch

OBJECTIVE

To review molecular mechanisms of lung cell activation by stretch.

DATA SOURCES

Published original and review articles.

DATA SUMMARY

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

CONCLUSION

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