Recruitment maneuver in experimental acute lung injury: the role of alveolar collapse and edema

The mitochondrial-derived peptide MOTS-c suppresses ferroptosis and alleviates acute lung injury induced by myocardial ischemia reperfusion via PPARγ signaling pathway

Acute lung injury (ALI) is a life-threatening complication of cardiac surgery that has a high rate of morbidity and mortality. Epithelial ferroptosis is believed to be involved in the pathogenesis of ALI. MOTS-c has been reported to play a role in regulating inflammation and sepsis-associated ALI. The purpose of this study is to observe the effect of MOTS-c on myocardial ischemia reperfusion (MIR)-induced ALI and ferroptosis. In humans, we used ELISA kits to investigate MOTS-c and malondialdehyde (MDA) levels in patients undergoing off-pump coronary artery bypass grafting (CABG). In vivo, we pretreated Sprague-Dawley rats with MOTS-c, Ferrostatin-1 and Fe-citrate(Ⅲ). We conducted Hematoxylin and Eosin (H&E) staining and detection of ferroptosis-related genes in MIR-induced ALI rats. In vitro, we evaluated the effect of MOTS-c on hypoxia regeneration (HR)-induced mouse lung epithelial-12 (MLE-12) ferroptosis and analyzed the expression of PPARγ through western blotting. We found that circulating MOTS-c levels were decreased in postoperative ALI patients after off-pump CABG, and that ferroptosis contributed to ALI induced by MIR in rats. MOTS-c suppressed ferroptosis and alleviated ALI induced by MIR, and the protective effect of MOTS-c- was dependent on PPARγ signaling pathway. Additionally, HR promoted ferroptosis in MLE-12 cells, and MOTS-c inhibited ferroptosis against HR through the PPARγ signaling pathway. These findings highlight the therapeutic potential of MOTS-c for improving postoperative ALI induced by cardiac surgery.

Effects of different fluid management on lung and kidney during pressure-controlled and pressure-support ventilation in experimental acute lung injury

Optimal fluid management is critical during mechanical ventilation to mitigate lung damage. Under normovolemia and protective ventilation, pulmonary tensile stress during pressure-support ventilation (PSV) results in comparable lung protection to compressive stress during pressure-controlled ventilation (PCV) in experimental acute lung injury (ALI). It is not yet known whether tensile stress can lead to comparable protection to compressive stress in ALI under a liberal fluid strategy (LF). A conservative fluid strategy (CF) was compared with LF during PSV and PCV on lungs and kidneys in an established model of ALI. Twenty-eight male Wistar rats received endotoxin intratracheally. After 24 h, they were treated with CF (minimum volume of Ringer's lactate to maintain normovolemia and mean arterial pressure ≥70 mmHg) or LF (~4 times higher than CF) combined with PSV or PCV (VT  = 6 ml/kg, PEEP = 3 cmH2 O) for 1 h. Nonventilated animals (n = 4) were used for molecular biology analyses. CF-PSV compared with LF-PSV: (1) decreased the diffuse alveolar damage score (10 [7.8-12] vs. 25 [23-31.5], p = 0.006), mainly due to edema in axial and alveolar parenchyma; (2) increased birefringence for occludin and claudin-4 in lung tissue and expression of zonula-occludens-1 and metalloproteinase-9 in lung. LF compared with CF reduced neutrophil gelatinase-associated lipocalin and interleukin-6 expression in the kidneys in PSV and PCV. In conclusion, CF compared with LF combined with PSV yielded less lung epithelial cell damage in the current model of ALI. However, LF compared with CF resulted in less kidney injury markers, regardless of the ventilatory strategy.

Ventilated postmortem computed tomography to evaluate the lungs of dogs with and without focal lung lesions

OBJECTIVE

To identify the optimal ventilation pressure for ventilated postmortem CT assessment of the lungs in cadaveric dogs and compare the optimal ventilation pressures between dogs with and without focal lung lesions.

SAMPLE

12 cadaveric dogs.

PROCEDURES

CT was performed with dogs positioned in sternal recumbency within 30 to 180 minutes after death. After orotracheal intubation, lungs were aerated to ventilation pressures of 0, 10, 15, 20, 25, 30, and 35 cm H₂O. Lung attenuation measurements were made at 5 predetermined anatomical locations with use of a multi-image analysis graphic user interface tool. Lungs were considered hyperaerated (-1000 to -901 HU), normo-aerated (-900 to -501 HU), poorly aerated (-500 to -101 HU), and nonaerated (-100 to 100 HU) on the basis of lung attenuation values. Optimal ventilation pressure was defined as the pressure at which the percentage of normo-aerated lung was greatest. For analysis, dogs were assigned to one group when focal lung lesions were evident and to another group when lesions were not evident.

RESULTS

Median optimal ventilation pressure was significantly higher for those dogs with lung lesions (35 cm H₂O), compared with those without (25 cm H₂O).

CONCLUSIONS AND CLINICAL RELEVANCE

A ventilation pressure of 35 cm H₂O may be considered for ventilated postmortem CT to determine the presence of focal lung lesions; however, further investigation is required.

Effects of Human Interleukin-10 on Ventilator-Associated Lung Injury in Rats

Ventilator-induced lung injury (VILI) is one of the most serious complications of mechanical ventilation (MV) and can increase the mortality of patients with acute respiratory distress syndrome (ARDS). This work aimed to test the hypothesis that the anti-inflammatory properties of human interleukin-10 (hIL-10) can reduce VILI. Thirty-six healthy male Sprague-Dawley rats were randomly assigned into three groups (n = 12) as follows: a control group, a VILI group, and a hIL-10 group. Lung function was evaluated by oxygenation index and pulmonary edema, and morphological changes associated with lung injury were assessed by HE staining and quantitative histological lung injury score. Malondialdehyde (MDA) and Superoxide dismutase (SOD) were measured, and the levels of various inflammatory cytokines were assessed in BALF and plasma. The oxygenation index in the VILI group decreased significantly relative to the control group and improved substantially in the hIL-10 group (P < 0.01). Compared to the control group, MDA production was stimulated (P < 0.01), and SOD activity rapidly declined (P < 0.01) in the VILI group. After hIL-10, MDA content was lower than that seen in the VILI group (P < 0.01), and SOD activity was enhanced (P < 0.01). The VILI group had the highest cytokine levels, compared to either the hIL-10 group or the control group (P < 0.05). High tidal volume MV can induce VILI. hIL-10 may regulate the inflammatory response in the lung tissue, improve lung tissue oxygenation, and inhibit oxidative stress, therefore reducing VILI in rats. These experiments reveal a potential new treatment option for VILI.

Effects of Obesity on Pulmonary Inflammation and Remodeling in Experimental Moderate Acute Lung Injury

Obese patients are at higher risk of developing acute respiratory distress syndrome (ARDS); however, their survival rates are also higher compared to those of similarly ill non-obese patients. We hypothesized that obesity would not only prevent lung inflammation, but also reduce remodeling in moderate endotoxin-induced acute lung injury (ALI). Obesity was induced by early postnatal overfeeding in Wistar rats in which the litter size was reduced to 3 pups/litter (Obese, n = 18); Control animals (n = 18) were obtained from unculled litters. On postnatal day 150, Control, and Obese animals randomly received E. coli lipopolysaccharide (ALI) or saline (SAL) intratracheally. After 24 h, echocardiography, lung function and morphometry, and biological markers in lung tissue were evaluated. Additionally, mediator expression in neutrophils and macrophages obtained from blood and bronchoalveolar lavage fluid (BALF) was analyzed. Compared to Control-SAL animals, Control-ALI rats showed no changes in echocardiographic parameters, increased lung elastance and resistance, higher monocyte phagocytic capacity, collagen fiber content, myeloperoxidase (MPO) activity, and levels of interleukin (IL-6), tumor necrosis factor (TNF)-α, transforming growth factor (TGF)-β, and type III (PCIII), and I (PCI) procollagen in lung tissue, as well as increased expressions of TNF-α and monocyte chemoattractant protein (MCP)-1 in blood and BALF neutrophils. Monocyte (blood) and macrophage (adipose tissue) phagocytic capacities were lower in Obese-ALI compared to Control-ALI animals, and Obese animals exhibited reduced neutrophil migration compared to Control. Obese-ALI animals, compared to Obese-SAL, exhibited increased interventricular septum thickness (p = 0.003) and posterior wall thickness (p = 0.003) and decreased pulmonary acceleration time to pulmonary ejection time ratio (p = 0.005); no changes in lung mechanics, IL-6, TNF-α, TGF-β, PCIII, and PCI in lung tissue; increased IL-10 levels in lung homogenate (p = 0.007); reduced MCP-1 expression in blood neutrophils (p = 0.009); decreased TNF-α expression in blood (p = 0.02) and BALF (p = 0.008) neutrophils; and increased IL-10 expression in monocytes (p = 0.004). In conclusion, after endotoxin challenge, obese rats showed less deterioration of lung function, secondary to anti-inflammatory and anti-fibrotic effects, as well as changes in neutrophil and monocyte/macrophage phenotype in blood and BALF compared to Control rats.

Ventilation in patients with intra-abdominal hypertension: what every critical care physician needs to know

The incidence of intra-abdominal hypertension (IAH) is high and still underappreciated by critical care physicians throughout the world. One in four to one in three patients will have IAH on admission, while one out of two will develop IAH within the first week of Intensive Care Unit stay. IAH is associated with high morbidity and mortality. Although considerable progress has been made over the past decades, some important questions remain regarding the optimal ventilation management in patients with IAH. An important first step is to measure intra-abdominal pressure (IAP). If IAH (IAP > 12 mmHg) is present, medical therapies should be initiated to reduce IAP as small reductions in intra-abdominal volume can significantly reduce IAP and airway pressures. Protective lung ventilation with low tidal volumes in patients with respiratory failure and IAH is important. Abdominal-thoracic pressure transmission is around 50%. In patients with IAH, higher positive end-expiratory pressure (PEEP) levels are often required to avoid alveolar collapse but the optimal PEEP in these patients is still unknown. During recruitment manoeuvres, higher opening pressures may be required while closely monitoring oxygenation and the haemodynamic response. During lung-protective ventilation, whilst keeping driving pressures within safe limits, higher plateau pressures than normally considered might be acceptable. Monitoring of the respiratory function and adapting the ventilatory settings during anaesthesia and critical care are of great importance. This review will focus on how to deal with the respiratory derangements in critically ill patients with IAH.

Driving-pressure-independent protective effects of open lung approach against experimental acute respiratory distress syndrome

Background

The open lung approach (OLA) reportedly has lung-protective effects against acute respiratory distress syndrome (ARDS). Recently, lowering of the driving pressure (ΔP), rather than improvement in lung aeration per se, has come to be considered as the primary lung-protective mechanism of OLA. However, the driving pressure-independent protective effects of OLA have never been evaluated in experimental studies. We here evaluated whether OLA shows protective effects against experimental ARDS even when the ΔP is not lowered.

Methods

Lipopolysaccharide was intratracheally administered to rats to establish experimental ARDS. After 24 h, rats were mechanically ventilated and randomly allocated to the OLA or control group. In the OLA group, 5 cmH₂O positive end-expiratory pressure (PEEP) and recruitment maneuver (RM) were applied. Neither PEEP nor RM was applied to the rats in the control group. Dynamic ΔP was kept at 15 cmH₂O in both groups. After 6 h of mechanical ventilation, rats in both groups received RM to inflate reversible atelectasis of the lungs. Arterial blood gas analysis, lung computed tomography, histological evaluation, and comprehensive biochemical analysis were performed.

Results

OLA significantly improved lung aeration, arterial oxygenation, and gas exchange. Even after RM in both groups, the differences in these parameters between the two groups persisted, indicating that the atelectasis-induced respiratory dysfunction observed in the control group is not an easily reversible functional problem. Lung histological damage was severe in the dorsal dependent area in both groups, but was attenuated by OLA. White blood cell counts, protein concentrations, and tissue injury markers in the broncho-alveolar lavage fluid (BALF) were higher in the control than in the OLA group. Furthermore, levels of CXCL-7, a platelet-derived chemokine, were higher in the BALF from the control group, indicating that OLA protects the lungs by suppressing platelet activation.

Conclusions

OLA shows protective effects against experimental ARDS, even when the ΔP is not decreased. In addition to reducing ΔP, maintaining lung aeration seems to be important for lung protection in ARDS.

Electronic supplementary material

The online version of this article (10.1186/s13054-018-2154-2) contains supplementary material, which is available to authorized users.

Lung Recruitment Maneuvers for Adult Patients with Acute Respiratory Distress Syndrome. A Systematic Review and Meta-Analysis

RATIONALE

In patients with acute respiratory distress syndrome (ARDS), lung recruitment maneuvers (LRMs) may prevent ventilator-induced lung injury and improve survival.

OBJECTIVES

To summarize the current evidence in support of the use of LRMs in adult patients with ARDS and to inform the recently published American Thoracic Society/European Society of Intensive Care Medicine/Society of Critical Care Medicine clinical practice guideline on mechanical ventilation in ARDS.

METHODS

We conducted a systematic review and meta-analysis of randomized trials comparing mechanical ventilation strategies with and without LRMs. Eligible trials were identified from among previously published systematic reviews and an updated literature search. Data on 28-day mortality, oxygenation, adverse events, and use of rescue therapy were collected, and results were pooled using random effects models weighted by inverse variance. Strength of evidence was assessed using the Grading of Recommendations Assessment, Development, and Evaluation methodology.

RESULTS

We screened 430 citations and previous systematic reviews and found six trials eligible for inclusion (n = 1,423 patients in total). The type of LRM varied widely between trials, and five of the trials involved a cointervention with a higher positive end-expiratory pressure (PEEP) ventilation strategy. Risk of bias was deemed high in one trial. In the primary analysis, the only trial without a cointervention showed that LRMs were associated with reduced mortality (one trial; risk ratio [RR], 0.62; 95% confidence interval [CI], 0.39-0.98; evidence grade = low). Meta-analysis of all six trials also suggested a significant mortality reduction (six trials; RR, 0.81; 95% CI, 0.69-0.95; evidence grade = moderate), and the use of a higher PEEP cointervention did not significantly modify the mortality effect (P = 0.27 for heterogeneity). LRMs were also associated with improved oxygenation after 24 hours (six trials; mean increase, 52 mm Hg; 95% CI, 23-81 mm Hg) and less frequent requirement for rescue therapy (three trials; RR, 0.65; 95% CI, 0.45-0.94). LRMs were not associated with an increased rate of barotrauma (four trials; RR, 0.84; 95% CI, 0.46-1.55). The rate of hemodynamic compromise was not significantly increased with LRMs (three trials; RR, 1.30; 95% CI, 0.92-1.78).

CONCLUSIONS

Randomized trials suggest that LRMs in combination with a higher PEEP ventilation strategy reduce mortality, but confidence in this finding is limited. Further trials are required to confirm benefit from LRMs in adults with ARDS.

Accelerated deflation promotes homogeneous airspace liquid distribution in the edematous lung

Edematous lungs contain regions with heterogeneous alveolar flooding. Liquid is trapped in flooded alveoli by a pressure barrier-higher liquid pressure at the border than in the center of flooded alveoli-that is proportional to surface tension, T Stress is concentrated between aerated and flooded alveoli, to a degree proportional to T Mechanical ventilation, by cyclically increasing T, injuriously exacerbates stress concentrations. Overcoming the pressure barrier to redistribute liquid more homogeneously between alveoli should reduce stress concentration prevalence and ventilation injury. In isolated rat lungs, we test whether accelerated deflation can overcome the pressure barrier and catapult liquid out of flooded alveoli. We generate a local edema model with normal T by microinfusing liquid into surface alveoli. We generate a global edema model with high T by establishing hydrostatic edema, which does not alter T, and then gently ventilating the edematous lungs, which increases T at 15 cmH₂O transpulmonary pressure by 52%. Thus ventilation of globally edematous lungs increases T, which should increase stress concentrations and, with positive feedback, cause escalating ventilation injury. In the local model, when the pressure barrier is moderate, accelerated deflation causes liquid to escape from flooded alveoli and redistribute more equitably. Flooding heterogeneity tends to decrease. In the global model, accelerated deflation causes liquid escape, but-because of elevated T-the liquid jumps to nearby, aerated alveoli. Flooding heterogeneity is unaltered. In pulmonary edema with normal T, early ventilation with accelerated deflation might reduce the positive feedback mechanism through which ventilation injury increases over time.NEW & NOTEWORTHY We introduce, in the isolated rat lung, a new model of pulmonary edema with elevated surface tension. We first generate hydrostatic edema and then ventilate gently to increase surface tension. We investigate the mechanical mechanisms through which 1) ventilation injures edematous lungs and 2) ventilation with accelerated deflation might lessen ventilation injury.

Variable ventilation improves pulmonary function and reduces lung damage without increasing bacterial translocation in a rat model of experimental pneumonia

Background

Variable ventilation has been shown to improve pulmonary function and reduce lung damage in different models of acute respiratory distress syndrome. Nevertheless, variable ventilation has not been tested during pneumonia. Theoretically, periodic increases in tidal volume (V_T) and airway pressures might worsen the impairment of alveolar barrier function usually seen in pneumonia and could increase bacterial translocation into the bloodstream. We investigated the impact of variable ventilation on lung function and histologic damage, as well as markers of lung inflammation, epithelial and endothelial cell damage, and alveolar stress, and bacterial translocation in experimental pneumonia.

Methods

Thirty-two Wistar rats were randomly assigned to receive intratracheal of Pseudomonas aeruginosa (PA) or saline (SAL) (n = 16/group). After 24-h, animals were anesthetized and ventilated for 2 h with either conventional volume-controlled (VCV) or variable volume-controlled ventilation (VV), with mean V_T = 6 mL/kg, PEEP = 5cmH₂O, and FiO₂ = 0.4. During VV, tidal volume varied randomly with a coefficient of variation of 30% and a Gaussian distribution. Additional animals assigned to receive either PA or SAL (n = 8/group) were not ventilated (NV) to serve as controls.

Results

In both SAL and PA, VV improved oxygenation and lung elastance compared to VCV. In SAL, VV decreased interleukin (IL)-6 expression compared to VCV (median [interquartile range]: 1.3 [0.3–2.3] vs. 5.3 [3.6–7.0]; p = 0.02) and increased surfactant protein-D expression compared to NV (2.5 [1.9–3.5] vs. 1.2 [0.8–1.2]; p = 0.0005). In PA, compared to VCV, VV reduced perivascular edema (2.5 [2.0–3.75] vs. 6.0 [4.5–6.0]; p < 0.0001), septum neutrophils (2.0 [1.0–4.0] vs. 5.0 [3.3–6.0]; p = 0.0008), necrotizing vasculitis (3.0 [2.0–5.5] vs. 6.0 [6.0–6.0]; p = 0.0003), and ultrastructural lung damage scores (16 [14–17] vs. 24 [14–27], p < 0.0001). Blood colony-forming-unit (CFU) counts were comparable (7 [0–28] vs. 6 [0–26], p = 0.77). Compared to NV, VCV, but not VV, increased expression amphiregulin, IL-6, and cytokine-induced neutrophil chemoattractant (CINC)-1 (2.1 [1.6–2.5] vs. 0.9 [0.7–1.2], p = 0.025; 12.3 [7.9–22.0] vs. 0.8 [0.6–1.9], p = 0.006; and 4.4 [2.9–5.6] vs. 0.9 [0.8–1.4], p = 0.003, respectively). Angiopoietin-2 expression was lower in VV compared to NV animals (0.5 [0.3–0.8] vs. 1.3 [1.0–1.5], p = 0.01).

Conclusion

In this rat model of pneumonia, VV improved pulmonary function and reduced lung damage as compared to VCV, without increasing bacterial translocation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-016-0476-7) contains supplementary material, which is available to authorized users.

Fast Versus Slow Recruitment Maneuver at Different Degrees of Acute Lung Inflammation Induced by Experimental Sepsis

BACKGROUND

Large tidal volume (VT) breaths or "recruitment maneuvers" (RMs) are used commonly to open collapsed lungs, but their effectiveness may depend on how the RM is delivered. We hypothesized that a stepped approach to RM delivery ("slow" RM) compared with a nonstepped ("fast" RM), when followed by decremental positive end-expiratory pressure (PEEP) titration to lowest dynamic elastance, would (1) yield a more homogeneous inflation of the lungs, thus reducing the PEEP obtained during post-RM titration; (2) produce less lung morphofunctional injury, regardless of the severity of sepsis-induced acute lung inflammation; and (3) result in less biological damage in severe, but not in moderate, acute lung inflammation.

METHODS

Sepsis was induced by cecal ligation and puncture surgery in 51 Wistar rats. After 48 hours, animals were anesthetized, mechanically ventilated (VT = 6 mL/kg), and stratified by PO2/fraction of inspired oxygen ratio into moderate (≥300) and severe (<300) acute lung inflammation groups. Each group was then subdivided randomly into 3 subgroups: (1) nonrecruited; (2) RM with continuous positive airway pressure (30 cm H2O for 30 seconds; CPAPRM or fast RM); and (3) RM with stepwise airway pressure increase (5 cm H2O/step, 8.5 seconds/step, 6 steps, 51 seconds; STEPRM or slow RM), with a maximum pressure hold for 10 seconds. All animals underwent decremental PEEP titration to determine the level of PEEP required to optimize dynamic compliance after RM and were then ventilated for 60 minutes with VT = 6 mL/kg, respiratory rate = 80 bpm, fraction of inspired oxygen = 0.4, and the newly adjusted PEEP for each animal. Respiratory mechanics, hemodynamics, and arterial blood gases were measured before and at the end of 60-minute mechanical ventilation. Lung histology and biological markers of inflammation and damage inflicted to endothelial cells were evaluated at the end of the 60-minute mechanical ventilation.

RESULTS

Respiratory system mean airway pressure was lower in STEPRM than that in CPAPRM. The total RM time was greater, and the RM rise angle was lower in STEPRM than that in CPAPRM. In both moderate and severe acute lung inflammation groups, STEPRM reduced total diffuse alveolar damage score compared with the score in nonrecruited rats. In moderate acute lung inflammation, STEPRM rats compared with CPAPRM rats had less endothelial cell damage and angiopoietin (Ang)-2 expression. In severe acute lung inflammation, STEPRM compared with CPAPRM reduced hyperinflation, endothelial cell damage, Ang-2, and intercellular adhesion molecule-1 expressions. RM rise angle correlated with Ang-2 expression.

CONCLUSIONS

Compared with CPAPRM, STEPRM reduced biological markers associated with endothelial cell damage and ultrastructural endothelial cell injury in both moderate and severe sepsis-induced acute lung inflammation.

Acute Lung Injury Causes Asynchronous Alveolar Ventilation That Can Be Corrected by Individual Sighs

RATIONALE

Improved ventilation strategies have been the mainstay for reducing mortality in acute respiratory distress syndrome. Their unique clinical effectiveness is, however, unmatched by our understanding of the underlying mechanobiology, and their impact on alveolar dynamics and gas exchange remains largely speculative.

OBJECTIVES

To assess changes in alveolar dynamics and associated effects on local gas exchange in experimental models of acute lung injury (ALI) and their responsiveness to sighs.

METHODS

Alveolar dynamics and local gas exchange were studied in vivo by darkfield microscopy and multispectral oximetry in experimental murine models of ALI induced by hydrochloric acid, Tween instillation, or in antibody-mediated transfusion-related ALI.

MEASUREMENTS AND MAIN RESULTS

Independent of injury mode, ALI resulted in asynchronous alveolar ventilation characteristic of alveolar pendelluft, which either spontaneously resolved or progressed to a complete cessation or even inversion of alveolar ventilation. The functional relevance of the latter phenomena was evident as impaired blood oxygenation in juxtaposed lung capillaries. Individual sighs (2 × 10 s at inspiratory plateau pressure of 30 cm H2O) largely restored normal alveolar dynamics and gas exchange in acid-induced ALI, yet not in Tween-induced surfactant depletion.

CONCLUSIONS

We describe for the first time in detail the different forms and temporal sequence of impaired alveolar dynamics in the acutely injured lung and report the first direct visualization of alveolar pendelluft. Moreover, we identify individual sighs as an effective strategy to restore intact alveolar ventilation by a mechanism independent of alveolar collapse and reopening.

Ventilator-Induced Lung Injury (VILI) in Acute Respiratory Distress Syndrome (ARDS): Volutrauma and Molecular Effects

Acute Respiratory Distress Syndrome (ARDS) is a clinical condition secondary to a variety of insults leading to a severe acute respiratory failure and high mortality in critically ill patients. Patients with ARDS generally require mechanical ventilation, which is another important factor that may increase the ALI (acute lung injury) by a series of pathophysiological mechanisms, whose common element is the initial volutrauma in the alveolar units, and forming part of an entity known clinically as ventilator-induced lung injury (VILI).

Injured lungs can be partially protected by optimal settings and ventilation modes, using low tidal volume (VT) values and high positive-end expiratory pressure (PEEP). The benefits in ARDS outcomes caused by these interventions have been confirmed by several prospective randomized controlled trials (RCTs) and are attributed to reduction in volutrauma.

The purpose of this article is to present an approach to VILI pathophysiology focused on the effects of volutrauma that lead to lung injury and the ‘mechanotransduction’ mechanism. A more complete understanding about the molecular effects that physical forces could have, is essential for a better assessment of existing strategies as well as the development of new therapeutic strategies to reduce the damage resulting from VILI, and thereby contribute to reducing mortality in ARDS.

Advances in ventilator-associated lung injury: prevention is the target

Mechanical ventilation (MV) is the main supportive treatment in respiratory failure due to different etiologies. However, MV might aggravate ventilator-associated lung injury (VALI). Four main mechanisms leading to VALI are: 1) increased stress and strain, induced by high tidal volume (VT); 2) increased shear stress, i.e. opening and closing, of previously atelectatic alveolar units; 3) distribution of perfusion and 4) biotrauma. In severe acute respiratory distress syndrome patients, low VT, higher levels of positive end expiratory pressure, long duration prone position and neuromuscular blockade within the first 48 hours are associated to a better outcome. VALI can also occur by using high VT in previously non injured lungs. We believe that prevention is the target to minimize injurious effects of MV. This review aims to describe pathophysiology of VALI, the possible prevention and treatment as well as monitoring MV to minimize VALI.

Epithelial and endothelial damage induced by mechanical ventilation modes

PURPOSE OF REVIEW

The adult respiratory distress syndrome (ARDS) is a common cause of respiratory failure with substantial impact on public health. Patients with ARDS generally require mechanical ventilation, which risks further lung damage. Recent improvements in ARDS outcomes have been attributed to reductions in deforming stress associated with lung protective mechanical ventilation modes and settings. The following review details the mechanics of the lung parenchyma at different spatial scales and the response of its resident cells to deforming stress in order to provide the biologic underpinnings of lung protective care.

RECENT FINDINGS

Although lung injury is typically viewed through the lens of altered barrier properties and mechanical ventilation-associated immune responses, in this review, we call attention to the importance of heterogeneity and the physical failure of the load bearing cell and tissue elements in the pathogenesis of ARDS. Specifically, we introduce a simple elastic network model to better understand the deformations of lung regions, intra-acinar alveoli and cells within a single alveolus, and consider the role of regional distension and interfacial stress-related injury for various ventilation modes.

SUMMARY

Heterogeneity of stiffness and intercellular and intracellular stress failure are fundamental components of ARDS and their development also depends on the ventilation mode.

Effects of sigh during pressure control and pressure support ventilation in pulmonary and extrapulmonary mild acute lung injury

INTRODUCTION

Sigh improves oxygenation and lung mechanics during pressure control ventilation (PCV) and pressure support ventilation (PSV) in patients with acute respiratory distress syndrome. However, so far, no study has evaluated the biological impact of sigh during PCV or PSV on the lung and distal organs in experimental pulmonary (p) and extrapulmonary (exp) mild acute lung injury (ALI).

METHODS

In 48 Wistar rats, ALI was induced by Escherichia coli lipopolysaccharide either intratracheally (ALIp) or intraperitoneally (ALIexp). After 24 hours, animals were anesthetized and mechanically ventilated with PCV or PSV with a tidal volume of 6 mL/kg, FiO2 = 0.4, and PEEP = 5 cmH2O for 1 hour. Both ventilator strategies were then randomly assigned to receive periodic sighs (10 sighs/hour, Sigh) or not (non-Sigh, NS). Ventilatory and mechanical parameters, arterial blood gases, lung histology, interleukin (IL)-1β, IL-6, caspase-3, and type III procollagen (PCIII) mRNA expression in lung tissue, and number of apoptotic cells in lung, liver, and kidney specimens were analyzed.

RESULTS

In both ALI etiologies: (1) PCV-Sigh and PSV-Sigh reduced transpulmonary pressure, and (2) PSV-Sigh reduced the respiratory drive compared to PSV-NS. In ALIp: (1) PCV-Sigh and PSV-Sigh decreased alveolar collapse as well as IL-1β, IL-6, caspase-3, and PCIII expressions in lung tissue, (2) PCV-Sigh increased alveolar-capillary membrane and endothelial cell damage, and (3) abnormal myofibril with Z-disk edema was greater in PCV-NS than PSV-NS. In ALIexp: (1) PSV-Sigh reduced alveolar collapse, but led to damage to alveolar-capillary membrane, as well as type II epithelial and endothelial cells, (2) PCV-Sigh and PSV-Sigh increased IL-1β, IL-6, caspase-3, and PCIII expressions, and (3) PCV-Sigh increased the number of apoptotic cells in the lung compared to PCV-NS.

CONCLUSIONS

In these models of mild ALIp and ALIexp, sigh reduced alveolar collapse and transpulmonary pressures during both PCV and PSV; however, improved lung protection only during PSV in ALIp.

Effects of early and late pneumothorax drainage on the development of pulmonary oedema

We analyzed the effects of pneumothorax duration and early or late drainage on lung histology and biological markers associated with inflammation, alveolar fluid clearance, and pulmonary oedema formation. Pneumothorax was induced by injecting air into the thorax of anaesthetized rats, which were randomized according to duration of pneumothorax [5 (PTX5) or 30 (PTX30)min] and further divided to be drained (D) or not (ND). ND rats were euthanized at 5 and 30min. In D groups, pneumothorax was drained and rats breathed spontaneously for 30min. PTX30-ND, compared to PTX5-ND, showed higher alveolar collapse and oedema, type III procollagen, caspase-3, epithelial sodium channel-α, and aquaporin (AQP)-1 mRNA expression, and epithelial and endothelial damage, with reduced cystic fibrosis transmembrane conductance regulator (CFTR) and AQP-3 expression. PTX5-D, compared to PTX30-D, showed less alveolar hyperinflation, oedema, and alveolar-capillary damage, with reduced interleukin-6, caspase-3, AQP-5, and Na,K-ATPase-α and -β expression, and increased CFTR expression. In conclusion, longer duration pneumothorax exacerbated lung damage, oedema, and inflammation.

Assessment of circulatory and pulmonary endothelin-1 levels in a lavage-induced surfactant-depleted lung injury rabbit model with repeated open endotracheal suctioning and hyperinflation

AIMS

Endothelin-1 (ET-1) is a mediator of various physiological and pathological processes, including vascular inflammation, cell proliferation and vasoconstriction. Attenuation of ET action using ET-1 antagonists reduces pulmonary vascular leakage and inflammation in several models of lung injuries and experimental acute respiratory distress syndrome (ARDS). Based on these earlier reports, the current study investigates the patterns of ET-1 levels in circulation and pulmonary tissues in an experimental model of lavage-induced surfactant-depleted lung injury. Additionally, we also test the effects of open endotracheal suctioning (OES) and hyperinflation (HI) as recruitment maneuver following OES on ET-1 levels.

MAIN METHODS

Briefly, 24 Japanese white rabbits were anesthetized and intubated. Normal saline was instilled into the lung and washed mildly. After instillation, rabbits were ventilated at definite settings at a total duration of 3 hours. OES and HI were performed every 15 minutes from the beginning of the protocol.

KEY FINDINGS

Here, we show that both circulatory and pulmonary ET-1 levels increased in models with lung injury induced by saline lavage compared to healthy control group. No further aggravation in expression of pulmonary ET-1 was seen after OES and HI, although OES and HI worsened arterial hypoxygenation and severity of lung injury. In contrast, circulatory ET-1 levels significantly decreased after OES and HI but were not associated with blood pressure changes.

SIGNIFICANCE

We conclude that in a saline lavage-induced lung injury model, both circulatory and pulmonary ET-1 levels increased. Further, OES and HI exerted differential effects on ET-1 expression at both circulatory and pulmonary levels.

Topical application of phosphatidyl-inositol-3,5-bisphosphate for acute lung injury in neonatal swine

Hypoxemic respiratory failure of the neonatal organism involves increased acid sphingomyelinase (aSMase) activity and production of ceramide, a second messenger of a pro-inflammatory pathway that promotes increased vascular permeability, surfactant alterations and alveolar epithelial apoptosis. We comparatively assessed the benefits of topical aSMase inhibition by either imipramine (Imi) or phosphatidylinositol-3,5-bisphosphate (PIP2) when administered into the airways together with surfactant (S) for fortification. In this translational study, a triple-hit acute lung injury model was used that entails repeated airway lavage, injurious ventilation and tracheal lipopolysaccharide instillation in newborn piglets subject to mechanical ventilation for 72 hrs. After randomization, we administered an air bolus (control), S, S+Imi, or S+PIP2. Only in the latter two groups we observed significantly improved oxygenation and ventilation, dynamic compliance and pulmonary oedema. S+Imi caused systemic aSMase suppression and ceramide reduction, whereas the S+PIP2 effect remained compartmentalized in the airways because of the molecule's bulky structure. The surfactant surface tensions improved by S+Imi and S+PIP2 interventions, but only to a minor extent by S alone. S+PIP2 inhibited the migration of monocyte-derived macrophages and granulocytes into airways by the reduction of CD14/CD18 expression on cell membranes and the expression of epidermal growth factors (amphiregulin and TGF-β1) and interleukin-6 as pro-fibrotic factors. Finally we observed reduced alveolar epithelial apoptosis, which was most apparent in S+PIP2 lungs. Exogenous surfactant “fortified” by PIP2, a naturally occurring surfactant component, improves lung function by topical suppression of aSMase, providing a potential treatment concept for neonates with hypoxemic respiratory failure.

Recruitment maneuvers modulate epithelial and endothelial cell response according to acute lung injury etiology

OBJECTIVE

To investigate the effects of the rate of increase in airway pressure and duration of lung recruitment maneuvers in experimental pulmonary and extrapulmonary acute lung injury.

DESIGN

Prospective, randomized, controlled experimental study.

SETTINGS

University research laboratory.

SUBJECTS

Fifty adult male Wistar rats.

INTERVENTIONS

Acute lung injury was induced by Escherichia coli lipopolysaccharide either intratracheally (pulmonary acute lung injury) or intraperitoneally (extrapulmonary acute lung injury). After 24 hours, animals were assigned to one of three different recruitment maneuvers, targeted to maximal airway pressure of 30 cm H2O: 1) continuous positive airway pressure for 30 seconds (CPAP-30); 2) stepwise airway pressure increase (5 cm H2O/step, 8.5 s at each step) over 51 seconds (STEP-51) to achieve a pressure-time product similar to that of CPAP-30; and 3) stepwise airway pressure increase (5 cm H2O/step, 5 s at each step) over 30 seconds with maximum pressure sustained for a further 30 seconds (STEP-30/30).

MEASUREMENTS AND MAIN RESULTS

All recruitment maneuvers reduced static lung elastance independent of acute lung injury etiology. In pulmonary acute lung injury, CPAP-30 yielded lower surfactant protein-B and higher type III procollagen expressions compared with STEP-30/30. In extrapulmonary acute lung injury, CPAP-30 and STEP-30/30 increased vascular cell adhesion molecule-1 expression, but the type of recruitment maneuver did not influence messenger ribonucleic acid expression of receptor for advanced glycation end products, surfactant protein-B, type III procollagen, and pro-caspase 3.

CONCLUSIONS

CPAP-30 worsened markers of potential epithelial cell damage in pulmonary acute lung injury, whereas both CPAP-30 and STEP-30/30 yielded endothelial injury in extrapulmonary acute lung injury. In both acute lung injury groups, recruitment maneuvers improved respiratory mechanics, but stepwise recruitment maneuver without sustained airway pressure appeared to associate with less biological impact on lungs.

Combined effects of ventilation mode and positive end-expiratory pressure on mechanics, gas exchange and the epithelium in mice with acute lung injury

The accepted protocol to ventilate patients with acute lung injury is to use low tidal volume (V(T)) in combination with recruitment maneuvers or positive end-expiratory pressure (PEEP). However, an important aspect of mechanical ventilation has not been considered: the combined effects of PEEP and ventilation modes on the integrity of the epithelium. Additionally, it is implicitly assumed that the best PEEP-V(T) combination also protects the epithelium. We aimed to investigate the effects of ventilation mode and PEEP on respiratory mechanics, peak airway pressures and gas exchange as well as on lung surfactant and epithelial cell integrity in mice with acute lung injury. HCl-injured mice were ventilated at PEEPs of 3 and 6 cmH(2)O with conventional ventilation (CV), CV with intermittent large breaths (CV(LB)) to promote recruitment, and a new mode, variable ventilation, optimized for mice (VV(N)). Mechanics and gas exchange were measured during ventilation and surfactant protein (SP)-B, proSP-B and E-cadherin levels were determined from lavage and lung homogenate. PEEP had a significant effect on mechanics, gas exchange and the epithelium. The higher PEEP reduced lung collapse and improved mechanics and gas exchange but it also down regulated surfactant release and production and increased epithelial cell injury. While CV(LB) was better than CV, VV(N) outperformed CV(LB) in recruitment, reduced epithelial injury and, via a dynamic mechanotransduction, it also triggered increased release and production of surfactant. For long-term outcome, selection of optimal PEEP and ventilation mode may be based on balancing lung physiology with epithelial injury.

Sepsis-induced acute lung injury (ALI) is milder in diabetic rats and correlates with impaired NFkB activation

Acute lung injury (ALI) develops in response to a direct insult to the lung or secondarily to a systemic inflammatory response, such as sepsis. There is clinical evidence that the incidence and severity of ALI induced by direct insult are lower in diabetics. In the present study we investigated whether the same occurs in ALI secondarily to sepsis and the molecular mechanisms involved. Diabetes was induced in male Wistar rats by alloxan and sepsis by caecal ligation and puncture surgery (CLP). Six hours later, the lungs were examined for oedema and cell infiltration in bronchoalveolar lavage. Alveolar macrophages (AMs) were cultured in vitro for analysis of IκB and p65 subunit of NFκB phosphorylation and MyD88 and SOCS-1 mRNA. Diabetic rats were more susceptible to sepsis than non-diabetics. In non-diabetic rats, the lung presented oedema, leukocyte infiltration and increased COX2 expression. In diabetic rats these inflammatory events were significantly less intense. To understand why diabetic rats despite being more susceptible to sepsis develop milder ALI, we examined the NFκB activation in AMs of animals with sepsis. Whereas in non-diabetic rats the phosphorylation of IκB and p65 subunit occurred after 6 h of sepsis induction, this did not occur in diabetics. Moreover, in AMs from diabetic rats the expression of MyD88 mRNA was lower and that of SOCS-1 mRNA was increased compared with AMs from non-diabetic rats. These results show that ALI secondary to sepsis is milder in diabetic rats and this correlates with impaired activation of NFκB, increased SOCS-1 and decreased MyD88 mRNA.

Time-dependent recruitment effects in ventilated healthy and lung-injured rats: "recruitment-memory"

We investigated the sustained effects of recruitment manoeuvres in terms of "recruitment memory" in healthy and lung injured rats. 46 ventilated rats were allocated to either the control (sham) or the lavage group. Two consecutive low-flow manoeuvres were performed before sham/lavage and hourly during a 2-h-observation period. The slopes of the inspiratory limbs of the two resulting pressure-volume loops were translated into compliance-volume curves. The difference between the two compliance curves was smaller after lavage (root-mean-square deviation: 0.065 ml/cm H2O control group, 0.038 ml/cm H2O lavage group; p<0.05) and stayed small during the whole experiment. In the control group, the deviation was small after sham manoeuvre but increased throughout the experiment. Compliance gain after recruitment was higher in the control group (0.1 ml/cm H2O) compared to the lavage group (0.02 ml/cm H2O, p<0.05). We conclude that lung lavage led to alveolar collapse not susceptible to recruitment manoeuvres. On the contrary in healthy lungs recruitment manoeuvres led to persistent lung recruitment which we interpret as recruitment memory.

Pathophysiology of ventilator-associated lung injury

PURPOSE OF REVIEW

Mechanical ventilation is essential for the support of critically ill patients, but may aggravate lung damage, leading to ventilator-associated lung injury (VALI). VALI results from a succession of events beginning with mechanical alteration of lung parenchyma, because of disproportionate stress and strain. The resulting structural tension initiates a biological inflammatory cascade; however, tension can reach the limits of stress, triggering the destruction of structures. This article reviews and discusses the ongoing research into the mechanisms of VALI and their implications for the management of ventilated patients.

RECENT FINDINGS

Several experimental and clinical studies have been performed to evaluate the contribution of pathogenic mechanical forces to organ and cellular deformation and the implications for guiding ventilator management in patients at risk for VALI. VALI may be attenuated by reducing tidal volume, but the key variable in determining pulmonary overdistension is transpulmonary pressure. Other parameters associated with the induction of VALI include positive end-expiratory pressure, inspiratory airflow and time, and respiratory frequency.

SUMMARY

How ventilation strategy, specific mechanisms of mechanotransduction, and their individual threshold values impact on VALI remains to be elucidated. In addition, clinical studies are required to evaluate the usefulness of individualized ventilator strategies based on lung mechanics.

Stress and strain within the lung

PURPOSE OF REVIEW

To describe the physiological meaning and the clinical application of the lung stress and strain concepts.

RECENT FINDINGS

The end-inspiratory plateau pressure and ratio of tidal volume/ideal body weight are inadequate surrogates for the end-inspiratory stress (equal to the transpulmonary pressure) and the end-inspiratory strain (change in lung volume relative to the resting volume). For a given plateau pressure or tidal volume/ideal body weight, stress and strain may vary largely due to the variability of chest wall elastance and the resting lung volume. The injurious limits of stress and strain in healthy lungs are reached when stress and strain reach the total lung capacity. This occurs when the resting lung volume (the baby lung in case of acute respiratory distress syndrome) is increased by two-fold to three-fold. As these limits are rarely reached in clinical practice and damage has been reported with stress and strain well below this upper limit, this implies the presence in the lung parenchyma of regions which act as stress raisers or pressure multipliers. These are primarily linked to the inhomogeneous distribution of local stress and strain.

SUMMARY

End-inspiratory stress and strain, as well as the lung inhomogeneity and the stress raisers, must be taken in account when setting mechanical ventilation.

Effects of different tidal volumes in pulmonary and extrapulmonary lung injury with or without intraabdominal hypertension

PURPOSE

We hypothesized that: (1) intraabdominal hypertension increases pulmonary inflammatory and fibrogenic responses in acute lung injury (ALI); (2) in the presence of intraabdominal hypertension, higher tidal volume reduces lung damage in extrapulmonary ALI, but not in pulmonary ALI.

METHODS

Wistar rats were randomly allocated to receive Escherichia coli lipopolysaccharide intratracheally (pulmonary ALI) or intraperitoneally (extrapulmonary ALI). After 24 h, animals were randomized into subgroups without or with intraabdominal hypertension (15 mmHg) and ventilated with positive end expiratory pressure = 5 cmH(2)O and tidal volume of 6 or 10 ml/kg during 1 h. Lung and chest wall mechanics, arterial blood gases, lung and distal organ histology, and interleukin (IL)-1β, IL-6, caspase-3 and type III procollagen (PCIII) mRNA expressions in lung tissue were analyzed.

RESULTS

With intraabdominal hypertension, (1) chest-wall static elastance increased, and PCIII, IL-1β, IL-6, and caspase-3 expressions were more pronounced than in animals with normal intraabdominal pressure in both ALI groups; (2) in extrapulmonary ALI, higher tidal volume was associated with decreased atelectasis, and lower IL-6 and caspase-3 expressions; (3) in pulmonary ALI, higher tidal volume led to higher IL-6 expression; and (4) in pulmonary ALI, liver, kidney, and villi cell apoptosis was increased, but not affected by tidal volume.

CONCLUSIONS

Intraabdominal hypertension increased inflammation and fibrogenesis in the lung independent of ALI etiology. In extrapulmonary ALI associated with intraabdominal hypertension, higher tidal volume improved lung morphometry with lower inflammation in lung tissue. Conversely, in pulmonary ALI associated with intraabdominal hypertension, higher tidal volume increased IL-6 expression.

Positive end-expiratory pressure and surfactant decrease lung injury during initiation of ventilation in fetal sheep

The initiation of ventilation in preterm, surfactant-deficient sheep without positive end-expiratory pressure (PEEP) causes airway injury and lung inflammation. We hypothesized that PEEP and surfactant treatment would decrease the lung injury from initiation of ventilation with high tidal volumes. Fetal sheep at 128-day gestational age were randomized to ventilation with: 1) no PEEP, no surfactant; 2) 8-cmH(2)O PEEP, no surfactant; 3) no PEEP + surfactant; 4) 8-cmH(2)O PEEP + surfactant; or 5) control (2-cmH(2)O continuous positive airway pressure) (n = 6-7/group). After maternal anesthesia and hysterotomy, the head and chest were exteriorized, and the fetus was intubated. While maintaining placental circulation, the fetus was ventilated for 15 min with a tidal volume escalating to 15 ml/kg using heated, humidified, 100% nitrogen. The fetus then was returned to the uterus, and tissue was collected after 30 min for evaluation of early markers of lung injury. Lambs receiving both surfactant and PEEP had increased dynamic compliance, increased static lung volumes, and decreased total protein and heat shock proteins 70 and 60 in bronchoalveolar lavage fluid compared with other groups. Ventilation, independent of PEEP or surfactant, increased mRNA expression of acute phase response genes and proinflammatory cytokine mRNA in the lung tissue compared with controls. PEEP decreased mRNA for cytokines (2-fold) compared with groups receiving no PEEP. Surfactant administration further decreased some cytokine mRNAs and changed the distribution of early growth response protein-1 expression. The use of PEEP during initiation of ventilation at birth decreased early mediators of lung injury. Surfactant administration changed the distribution of injury and had a moderate additive protective effect.

Efficacy and safety of recruitment maneuvers in acute respiratory distress syndrome

Recruitment maneuvers (RM) consist of a ventilatory strategy that increases the transpulmonary pressure transiently to reopen the recruitable lung units in acute respiratory distress syndrome (ARDS). The rationales to use RM in ARDS are that there is a massive loss of aerated lung and that once the end-inspiratory pressure surpasses the regional critical opening pressure of the lung units, those units are likely to reopen. There are different methods to perform RM when using the conventional ICU ventilator. The three RM methods that are mostly used and investigated are sighs, sustained inflation, and extended sigh. There is no standardization of any of the above RM. Meta-analysis recommended not to use RM in routine in stable ARDS patients but to run them in case of life-threatening hypoxemia. There are some concerns regarding the safety of RM in terms of hemodynamics preservation and lung injury as well. The rapid rising in pressure can be a factor that explains the potential harmful effects of the RM. In this review, we describe the balance between the beneficial effects and the harmful consequences of RM. Recent animal studies are discussed.