Ventilator-induced lung injury, cytokines, PEEP, and mortality: implications for practice and for clinical trials

Modeling Ventilator-Induced Lung Injury and Neutrophil Infiltration to Infer Injury Interdependence

Acute respiratory distress syndrome (ARDS) and ventilator-induced lung injury (VILI) are heterogeneous conditions. The spatiotemporal evolution of these heterogeneities is complex, and it is difficult to elucidate the mechanisms driving its progression. Through previous quantitative analyses, we explored the distributions of cellular injury and neutrophil infiltration in experimental VILI and discovered that VILI progression is characterized by both the formation of new injury in quasi-random locations and the expansion of existing injury clusters. Distributions of neutrophil infiltration do not correlate with cell injury progression and suggest a systemic response. To further examine the dynamics of VILI, we have developed a novel computational model that simulates damage (cellular injury progression and neutrophil infiltration) using a stochastic approach. Optimization of the model parameters to fit experimental data reveals that the range and strength of interdependence between existing and new damaged regions both increase as mechanical ventilation patterns become more injurious. The interdependence of cellular injury can be attributed to mechanical tethering forces, while the interdependence of neutrophils is likely due to longer-range cell signaling pathways.

Spatiotemporal distribution of cellular injury and leukocytes during the progression of ventilator-induced lung injury

Supportive mechanical ventilation is a necessary lifesaving treatment for acute respiratory distress syndrome (ARDS). This intervention often leads to injury exacerbation by ventilator-induced lung injury (VILI). Patterns of injury in ARDS and VILI are recognized to be heterogeneous; however, quantification of these injury distributions remains incomplete. Developing a more detailed understanding of injury heterogeneity, particularly how it varies in space and time, can help elucidate the mechanisms of VILI pathogenesis. Ultimately, this knowledge can be used to develop protective ventilation strategies that slow disease progression. To expand existing knowledge of VILI heterogeneity, we document the spatial evolution of cellular injury distribution and leukocyte infiltration, on the micro- and macroscales, during protective and injurious mechanical ventilation. We ventilated naïve mice using either high inspiratory pressure and zero positive end-expiratory pressure ventilation or low tidal volume with positive end-expiratory pressure. Distributions of cellular injury, identified with propidium iodide staining, were microscopically analyzed at three levels of injury severity. Cellular injury initiated in diffuse, quasi-random patterns, and progressed through expansion of high-density regions of injured cells termed "injury clusters." The density profile of the expanding injury regions suggests that stress shielding occurs, protecting the already injured regions from further damage. Spatial distribution of leukocytes did not correlate with that of cellular injury or ventilation-induced changes in lung function. These results suggest that protective ventilation protocols should protect the interface between healthy and injured regions to stymie injury propagation.

Resolvin D1 Alleviates Ventilator-Induced Lung Injury in Mice by Activating PPARγ/NF-κB Signaling Pathway

As one of the basic treatment modalities in the intensive care unit (ICU), mechanical ventilation can cause or aggravate acute lung injury or ventilator-induced lung injury (VILI). Resolvin D1 (RvD1) is an endogenous polyunsaturated fatty acid derivative with strong anti-inflammatory action. In this study, we explored if RvD1 possesses a protective effect on VILI. Mice were ventilated with high tidal volume (40 mL/kg, HV_T) for 4 h and were then intraperitoneally administered RvD1 at the beginning of high tidal volume ventilation and given GW9662 (a PPAR-γ antagonist) intraperitoneally 30 min before ventilation. RvD1 attenuated VILI, as evidenced by improved oxygenation and reduced histological injury, compared with HV_T -induced lung injury. Similarly, it could ameliorate neutrophil accumulation and production of proinflammatory cytokines in lung tissue. In contrast, the protective effect of RvD1 on lung tissue could be reversed by GW9662. RvD1 mitigated VILI by activating peroxisome proliferator-activated receptor gamma (PPAR-γ) and inhibiting nuclear factor-kappa B (NF-κB) signaling pathways in mice. In conclusion, RvD1 could reduce the inflammatory response in VILI by activating PPAR-γ and inhibiting NF-κB signaling pathways.

TRPV4 inhibition attenuates stretch-induced inflammatory cellular responses and lung barrier dysfunction during mechanical ventilation

Mechanical ventilation is an important tool for supporting critically ill patients but may also exert pathological forces on lung cells leading to Ventilator-Induced Lung Injury (VILI). We hypothesised that inhibition of the force-sensitive transient receptor potential vanilloid (TRPV4) ion channel may attenuate the negative effects of mechanical ventilation. Mechanical stretch increased intracellular Ca²⁺ influx and induced release of pro-inflammatory cytokines in lung epithelial cells that was partially blocked by about 30% with the selective TRPV4 inhibitor GSK2193874, but nearly completely blocked with the pan-calcium channel blocker ruthenium red, suggesting the involvement of more than one calcium channel in the response to mechanical stress. Mechanical stretch also induced the release of pro-inflammatory cytokines from M1 macrophages, but in contrast this was entirely dependent upon TRPV4. In a murine ventilation model, TRPV4 inhibition attenuated both pulmonary barrier permeability increase and pro-inflammatory cytokines release due to high tidal volume ventilation. Taken together, these data suggest TRPV4 inhibitors may have utility as a prophylactic pharmacological treatment to improve the negative pathological stretch-response of lung cells during ventilation and potentially support patients receiving mechanical ventilation.

Microscale distribution and dynamic surface tension of pulmonary surfactant normalize the recruitment of asymmetric bifurcating airways

We investigate the influence of bifurcation geometry, asymmetry of daughter airways, surfactant distribution, and physicochemical properties on the uniformity of airway recruitment of asymmetric bifurcating airways. To do so, we developed microfluidic idealized in vitro models of bifurcating airways, through which we can independently evaluate the impact of carina location and daughter airway width and length. We explore the uniformity of recruitment and its relationship to the dynamic surface tension of the lining fluid and relate this behavior to the hydraulic (PHyd) and capillary (PCap) pressure drops. These studies demonstrate the extraordinary importance of PCap in stabilizing reopening, even in highly asymmetric systems. The dynamic surface tension of pulmonary surfactant is integral to this stability because it modulates PCap in a velocity-dependent manner. Furthermore, the surfactant distribution at the propagating interface can have a very large influence on recruitment stability by focusing surfactant preferentially to specific daughter airways. This implies that modification of the surfactant distribution through novel modes of ventilation could be useful in inducing uniformly recruited lungs, aiding in gas exchange, and reducing ventilator-induced lung injury.

NEW & NOTEWORTHY The dynamic surface tension of pulmonary surfactant is integral to the uniformity of asymmetric bifurcation airway recruitments because it modulates capillary pressure drop in a velocity-dependent manner. Also, the surfactant distribution at the propagating interface can have a very large influence on recruitment stability by focusing surfactant preferentially to specific daughter airways. This implies that modification of the surfactant distribution through novel modes of ventilation could be useful in inducing uniformly recruited lungs, reducing ventilator-induced lung injury.

Prevention of ventilator-induced lung edema by inhalation of nanoparticles releasing ruthenium red

The acute respiratory distress syndrome (ARDS), a devastating lung disease that has no cure, is exacerbated by life-supportive mechanical ventilation that worsens lung edema and inflammation through the syndrome of ventilator-induced lung injury. Recently, the membrane ion channel transient receptor potential vanilloid 4 (TRPV4) on alveolar macrophages was shown to mediate murine lung vascular permeability induced by high-pressure mechanical ventilation. The objective of this study was to determine whether inhalation of nanoparticles (NPs) containing the TRPV4 inhibitor ruthenium red (RR) prevents ventilator-induced lung edema in mice. Poly-lactic-co-glycolic acid NPs containing RR were evaluated in vitro for their ability to block TRPV4-mediated calcium signaling in alveolar macrophages and capillary endothelial cells. Lungs from adult C57BL6 mice treated with nebulized NPs were then used in ex vivo ventilation perfusion experiments to assess the ability of the NPs to prevent high-pressure mechanical ventilation-induced lung edema. Poly-lactic-co-glycolic acid NPs (300 nm) released RR for 150 hours in vitro, and blocked TRPV4-mediated calcium signaling in cells up to 7 days after phagocytosis. Inhaled NPs deposited in alveoli of spontaneously breathing mice were rapidly phagocytosed by alveolar macrophages, and blocked increased vascular permeability from high-pressure mechanical ventilation for 72 hours in ex vivo ventilation perfusion experiments. These data offer proof of principle that inhalation of NPs containing a TRPV4 inhibitor prevents ventilator damage for several days, and imply that this novel drug delivery strategy could be used to target alveolar macrophages in patients at risk of ventilator-induced lung injury before initiating mechanical ventilation.

Volutrauma, but not atelectrauma, induces systemic cytokine production by lung-marginated monocytes

OBJECTIVES

Ventilator-induced lung injury has substantive impact on mortality of patients with acute respiratory distress syndrome. Although low tidal volume ventilation has been shown to reduce mortality, clinical benefits of open-lung strategy are controversial. In this study, we investigated the impact of two distinct forms of ventilator-induced lung injury, i.e., volutrauma and atelectrauma, on the progression of lung injury and inflammation, in particular alveolar and systemic cytokine production.

DESIGN

Ex vivo study.

SETTING

University research laboratory.

SUBJECTS

C57BL/6 mice.

INTERVENTIONS

Isolated, buffer-perfused lungs were allocated to one of three ventilatory protocols for 3 hours: control group received low tidal volume (7 mL/kg) with positive end-expiratory pressure (5 cm H2O) and regular sustained inflation; high-stretch group received high tidal volume (30-32 mL/kg) with positive end-expiratory pressure (3 cm H2O) and sustained inflation; and atelectasis group received the same tidal volume as control but neither positive end-expiratory pressure nor sustained inflation.

MEASUREMENTS AND MAIN RESULTS

Both injurious ventilatory protocols developed comparable levels of physiological injury and pulmonary edema, measured by respiratory system mechanics and lavage fluid protein. High-stretch induced marked increases in proinflammatory cytokines in perfusate and lung lavage fluid, compared to control. In contrast, atelectasis had no effect on perfusate cytokines compared to control but did induce some up-regulation of lavage cytokines. Depletion of monocytes marginated within the lung microvasculature, achieved by pretreating mice with i.v. liposome-encapsulated clodronate, significantly attenuated perfusate cytokine levels, especially tumor necrosis factor, in the high-stretch, but not atelectasis group.

CONCLUSIONS

Volutrauma (high-stretch), but not atelectrauma (atelectasis), directly activates monocytes within the pulmonary vasculature, leading to cytokine release into systemic circulation. We postulate this as a potential explanation why open-lung strategy has limited mortality benefits in ventilated critically ill patients.

The unusual symmetric reopening effect induced by pulmonary surfactant

This study investigates the stability of a finger of air as it propagates into a liquid-filled model of a liquid-filled model of a pulmonary bifurcation. We seek to elucidate the stability characteristics of the reopening of daughter airways, an event that may be important to the treatment of acute lung disease. To do so, we investigated the symmetry of reopening under conditions of nearly constant surface tension with 1) purified H2O or 2) an anionic surfactant (sodium dodecyl sulfate). Dynamic surface tension was investigated using pulmonary surfactant (Infasurf) with and without the presence of albumin. Flow visualization was accomplished using a microparticle image velocimetry (μ-PIV)/shadowgraph system through which we measured 1) the propagation velocity of the finger of air that reopens each daughter branch, and 2) the instantaneous and averaged velocity field of liquid phase surrounding the tip of the propagating bubble. Only pulmonary surfactant demonstrated the ability of maintaining a nearly symmetric propagation in the daughter channels, which is likely to lead to homogeneous airway reopening. In contrast, when pulmonary surfactant was inactivated by albumin or when the system was held at a nearly constant surface tension, reopening occurred asymmetrically. Our analysis suggests that Infasurf's dynamic surface tension qualities are important to stabilize the removal of liquid obstructions. This demonstrates a new important function of pulmonary surfactant for airway reopening of a multibranched network.

The evolution of extracorporeal life support as a bridge to lung transplantation

The use of extracorporeal membrane oxygenation (ECMO) as a bridge to lung transplantation was reported for the first time more than three decades ago; nevertheless, its use in lung transplantation was largely abandoned because of poor patient survival and frequent complications. The outcomes of patients bridged to lung transplantation using ECMO have substantially improved in the last 5 years. Recent advances in extracorporeal life support technology now allow patients with end-stage lung disease to be successfully supported for prolonged periods of time, preventing the use of mechanical ventilation and facilitating physical rehabilitation and ambulation while the patients awaits lung transplantation. This review briefly describes the evolution of ECMO use in lung transplantation and summarizes the available technology and current approaches to provide ECMO support.

Extracorporeal life support as a bridge to lung transplantation

Patients who are excellent candidates for lung transplantation often die on the waiting list because they are too sick to survive until an organ becomes available. Improvements in lung transplant outcomes, patient selection, and artificial lung device technologies have made it possible to bridge these patients to successful life-saving transplantation. Extracorporeal life support (ECLS) should be tailored to minimize morbidity and provide the appropriate mode and level of cardiopulmonary support for each patient's physiologic requirements. Novel device refinements and further development of ECLS in an ambulatory and simplified manner will help maintain these patients in better condition until transplantation.

A mathematical model of pulmonary gas exchange under inflammatory stress

During a severe local or systemic inflammatory response, immune mediators target lung tissue. This process may lead to acute lung injury and impaired diffusion of gas molecules. Although several mathematical models of gas exchange have been described, none simulate acute lung injury following inflammatory stress. In view of recent laboratory and clinical progress in the understanding of the pathophysiology of acute lung injury, such a mathematical model would be useful. We first derived a partial differential equations model of gas exchange on a small physiological unit of the lung ( approximately 25 alveoli), which we refer to as a respiratory unit (RU). We next developed a simple model of the acute inflammatory response and implemented its effects within a RU, creating a single RU model. Linking multiple RUs with various ventilation/perfusion ratios and taking into account pulmonary venous blood remixing yielded our lung-scale model. Using the lung-scale model, we explored the predicted effects of inflammation on ventilation/perfusion distribution and the resulting pulmonary venous partial pressure oxygen level during systemic inflammatory stresses. This model represents a first step towards the development of anatomically faithful models of gas exchange and ventilation under a broad range of local and systemic inflammatory stimuli resulting in acute lung injury, such as infection and mechanical strain of lung tissue.

Positive pressure ventilation: what is the real cost?

Positive pressure ventilation is a radical departure from the physiology of breathing spontaneously. The immediate physiological consequences of positive pressure ventilation such as haemodynamic changes are recognized, studied, and understood. There are other significant physiological interactions which are less obvious, more insidious, and may only produce complications if ventilation is prolonged. The interaction of positive pressure with airway resistance and alveolar compliance affects distribution of gas flow within the lung. The result is a wide range of ventilation efficacy throughout different areas of the lung, but the pressure differentials between alveolus and interstitium also influence capillary perfusion. The hydrostatic forces across the capillaries associated with the effects of raised venous pressures compound these changes resulting in interstitial fluid sequestration. This is increased by impaired lymphatic drainage which is secondary to raised intrathoracic pressure but also influenced by raised central venous pressure. Ventilation and PEEP promulgate further physiological derangement. In theory, avoiding these physiological disturbances in a rested lung may be better for the lung and other organs. An alternative to positive pressure ventilation might be to investigate oxygen supplementation of a physiologically neutral and rested lung. Abandoning heroic ventilation would be a massive departure from current practice but might be a more rationale approach to future practice.

Ventilator strategies for posttraumatic acute respiratory distress syndrome: airway pressure release ventilation and the role of spontaneous breathing in critically ill patients

PURPOSE OF REVIEW

Patients who experience severe trauma are at increased risk for the development of acute lung injury and acute respiratory distress syndrome. The management strategies used to treat respiratory failure in this patient population should be comprehensive. Current trends in the management of acute lung injury and acute respiratory distress syndrome consist of maintaining acceptable gas exchange while limiting ventilator-associated lung injury.

RECENT FINDINGS

Currently, two distinct forms of ventilator-associated lung injury are recognized to produce alveolar stress failure and have been termed low-volume lung injury (intratidal alveolar recruitment and derecruitment) and high-volume lung injury (alveolar stretch and overdistension). Pathologically, alveolar stress failure from low- and high-volume ventilation can produce lung injury in animal models and is termed ventilator-induced lung injury. The management goal in acute lung injury and acute respiratory distress syndrome challenges clinicians to achieve the optimal balance that both limits the forms of alveolar stress failure and maintains effective gas exchange. The integration of new ventilator modes that include the augmentation of spontaneous breathing during mechanical ventilation may be beneficial and may improve the ability to attain these goals.

SUMMARY

Airway pressure release ventilation is a mode of mechanical ventilation that maintains lung volume to limit intra tidal recruitment /derecruitment and improves gas exchange while limiting over distension. Clinical and experimental data demonstrate improvements in arterial oxygenation, ventilation-perfusion matching (less shunt and dead space ventilation), cardiac output, oxygen delivery, and lower airway pressures during airway pressure release ventilation. Mechanical ventilation with airway pressure release ventilation permits spontaneous breathing throughout the entire respiratory cycle, improves patient comfort, reduces the use of sedation, and may reduce ventilator days.

Inhalation injury assessed by score does not contribute to the development of acute respiratory distress syndrome in burn victims

OBJECTIVE

To establish the incidence, mortality, and time of onset of acute respiratory distress syndrome (ARDS) in relation to extent of burn and inhalation injury in patients who required mechanical ventilation.

DESIGN

Data about burn and inhalation injury were recorded prospectively whereas ARDS and multiple organ dysfunction were assessed by review of patient charts.

SETTING

National burn intensive care unit at Linkoping University Hospital, Sweden (a tertiary referral hospital).

PATIENTS

Between 1993 and 1999, we studied all patients with thermal injury (n=553) who required mechanical ventilation for more than two days (n=91).

MEASUREMENTS AND RESULTS

Out of the thirty-six burn victims who developed ARDS (40%), 25 (70%) did so early post burn (in less than 6 days). Patients with ARDS had higher multiple organ dysfunction scores (mean 10.5) than those who did not develop ARDS (mean 5.6) (p<0.01). The probable presence of inhalation injury as assessed by an inhalation lung injury score (ILIS) did not contribute to the development of ARDS. Mortality tended to be higher in patients who developed ARDS (14%) compared to those who did not (6%, p=0.2).

CONCLUSIONS

In our burn patients the incidence of ARDS was high whereas mortality was low. We found no association between inhalation injury as assessed using the ILIS and development of ARDS. Our data support a multi-factorial origin of ARDS in burn victims as a part of a multiple organ failure event.

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

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