Capturing the multifactorial nature of ARDS - "Two-hit" approach to model murine acute lung injury

Lactate produced by alveolar type II cells suppresses inflammatory alveolar macrophages in acute lung injury

Alveolar inflammation is a hallmark of acute lung injury (ALI), and its clinical correlate is acute respiratory distress syndrome-and it is as a result of interactions between alveolar type II cells (ATII) and alveolar macrophages (AM). In the setting of acute injury, the microenvironment of the intra-alveolar space is determined in part by metabolites and cytokines and is known to shape the AM phenotype. In response to ALI, increased glycolysis is observed in AT II cells, mediated by the transcription factor hypoxia-inducible factor (HIF) 1α, which has been shown to decrease inflammation. We hypothesized that in acute lung injury, lactate, the end product of glycolysis, produced by ATII cells shifts AMs toward an anti-inflammatory phenotype, thus mitigating ALI. We found that local intratracheal delivery of lactate improved ALI in two different mouse models. Lactate shifted cytokine expression of murine AMs toward increased IL-10, while decreasing IL-1 and IL-6 expression. Mice with ATII-specific deletion of Hif1a and mice treated with an inhibitor of lactate dehydrogenase displayed exacerbated ALI and increased inflammation with decreased levels of lactate in the bronchoalveolar lavage fluid; however, all those parameters improved with intratracheal lactate. When exposed to LPS (to recapitulate an inflammatory stimulus as it occurs in ALI), human primary AMs co-cultured with alveolar epithelial cells had reduced inflammatory responses. Taken together, these studies reveal an innate protective pathway, in which lactate produced by ATII cells shifts AMs toward an anti-inflammatory phenotype and dampens excessive inflammation in ALI.

Synthetic surfactant with a combined SP-B and SP-C analogue is efficient in rabbit models of adult and neonatal respiratory distress syndrome

Respiratory distress syndrome (RDS) in premature infants is caused by insufficient amounts of endogenous lung surfactant and is efficiently treated with replacement therapy using animal-derived surfactant preparations. On the other hand, adult/acute RDS (ARDS) occurs secondary to for example, sepsis, aspiration of gastric contents, and multitrauma and is caused by alveolar endothelial damage, leakage of plasma components into the airspaces and inhibition of surfactant activity. Instillation of surfactant preparations in ARDS has so far resulted in very limited treatment effects, partly due to inactivation of the delivered surfactants in the airspace. Here, we develop a combined surfactant protein B (SP-B) and SP-C peptide analogue (Combo) that can be efficiently expressed and purified from Escherichia coli without any solubility or purification tag. NMR spectroscopy shows that Combo peptide forms α-helices both in organic solvents and in lipid micelles, which coincide with the helical regions described for the isolated SP-B and SP-C parts. Artificial Combo surfactant composed of synthetic dipalmitoylphosphatidylcholine:palmitoyloleoylphosphatidylglycerol, 1:1, mixed with 3 weights % relative to total phospholipids of Combo peptide efficiently improves tidal volumes and lung gas volumes at end-expiration in a premature rabbit fetus model of RDS. Combo surfactant also improves oxygenation and respiratory parameters and lowers cytokine release in an acid instillation-induced ARDS adult rabbit model. Combo surfactant is markedly more resistant to inhibition by albumin and fibrinogen than a natural-derived surfactant in clinical use for the treatment of RDS. These features of Combo surfactant make it attractive for the development of novel therapies against human ARDS.

Synergistic Effects of Resistive Breathing on Endotoxin-Induced Lung Injury in Mice

Introduction

Resistive breathing (RB) is characterized by forceful contractions of the inspiratory muscles, mainly the diaphragm, resulting in large negative intrathoracic pressure and mechanical stress imposed on the lung. We have shown that RB induces lung injury in healthy animals. Whether RB exerts additional injurious effects when added to pulmonary or extrapulmonary lung injury is unknown. Our aim was to study the synergistic effect of RB on lipopolysaccharide (LPS)-induced lung injury.

Methods

C57BL/6 mice inhaled an LPS aerosol (10mg/3mL) or received an intraperitoneal injection of LPS (10 mg/kg). Mice were then anaesthetized, the trachea was surgically exposed, and a nylon band of a specified length was sutured around the trachea, to provoke a reduction of the surface area at 50%. RB through tracheal banding was applied for 24 hours. Respiratory system mechanics were measured, BAL was performed, and lung sections were evaluated for histological features of lung injury.

Results

LPS inhalation increased BAL cellularity, mainly neutrophils (p < 0.001 to ctr), total protein and IL-6 in BAL (p < 0.001 and p < 0.001, respectively) and increased the lung injury score (p = 0.001). Lung mechanics were not altered. Adding RB to inhaled LPS further increased BAL cellularity (p < 0.001 to LPS inh.), total protein (p = 0.016), lung injury score (p = 0.001) and increased TNFa levels in BAL (p = 0.011). Intraperitoneal LPS increased BAL cellularity, mainly macrophages (p < 0.001 to ctr.), total protein levels (p = 0.017), decreased static compliance (p = 0.004) and increased lung injury score (p < 0.001). Adding RB further increased histological features of lung injury (p = 0.022 to LPS ip).

Conclusion

Resistive breathing exerts synergistic injurious effects when combined with inhalational LPS-induced lung injury, while the additive effect on extrapulmonary lung injury is less prominent.

Effects of atelectatic areas on the surrounding lung tissue during mechanical ventilation in an experimental model of acute lung injury induced by lipopolysaccharide

OBJECTIVE

To assess the effect of atelectasis during mechanical ventilation on the periatelectatic and normal lung regions in a model of atelectasis in rats with acute lung injury induced by lipopolysaccharide.

METHODS

Twenty-four rats were randomized into the following four groups, each with 6 animals: the Saline-Control Group, Lipopolysaccharide Control Group, Saline-Atelectasis Group, and Lipopolysaccharide Atelectasis Group. Acute lung injury was induced by intraperitoneal injection of lipopolysaccharide. After 24 hours, atelectasis was induced by bronchial blocking. The animals underwent mechanical ventilation for two hours with protective parameters, and respiratory mechanics were monitored during this period. Thereafter, histologic analyses of two regions of interest, periatelectatic areas and the normally-aerated lung contralateral to the atelectatic areas, were performed.

RESULTS

The lung injury score was significantly higher in the Lipopolysaccharide Control Group (0.41 ± 0.13) than in the Saline Control Group (0.15 ± 0.51), p < 0.05. Periatelectatic regions showed higher lung injury scores than normally-aerated regions in both the Saline-Atelectasis (0.44 ± 0.06 x 0.27 ± 0.74 p < 0.05) and Lipopolysaccharide Atelectasis (0.56 ± 0.09 x 0.35 ± 0.04 p < 0.05) Groups. The lung injury score in the periatelectatic regions was higher in the Lipopolysaccharide Atelectasis Group (0.56 ± 0.09) than in the periatelectatic region of the Saline-Atelectasis Group (0.44 ± 0.06), p < 0.05.

CONCLUSION

Atelectasis may cause injury to the surrounding tissue after a period of mechanical ventilation with protective parameters. Its effect was more significant in previously injured lungs.

AA, Kudriaeva AA, Bugrimov DY, Krasnorutskaya ON, Murashev AN. Derinat® has an immunomodulatory and anti-inflammatory effect on the model of acute lung injury in male SD rats

To simulate acute lung injury (ALI) in SD male rats they we administered intratracheally with lipopolysaccharide (LPS) followed by hyperventilation of the lungs (HVL), which lead to functional changes in the respiratory system and an increase in the blood serum concentration of inflammatory cytokines. LPS + HVL after 4 h lead to pronounced histological signs of lung damage. We have studied the effectiveness of Derinat^® when administered intramuscularly at dose of 7.5 mg/kg for 8 days in the ALI model. Derinat^® administration lead to an increase in the concentration of most of the studied cytokines in a day. In the ALI model the administration of Derinat^® returned the concentration of cytokines to its original values already 48 h after LPS + HVL, and also normalized the parameters of pulmonary respiration in comparison with animals without treatment. By the eighth day after LPS + HVL, respiratory parameters and cytokine levels, as well as biochemical and hematological parameters did not differ between groups, while histological signs of residual effects of lung damage were found in all animals, and were more pronounced in Derinat^® group, which may indicate stimulation of the local immune response. Thus, the administration of Derinat^® stimulates the immune response, has a pronounced protective effect against cytokinemia and respiratory failure caused by ALI, has immunomodulatory effect, and also stimulates a local immune response in lung tissues. Thus, Derinat^® is a promising treatment for ALI.

HIF1A-dependent induction of alveolar epithelial PFKFB3 dampens acute lung injury

Acute lung injury (ALI) is a severe form of lung inflammation causing acute respiratory distress syndrome in patients. ALI pathogenesis is closely linked to uncontrolled alveolar inflammation. We hypothesize that specific enzymes of the glycolytic pathway could function as key regulators of alveolar inflammation. Therefore, we screened isolated alveolar epithelia from mice exposed to ALI induced by injurious ventilation to assess their metabolic responses. These studies pointed us toward a selective role for isoform 3 of the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFKFB3). Pharmacologic inhibition or genetic deletion of Pfkfb3 in alveolar epithelia (Pfkfb3loxP/loxP SPC-ER-Cre+ mice) was associated with profound increases in ALI during injurious mechanical ventilation or acid instillation. Studies in genetic models linked Pfkfb3 expression and function to Hif1a. Not only did intratracheal pyruvate instillation reconstitute Pfkfb3loxP/loxP or Hif1aloxP/loxP SPC-ER-Cre+ mice, but pyruvate was also effective in ALI treatment of wild-type mice. Finally, proof-of-principle studies in human lung biopsies demonstrated increased PFKFB3 staining in injured lungs and colocalized PFKFB3 to alveolar epithelia. These studies reveal a specific role for PFKFB3 in counterbalancing alveolar inflammation and lay the groundwork for novel metabolic therapeutic approaches during ALI.

Efficacy of surfactant therapy of ARDS induced by hydrochloric acid aspiration followed by ventilator-induced lung injury - an animal study

The development of acute respiratory distress syndrome (ARDS) is known to be independently attributable to aspiration-induced lung injury. Mechanical ventilation as a high pressure/volume support to maintain sufficient oxygenation of a patient could initiate ventilator-induced lung injury (VILI) and thus contribute to lung damage. Although these phenomena are rare in the clinic, they could serve as the severe experimental model of alveolar-capillary membrane deterioration. Lung collapse, diffuse inflammation, alveolar epithelial and endothelial damage, leakage of fluid into the alveoli, and subsequent inactivation of pulmonary surfactant, leading to respiratory failure. Therefore, exogenous surfactant could be considered as a therapy to restore lung function in experimental ARDS. This study aimed to investigate the effect of modified porcine surfactant in animal model of severe ARDS (P/F ratio </=13.3 kPa) induced by intratracheal instillation of hydrochloric acid (HCl, 3 ml/kg, pH 1.25) followed by VILI (V(T) 20 ml/kg). Adult rabbits were divided into three groups: untreated ARDS, model treated with a bolus of poractant alfa (Curosurf®, 2.5 ml/kg, 80 mg phospholipids/ml), and healthy ventilated animals (saline), which were oxygen-ventilated for an additional 4 h. The lung function parameters, histological appearance, degree of lung edema and levels of inflammatory and oxidative markers in plasma were evaluated. Whereas surfactant therapy with poractant alfa improved lung function, attenuated inflammation and lung edema, and partially regenerated significant changes in lung architecture compared to untreated controls. This study indicates a potential of exogenous surfactant preparation in the treatment of experimental ARDS.

Lipopolysaccharide-induced murine lung injury results in long-term pulmonary changes and downregulation of angiogenic pathways

Acute respiratory distress syndrome is the most severe form of acute lung injury (ALI) and is associated with significant mortality. Lipopolysaccharide (LPS)-induced injury is a valuable murine model of ALI but there is a paucity of data on lung regeneration and the role of angiogenic signaling involving vascular endothelial growth factor (VEGF). Eight-week-old male C57BL/6J mice were randomized to receive intratracheal instillation of either LPS or isovolumetric phosphate buffered saline as a vehicle control. Mice were observed at a single follow-up time-point that was either short-term (24 h or 4 days) or long-term (7 days or 4 weeks). On pulmonary function testing, LPS-treated mice had increased compliance at 4 weeks post-instillation, which correlated with decreased vascularization and with time-dependent, progressive decrease in alveolarization. Treadmill exercise tolerance testing demonstrated impaired performance at 24 h, 4 days and 4 weeks following LPS exposure. On lung protein analysis, LPS instillation decreased VEGF expression at up to 4 weeks, and decreased activation of its key receptor, VEGFR2 at 7 days and 4 weeks post-instillation. Together, these data provide insight on long-term pulmonary functional outcomes 4 weeks after ALI and identify angiogenic proteins as possible therapeutic targets following lung injury.

Antioxidants as Therapeutic Agents in Acute Respiratory Distress Syndrome (ARDS) Treatment-From Mice to Men

Acute respiratory distress syndrome (ARDS) is a major cause of patient mortality in intensive care units (ICUs) worldwide. Considering that no causative treatment but only symptomatic care is available, it is obvious that there is a high unmet medical need for a new therapeutic concept. One reason for a missing etiologic therapy strategy is the multifactorial origin of ARDS, which leads to a large heterogeneity of patients. This review summarizes the various kinds of ARDS onset with a special focus on the role of reactive oxygen species (ROS), which are generally linked to ARDS development and progression. Taking a closer look at the data which already have been established in mouse models, this review finally proposes the translation of these results on successful antioxidant use in a personalized approach to the ICU patient as a potential adjuvant to standard ARDS treatment.

A Peptide Inhibitor of Peroxiredoxin 6 Phospholipase A2 Activity Significantly Protects against Lung Injury in a Mouse Model of Ventilator Induced Lung Injury (VILI)

Ventilator induced lung injury (VILI) is a lung injury syndrome associated with mechanical ventilation, most frequently for treatment of Acute Lung Injury (ALI), and generally secondary to the use of greater than physiologic tidal volumes. To reproduce this syndrome experimentally, C57Bl/6 mice were intubated and ventilated with low (4 mL/Kg body weight) or high (12 mL/Kg) tidal volume for 6 h. Lung parameters with low volume ventilation were unchanged from non-ventilated (control) mice. High tidal volume ventilation resulted in marked lung injury with increased neutrophils in the bronchoalveolar lavage fluid (BALf) indicating lung inflammation, increase in both protein in BALf and lung dry/wet weight indicating lung edema, increased lung thiobarbituric acid reactive substances (TBARS) and 8-isoprostanes indicating lung lipid peroxidation, and increased lung protein carbonyls indicating protein oxidation. Either intratracheal or intravenous pretreatment of mice with a 9 amino acid peptide called peroxiredoxin 6 inhibitor peptide-2 (PIP-2) significantly reduced all parameters of lung injury by ~50–80%. PIP-2 inhibits NADPH oxidase type 2 (NOX2) activation. We propose that PIP-2 does not affect the mechanically induced lung damage component of VILI but does significantly reduce the secondary inflammatory component.

[Intraoperative Ventilation in Adults]

Avoiding postoperative pulmonary complications (PPC) is an important goal for anesthesiologists during general anesthesia, and ventilation strategies may play a role. It seems reasonable to apply knowledge from lessons we learned from ventilation of intensive care unit patients aiming at avoiding ventilator associated lung injury. Ventilation associated lung injuries occur frequently and are associated with substantial morbidity and mortality. Strategies of lung protective ventilation, like lower tidal volumes and the use of positive end-expiatory pressure (PEEP), can usually be transferred safely to perioperative ventilation, although some issues such as hemodynamic side effects must be considered. For some reasons, however, current evidence is conflicting and there is no consensus on ventilatory perioperative management to avoid PPCs so far. This paper briefly summarizes physiological backgrounds in a functional context, current evidence, and provides some recommendations at "expert" opinion level for perioperative ventilation procedures.Especially in patients at risk and/or during surgery with higher surgical trauma and inflammation, we recommend limiting tidal volume to 6 - 8 ml/kg predicted body weight and the use of PEEP, which should be individualized e.g. by minimizing driving pressure. Recruitment maneuvers may be considered and should be carried out by using the ventilator.Obese patients are an increasing entity and can be challenging during anesthesia and ventilation. From a physiological point of view, these patients require much higher ventilation pressures as currently used, although recent evidence is not in favor of using moderately higher PEEP, which is matter of discussion.

Treatment with senicapoc in a porcine model of acute respiratory distress syndrome

Background

Senicapoc is a potent and selective blocker of KCa3.1, a calcium-activated potassium channel of intermediate conductance. In the present study, we investigated whether there is a beneficial effect of senicapoc in a large animal model of acute respiratory distress syndrome (ARDS). The primary end point was the PaO₂/FiO₂ ratio.

Methods

ARDS was induced in female pigs (42–49 kg) by repeated lung lavages followed by injurious mechanical ventilation. Animals were then randomly assigned to vehicle (n = 9) or intravenous senicapoc (10 mg, n = 9) and received lung-protective ventilation for 6 h.

Results

Final senicapoc plasma concentrations were 67 ± 18 nM (n = 9). Senicapoc failed to change the primary endpoint PaO₂/FiO₂ ratio (senicapoc, 133 ± 23 mmHg; vehicle, 149 ± 68 mmHg). Lung compliance remained similar in the two groups. Senicapoc reduced the level of white blood cells and neutrophils, while the proinflammatory cytokines TNFα, IL-1β, and IL-6 in the bronchoalveolar lavage fluid were unaltered 6 h after induction of the lung injury. Senicapoc-treatment reduced the level of neutrophils in the alveolar space but with no difference between groups in the cumulative lung injury score. Histological analysis of pulmonary hemorrhage indicated a positive effect of senicapoc on alveolar–capillary barrier function, but this was not supported by measurements of albumin content and total protein in the bronchoalveolar lavage fluid.

Conclusions

In summary, senicapoc failed to improve the primary endpoint PaO₂/FiO₂ ratio, but reduced pulmonary hemorrhage and the influx of neutrophils into the lung. These findings open the perspective that blocking KCa3.1 channels is a potential treatment to reduce alveolar neutrophil accumulation and improve long-term outcome in ARDS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40635-021-00381-z.

Targeting alveolar-specific succinate dehydrogenase A attenuates pulmonary inflammation during acute lung injury

Acute lung injury (ALI) is an inflammatory lung disease, which manifests itself in patients as acute respiratory distress syndrome (ARDS). Previous studies have implicated alveolar-epithelial succinate in ALI protection. Therefore, we hypothesized that targeting alveolar succinate dehydrogenase SDH A would result in elevated succinate levels and concomitant lung protection. Wild-type (WT) mice or transgenic mice with targeted alveolar-epithelial Sdha or hypoxia-inducible transcription factor Hif1a deletion were exposed to ALI induced by mechanical ventilation. Succinate metabolism was assessed in alveolar-epithelial via mass spectrometry as well as redox measurements and evaluation of lung injury. In WT mice, ALI induced by mechanical ventilation decreased SDHA activity and increased succinate in alveolar-epithelial. In vitro, cell-permeable succinate decreased epithelial inflammation during stretch injury. Mice with inducible alveolar-epithelial Sdha deletion (Sdha^loxp/loxp SPC-CreER mice) revealed reduced lung inflammation, improved alveolar barrier function, and attenuated histologic injury. Consistent with a functional role of succinate to stabilize HIF, Sdha^loxp/loxp SPC-CreER experienced enhanced Hif1a levels during hypoxia or ALI. Conversely, Hif1a^loxp/loxp SPC-CreER showed increased inflammation with ALI induced by mechanical ventilation. Finally, wild-type mice treated with intra-tracheal dimethlysuccinate were protected during ALI. These data suggest that targeting alveolar-epithelial SDHA dampens ALI via succinate-mediated stabilization of HIF1A. Translational extensions of our studies implicate succinate treatment in attenuating alveolar inflammation in patients suffering from ARDS.

Oxycodone attenuates vascular leak and lung inflammation in a clinically relevant two-hit rat model of acute lung injury

BACKGROUND

Oxycodone is a synthetic opioid receptor agonist that exerts antinociceptive activity via κ-, μ- and δ-opioid receptors (KOR, MOR and DOR, respectively). Activation of MOR has been reported to provide protection against acute lung injury (ALI). We hypothesized that pretreatment with oxycodone would attenuate lung injury at the level of alveolar tight junctions (TJs) and aquaporins (AQPs) and investigated this possibility in a two-hit model of ALI induced by lipopolysaccharide (LPS) and mechanical ventilation (MV).

METHOD

Male Sprague Dawley rats and A59 cells were divided into 6 groups: the control group, ALI group, oxycodone-pretreated group, and oxycodone/κ-, μ-, or δ-opioid receptor antagonist-pretreated groups. The rats were pretreated with oxycodone 30 min before intravenous injection of LPS and then allowed to recover for 24 h prior to MV, establishing a two-hit model of ALI. The cells were similarly treated with oxycodone (with or without antagonists) 30 min after exposure to lipopolysaccharide. The cells were cyclically stretched 24 h later to mirror the in vivo MV protocol.

RESULTS

Oxycodone alleviated the histological lung changes in the rats with ALI and decreased pulmonary microvascular permeability both in vivo and in vitro. Oxycodone upregulated the expression of claudin-5, ZO-1, AQP1, and AQP5 but downregulated the expression of TNF-α, IL-1β, TLR4, NF-κB, MMP9, and caspase-3 and suppressed endothelial apoptosis in vivo and in vitro. These protective effects of oxycodone were partly eliminated by KOR and MOR antagonists but not by DOR antagonists.

CONCLUSION

Oxycodone pretreatment appears to act via κ- and μ-opioid receptors to ameliorate LPS- and MV-induced lung injury by suppressing inflammation and apoptosis, and this protective effect might be mediated through the inhibition of the TLR4/NF-κB pathways.

Inflammation, Thrombosis, and Destruction: The Three-Headed Cerberus of Trauma- and SARS-CoV-2-Induced ARDS

Physical trauma can be considered an unrecognized “pandemic” because it can occur anywhere and affect anyone and represents a global burden. Following severe tissue trauma, patients frequently develop acute lung injury (ALI) and/or acute respiratory distress syndrome (ARDS) despite modern surgical and intensive care concepts. The underlying complex pathophysiology of life-threatening ALI/ARDS has been intensively studied in experimental and clinical settings. However, currently, the coronavirus family has become the focus of ALI/ARDS research because it represents an emerging global public health threat. The clinical presentation of the infection is highly heterogeneous, varying from a lack of symptoms to multiple organ dysfunction and mortality. In a particular subset of patients, the primary infection progresses rapidly to ALI and ARDS. The pathophysiological mechanisms triggering and driving severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-induced ALI/ARDS are still poorly understood. Although it is also generally unknown whether insights from trauma-induced ARDS may be readily translated to SARS-CoV-2-associated ARDS, it was still recommended to treat coronavirus-positive patients with ALI/ARDS with standard protocols for ALI/ARDS. However, this strategy was questioned by clinical scientists, because it was documented that some severely hypoxic SARS-CoV-2-infected patients exhibited a normal respiratory system compliance, a phenomenon rarely observed in ARDS patients with another underlying etiology. Therefore, coronavirus-induced ARDS was defined as a specific ARDS phenotype, which accordingly requires an adjusted therapeutic approach. These suggestions reflect previous attempts of classifying ARDS into different phenotypes that might overall facilitate ARDS diagnosis and treatment. Based on the clinical data from ARDS patients, two major phenotypes have been proposed: hyper- and hypo-inflammatory. Here, we provide a comparative review of the pathophysiological pathway of trauma-/hemorrhagic shock-induced ARDS and coronavirus-induced ARDS, with an emphasis on the crucial key points in the pathogenesis of both these ARDS forms. Therefore, the manifold available data on trauma-/hemorrhagic shock-induced ARDS may help to better understand coronavirus-induced ARDS.

Understanding the role of neutrophils in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is difficult to treat and is associated with a high mortality rate. The most severe form of coronavirus disease 2019 (COVID-19) also leads to life-threatening ARDS. Neutrophil counts are positively correlated with disease severity in ARDS. Neutrophil activation not only plays a significant role in immune defense against infections, but also causes tissue damage and leads to inflammatory diseases. Activated neutrophils rapidly migrate to inflamed lung tissue, releasing toxic granular contents and generating neutrophil extracellular traps. In the last few decades, it has become apparent that neutrophils occupy a central role in ARDS pathology. In this review, we summarize the neutrophil inflammatory responses and their relationships to ARDS. According to the current literature, understanding the function of neutrophils may be helpful in the treatment of ARDS.

P2 Purinergic Signaling in the Distal Lung in Health and Disease

The distal lung provides an intricate structure for gas exchange in mammalian lungs. Efficient gas exchange depends on the functional integrity of lung alveoli. The cells in the alveolar tissue serve various functions to maintain alveolar structure, integrity and homeostasis. Alveolar epithelial cells secrete pulmonary surfactant, regulate the alveolar surface liquid (ASL) volume and, together with resident and infiltrating immune cells, provide a powerful host-defense system against a multitude of particles, microbes and toxicants. It is well established that all of these cells express purinergic P2 receptors and that purinergic signaling plays important roles in maintaining alveolar homeostasis. Therefore, it is not surprising that purinergic signaling also contributes to development and progression of severe pathological conditions like pulmonary inflammation, acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and pulmonary fibrosis. Within this review we focus on the role of P2 purinergic signaling in the distal lung in health and disease. We recapitulate the expression of P2 receptors within the cells in the alveoli, the possible sources of ATP (adenosine triphosphate) within alveoli and the contribution of purinergic signaling to regulation of surfactant secretion, ASL volume and composition, as well as immune homeostasis. Finally, we summarize current knowledge of the role for P2 signaling in infectious pneumonia, ALI/ARDS and idiopathic pulmonary fibrosis (IPF).

COVID-19-associated acute respiratory distress syndrome: is a different approach to management warranted?

The COVID-19 pandemic has seen a surge of patients with acute respiratory distress syndrome (ARDS) in intensive care units across the globe. As experience of managing patients with COVID-19-associated ARDS has grown, so too have efforts to classify patients according to respiratory system mechanics, with a view to optimising ventilatory management. Personalised lung-protective mechanical ventilation reduces mortality and has become the mainstay of treatment in ARDS. In this Viewpoint, we address ventilatory strategies in the context of recent discussions on phenotypic heterogeneity in patients with COVID-19-associated ARDS. Although early reports suggested that COVID-19-associated ARDS has distinctive features that set it apart from historical ARDS, emerging evidence indicates that the respiratory system mechanics of patients with ARDS, with or without COVID-19, are broadly similar. In the absence of evidence to support a shift away from the current paradigm of ventilatory management, we strongly recommend adherence to evidence-based management, informed by bedside physiology, as resources permit.

Static and Dynamic Contributors to Ventilator-induced Lung Injury in Clinical Practice. Pressure, Energy, and Power

Ventilation is inherently a dynamic process. The present-day clinical practice of concentrating on the static inflation characteristics of the individual tidal cycle (plateau pressure, positive end-expiratory pressure, and their difference [driving pressure, the ratio of Vt to compliance]) does not take into account key factors shown experimentally to influence ventilator-induced lung injury (VILI). These include rate of airway pressure change (influenced by flow amplitude, inspiratory time fraction, and inspiratory inflation contour) and cycling frequency. Energy must be expended to cause injury, and the product of applied stress and resulting strain determines the energy delivered to the lungs per breathing cycle. Understanding the principles of VILI energetics may provide valuable insights and guidance to intensivists for safer clinical practice. In this interpretive review, we highlight that the injuring potential of the inflation pattern depends upon tissue vulnerability, the number of intolerable high-energy cycles applied in unit time (mechanical power), and the duration of that exposure. Yet, as attractive as this energy/power hypothesis for encapsulating the drivers of VILI may be for clinical applications, we acknowledge that even these all-inclusive and measurable ergonomic parameters (energy per cycle and power) are still too bluntly defined to pinpoint the precise biophysical link between ventilation strategy and tissue injury.

Systems Biology ARDS Research with a Focus on Metabolomics

Acute respiratory distress syndrome (ARDS) is a clinical syndrome that inflicts a considerably heavy toll in terms of morbidity and mortality. While there are multitudes of conditions that can lead to ARDS, the vast majority of ARDS cases are caused by a relatively small number of diseases, especially sepsis and pneumonia. Currently, there is no clinically agreed upon reliable diagnostic test for ARDS, and the detection or diagnosis of ARDS is based on a constellation of laboratory and radiological tests in the absence of evidence of left ventricular dysfunction, as specified by the Berlin definition of ARDS. Virtually all the ARDS biomarkers to date have been proven to be of very limited clinical utility. Given the heterogeneity of ARDS due to the wide variation in etiology, clinical and molecular manifestations, there is a current scientific consensus agreement that ARDS is not just a single entity but rather a spectrum of conditions that need further study for proper classification, the identification of reliable biomarkers and the adequate institution of therapeutic targets. This scoping review aims to elucidate ARDS omics research, focusing on metabolomics and how metabolomics can boost the study of ARDS biomarkers and help to facilitate the identification of ARDS subpopulations.

Strain, strain rate, and mechanical power: An optimization comparison for oscillatory ventilation

The purpose of this study was to assess the potential for optimization of mechanical ventilator waveforms using multiple frequencies of oscillatory flow delivered simultaneously to minimize the risk of ventilator-induced lung injury (VILI) associated with regional strain, strain rate, and mechanical power. Optimization was performed using simulations of distributed oscillatory flow and gas transport in a computational model of anatomically derived branching airway segments and viscoelastic terminal acini under healthy and injured conditions. Objective functions defined by regional strain or strain rate were minimized by single-frequency ventilation waveforms using the highest or lowest frequencies available, respectively. However, a mechanical power objective function was minimized by a combination of multiple frequencies delivered simultaneously. This simulation study thus demonstrates the potential for multifrequency oscillatory ventilation to reduce regional mechanical power in comparison to single-frequency ventilation, and thereby reduce the risk of VILI.

Correlation analysis between mechanical power, transforming growth factor-β1, and connective tissue growth factor levels in acute respiratory distress syndrome patients and their clinical significance in pulmonary structural remodeling

BACKGROUND

To identify the clinical correlations between mechanical power and transforming growth factor-β1 (TGF-β1) and connective tissue growth factor (CTGF) in acute respiratory distress syndrome (ARDS) patients, their clinical significance in pulmonary structural remodeling in ARDS patients was investigated.

METHODS

Ninety-five patients with moderate or severe ARDS, who required mechanical ventilation therapy, were randomly selected among hospitalized patients from January 2017 to February 2019. Their mechanical power was monitored and recorded, the TGF-β1 and CTGF levels were detected by enzyme-linked immunosorbent assay (ELISA), their relevance was analyzed, and the relationship between mechanical power and 28-day survival rate was investigated. According to the high-resolution computed tomography (HRCT) examination, the patients were divided into an ARDS group and an ARDS pulmonary fibrosis (ARDS-PF) group. The differences in mechanical power, TGF-β1, and CTGF between the 2 groups were compared, and the significance of TGF-β1 and CTGF in the diagnosis of ARDS pulmonary interstitial fibrosis were evaluated.

RESULTS

A significant positive correlation between mechanical power and serum TGF-β1 and CTGF in patients with ARDS was found and the correlation coefficients were 0.424 and 0.581, respectively. The difference between mechanical power and 28-day survival rate was statistically significant (P < .05), while the area under the receiver operating characteristic curves of TGF-β1 and CTGF for the diagnosis of ARDS pulmonary fibrosis was 0.838 and 0.884, respectively (P < .05).

CONCLUSION

A significant correlation between mechanical power and serum fibrosis biomarkers TGF-β1 and CTGF in ARDS patients was found, and its level was related to the survival prognosis of patients. Mechanical power, TGF-β1, and CTGF were clinically evaluated for the assessment of lung structural remodeling, such as ARDS pulmonary fibrosis. This study has particular significance to the early prevention of ventilator-induced lung injury and pulmonary fibrosis in patients with ARDS receiving mechanical ventilation.