Recurrent recruitment manoeuvres improve lung mechanics and minimize lung injury during mechanical ventilation of healthy mice

Collagen Tubular Airway-on-Chip for Extended Epithelial Culture and Investigation of Ventilation Dynamics

The lower respiratory tract is a hierarchical network of compliant tubular structures that are made from extracellular matrix proteins with a wall lined by an epithelium. While microfluidic airway-on-a-chip models incorporate the effects of shear and stretch on the epithelium, week-long air-liquid-interface culture at physiological shear stresses, the circular cross-section, and compliance of native airway walls have yet to be recapitulated. To overcome these limitations, a collagen tube-based airway model is presented. The lumen is lined with a confluent epithelium during two-week continuous perfusion with warm, humid air while presenting culture medium from the outside and compensating for evaporation. The model recapitulates human small airways in extracellular matrix composition and mechanical microenvironment, allowing for the first time dynamic studies of elastocapillary phenomena associated with regular breathing and mechanical ventilation, as well as their impacts on the epithelium. A case study reveales increasing damage to the epithelium during repetitive collapse and reopening cycles as opposed to overdistension, suggesting expiratory flow resistance to reduce atelectasis. The model is expected to promote systematic comparisons between different clinically used ventilation strategies and, more broadly, to enhance human organ-on-a-chip platforms for a variety of tubular tissues.

Sigh Ventilation in Patients With Trauma: The SiVent Randomized Clinical Trial

Importance

Among patients receiving mechanical ventilation, tidal volumes with each breath are often constant or similar. This may lead to ventilator-induced lung injury by altering or depleting surfactant. The role of sigh breaths in reducing ventilator-induced lung injury among trauma patients at risk of poor outcomes is unknown.

Objective

To determine whether adding sigh breaths improves clinical outcomes.

Design, Setting, and Participants

A pragmatic, randomized trial of sigh breaths plus usual care conducted from 2016 to 2022 with 28-day follow-up in 15 academic trauma centers in the US. Inclusion criteria were age older than 18 years, mechanical ventilation because of trauma for less than 24 hours, 1 or more of 5 risk factors for developing acute respiratory distress syndrome, expected duration of ventilation longer than 24 hours, and predicted survival longer than 48 hours.

Interventions

Sigh volumes producing plateau pressures of 35 cm H2O (or 40 cm H2O for inpatients with body mass indexes >35) delivered once every 6 minutes. Usual care was defined as the patient's physician(s) treating the patient as they wished.

Main Outcomes and Measures

The primary outcome was ventilator-free days. Prespecified secondary outcomes included all-cause 28-day mortality.

Results

Of 5753 patients screened, 524 were enrolled (mean [SD] age, 43.9 [19.2] years; 394 [75.2%] were male). The median ventilator-free days was 18.4 (IQR, 7.0-25.2) in patients randomized to sighs and 16.1 (IQR, 1.1-24.4) in those receiving usual care alone (P = .08). The unadjusted mean difference in ventilator-free days between groups was 1.9 days (95% CI, 0.1 to 3.6) and the prespecified adjusted mean difference was 1.4 days (95% CI, -0.2 to 3.0). For the prespecified secondary outcome, patients randomized to sighs had 28-day mortality of 11.6% (30/259) vs 17.6% (46/261) in those receiving usual care (P = .05). No differences were observed in nonfatal adverse events comparing patients with sighs (80/259 [30.9%]) vs those without (80/261 [30.7%]).

Conclusions and Relevance

In a pragmatic, randomized trial among trauma patients receiving mechanical ventilation with risk factors for developing acute respiratory distress syndrome, the addition of sigh breaths did not significantly increase ventilator-free days. Prespecified secondary outcome data suggest that sighs are well-tolerated and may improve clinical outcomes.

Trial Registration

ClinicalTrials.gov Identifier: NCT02582957.

More airway smooth muscle in males versus females in a mouse model of asthma: A blessing in disguise?

NEW FINDINGS

What is the central question of this study? The lung response to inhaled methacholine is reputed to be greater in male than in female mice. The underpinnings of this sex disparity are ill defined. What is the main finding and its importance? We demonstrated that male airways exhibit a greater content of airway smooth muscle than female airways. We also found that, although a more muscular airway tree in males might contribute to their greater responsiveness to inhaled methacholine than females, it might also curb the heterogeneity in small airway narrowing.

ABSTRACT

Mouse models are helpful in unveiling the mechanisms underlying sex disparities in asthma. In comparison to their female counterparts, male mice are hyperresponsive to inhaled methacholine, a cardinal feature of asthma that contributes to its symptoms. The physiological details and the structural underpinnings of this hyperresponsiveness in males are currently unknown. Herein, BALB/c mice were exposed intranasally to either saline or house dust mite once daily for 10 consecutive days to induce experimental asthma. Twenty-four hours after the last exposure, respiratory mechanics were measured at baseline and after a single dose of inhaled methacholine that was adjusted to trigger the same degree of bronchoconstriction in both sexes (it was twice as high in females). Bronchoalveolar lavages were then collected, and the lungs were processed for histology. House dust mite increased the number of inflammatory cells in bronchoalveolar lavages to the same extent in both sexes (asthma, P = 0.0005; sex, P = 0.96). The methacholine response was also markedly increased by asthma in both sexes (e.g., P = 0.0002 for asthma on the methacholine-induced bronchoconstriction). However, for a well-matched bronchoconstriction between sexes, the increase in hysteresivity, an indicator of airway narrowing heterogeneity, was attenuated in males for both control and asthmatic mice (sex, P = 0.002). The content of airway smooth muscle was not affected by asthma but was greater in males (asthma, P = 0.31; sex, P < 0.0001). These results provide further insights regarding an important sex disparity in mouse models of asthma. The increased amount of airway smooth muscle in males might contribute functionally to their greater methacholine response and, possibly, to their decreased propensity for airway narrowing heterogeneity.

Optimized ventilation strategy for surgery on patients with obesity from the perspective of lung protection: A network meta-analysis

Objectives

New ventilation modes have been proposed to support the perioperative treatment of patients with obesity, but there is a lack of consensus regarding the optimal strategy. Therefore, a network meta-analysis update of 13 ventilation strategies was conducted to determine the optimal mode of mechanical ventilation as a protective ventilation strategy decreases pulmonary atelectasis caused by inflammation.

Methods

The following databases were searched: MEDLINE; Cochrane Library; Embase; CINAHL; Google Scholar; and Web of Science for randomized controlled trials of mechanical ventilation in patients with obesity published up to May 1, 2022.

Results

Volume-controlled ventilation with individualized positive end-expiratory pressure and a recruitment maneuver (VCV+PEEPind+RM) was found to be the most effective strategy for improving ratio of the arterial O2 partial pressure to the inspiratory O2 concentration (PaO2/FiO2), and superior to pressure-controlled ventilation (PCV), volume-controlled ventilation (VCV), volume-controlled ventilation with recruitment maneuver (VCV+RM), volume-controlled ventilation with low positive end-expiratory pressure (VCV+lowPEEP), volume-controlled ventilation with lower positive expiratory end pressure (PEEP) and recruitment maneuver (VCV+lowPEEP+RM), and the mean difference [MD], the 95% confidence intervals [CIs] and [quality of evidence] were: 162.19 [32.94, 291.45] [very low]; 180.74 [59.22, 302.27] [low]; 171.07 [40.60, 301.54] [very low]; 135.14 [36.10, 234.18] [low]; and 139.21 [27.08, 251.34] [very low]. Surface under the cumulative ranking curve (SUCRA) value showed VCV+PEEPind+RM was the best strategy for improving PaO2/FiO2 (SUCRA: 0.963). VCV with high positive PEEP and recruitment maneuver (VCV+highPEEP+RM) was more effective in decreasing postoperative pulmonary atelectasis than the VCV+lowPEEP+RM strategy. It was found that volume-controlled ventilation with high positive expiratory end pressure (VCV+highPEEP), risk ratio [RR] [95% CIs] and [quality of evidence], 0.56 [0.38, 0.81] [moderate], 0.56 [0.34, 0.92] [moderate]. SUCRA value ranked VCV+highPEEP+RM the best strategy for improving postoperative pulmonary atelectasis intervention (SUCRA: 0.933). It should be noted that the quality of evidence was in all cases very low or only moderate.

Conclusions

This research suggests that VCV+PEEPind+RM is the optimal ventilation strategy for patients with obesity and is more effective in increasing PaO2/FiO2, improving lung compliance, and among the five ventilation strategies for postoperative atelectasis, VCV+highPEEP+RM had the greatest potential to reduce atelectasis caused by inflammation.

Systematic Review Registration

https://www.crd.york.ac.uk/PROSPERO/, identifier CRD42021288941.

Effects of airway smooth muscle contraction and inflammation on lung tissue compliance

There are renewed interests in using the parameter K of Salazar-Knowles' equation to assess lung tissue compliance. K either decreases or increases when the lung's parenchyma stiffens or loosens, respectively. However, whether K is affected by other common features of respiratory diseases, such as inflammation and airway smooth muscle (ASM) contraction, is unknown. Herein, male C57BL/6 mice were treated intranasally with either saline or lipopolysaccharide (LPS) at 1 mg/Kg to induce pulmonary inflammation. They were then subjected to either a multiple or a single-dose challenge with methacholine to activate ASM to different degrees. A quasi-static pressure-driven partial pressure-volume maneuver was performed before and after methacholine. The Salazar-Knowles' equation was then fitted to the deflation limb of the P-V loop to obtain K, as well as the parameter A, an estimate of lung volume (inspiratory capacity). The fitted curve was also used to derive the quasi-static elastance (E_st) at 5 cmH₂O. The results demonstrate that LPS and both methacholine challenges increased E_st. LPS also decreased A, but did not affect K. In contradistinction, methacholine decreased both A and K in the multiple-dose challenge, while it decreased K but not A in the single-dose challenge. These results suggest that LPS increases E_st by reducing the open lung volume (A) and without affecting tissue compliance (K), while methacholine increases E_st by decreasing tissue compliance with or without affecting lung volume. We conclude that lung tissue compliance, assessed using the parameter K of Salazar-Knowles' equation, is insensitive to inflammation but sensitive to ASM contraction.

Translational Role of Rodent Models to Study Ventilator-Induced Lung Injury

Mechanical ventilation is an important part of medical care in intensive care units and operating rooms to support respiration. While it is a critical component of medical care, it is well known that mechanical ventilation itself can be injurious to the lungs. Despite a large number of clinical and preclinical studies that have been done so far, there still exists a gap of knowledge regarding how to ventilate patients mechanically without increasing lung injury. Here, we will review what we have learned so far from preclinical and clinical studies and consider how to use preclinical models of ventilation-induced lung injury that better recapitulate the clinical scenarios.

Constant Tidal Volume Ventilation and Surfactant Dysfunction: An Overlooked Cause of Ventilator-Induced Lung Injury

Ventilator-induced lung injury (VILI) is currently ascribed to volutrauma and/or atelectrauma but the effect of constant tidal volume ventilation (CVTV) has received little attention. This Perspective summarizes the literature documenting that CVTV causes VILI and reviews the mechanisms by which it occurs. Surfactant is continuously inactivated, depleted, displaced or desorbed as a function of the duration of ventilation, the tidal volume, the level of PEEP and possibly the respiratory rate. Accordingly, surfactant must be continuously replenished and secretion primarily depends on intermittent delivery of large ventilatory excursions. The surfactant abnormalities resulting from CVTV result in atelectasis and VILI. While surfactant secretion is reduced by the absence of intermittent deep breaths continuous administration of large tidal volumes depletes surfactant and impairs subsequent secretion. Low or normal lung volumes result in desorption of surfactant. PEEP can be protective by reducing surface film collapse and subsequent film rupture on re-expansion, and/or by reducing surfactant displacement into the airways, but PEEP can also down-regulate surfactant release. Conclusions: The effect of CVTV on surfactant is complex. If attention is not paid to facilitating surfactant secretion and limiting its inactivation, depletion, desorption or displacement surface tension will increase and atelectasis and VILI will occur.

Double-chamber plethysmography versus oscillometry to detect baseline airflow obstruction in a model of asthma in two mouse strains

AIM OF THE STUDY

The current gold standard to assess respiratory mechanics in mice is oscillometry, a technique from which several readouts of the respiratory system can be deduced, such as resistance and elastance. However, these readouts are often not altered in mouse models of asthma. This is in stark contrast with humans, where asthma is generally associated with alterations when assessed by either oscillometry or other techniques. In the present study, we have used double-chamber plethysmography (DCP) to evaluate the breathing pattern and the degree of airflow obstruction in a mouse model of asthma.

MATERIALS AND METHODS

Female C57BL/6 and BALB/c mice were studied at day 1 using DCP, as well as at day 11 using both DCP and oscillometry following a once-daily exposure to either house-dust mite (HDM) or saline for 10 consecutive days.

RESULTS

All DCP readouts used to describe either the breathing pattern (e.g., tidal volume and breathing frequency) or the degree of airflow obstruction (e.g., specific airway resistance) were different between mouse strains at day 1. Most of these strain differences persisted at day 11. Most oscillometric readouts (e.g., respiratory system resistance and elastance) were also different between strains. Changes caused by HDM were obvious with DCP, including decreases in tidal volume, minute ventilation, inspiratory time and mid-tidal expiratory flow and an increase in specific airway resistance. HDM also caused some strain specific alterations in breathing pattern, including increases in expiratory time and end inspiratory pause, which were only observed in C57BL/6 mice. Oscillometry also detected a small but significant increase in tissue elastance in HDM versus saline-exposed mice.

CONCLUSIONS

DCP successfully identified differences between C57BL/6 and BALB/c mice, as well as alterations in mice from both strains exposed to HDM. We conclude that, depending on the study purpose, DCP may sometimes outweigh oscillometry.

Reference Gene Selection for Gene Expression Analyses in Mouse Models of Acute Lung Injury

qRT-PCR still remains the most widely used method for quantifying gene expression levels, although newer technologies such as next generation sequencing are becoming increasingly popular. A critical, yet often underappreciated, problem when analysing qRT-PCR data is the selection of suitable reference genes. This problem is compounded in situations where up to 25% of all genes may change (e.g., due to leukocyte invasion), as is typically the case in ARDS. Here, we examined 11 widely used reference genes for their suitability in commonly used models of acute lung injury (ALI): ventilator-induced lung injury (VILI), in vivo and ex vivo, lipopolysaccharide plus mechanical ventilation (MV), and hydrochloric acid plus MV. The stability of reference gene expression was determined using the NormFinder, BestKeeper, and geNorm algorithms. We then proceeded with the geNorm results because this is the only algorithm that provides the number of reference genes required to achieve normalisation. We chose interleukin-6 (Il-6) and C-X-C motif ligand 1 (Cxcl-1) as the genes of interest to analyse and demonstrate the impact of inappropriate normalisation. Reference gene stability differed between the ALI models and even within the subgroup of VILI models, no common reference gene index (RGI) could be determined. NormFinder, BestKeeper, and geNorm produced slightly different, but comparable results. Inappropriate normalisation of Il-6 and Cxcl1 gene expression resulted in significant misinterpretation in all four ALI settings. In conclusion, choosing an inappropriate normalisation strategy can introduce different kinds of bias such as gain or loss as well as under- or overestimation of effects, affecting the interpretation of gene expression data.

The Role of Macrophages During Mammalian Tissue Remodeling and Regeneration Under Infectious and Non-Infectious Conditions

Several infectious pathologies in humans, such as tuberculosis or SARS-CoV-2, are responsible for tissue or lung damage, requiring regeneration. The regenerative capacity of adult mammals is limited to few organs. Critical injuries of non-regenerative organs trigger a repair process that leads to a definitive architectural and functional disruption, while superficial wounds result in scar formation. Tissue lesions in mammals, commonly studied under non-infectious conditions, trigger cell death at the site of the injury, as well as the production of danger signals favouring the massive recruitment of immune cells, particularly macrophages. Macrophages are also of paramount importance in infected injuries, characterized by the presence of pathogenic microorganisms, where they must respond to both infection and tissue damage. In this review, we compare the processes implicated in the tissue repair of non-infected versus infected injuries of two organs, the skeletal muscles and the lungs, focusing on the primary role of macrophages. We discuss also the negative impact of infection on the macrophage responses and the possible routes of investigation for new regenerative therapies to improve the recovery state as seen with COVID-19 patients.

Metabolic monitoring via on-line analysis of 13C-enriched carbon dioxide in exhaled mouse breath using substrate-integrated hollow waveguide infrared spectroscopy and luminescence sensing combined with Bayesian sampling

In studies that target specific functions or organs, the response is often overlaid by indirect effects of the intervention on global metabolism. The metabolic side of these interactions can be assessed based on total energy expenditure (TEE) and the contributions of the principal energy sources, carbohydrates, proteins and fat to whole body CO₂ production. These parameters can be identified from indirect calorimetry using respiratory oxygen intake and CO₂ dioxide production data that are combined with the response of the ¹³CO₂ release in the expired air and the glucose tracer enrichment in plasma following a ¹³C glucose stable isotope infusion. This concept is applied to a mouse protocol involving anesthesia, mechanical respiration, a disease model, like hemorrhage and therapeutic intervention. It faces challenges caused by a small sample size for both breath and plasma as well as changes in metabolic parameters caused by disease and intervention. Key parameters are derived from multiple measurements, all afflicted with errors that may accumulate leading to unrealistic values. To cope with these challenges, a sensitive on-line breath analysis system based on substrate-integrated hollow waveguide infrared spectroscopy and luminescence (iHWG-IR-LS) was used to monitor gas exchange values. A Bayesian statistical model is developed that uses established equations for indirect calorimetry to predict values for respiratory gas exchange and tracer data that are consistent with the corresponding measurements and also provides statistical error bands for these parameters. With this new methodology, it was possible to estimate important metabolic parameters (respiratory quotient (RQ), relative contribution of carbohydrate, protein and fat oxidation f_carb, f_fat and f_prot , total energy expenditure TEE) in a resolution never available before for a minimal invasive protocol of mice under anesthesia.

Drug class effects on respiratory mechanics in animal models: access and applications

Assessment of respiratory mechanics extends from basic research and animal modeling to clinical applications in humans. However, to employ the applications in human models, it is desirable and sometimes mandatory to study non-human animals first. To acquire further precise and controlled signals and parameters, the animals studied must be further distant from their spontaneous ventilation. The majority of respiratory mechanics studies use positive pressure ventilation to model the respiratory system. In this scenario, a few drug categories become relevant: anesthetics, muscle blockers, bronchoconstrictors, and bronchodilators. Hence, the main objective of this study is to briefly review and discuss each drug category, and the impact of a drug on the assessment of respiratory mechanics. Before and during the positive pressure ventilation, the experimental animal must be appropriately sedated and anesthetized. The sedation will lower the pain and distress of the studied animal and the plane of anesthesia will prevent the pain. With those drugs, a more controlled procedure is carried out; further, because many anesthetics depress the respiratory system activity, a minimum interference of the animal's respiration efforts are achieved. The latter phenomenon is related to muscle blockers, which aim to minimize respiratory artifacts that may interfere with forced oscillation techniques. Generally, the respiratory mechanics are studied under appropriate anesthesia and muscle blockage. The application of bronchoconstrictors is prevalent in respiratory mechanics studies. To verify the differences among studied groups, it is often necessary to challenge the respiratory system, for example, by pharmacologically inducing bronchoconstriction. However, the selected bronchoconstrictor, doses, and administration can affect the evaluation of respiratory mechanics. Although not prevalent, studies have applied bronchodilators to return (airway resistance) to the basal state after bronchoconstriction. The drug categories can influence the mathematical modeling of the respiratory system, systemic conditions, and respiratory mechanics outcomes.

Physiologically variable ventilation reduces regional lung inflammation in a pediatric model of acute respiratory distress syndrome

BACKGROUND

Benefits of variable mechanical ventilation based on the physiological breathing pattern have been observed both in healthy and injured lungs. These benefits have not been characterized in pediatric models and the effect of this ventilation mode on regional distribution of lung inflammation also remains controversial. Here, we compare structural, molecular and functional outcomes reflecting regional inflammation between PVV and conventional pressure-controlled ventilation (PCV) in a pediatric model of healthy lungs and acute respiratory distress syndrome (ARDS).

METHODS

New-Zealand White rabbit pups (n = 36, 670 ± 20 g [half-width 95% confidence interval]), with healthy lungs or after induction of ARDS, were randomized to five hours of mechanical ventilation with PCV or PVV. Regional lung aeration, inflammation and perfusion were assessed using x-ray computed tomography, positron-emission tomography and single-photon emission computed tomography, respectively. Ventilation parameters, blood gases and respiratory tissue elastance were recorded hourly.

RESULTS

Mechanical ventilation worsened respiratory elastance in healthy and ARDS animals ventilated with PCV (11 ± 8%, 6 ± 3%, p < 0.04), however, this trend was improved by PVV (1 ± 4%, - 6 ± 2%). Animals receiving PVV presented reduced inflammation as assessed by lung normalized [¹⁸F]fluorodeoxyglucose uptake in healthy (1.49 ± 0.62 standardized uptake value, SUV) and ARDS animals (1.86 ± 0.47 SUV) compared to PCV (2.33 ± 0.775 and 2.28 ± 0.3 SUV, respectively, p < 0.05), particularly in the well and poorly aerated lung zones. No benefit of PVV could be detected on regional blood perfusion or blood gas parameters.

CONCLUSIONS

Variable ventilation based on a physiological respiratory pattern, compared to conventional pressure-controlled ventilation, reduced global and regional inflammation in both healthy and injured lungs of juvenile rabbits.

Reevaluation of Lung Injury in TNF-Induced Shock: The Role of the Acid Sphingomyelinase

Tumor necrosis factor (TNF) is a well-known mediator of sepsis. In many cases, sepsis results in multiple organ injury including the lung with acute respiratory distress syndrome (ARDS). More than 20-year-old studies have suggested that TNF may be directly responsible for organ injury during sepsis. However, these old studies are inconclusive, because they relied on human rather than conspecific TNF, which was contaminated with endotoxin in most studies. In this study, we characterized the direct effects of intravenous murine endotoxin-free TNF on cardiovascular functions and organ injury in mice with a particular focus on the lungs. Because of the relevance of the acid sphingomyelinase in sepsis, ARDS, and caspase-independent cell death, we also included acid sphingomyelinase-deficient (ASM^−/−) mice. ASM^−/− and wild-type (WT) mice received 50 μg endotoxin-free murine TNF intravenously alone or in combination with the pan-caspase inhibitor carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone (zVAD) and were ventilated at low tidal volume while lung mechanics were followed. Blood pressure was stabilized by intra-arterial fluid support, and body temperature was kept at 37°C to delay lethal shock and to allow investigation of blood gases, lung histopathology, proinflammatory mediators, and microvascular permeability 6 hours after TNF application. Besides the lungs, also the kidneys and liver were examined. TNF elicited the release of inflammatory mediators and a high mortality rate, but failed to injure the lungs, kidneys, or liver of healthy mice significantly within 6 hours. Mortality in WT mice was most likely due to sepsis-like shock, as indicated by metabolic acidosis, high procalcitonin levels, and cardiovascular failure. ASM^−/− mice were protected from TNF-induced hypotension and reflex tachycardia and also from mortality. In WT mice, intravenous exogenous TNF does not cause organ injury but induces a systemic inflammatory response with cardiovascular failure, in which the ASM plays a role.

From bedside to bench: lung ultrasound for the assessment of pulmonary edema in animal models

Traditionally, the lung has been excluded from the ultrasound organ repertoire and, hence, the application of lung ultrasound (LUS) was largely limited to a few enthusiastic clinicians. Yet, in the last decades, the recognition of the previously untapped diagnostic potential of LUS in intensive care medicine has fueled its widespread use as a rapid, non-invasive and radiation-free bedside approach with excellent diagnostic accuracy for many of the most common causes of acute respiratory failure, e.g., cardiogenic pulmonary edema, pneumonia, pleural effusion and pneumothorax. Its increased clinical use has also incited attention for the potential usefulness of LUS in preclinical studies with small animal models mimicking lung congestion and pulmonary edema formation. Application of LUS to small animal models of pulmonary edema may save time, is cost-effective, and may reduce the number of experimental animals due to the possibility of serial evaluations in the same animal as compared with traditional end-point measurements. This review provides an overview of the emerging field of LUS with a specific focus on its application in animal models and highlights future perspectives for LUS in preclinical research.

Relation between Respiratory Mechanics, Inflammation, and Survival in Experimental Mechanical Ventilation

Low-tidal volume (Vt) ventilation might protect healthy lungs from volutrauma but lead to inflammation resulting from other mechanisms, namely alveolar derecruitment and the ensuing alveolar collapse and tidal reexpansion. We hypothesized that the different mechanisms of low- and high-volume injury would be reflected in different mechanical properties being associated with development of pulmonary inflammation and mortality: an increase of hysteresis, reflecting progressive alveolar derecruitment, at low Vt; an increase of elastance, as a result of overdistension, at higher Vt. Mice were allocated to "protective" (6 ml/kg) or "injurious" (15-20 ml/kg) Vt groups and ventilated for 16 hours or until death. We measured elastance and hysteresis; pulmonary IL-6, IL-1β, and MIP-2 (macrophage inflammatory protein 2); wet-to-dry ratio; and blood gases. Survival was greater in the protective group (60%) than in the injurious group (25%). Nonsurvivors showed increased pulmonary cytokines, particularly in the injurious group, with the increase of elastance reflecting IL-6 concentration. Survivors instead showed only modest increases of cytokines, independent of Vt and unrelated to the increase of elastance. No single lung strain threshold could discriminate survivors from nonsurvivors. Hysteresis increased faster in the protective group, but, contrary to our hypothesis, its change was inversely related to the concentration of cytokines. In this model, significant mortality associated with pulmonary inflammation occurred even for strain values as low as about 0.8. Low Vt improved survival. The accompanying increase of hysteresis was not associated with greater inflammation.

Adiponectin deficiency induces mitochondrial dysfunction and promotes endothelial activation and pulmonary vascular injury

Adiponectin (APN), an adipocyte-derived adipokine, has been shown to limit lung injury originating from endothelial cell (EC) damage. Previously we reported that obese mice with low circulatory APN levels exhibited pulmonary vascular endothelial dysfunction. This study was designed to investigate the cellular and molecular mechanisms underlying the pulmonary endothelium-dependent protective effects of APN. Our results demonstrated that in APN-/- mice, there was an inherent state of endothelium mitochondrial dysfunction that could contribute to endothelial activation and increased susceptibility to LPS-induced acute lung injury (ALI). We noted that APN-/- mice showed decreased expression of mitochondrial biogenesis regulatory protein peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α) and its downstream proteins nuclear respiratory factor 1, transcription factor A, mitochondrial, and Sirtuin (Sirt)3 and Sirt1 expression in whole lungs and in freshly isolated lung ECs from these mice at baseline and subjected to LPS-induced ALI. We further showed that treating APN-/- mice with PGC-1α activator pyrroloquinoline quinone enhances mitochondrial biogenesis and function in lung endothelium and attenuation of ALI. These results suggest that the pulmonary endothelium-protective properties of APN are mediated, at least in part, by an enhancement of mitochondrial biogenesis through a mechanism involving PGC-1α activation.-Shah, D., Torres, C., Bhandari, V. Adiponectin deficiency induces mitochondrial dysfunction and promotes endothelial activation and pulmonary vascular injury.

Alveolar dynamics during mechanical ventilation in the healthy and injured lung

Mechanical ventilation is a life-saving therapy in patients with acute respiratory distress syndrome (ARDS). However, mechanical ventilation itself causes severe co-morbidities in that it can trigger ventilator-associated lung injury (VALI) in humans or ventilator-induced lung injury (VILI) in experimental animal models. Therefore, optimization of ventilation strategies is paramount for the effective therapy of critical care patients. A major problem in the stratification of critical care patients for personalized ventilation settings, but even more so for our overall understanding of VILI, lies in our limited insight into the effects of mechanical ventilation at the actual site of injury, i.e., the alveolar unit. Unfortunately, global lung mechanics provide for a poor surrogate of alveolar dynamics and methods for the in-depth analysis of alveolar dynamics on the level of individual alveoli are sparse and afflicted by important limitations. With alveolar dynamics in the intact lung remaining largely a "black box," our insight into the mechanisms of VALI and VILI and the effectiveness of optimized ventilation strategies is confined to indirect parameters and endpoints of lung injury and mortality.In the present review, we discuss emerging concepts of alveolar dynamics including alveolar expansion/contraction, stability/instability, and opening/collapse. Many of these concepts remain still controversial, in part due to limitations of the different methodologies applied. We therefore preface our review with an overview of existing technologies and approaches for the analysis of alveolar dynamics, highlighting their individual strengths and limitations which may provide for a better appreciation of the sometimes diverging findings and interpretations. Joint efforts combining key technologies in identical models to overcome the limitations inherent to individual methodologies are needed not only to provide conclusive insights into lung physiology and alveolar dynamics, but ultimately to guide critical care patient therapy.

Variable Ventilation Is Equally Effective as Conventional Pressure Control Ventilation for Optimizing Lung Function in a Rabbit Model of ARDS

Background

Introducing mathematically derived variability (MVV) into the otherwise monotonous conventional mechanical ventilation has been suggested to improve lung recruitment and gas exchange. Although the application of a ventilation pattern based on variations in physiological breathing (PVV) is beneficial for healthy lungs, its value in the presence of acute respiratory distress syndrome (ARDS) has not been characterized. We therefore aimed at comparing conventional pressure-controlled ventilation with (PCS) or without regular sighs (PCV) to MVV and PVV at two levels of positive end-expiratory pressure (PEEP) in a model of severe ARDS.

Methods

Anesthetised rabbits (n = 54) were mechanically ventilated and severe ARDS (PaO₂/FiO₂ ≤ 150 mmHg) was induced by combining whole lung lavage, i.v. endotoxin and injurious ventilation. Rabbits were then randomly assigned to be ventilated with PVV, MVV, PCV, or PCS for 5 h while maintaining either 6 or 9 cmH₂O PEEP. Ventilation parameters, blood gas indices and respiratory mechanics (tissue damping, G, and elastance, H) were recorded hourly. Serum cytokine levels were assessed with ELISA and lung histology was analyzed.

Results

Although no progression of lung injury was observed after 5 h of ventilation at PEEP 6 cmH₂O with PVV and PCV, values for G (58.8 ± 71.1[half-width of 95% CI]% and 40.8 ± 39.0%, respectively), H (54.5 ± 57.2%, 50.7 ± 28.3%), partial pressure of carbon-dioxide (PaCO₂, 43.9 ± 23.8%, 46.2 ± 35.4%) and pH (−4.6 ± 3.3%, −4.6 ± 2.2%) worsened with PCS and MVV. Regardless of ventilation pattern, application of a higher PEEP improved lung function and precluded progression of lung injury and inflammation. Histology lung injury scores were elevated in all groups with no difference between groups at either PEEP level.

Conclusion

At moderate PEEP, variable ventilation based on a pre-recorded physiological breathing pattern protected against progression of lung injury equally to the conventional pressure-controlled mode, whereas mathematical variability or application of regular sighs caused worsening in lung mechanics. This outcome may be related to the excessive increases in peak inspiratory pressure with the latter ventilation modes. However, a greater benefit on respiratory mechanics and gas exchange could be obtained by elevating PEEP, compared to the ventilation mode in severe ARDS.

Preclinical septic shock research: why we need an animal ICU

Animal experiments are widely used in preclinical medical research with the goal of disease modeling and exploration of novel therapeutic approaches. In the context of sepsis and septic shock, the translation into clinical practice has been disappointing. Classical animal models of septic shock usually involve one-sex-one-age animal models, mostly in mice or rats, contrasting with the heterogeneous population of septic shock patients. Many other factors limit the reliability of preclinical models and may contribute to preclinical research failure in critical care, including the host specificity of several pathogens, the fact that laboratory animals are raised in pathogen-free facilities and that organ support techniques are either absent or minimal. Advanced animal models have been developed with the aim of improving the clinical translatability of experimental findings. So-called animal ICUs refer to the preclinical investigation of adult or even aged animals of either sex, using—in case of rats and mice—miniaturized equipment allowing for reproducing an ICU environment at a small animal scale and integrating chronic comorbidities to more closely reflect the clinical conditions studied. Strength and limitations of preclinical animal models designed to decipher the mechanisms involved in septic cardiomyopathy are discussed. This article reviews the current status and the challenges of setting up an animal ICU.

Retrograde perfusion in isolated perfused mouse lungs-Feasibility and effects on cytokine levels and pulmonary oedema formation

Retrograde lung vascular perfusion can appear in high-risk surgeries. The present report is the first to study long-term retrograde perfusion of isolated perfused mouse lungs (IPLs) and to use the tyrosine kinase ephB4 and its ligand ephrinB2 as potential markers for acute lung injury. Mouse lungs were subjected to anterograde or retrograde perfusion with normal-pressure ventilation (NV) or high-pressure ventilation (=overventilation, OV) for 4 hours. Outcome parameters were cytokine, ephrinB2 and ephB4 levels in perfusate samples and bronchoalveolar lavage (BAL), and the wet-to-dry ratio. Anterograde perfusion was feasible for 4 hours, while lungs receiving retrograde perfusion presented considerable collapse rates. Retrograde perfusion resulted in an increased wet-to-dry ratio when combined with high-pressure ventilation; other physiological parameters were not affected. Cytokine levels in BAL and perfusate, as well as levels of soluble ephB4 in BAL were increased in OV, while soluble ephrinB2 BAL levels were increased in retrograde perfusion. BAL levels of ephrinB2 and ephB4 were also determined in vivo, including mice ventilated for 7 hours with normal-volume ventilation (NVV) or high-volume ventilation (HVV) with increased levels of ephB4 in HVV BAL compared to NVV. Retrograde perfusion in IPL is limited as a routine method to investigate effects due to collapse for yet unclear reasons. If successful, retrograde perfusion has an influence on pulmonary oedema formation. In BAL, ephrinB2 seems to be up-regulated by flow reversal, while ephB4 is a marker for acute lung injury.

Initiation of LPS-induced pulmonary dysfunction and its recovery occur independent of T cells

Background

The acute respiratory distress syndrome (ARDS) is a serious disease in critically ill patients that is characterized by pulmonary dysfunctions, hypoxemia and significant mortality. Patients with immunodeficiency (e.g. SCID with T and B cell deficiency) are particularly susceptible to the development of severe ARDS. However, the role of T cells on pulmonary dysfunctions in immune-competent patients with ARDS is only incompletely understood.

Methods

Wild-type (wt) and RAG2^−/− mice (lymphocyte deficient) received intratracheal instillations of LPS (4 mg/kg) or saline. On day 1, 4 and 10 lung mechanics and bronchial hyperresponsiveness towards acetylcholine were measured with the flexiVent ventilation set-up. The bronchoalveolar lavage fluid (BALF) was examined for leukocytes (FACS analysis) and pro-inflammatory cytokines (ELISA).

Results

In wt mice, lung mechanics, body weight and body temperature deteriorated in the LPS-group during the early phase (up to d4); these alterations were accompanied by increased leukocyte numbers and inflammatory cytokine levels in the BALF. During the late phase (day 10), both lung mechanics and the cell/cytokine homeostasis recovered in LPS-treated wt mice. RAG2^−/− mice experienced changes in body weight, lung mechanics, BAL neutrophil numbers, BAL inflammatory cytokines levels that were comparable to wt mice.

Conclusion

Following LPS instillation, lung mechanics deteriorate within the first 4 days and recover towards day 10. This response is not altered by the lack of T lymphocytes suggesting that T cells play only a minor role for the initiation, propagation or recovery of LPS-induced lung dysfunctions or function of T lymphocytes can be compensated by other immune cells, such as alveolar macrophages.

Electronic supplementary material

The online version of this article (10.1186/s12890-018-0741-2) contains supplementary material, which is available to authorized users.

Advanced Photonic Sensors Based on Interband Cascade Lasers for Real-Time Mouse Breath Analysis

A multiparameter gas sensor based on distributed feedback interband cascade lasers emitting at 4.35 μm and ultrafast electro-spun luminescence oxygen sensors has been developed for the quantification and continuous monitoring of ¹³CO₂/¹²CO₂ isotopic ratio changes and oxygen in exhaled mouse breath samples. Mid-infrared absorption spectra for quantitatively monitoring the enrichment of ¹³CO₂ levels were recorded in a miniaturized dual-channel substrate-integrated hollow waveguide using balanced ratiometric detection, whereas luminescence quenching was used for synchronously detecting exhaled oxygen levels. Allan variance analysis verified a CO₂ measurement precision of 1.6‰ during a 480 s integration time. Routine online monitoring of exhaled mouse breath was performed in 14 mechanically ventilated and instrumented mice and demonstrated the feasibility of online isotope-selective exhaled breath analysis within microliters of probed gas samples using the reported combined sensor platform.

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.

Inflammatory processes during acute respiratory distress syndrome: a complex system

PURPOSE OF REVIEW

ARDS is a severe pulmonary disease characterized by inflammation. However, inflammation-directed therapies have yet failed to improve the outcome in ARDS patients. One of the reasons may be the underestimated complexity of inflammation. Here, we summarize recent insights into the complex interrelations between inflammatory circuits.

RECENT FINDINGS

Gene expression analysis from animal models or from patients with ARDS, sepsis or trauma show an enormous number of differentially expressed genes with highly significant overlaps between the various conditions. These similarities, however, should not obscure the complexity of inflammation. We suggest to consider inflammation in ARDS as a system controlled by scale-free networks of genome-wide molecular interaction with hubs (e.g. NFκB, C/EBPβ, ATF3), exhibiting nonlinear emergence and the ability to adapt, meaning for instance that mild and life-threatening inflammation in ARDS are distinct processes. In order to comprehend this complex system, it seems necessary to combine model-driven simulations, data-driven modelling and hypothesis-driven experimental studies. Recent experimental studies have illustrated how several regulatory circuits interact during pulmonary inflammation, including the resolution of inflammation, the inflammasome, autophagy and apoptosis.

SUMMARY

We suggest that therapeutic interventions in ARDS should be based on a systems approach to inflammation.

Difficulties in modelling ARDS (2017 Grover Conference Series)

Fifty years after the first description of acute respiratory distress syndrome (ARDS), none of the many positive drug studies in animal models have been confirmed in clinical trials and translated into clinical practice. This bleak outcome of so many animal experiments shows how difficult it is to model ARDS. Lungs from patients are characterized by hyperinflammation, permeability edema, and hypoxemia; accordingly, this is what most models aim to reproduce. However, in animal models it is very easy to cause inflammation in the lungs, but difficult to cause hypoxemia. Often - and not unlike in patients - models with hypoxemia are accompanied by cardiovascular failure that necessitates fluid support and ventilation, raising the question as to the role of intensive care measures in models of ARDS. In our opinion, there are two major arguments in favor of modelling intensive care medicine in models of ARDS: (1) preventing death from shock; and (2) modelling ventilation and other ICU measures as a second hit. The preferable predictive endpoints in any model of ARDS remain unclear. At present, the best recommendation is to use endpoints that can be compared across studies (i.e. PaO₂/FiO₂ ratio, compliance, wet-to-dry weight ratio) rather than percentage data. Another important and often overlooked issue is the fact that the thermoneutral environmental temperatures for mice and rats are 30℃ and 28℃, respectively; thus, at room temperature (20-22℃) they suffer from cold stress with the associated significant metabolic changes. While, by definition, any model is an abstraction, we suggest that clinically relevant models of ARDS will have to closer recapitulate important properties of the disease while taking into account species-specific confounders.

Role of the Purinergic Receptor P2XR4 After Blunt Chest Trauma in Cigarette Smoke-Exposed Mice

Both acute and chronic lung injury are associated with up-regulation of the pulmonary expression of the purinergic receptors P2XR4 and P2XR7. Genetic deletion or blockade of P2XR7 attenuated pulmonary hyperinflammation, but simultaneous P2XR4 up-regulation compensated for P2XR7 deletion. Therefore, we tested the hypothesis whether genetic P2XR4 deletion would attenuate the pulmonary inflammatory response and thereby improve organ function after blunt chest trauma in mice with and without pretraumatic cigarette smoke (CS) exposure.After 3 weeks to 4 weeks of exposure to CS, anesthetized wildtype or P2XR4 mice (n = 32) underwent a blast wave-induced blunt chest trauma followed by 4 h of lung-protective mechanical ventilation, fluid resuscitation, and noradrenaline support to maintain mean arterial pressure >55 mm Hg. Hemodynamics, lung mechanics, gas exchange, and acid-base status were measured together with blood and tissue cytokine and chemokine concentrations, heme oxygenase-1, B-cell lymphoma-extra large (Bcl-xL), endogenous nuclear factor-κB inhibitor (IκBα) expression, nitrotyrosine formation, purinergic receptor expression, and histological scoring.Despite a significant increase in the histopathology score in both CS-exposed groups, neither CS exposure nor P2XR4 deletion had any significant effect on post-traumatic pulmonary function and inflammatory response. However, P2XR4 deletion was associated with attenuated impairment of glucose homeostasis and acid-base-status after CS exposure and chest trauma.In conclusion, genetic P2XR4 deletion failed to attenuate the acute post-traumatic pulmonary inflammatory response. The improved glucose homeostasis and acid-base-status after CS exposure in the P2XR4 group was possibly due to less alveolar hypoxia-induced right ventricular remodeling resulting in preserved liver metabolic capacity.

Impact of hyperglycemia on cystathionine-γ-lyase expression during resuscitated murine septic shock

BACKGROUND

Cystathionine-γ-lyase (CSE) was shown to have a regulatory role in glucose metabolism. Circulatory shock can induce metabolic stress, thereby leading to hyperglycemia and mitochondrial dysfunction. In vitro data suggest an effect of high glucose on CSE expression. Therefore, the aim of this study was to investigate the effects of hyperglycemia on CSE expression in resuscitated murine septic shock.

METHODS

Normo- (80-150 mg/dl) and hyperglycemic (>200 mg/dl) male C57/BL6J mice (n = 5-6 per group) underwent cecal ligation and puncture (CLP)-induced polymicrobial sepsis or sham procedure (n = 6 per group) and, 15 h afterwards, were anesthetized again, surgically instrumented and received intensive care treatment, including antibiotics, lung protective mechanical ventilation, circulatory support, and intravenous (i.v.) glucose infusion (50% as stable-isotope labeled 1,2,3,4,5,6-13C6 glucose). Blood and breath gas were sampled hourly to quantify parameters of glucose metabolism. 5 h later, mice were sacrificed and organs were harvested. The liver mitochondrial respiratory activity was determined via high resolution respirometry; CSE, peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α), and adipocyte differentiation-related protein (ADRP) expression was immunohistochemically investigated.

RESULTS

In sepsis combined with hyperglycemia the least CSE and PGC1α expression could be detected, along with reduced mitochondrial respiratory activity, and enhanced ADRP expression, a marker of lipid droplet formation, in the liver. A novel in vivo finding is the CSE translocation from the cytosol to the nucleus triggered by metabolic stress.

CONCLUSIONS

A relationship between CSE and glucose metabolism was established, which, when dysregulated, may contribute to fatty liver disease and hepatic steatosis.

Effects of positive end-expiratory pressure and recruitment maneuvers in a ventilator-induced injury mouse model

Background

Positive-pressure mechanical ventilation is an essential therapeutic intervention, yet it causes the clinical syndrome known as ventilator-induced lung injury. Various lung protective mechanical ventilation strategies have attempted to reduce or prevent ventilator-induced lung injury but few modalities have proven effective. A model that isolates the contribution of mechanical ventilation on the development of acute lung injury is needed to better understand biologic mechanisms that lead to ventilator-induced lung injury.

Objectives

To evaluate the effects of positive end-expiratory pressure and recruitment maneuvers in reducing lung injury in a ventilator-induced lung injury murine model in short- and longer-term ventilation.

Methods

5–12 week-old female BALB/c mice (n = 85) were anesthetized, placed on mechanical ventilation for either 2 hrs or 4 hrs with either low tidal volume (8 ml/kg) or high tidal volume (15 ml/kg) with or without positive end-expiratory pressure and recruitment maneuvers.

Results

Alteration of the alveolar-capillary barrier was noted at 2 hrs of high tidal volume ventilation. Standardized histology scores, influx of bronchoalveolar lavage albumin, proinflammatory cytokines, and absolute neutrophils were significantly higher in the high-tidal volume ventilation group at 4 hours of ventilation. Application of positive end-expiratory pressure resulted in significantly decreased standardized histology scores and bronchoalveolar absolute neutrophil counts at low- and high-tidal volume ventilation, respectively. Recruitment maneuvers were essential to maintain pulmonary compliance at both 2 and 4 hrs of ventilation.

Conclusions

Signs of ventilator-induced lung injury are evident soon after high tidal volume ventilation (as early as 2 hours) and lung injury worsens with longer-term ventilation (4 hrs). Application of positive end-expiratory pressure and recruitment maneuvers are protective against worsening VILI across all time points. Dynamic compliance can be used guide the frequency of recruitment maneuvers to help ameloriate ventilator-induced lung injury.

One-hit Models of Ventilator-induced Lung Injury: Benign Inflammation versus Inflammation as a By-product

BACKGROUND

One important explanation for the detrimental effects of conventional mechanical ventilation is the biotrauma hypothesis that ventilation may trigger proinflammatory responses that subsequently cause lung injury. This hypothesis has frequently been studied in so-called one-hit models (overventilation of healthy lungs) that so far have failed to establish an unequivocal link between inflammation and hypoxemic lung failure. This study was designed to develop a one-hit biotrauma model.

METHODS

Mice (six per group) were ventilated for up to 7 h (positive end-expiratory pressure 2 cm H2O) and received 300 μl/h fluid support. Series_1: initial plateau pressures of 10, 24, 27, or 30 cm H2O. Series_2: ventilation with pressure release at 34 cm H2O and initial plateau pressure of 10, 24, 27, or 30 cm H2O. To study the significance of inflammation, the latter groups were also pretreated with the steroid dexamethasone.

RESULTS

Within 7 h, 20 of 24 mice ventilated with plateau pressure of 27 cm H2O or more died of a catastrophic lung failure characterized by strongly increased proinflammatory markers and a precipitous decrease in pulmonary compliance, blood pressure, and oxygenation. Pretreatment with dexamethasone reduced inflammation, but prolonged median survival time by 30 min.

CONCLUSIONS

Our findings demonstrate a sharp distinction between ventilation with 24 cm H2O that was well tolerated and ventilation with 27 cm H2O that was lethal for most animals due to catastrophic lung failure. In the former case, inflammation was benign and in the latter, a by-product that only accelerated lung failure. The authors suggest that biotrauma-when defined as a ventilation-induced and inflammation-dependent hypoxemia-is difficult to study in murine one-hit models of ventilation, at least not within 7 h. (Anesthesiology 2017; 126:909-22).

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.

Biological Impact of Transpulmonary Driving Pressure in Experimental Acute Respiratory Distress Syndrome

Background:

Ventilator-induced lung injury has been attributed to the interaction of several factors: tidal volume (VT), positive end-expiratory pressure (PEEP), transpulmonary driving pressure (difference between transpulmonary pressure at end-inspiration and end-expiration, ΔP,L), and respiratory system plateau pressure (Pplat,rs).

Methods:

Forty-eight Wistar rats received Escherichia coli lipopolysaccharide intratracheally. After 24 h, animals were randomized into combinations of VT and PEEP, yielding three different ΔP,L levels: ΔP,LLOW (VT = 6 ml/kg, PEEP = 3 cm H2O); ΔP,LMEAN (VT = 13 ml/kg, PEEP = 3 cm H2O or VT = 6 ml/kg, PEEP = 9.5 cm H2O); and ΔP,LHIGH (VT = 22 ml/kg, PEEP = 3 cm H2O or VT = 6 ml/kg, PEEP = 11 cm H2O). In other groups, at low VT, PEEP was adjusted to obtain a Pplat,rs similar to that achieved with ΔP,LMEAN and ΔP,LHIGH at high VT.

Results:

At ΔP,LLOW, expressions of interleukin (IL)-6, receptor for advanced glycation end products (RAGE), and amphiregulin were reduced, despite morphometric evidence of alveolar collapse. At ΔP,LHIGH (VT = 6 ml/kg and PEEP = 11 cm H2O), lungs were fully open and IL-6 and RAGE were reduced compared with ΔP,LMEAN (27.4 ± 12.9 vs. 41.6 ± 14.1 and 0.6 ± 0.2 vs. 1.4 ± 0.3, respectively), despite increased hyperinflation and amphiregulin expression. At ΔP,LMEAN (VT = 6 ml/kg and PEEP = 9.5 cm H2O), when PEEP was not high enough to keep lungs open, IL-6, RAGE, and amphiregulin expression increased compared with ΔP,LLOW (41.6 ± 14.1 vs. 9.0 ± 9.8, 1.4 ± 0.3 vs. 0.6 ± 0.2, and 6.7 ± 0.8 vs. 2.2 ± 1.0, respectively). At Pplat,rs similar to that achieved with ΔP,LMEAN and ΔP,LHIGH, higher VT and lower PEEP reduced IL-6 and RAGE expression.

Conclusion:

In the acute respiratory distress syndrome model used in this experiment, two strategies minimized ventilator-induced lung injury: (1) low VT and PEEP, yielding low ΔP,L and Pplat,rs; and (2) low VT associated with a PEEP level sufficient to keep the lungs open.

Blunt Chest Trauma in Mice after Cigarette Smoke-Exposure: Effects of Mechanical Ventilation with 100% O2

Cigarette smoking (CS) aggravates post-traumatic acute lung injury and increases ventilator-induced lung injury due to more severe tissue inflammation and apoptosis. Hyper-inflammation after chest trauma is due to the physical damage, the drop in alveolar PO₂, and the consecutive hypoxemia and tissue hypoxia. Therefore, we tested the hypotheses that 1) CS exposure prior to blunt chest trauma causes more severe post-traumatic inflammation and thereby aggravates lung injury, and that 2) hyperoxia may attenuate this effect. Immediately after blast wave-induced blunt chest trauma, mice (n=32) with or without 3-4 weeks of CS exposure underwent 4 hours of pressure-controlled, thoraco-pulmonary compliance-titrated, lung-protective mechanical ventilation with air or 100 % O₂. Hemodynamics, lung mechanics, gas exchange, and acid-base status were measured together with blood and tissue cytokine and chemokine concentrations, heme oxygenase-1 (HO-1), activated caspase-3, and hypoxia-inducible factor 1-α (HIF-1α) expression, nuclear factor-κB (NF-κB) activation, nitrotyrosine formation, purinergic receptor 2X₄ (P2XR₄) and 2X₇ (P2XR₇) expression, and histological scoring. CS exposure prior to chest trauma lead to higher pulmonary compliance and lower PaO₂ and Horovitz-index, associated with increased tissue IL-18 and blood MCP-1 concentrations, a 2-4-fold higher inflammatory cell infiltration, and more pronounced alveolar membrane thickening. This effect coincided with increased activated caspase-3, nitrotyrosine, P2XR₄, and P2XR₇ expression, NF-κB activation, and reduced HIF-1α expression. Hyperoxia did not further affect lung mechanics, gas exchange, pulmonary and systemic cytokine and chemokine concentrations, or histological scoring, except for some patchy alveolar edema in CS exposed mice. However, hyperoxia attenuated tissue HIF-1α, nitrotyrosine, P2XR₇, and P2XR₄ expression, while it increased HO-1 formation in CS exposed mice. Overall, CS exposure aggravated post-traumatic inflammation, nitrosative stress and thereby organ dysfunction and injury; short-term, lung-protective, hyperoxic mechanical ventilation have no major beneficial effect despite attenuation of nitrosative stress, possibly due to compensation of by regional alveolar hypoxia and/or consecutive hypoxemia, resulting in down-regulation of HIF-1α expression.

Intraoperative mechanical ventilation strategies for obese patients: a systematic review and network meta-analysis

Several intraoperative ventilation strategies are available for obese patients. However, the same ventilation interventions have exhibited different effects on PaO2 /FIO2 concerning obese patients in different trials, and the issue remains controversial. Therefore, we conducted a network meta-analysis to identify the optimal mechanical ventilation strategy. We searched the Cochrane Central Register of Controlled Trials (CENTRAL) in the Cochrane Library, Embase, MEDLINE, CINAHL and Web of Science for studies published up to June 2014, and the PaO2 /FIO2 in obese patients given different mechanical ventilation strategies was assessed. We assessed the studies for eligibility and extracted data and then pooled the data and used a Bayesian fixed-effect model to combine direct comparisons with indirect evidence. Eligible studies evaluated different ventilation strategies for obese patients and reported the intraoperative PaO2 /FIO2 ratio, atelectasis and pulmonary compliance. Thirteen randomized controlled trials were included for network meta-analysis, including 476 patients who received 1 of 12 ventilation strategies. Volume-controlled ventilation with higher PEEP plus single recruitment manoeuvres (VCV + higher PEEP + single RM) was associated with the highest PaO2 /FiO2 ratio, improving intraoperative pulmonary compliance and reducing the incidence of intraoperative atelectasis.

Kinetic profiling of in vivo lung cellular inflammatory responses to mechanical ventilation

Mechanical ventilation, through overdistension of the lung, induces substantial inflammation that is thought to increase mortality among critically ill patients. The mechanotransduction processes involved in converting lung distension into inflammation during this ventilator-induced lung injury (VILI) remain unclear, although many cell types have been shown to be involved in its pathogenesis. This study aimed to identify the profile of in vivo lung cellular activation that occurs during the initiation of VILI. This was achieved using a flow cytometry-based method to quantify the phosphorylation of several markers (p38, ERK1/2, MAPK-activated protein kinase 2, and NF-κB) of inflammatory pathway activation within individual cell types. Anesthetized C57BL/6 mice were ventilated with low (7 ml/kg), intermediate (30 ml/kg), or high (40 ml/kg) tidal volumes for 1, 5, or 15 min followed by immediate fixing and processing of the lungs. Surprisingly, the pulmonary endothelium was the cell type most responsive to in vivo high-tidal-volume ventilation, demonstrating activation within just 1 min, followed by the alveolar epithelium. Alveolar macrophages were the slowest to respond, although they still demonstrated activation within 5 min. This order of activation was specific to VILI, since intratracheal lipopolysaccharide induced a very different pattern. These results suggest that alveolar macrophages may become activated via a secondary mechanism that occurs subsequent to activation of the parenchyma and that the lung cellular activation mechanism may be different between VILI and lipopolysaccharide. Our data also demonstrate that even very short periods of high stretch can promote inflammatory activation, and, importantly, this injury may be immediately manifested within the pulmonary vasculature.

Increasing the inspiratory time and I:E ratio during mechanical ventilation aggravates ventilator-induced lung injury in mice

Introduction

Lung-protective ventilation reduced acute respiratory distress syndrome (ARDS) mortality. To minimize ventilator-induced lung injury (VILI), tidal volume is limited, high plateau pressures are avoided, and positive end-expiratory pressure (PEEP) is applied. However, the impact of specific ventilatory patterns on VILI is not well defined. Increasing inspiratory time and thereby the inspiratory/expiratory ratio (I:E ratio) may improve oxygenation, but may also be harmful as the absolute stress and strain over time increase. We thus hypothesized that increasing inspiratory time and I:E ratio aggravates VILI.

Methods

VILI was induced in mice by high tidal-volume ventilation (HV_T 34 ml/kg). Low tidal-volume ventilation (LV_T 9 ml/kg) was used in control groups. PEEP was set to 2 cm H₂O, FiO₂ was 0.5 in all groups. HV_T and LV_T mice were ventilated with either I:E of 1:2 (LV_T 1:2, HV_T 1:2) or 1:1 (LV_T 1:1, HV_T 1:1) for 4 hours or until an alternative end point, defined as mean arterial blood pressure below 40 mm Hg. Dynamic hyperinflation due to the increased I:E ratio was excluded in a separate group of animals. Survival, lung compliance, oxygenation, pulmonary permeability, markers of pulmonary and systemic inflammation (leukocyte differentiation in lung and blood, analyses of pulmonary interleukin-6, interleukin-1β, keratinocyte-derived chemokine, monocyte chemoattractant protein-1), and histopathologic pulmonary changes were analyzed.

Results

LV_T 1:2 or LV_T 1:1 did not result in VILI, and all individuals survived the ventilation period. HV_T 1:2 decreased lung compliance, increased pulmonary neutrophils and cytokine expression, and evoked marked histologic signs of lung injury. All animals survived. HV_T 1:1 caused further significant worsening of oxygenation, compliance and increased pulmonary proinflammatory cytokine expression, and pulmonary and blood neutrophils. In the HV_T 1:1 group, significant mortality during mechanical ventilation was observed.

Conclusion

According to the “baby lung” concept, mechanical ventilation-associated stress and strain in overinflated regions of ARDS lungs was simulated by using high tidal-volume ventilation. Increase of inspiratory time and I:E ratio significantly aggravated VILI in mice, suggesting an impact of a “stress/strain × time product” for the pathogenesis of VILI. Thus increasing the inspiratory time and I:E ratio should be critically considered.

Electronic supplementary material

The online version of this article (doi:10.1186/s13054-015-0759-2) contains supplementary material, which is available to authorized users.

Interplay between nuclear factor erythroid 2-related factor 2 and amphiregulin during mechanical ventilation

Mechanical ventilation (MV) elicits complex and clinically relevant cellular responses in the lungs. The current study was designed to define the role of the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2), a major regulator of the cellular antioxidant defense system, in the pulmonary response to MV. Nrf2 activity was quantified in ventilated isolated perfused mouse lungs (IPL). Regulation of amphiregulin (AREG) was investigated in BEAS-2B cells with inactivated Nrf2 or Keap1, the inhibitor of Nrf2, using a luciferase vector with AREG promoter. AREG-dependent Nrf2 activity was examined in BEAS-2B cells, murine precision-cut lung slices (PCLS), and IPL. Finally, Nrf2 knockout and wild-type mice were ventilated to investigate the interplay between Nrf2 and AREG during MV in vivo. Lung functions and inflammatory parameters were measured. Nrf2 was activated in a ventilation-dependent manner. The knockdown of Nrf2 and Keap1 via short hairpin RNA in BEAS-2B cells and an EMSA with lung tissue revealed that AREG is regulated by Nrf2. Conversely, AREG application induced a significant Nrf2 activation in BEAS-2B cells, PCLS, and IPL. The signal transduction of ventilation-induced Nrf2 activation was shown to be p38 MAP kinase-dependent. In vivo ventilation experiments indicated that AREG is regulated by Nrf2 during MV. We conclude that Areg expression is regulated by Nrf2. During high-pressure ventilation, Nrf2 becomes activated and induces AREG, leading to a positive feedback loop between Nrf2 and AREG, which involves the p38 MAPK and results in the expression of cytoprotective genes.

Ventilation with lower tidal volumes for critically ill patients without the acute respiratory distress syndrome: a systematic translational review and meta-analysis

PURPOSE OF REVIEW

There is convincing evidence for benefit from lung-protective mechanical ventilation with lower tidal volumes in patients with the acute respiratory distress syndrome (ARDS). It is uncertain whether this strategy benefits critically ill patients without ARDS as well. This manuscript systematically reviews recent preclinical studies of ventilation in animals with uninjured lungs, and clinical trials of ventilation in ICU patients without ARDS on the association between tidal volume size and pulmonary complications and outcome.

RECENT FINDINGS

Successive preclinical studies almost without exception show that ventilation with lower tidal volumes reduces the injurious effects of ventilation in animals with uninjured lungs. This finding is in line with results from recent trials in ICU patients without ARDS, demonstrating that ventilation with lower tidal volumes has a strong potential to prevent development of pulmonary complications and maybe even to improve survival. However, evidence mostly comes from nonrandomized clinical trials, and concerns are expressed regarding unselected use of lower tidal volumes in the ICU, that is, in all ventilated critically ill patients, since this strategy could also increase needs for sedation and/or neuromuscular blockade, and maybe even cause respiratory muscle fatigue. These all then could in fact worsen outcome, possibly counteracting the beneficial effects of ventilation with lower tidal volumes.

SUMMARY

Ventilation with lower tidal volumes protects against pulmonary complications, but well-powered randomized controlled trials are urgently needed to determine whether this ventilation strategy truly benefits all ventilated ICU patients without ARDS.

Effects of various modes of mechanical ventilation in normal rats

BACKGROUND

Recent studies in healthy mice and rats have reported that positive pressure ventilation delivered with physiological tidal volumes at normal end-expiratory volume worsens lung mechanics and induces cytokine release, thus suggesting that detrimental effects are due to positive pressure ventilation per se. The aim of this study in healthy animals is to assess whether these adverse outcomes depend on the mode of mechanical ventilation.

METHODS

Rats were subjected to 4 h of spontaneous, positive pressure, and whole-body or thorax-only negative pressure ventilation (N = 8 per group). In all instances the ventilatory pattern was that of spontaneous breathing. Lung mechanics, cytokines concentration in serum and broncho-alveolar lavage fluid, lung wet-to-dry ratio, and histology were assessed. Values from eight animals euthanized shortly after anesthesia served as control.

RESULTS

No evidence of mechanical ventilation-dependent lung injury was found in terms of lung mechanics, histology, or wet-to-dry ratio. Relative to control, cytokine levels and recruitment of polymorphonuclear leucocytes increased slightly, and to the same extent with spontaneous, positive pressure, and whole-body negative pressure ventilation. Thorax-only negative pressure ventilation caused marked chest and lung distortion, reversible increase of lung elastance, and higher polymorphonuclear leucocyte count and cytokine levels.

CONCLUSION

Both positive and negative pressure ventilation performed with tidal volumes and timing of spontaneous, quiet breathing neither elicit an inflammatory response nor cause morpho-functional alterations in normal animals, thus supporting the notion of the presence of a critical volume threshold above which acute lung injury ensues. Distortion of lung parenchyma can induce an inflammatory response, even in the absence of volotrauma.

Negative pressure ventilation and positive pressure ventilation promote comparable levels of ventilator-induced diaphragmatic dysfunction in rats

BACKGROUND

Mechanical ventilation is a life-saving intervention for patients with respiratory failure. Unfortunately, a major complication associated with prolonged mechanical ventilation is ventilator-induced diaphragmatic atrophy and contractile dysfunction, termed ventilator-induced diaphragmatic dysfunction (VIDD). Emerging evidence suggests that positive pressure ventilation (PPV) promotes lung damage (ventilator-induced lung injury [VILI]), resulting in the release of signaling molecules that foster atrophic signaling in the diaphragm and the resultant VIDD. Although a recent report suggests that negative pressure ventilation (NPV) results in less VILI than PPV, it is unknown whether NPV can protect against VIDD. Therefore, the authors tested the hypothesis that compared with PPV, NPV will result in a lower level of VIDD.

METHODS

Adult rats were randomly assigned to one of three experimental groups (n = 8 each): (1) acutely anesthetized control (CON), (2) 12 h of PPV, and (3) 12 h of NPV. Dependent measures included indices of VILI, diaphragmatic muscle fiber cross-sectional area, diaphragm contractile properties, and the activity of key proteases in the diaphragm.

RESULTS

Our results reveal that no differences existed in the degree of VILI between PPV and NPV animals as evidenced by VILI histological scores (CON = 0.082 ± 0.001; PPV = 0.22 ± 0.04; NPV = 0.25 ± 0.02; mean ± SEM). Both PPV and NPV resulted in VIDD. Importantly, no differences existed between PPV and NPV animals in diaphragmatic fiber cross-sectional area, contractile properties, and the activation of proteases.

CONCLUSION

These results demonstrate that NPV and PPV result in similar levels of VILI and that NPV and PPV promote comparable levels of VIDD in rats.

Ventilation strategies in obese patients undergoing surgery: a quantitative systematic review and meta-analysis

BACKGROUND

Pathophysiological changes due to obesity may complicate mechanical ventilation during general anaesthesia. The ideal ventilation strategy is expected to optimize gas exchange and pulmonary mechanics and to reduce the risk of respiratory complications.

METHODS

Systematic search (databases, bibliographies, to March 2012, all languages) was performed for randomized trials testing intraoperative ventilation strategies in obese patients (BMI ≥ 30 kg m(-2)), and reporting on gas exchange, pulmonary mechanics, or pulmonary complications. Meta-analyses were performed when data from at least three studies or 100 patients could be combined.

RESULTS

Thirteen studies (505 obese surgical patients) reported on a variety of ventilation strategies: pressure- or volume-controlled ventilation (PCV, VCV), various tidal volumes, and different PEEP or recruitment manoeuvres (RM), and combinations thereof. Definitions and reporting of endpoints were inconsistent. In five trials (182 patients), RM added to PEEP compared with PEEP alone improved intraoperative PaO2/FIO2 ratio [weighted mean difference (WMD), 16.2 kPa; 95% confidence interval (CI), 8.0-24.4] and increased respiratory system compliance (WMD, 14 ml cm H(2)O(-1); 95% CI, 8-20). Arterial pressure remained unchanged. In four trials (100 patients) comparing PCV with VCV, there was no difference in PaO2/FIO2 ratio, tidal volume, or arterial pressure. Comparison of further ventilation strategies or combination of other outcomes was not feasible. Data on postoperative complications were seldom reported.

CONCLUSIONS

The ideal intraoperative ventilation strategy in obese patients remains obscure. There is some evidence that RM added to PEEP compared with PEEP alone improves intraoperative oxygenation and compliance without adverse effects. There is no evidence of any difference between PCV and VCV.

Comparison of recruitment manoeuvres in ventilated sheep with acute respiratory distress syndrome

BACKGROUND

Recruitment manoeuvres are widely used in clinical practice to open the lung and prevent lung injury by derecruitment, although the evidence is still discussed. In this study two different recruitment manoeuvres were compared to no recruitment manoeuvres (control) in ventilated sheep with acute respiratory distress syndrome (ARDS), induced by lung lavage.

METHODS

We performed a prospective, randomised study in 26 ventilated sheep with ARDS, to evaluate the effect of two different recruitment manoeuvres on gas exchange, blood pressure and lung injury. The two different recruitment manoeuvres, the high pressure recruitment manoeuvre (HPRM), with high peak pressure, and the smooth and moderate recruitment manoeuvre (SMRM), with lower peak pressure, were compared to controls (no recruitment) after disconnection. Oxygenation index and ventilation efficacy index were calculated to evaluate gas exchange. Lung injury was assessed by inflammatory response in broncho-alveolar lavage fluid (BALF) and blood and histology of the lung.

RESULTS

Oxygenation index improved significantly after both recruitment manoeuvres compared with controls, but no significant difference was found between the recruitment manoeuvres. Blood pressure decreased after HPRM but not after SMRM. HPRM induced a higher number of total cells and more neutrophils in the BALF. In the histology of the lung, mean alveolar size was increased in the dorsocranial region of the lung of SMRM compared to controls.

CONCLUSION

Recruitment manoeuvres improved oxygenation, but SMRM was superior, with respect to hemodynamics and pulmonary inflammation, in ventilated sheep suffering from ARDS induced by lung lavage.