Visualizing the Propagation of Acute Lung Injury

Airway pressure release ventilation for lung protection in acute respiratory distress syndrome: an alternative way to recruit the lungs

PURPOSE OF REVIEW

Airway pressure release ventilation (APRV) is a modality of ventilation in which high inspiratory continuous positive airway pressure (CPAP) alternates with brief releases. In this review, we will discuss the rationale for APRV as a lung protective strategy and then provide a practical introduction to initiating APRV using the time-controlled adaptive ventilation (TCAV) method.

RECENT FINDINGS

APRV using the TCAV method uses an extended inspiratory time and brief expiratory release to first stabilize and then gradually recruit collapsed lung (over hours/days), by progressively 'ratcheting' open a small volume of collapsed tissue with each breath. The brief expiratory release acts as a 'brake' preventing newly recruited units from re-collapsing, reversing the main drivers of ventilator-induced lung injury (VILI). The precise timing of each release is based on analysis of expiratory flow and is set to achieve termination of expiratory flow at 75% of the peak expiratory flow. Optimization of the release time reflects the changes in elastance and, therefore, is personalized (i.e. conforms to individual patient pathophysiology), and adaptive (i.e. responds to changes in elastance over time).

SUMMARY

APRV using the TCAV method is a paradigm shift in protective lung ventilation, which primarily aims to stabilize the lung and gradually reopen collapsed tissue to achieve lung homogeneity eliminating the main mechanistic drivers of VILI.

Time-Controlled Adaptive Ventilation (TCAV): a personalized strategy for lung protection

Acute respiratory distress syndrome (ARDS) alters the dynamics of lung inflation during mechanical ventilation. Repetitive alveolar collapse and expansion (RACE) predisposes the lung to ventilator-induced lung injury (VILI). Two broad approaches are currently used to minimize VILI: (1) low tidal volume (LVT) with low-moderate positive end-expiratory pressure (PEEP); and (2) open lung approach (OLA). The LVT approach attempts to protect already open lung tissue from overdistension, while simultaneously resting collapsed tissue by excluding it from the cycle of mechanical ventilation. By contrast, the OLA attempts to reinflate potentially recruitable lung, usually over a period of seconds to minutes using higher PEEP used to prevent progressive loss of end-expiratory lung volume (EELV) and RACE. However, even with these protective strategies, clinical studies have shown that ARDS-related mortality remains unacceptably high with a scarcity of effective interventions over the last two decades. One of the main limitations these varied interventions demonstrate to benefit is the observed clinical and pathologic heterogeneity in ARDS. We have developed an alternative ventilation strategy known as the Time Controlled Adaptive Ventilation (TCAV) method of applying the Airway Pressure Release Ventilation (APRV) mode, which takes advantage of the heterogeneous time- and pressure-dependent collapse and reopening of lung units. The TCAV method is a closed-loop system where the expiratory duration personalizes VT and EELV. Personalization of TCAV is informed and tuned with changes in respiratory system compliance (CRS) measured by the slope of the expiratory flow curve during passive exhalation. Two potentially beneficial features of TCAV are: (i) the expiratory duration is personalized to a given patient's lung physiology, which promotes alveolar stabilization by halting the progressive collapse of alveoli, thereby minimizing the time for the reopened lung to collapse again in the next expiration, and (ii) an extended inspiratory phase at a fixed inflation pressure after alveolar stabilization gradually reopens a small amount of tissue with each breath. Subsequently, densely collapsed regions are slowly ratcheted open over a period of hours, or even days. Thus, TCAV has the potential to minimize VILI, reducing ARDS-related morbidity and mortality.

Ratchet recruitment in the acute respiratory distress syndrome: lessons from the newborn cry

Patients with acute respiratory distress syndrome (ARDS) have few treatment options other than supportive mechanical ventilation. The mortality associated with ARDS remains unacceptably high, and mechanical ventilation itself has the potential to increase mortality further by unintended ventilator-induced lung injury (VILI). Thus, there is motivation to improve management of ventilation in patients with ARDS. The immediate goal of mechanical ventilation in ARDS should be to prevent atelectrauma resulting from repetitive alveolar collapse and reopening. However, a long-term goal should be to re-open collapsed and edematous regions of the lung and reduce regions of high mechanical stress that lead to regional volutrauma. In this paper, we consider the proposed strategy used by the full-term newborn to open the fluid-filled lung during the initial breaths of life, by ratcheting tissues opened over a series of initial breaths with brief expirations. The newborn's cry after birth shares key similarities with the Airway Pressure Release Ventilation (APRV) modality, in which the expiratory duration is sufficiently short to minimize end-expiratory derecruitment. Using a simple computational model of the injured lung, we demonstrate that APRV can slowly open even the most recalcitrant alveoli with extended periods of high inspiratory pressure, while reducing alveolar re-collapse with brief expirations. These processes together comprise a ratchet mechanism by which the lung is progressively recruited, similar to the manner in which the newborn lung is aerated during a series of cries, albeit over longer time scales.

First Stabilize and then Gradually Recruit: A Paradigm Shift in Protective Mechanical Ventilation for Acute Lung Injury

Acute respiratory distress syndrome (ARDS) is associated with a heterogeneous pattern of injury throughout the lung parenchyma that alters regional alveolar opening and collapse time constants. Such heterogeneity leads to atelectasis and repetitive alveolar collapse and expansion (RACE). The net effect is a progressive loss of lung volume with secondary ventilator-induced lung injury (VILI). Previous concepts of ARDS pathophysiology envisioned a two-compartment system: a small amount of normally aerated lung tissue in the non-dependent regions (termed "baby lung"); and a collapsed and edematous tissue in dependent regions. Based on such compartmentalization, two protective ventilation strategies have been developed: (1) a "protective lung approach" (PLA), designed to reduce overdistension in the remaining aerated compartment using a low tidal volume; and (2) an "open lung approach" (OLA), which first attempts to open the collapsed lung tissue over a short time frame (seconds or minutes) with an initial recruitment maneuver, and then stabilize newly recruited tissue using titrated positive end-expiratory pressure (PEEP). A more recent understanding of ARDS pathophysiology identifies regional alveolar instability and collapse (i.e., hidden micro-atelectasis) in both lung compartments as a primary VILI mechanism. Based on this understanding, we propose an alternative strategy to ventilating the injured lung, which we term a "stabilize lung approach" (SLA). The SLA is designed to immediately stabilize the lung and reduce RACE while gradually reopening collapsed tissue over hours or days. At the core of SLA is time-controlled adaptive ventilation (TCAV), a method to adjust the parameters of the airway pressure release ventilation (APRV) modality. Since the acutely injured lung at any given airway pressure requires more time for alveolar recruitment and less time for alveolar collapse, SLA adjusts inspiratory and expiratory durations and inflation pressure levels. The TCAV method SLA reverses the open first and stabilize second OLA method by: (i) immediately stabilizing lung tissue using a very brief exhalation time (≤0.5 s), so that alveoli simply do not have sufficient time to collapse. The exhalation duration is personalized and adaptive to individual respiratory mechanical properties (i.e., elastic recoil); and (ii) gradually recruiting collapsed lung tissue using an inflate and brake ratchet combined with an extended inspiratory duration (4-6 s) method. Translational animal studies, clinical statistical analysis, and case reports support the use of TCAV as an efficacious lung protective strategy.

Ventilation during ex vivo lung perfusion, a review

Evidence suggests that ventilation during ex vivo lung perfusion (EVLP) with a 'one-size-fits-all' strategy has the potential to cause lung injury which may only become clinically relevant in marginal lung allografts. EVLP induced- or accelerated lung injury is a dynamic and cumulative process reflecting the interplay of a number of factors. Stress and strain in lung tissue caused by positive pressure ventilation may be exacerbated by the altered properties of lung tissue in an EVLP setting. Any pre-existing injury may alter the ability of lung allografts to accommodate set ventilation and perfusion techniques on EVLP leading to further injury. This review will examine the effects of ventilation on donor lungs in the setting of EVLP. A framework for developing a protective ventilation technique will be proposed.

Unshrinking the baby lung to calm the VILI vortex

A hallmark of ARDS is progressive shrinking of the ‘baby lung,’ now referred to as the ventilator-induced lung injury (VILI) ‘vortex.’ Reducing the risk of the VILI vortex is the goal of current ventilation strategies; unfortunately, this goal has not been achieved nor has mortality been reduced. However, the temporal aspects of a mechanical breath have not been considered. A brief expiration prevents alveolar collapse, and an extended inspiration can recruit the atelectatic lung over hours. Time-controlled adaptive ventilation (TCAV) is a novel ventilator approach to achieve these goals, since it considers many of the temporal aspects of dynamic lung mechanics.

Imatinib alleviates lung injury and prolongs survival in ventilated rats

Imatinib, a tyrosine kinase inhibitor, attenuates pulmonary edema and inflammation in lung injury. However, the physiological effects of this drug and their impact on outcomes are poorly characterized. Using serial computed tomography (CT), we tested the hypothesis that imatinib reduces injury severity and improves survival in ventilated rats. Hydrochloric acid (HCl) was instilled in the trachea (pH 1.5, 2.5 mL/kg) of anesthetized, intubated supine rats. Animals were randomized (n = 17 each group) to receive intraperitoneal imatinib or vehicle immediately prior to HCl. All rats then received mechanical ventilation. CT was performed hourly for 4 h. Images were quantitatively analyzed to assess the progression of radiological abnormalities. Injury severity was confirmed via hourly blood gases, serum biomarkers, bronchoalveolar lavage (BAL), and histopathology. Serial blood drug levels were measured in a subset of rats. Imatinib reduced mortality while delaying functional and radiological injury progression: out of 17 rats per condition, 2 control vs. 8 imatinib-treated rats survived until the end of the experiment (P = 0.02). Imatinib attenuated edema after lung injury (P < 0.05), and survival time in both groups was negatively correlated with increased lung mass (R2 = 0.70) as well as other physiological and CT parameters. Capillary leak (BAL protein concentration) was significantly lower in the treated group (P = 0.04). Peak drug concentration was reached after 70 min, and the drug half-life was 150 min. Imatinib decreased both mortality and lung injury severity in mechanically ventilated rats. Pharmacological inhibition of edema could be used during mechanical ventilation to improve the severity and outcome of lung injury.

Effects of the Prone Position on Regional Neutrophilic Lung Inflammation According to 18F-FDG Pet in an Experimental Ventilator-Induced Lung Injury Model

ABSTRACT

Ventilator-induced lung injury (VILI) can be life-threatening and it is important to prevent the development of VILI. It remains unclear whether the prone position affects neutrophilic inflammation in the lung regions in vivo, which plays a crucial role in the pathogenesis of VILI. This study aimed to assess the relationship between the use of the prone position and the development of VILI-associated regional neutrophilic lung inflammation. Regional neutrophilic lung inflammation and lung aeration during low tidal volume mechanical ventilation were assessed using in vivo 2-deoxy-2-[(18)F] fluoro-D-glucose (18F-FDG) positron emission tomography and computed tomography in acutely experimentally injured rabbit lungs (lung injury induced by lung lavage and excessive ventilation). Direct comparisons were made among three groups: control, supine, and prone positions. After approximately 7 h, tissue-normalized 18F-FDG uptake differed significantly between the supine and prone positions (SUP: 0.038 ± 0.014 vs. PP: 0.029 ± 0.008, P = 0.038), especially in the ventral region (SUP: 0.052 ± 0.013 vs. PP: 0.026 ± 0.007, P = 0.003). The use of the prone position reduced lung inhomogeneities, which was demonstrated by the correction of the disproportionate rate of voxel gas over the given lung region. The progression of neutrophilic inflammation was affected by the interaction between the total strain (for aeration) and the inhomogeneity. The prone position is effective in slowing down the progression of VILI-associated neutrophilic inflammation. Under low-tidal-volume ventilation, the main drivers of its effect may be homogenization of lung tissue and that of mechanical forces.

Pulmonary Interstitial Matrix and Lung Fluid Balance From Normal to the Acutely Injured Lung

This review analyses the mechanisms by which lung fluid balance is strictly controlled in the air-blood barrier (ABB). Relatively large trans-endothelial and trans-epithelial Starling pressure gradients result in a minimal flow across the ABB thanks to low microvascular permeability aided by the macromolecular structure of the interstitial matrix. These edema safety factors are lost when the integrity of the interstitial matrix is damaged. The result is that small Starling pressure gradients, acting on a progressively expanding alveolar barrier with high permeability, generate a high transvascular flow that causes alveolar flooding in minutes. We modeled the trans-endothelial and trans-epithelial Starling pressure gradients under control conditions, as well as under increasing alveolar pressure (Palv) conditions of up to 25 cmH2O. We referred to the wet-to-dry weight (W/D) ratio, a specific index of lung water balance, to be correlated with the functional state of the interstitial structure. W/D averages ∼5 in control and might increase by up to ∼9 in severe edema, corresponding to ∼70% loss in the integrity of the native matrix. Factors buffering edemagenic conditions include: (i) an interstitial capacity for fluid accumulation located in the thick portion of ABB, (ii) the increase in interstitial pressure due to water binding by hyaluronan (the "safety factor" opposing the filtration gradient), and (iii) increased lymphatic flow. Inflammatory factors causing lung tissue damage include those of bacterial/viral and those of sterile nature. Production of reactive oxygen species (ROS) during hypoxia or hyperoxia, or excessive parenchymal stress/strain [lung overdistension caused by patient self-induced lung injury (P-SILI)] can all cause excessive inflammation. We discuss the heterogeneity of intrapulmonary distribution of W/D ratios. A W/D ∼6.5 has been identified as being critical for the transition to severe edema formation. Increasing Palv for W/D > 6.5, both trans-endothelial and trans-epithelial gradients favor filtration leading to alveolar flooding. Neither CT scan nor ultrasound can identify this initial level of lung fluid balance perturbation. A suggestion is put forward to identify a non-invasive tool to detect the earliest stages of perturbation of lung fluid balance before the condition becomes life-threatening.

Use of Organ Dysfunction as a Primary Outcome Variable Following Cecal Ligation and Puncture: Recommendations for Future Studies

Outcomes variables for research on sepsis have centered on mortality and changes in the host immune response. However, a recent task force (Sepsis-3) revised the definition of sepsis to "life-threatening organ dysfunction caused by a dysregulated host response to infection." This new definition suggests that human studies should focus on organ dysfunction. The appropriate criteria for organ dysfunction in either human sepsis or animal models are, however, poorly delineated, limiting the potential for translation. Further, in many systems, the difference between "dysfunction" and "injury" may not be clear. In this review, we identify criteria for organ dysfunction and/or injury in human sepsis and in rodents subjected to cecal ligation and puncture (CLP), the most commonly used animal model of sepsis. We further examine instances where overlap between human sepsis and CLP is sufficient to identify translational endpoints. Additional verification may demonstrate that these endpoints are applicable to other animals and to other sepsis models, for example, pneumonia. We believe that the use of these proposed measures of organ dysfunction will facilitate mechanistic studies on the pathobiology of sepsis and enhance our ability to develop animal model platforms to evaluate therapeutic approaches to human sepsis.

Imaging atelectrauma in Ventilator-Induced Lung Injury using 4D X-ray microscopy

Mechanical ventilation can damage the lungs, a condition called Ventilator-Induced Lung Injury (VILI). However, the mechanisms leading to VILI at the microscopic scale remain poorly understood. Here we investigated the within-tidal dynamics of cyclic recruitment/derecruitment (R/D) using synchrotron radiation phase-contrast imaging (PCI), and the relation between R/D and cell infiltration, in a model of Acute Respiratory Distress Syndrome in 6 anaesthetized and mechanically ventilated New-Zealand White rabbits. Dynamic PCI was performed at 22.6 µm voxel size, under protective mechanical ventilation [tidal volume: 6 ml/kg; positive end-expiratory pressure (PEEP): 5 cmH₂O]. Videos and quantitative maps of within-tidal R/D showed that injury propagated outwards from non-aerated regions towards adjacent regions where cyclic R/D was present. R/D of peripheral airspaces was both pressure and time-dependent, occurring throughout the respiratory cycle with significant scatter of opening/closing pressures. There was a significant association between R/D and regional lung cellular infiltration (p = 0.04) suggesting that tidal R/D of the lung parenchyma may contribute to regional lung inflammation or capillary-alveolar barrier dysfunction and to the progression of lung injury. PEEP may not fully mitigate this phenomenon even at high levels. Ventilation strategies utilizing the time-dependence of R/D may be helpful in reducing R/D and associated injury.

Metabolic Imaging and Biological Assessment: Platforms to Evaluate Acute Lung Injury and Inflammation

Pulmonary inflammation is a hallmark of several pulmonary disorders including acute lung injury and acute respiratory distress syndrome. Moreover, it has been shown that patients with hyperinflammatory phenotype have a significantly higher mortality rate. Despite this, current therapeutic approaches focus on managing the injury rather than subsiding the inflammatory burden of the lung. This is because of the lack of appropriate non-invasive biomarkers that can be used clinically to assess pulmonary inflammation. In this review, we discuss two metabolic imaging tools that can be used to non-invasively assess lung inflammation. The first method, Positron Emission Tomography (PET), is widely used in clinical oncology and quantifies flux in metabolic pathways by measuring uptake of a radiolabeled molecule into the cells. The second method, hyperpolarized ¹³C MRI, is an emerging tool that interrogates the branching points of the metabolic pathways to quantify the fate of metabolites. We discuss the differences and similarities between these techniques and discuss their clinical applications.

Mechanical Ventilation Lessons Learned From Alveolar Micromechanics

Morbidity and mortality associated with lung injury remains disappointingly unchanged over the last two decades, in part due to the current reliance on lung macro-parameters set on the ventilator instead of considering the micro-environment and the response of the alveoli and alveolar ducts to ventilator adjustments. The response of alveoli and alveolar ducts to mechanical ventilation modes cannot be predicted with current bedside methods of assessment including lung compliance, oxygenation, and pressure-volume curves. Alveolar tidal volumes (Vt) are less determined by the Vt set on the mechanical ventilator and more dependent on the number of recruited alveoli available to accommodate that Vt and their heterogeneous mechanical properties, such that high lung Vt can lead to a low alveolar Vt and low Vt can lead to high alveolar Vt. The degree of alveolar heterogeneity that exists cannot be predicted based on lung calculations that average the individual alveolar Vt and compliance. Finally, the importance of time in promoting alveolar stability, specifically the inspiratory and expiratory times set on the ventilator, are currently under-appreciated. In order to improve outcomes related to lung injury, the respiratory physiology of the individual patient, specifically at the level of the alveolus, must be targeted. With experimental data, this review highlights some of the known mechanical ventilation adjustments that are helpful or harmful at the level of the alveolus.

A Physiologically Informed Strategy to Effectively Open, Stabilize, and Protect the Acutely Injured Lung

Acute respiratory distress syndrome (ARDS) causes a heterogeneous lung injury and remains a serious medical problem, with one of the only treatments being supportive care in the form of mechanical ventilation. It is very difficult, however, to mechanically ventilate the heterogeneously damaged lung without causing secondary ventilator-induced lung injury (VILI). The acutely injured lung becomes time and pressure dependent, meaning that it takes more time and pressure to open the lung, and it recollapses more quickly and at higher pressure. Current protective ventilation strategies, ARDSnet low tidal volume (LVt) and the open lung approach (OLA), have been unsuccessful at further reducing ARDS mortality. We postulate that this is because the LVt strategy is constrained to ventilating a lung with a heterogeneous mix of normal and focalized injured tissue, and the OLA, although designed to fully open and stabilize the lung, is often unsuccessful at doing so. In this review we analyzed the pathophysiology of ARDS that renders the lung susceptible to VILI. We also analyzed the alterations in alveolar and alveolar duct mechanics that occur in the acutely injured lung and discussed how these alterations are a key mechanism driving VILI. Our analysis suggests that the time component of each mechanical breath, at both inspiration and expiration, is critical to normalize alveolar mechanics and protect the lung from VILI. Animal studies and a meta-analysis have suggested that the time-controlled adaptive ventilation (TCAV) method, using the airway pressure release ventilation mode, eliminates the constraints of ventilating a lung with heterogeneous injury, since it is highly effective at opening and stabilizing the time- and pressure-dependent lung. In animal studies it has been shown that by “casting open” the acutely injured lung with TCAV we can (1) reestablish normal expiratory lung volume as assessed by direct observation of subpleural alveoli; (2) return normal parenchymal microanatomical structural support, known as alveolar interdependence and parenchymal tethering, as assessed by morphometric analysis of lung histology; (3) facilitate regeneration of normal surfactant function measured as increases in surfactant proteins A and B; and (4) significantly increase lung compliance, which reduces the pathologic impact of driving pressure and mechanical power at any given tidal volume.

Multi-resolution convolutional neural networks for fully automated segmentation of acutely injured lungs in multiple species

Segmentation of lungs with acute respiratory distress syndrome (ARDS) is a challenging task due to diffuse opacification in dependent regions which results in little to no contrast at the lung boundary. For segmentation of severely injured lungs, local intensity and texture information, as well as global contextual information, are important factors for consistent inclusion of intrapulmonary structures. In this study, we propose a deep learning framework which uses a novel multi-resolution convolutional neural network (ConvNet) for automated segmentation of lungs in multiple mammalian species with injury models similar to ARDS. The multi-resolution model eliminates the need to tradeoff between high-resolution and global context by using a cascade of low-resolution to high-resolution networks. Transfer learning is used to accommodate the limited number of training datasets. The model was initially pre-trained on human CT images, and subsequently fine-tuned on canine, porcine, and ovine CT images with lung injuries similar to ARDS. The multi-resolution model was compared to both high-resolution and low-resolution networks alone. The multi-resolution model outperformed both the low- and high-resolution models, achieving an overall mean Jacaard index of 0.963 ± 0.025 compared to 0.919 ± 0.027 and 0.950 ± 0.036, respectively, for the animal dataset (N=287). The multi-resolution model achieves an overall average symmetric surface distance of 0.438 ± 0.315 mm, compared to 0.971 ± 0.368 mm and 0.657 ± 0.519 mm for the low-resolution and high-resolution models, respectively. We conclude that the multi-resolution model produces accurate segmentations in severely injured lungs, which is attributed to the inclusion of both local and global features.

Imaging the Injured Lung: Mechanisms of Action and Clinical Use

Acute respiratory distress syndrome (ARDS) consists of acute hypoxemic respiratory failure characterized by massive and heterogeneously distributed loss of lung aeration caused by diffuse inflammation and edema present in interstitial and alveolar spaces. It is defined by consensus criteria, which include diffuse infiltrates on chest imaging-either plain radiography or computed tomography. This review will summarize how imaging sciences can inform modern respiratory management of ARDS and continue to increase the understanding of the acutely injured lung. This review also describes newer imaging methodologies that are likely to inform future clinical decision-making and potentially improve outcome. For each imaging modality, this review systematically describes the underlying principles, technology involved, measurements obtained, insights gained by the technique, emerging approaches, limitations, and future developments. Finally, integrated approaches are considered whereby multimodal imaging may impact management of ARDS.

Prevention and treatment of acute lung injury with time-controlled adaptive ventilation: physiologically informed modification of airway pressure release ventilation

Mortality in acute respiratory distress syndrome (ARDS) remains unacceptably high at approximately 39%. One of the only treatments is supportive: mechanical ventilation. However, improperly set mechanical ventilation can further increase the risk of death in patients with ARDS. Recent studies suggest that ventilation-induced lung injury (VILI) is caused by exaggerated regional lung strain, particularly in areas of alveolar instability subject to tidal recruitment/derecruitment and stress-multiplication. Thus, it is reasonable to expect that if a ventilation strategy can maintain stable lung inflation and homogeneity, regional dynamic strain would be reduced and VILI attenuated. A time-controlled adaptive ventilation (TCAV) method was developed to minimize dynamic alveolar strain by adjusting the delivered breath according to the mechanical characteristics of the lung. The goal of this review is to describe how the TCAV method impacts pathophysiology and protects lungs with, or at high risk of, acute lung injury. We present work from our group and others that identifies novel mechanisms of VILI in the alveolar microenvironment and demonstrates that the TCAV method can reduce VILI in translational animal ARDS models and mortality in surgical/trauma patients. Our TCAV method utilizes the airway pressure release ventilation (APRV) mode and is based on opening and collapsing time constants, which reflect the viscoelastic properties of the terminal airspaces. Time-controlled adaptive ventilation uses inspiratory and expiratory time to (1) gradually “nudge” alveoli and alveolar ducts open with an extended inspiratory duration and (2) prevent alveolar collapse using a brief (sub-second) expiratory duration that does not allow time for alveolar collapse. The new paradigm in TCAV is configuring each breath guided by the previous one, which achieves real-time titration of ventilator settings and minimizes instability induced tissue damage. This novel methodology changes the current approach to mechanical ventilation, from arbitrary to personalized and adaptive. The outcome of this approach is an open and stable lung with reduced regional strain and greater lung protection.

Lessons learned in acute respiratory distress syndrome from the animal laboratory

Since the description of the acute respiratory distress syndrome (ARDS) in 1967, investigators have struggled to reproduce the syndrome in the animal laboratory. While several different models of experimental acute lung injury (ALI) have been developed, none completely capture the inciting etiologies, initial inflammation, heterogeneity, and resolution of human ARDS. This potentially has contributed to the poor translation of potential therapeutics between animal ALI models and human ARDS. It was only recently that standardized criteria were suggested for what makes an ALI model comparable to human ARDS. Nevertheless, despite model heterogeneity, these models have contributed substantially to our understanding of the syndrome. From the initial studies identifying the risks of mechanical ventilation to the identification of potentially targetable inflammatory mediators, to modern studies focusing on regional heterogeneity and novel molecular pathways, animal models continue to inform our understanding of ARDS. This review will cover several major lessons learned from animal models of ALI, and provide some direction for future studies in this field.

Unstable Inflation Causing Injury. Insight from Prone Position and Paired Computed Tomography Scans

RATIONALE

It remains unclear how prone positioning improves survival in acute respiratory distress syndrome. Using serial computed tomography (CT), we previously reported that "unstable" inflation (i.e., partial aeration with large tidal density swings, indicating increased local strain) is associated with injury progression.

OBJECTIVES

We prospectively tested whether prone position contains the early propagation of experimental lung injury by stabilizing inflation.

METHODS

Injury was induced by tracheal hydrochloric acid in rats; after randomization to supine or prone position, injurious ventilation was commenced using high tidal volume and low positive end-expiratory pressure. Paired end-inspiratory (EI) and end-expiratory (EE) CT scans were acquired at baseline and hourly up to 3 hours. Each sequential pair (EI, EE) of CT images was superimposed in parametric response maps to analyze inflation. Unstable inflation was then measured in each voxel in both dependent and nondependent lung. In addition, five pigs were imaged (EI and EE) prone versus supine, before and (1 hour) after hydrochloric acid aspiration.

MEASUREMENTS AND MAIN RESULTS

In rats, prone position limited lung injury propagation and increased survival (11/12 vs. 7/12 supine; P = 0.01). EI-EE densities, respiratory mechanics, and blood gases deteriorated more in supine versus prone rats. At baseline, more voxels with unstable inflation occurred in dependent versus nondependent regions when supine (41 ± 6% vs. 18 ± 7%; P < 0.01) but not when prone. In supine pigs, unstable inflation predominated in dorsal regions and was attenuated by prone positioning.

CONCLUSIONS

Prone position limits the radiologic progression of early lung injury. Minimizing unstable inflation in this setting may alleviate the burden of acute respiratory distress syndrome.

Looking closer at acute respiratory distress syndrome: the role of advanced imaging techniques

PURPOSE OF REVIEW

Advanced imaging techniques have provided invaluable insights in understanding of acute respiratory distress syndrome (ARDS) and the effect of therapeutic strategies, thanks to the possibility of gaining regional information and moving from simple 'anatomical' information to in-vivo functional imaging.

RECENT FINDINGS

Computed tomography (CT) led to the understanding of several ARDS mechanisms and interaction with mechanical ventilation. It is nowadays frequently part of routine diagnostic workup, often leading to treatment changes. Moreover, CT is a reference for novel techniques both in clinical and preclinical studies. Bedside transthoracic lung ultrasound allows semiquantitative regional analysis of lung aeration, identifies ARDS lung morphology and response to therapeutic maneuvers. Electrical impedance tomography is a radiation-free, functional, bedside, imaging modality which allows a real-time monitoring of regional ventilation. Finally, positron emission tomography (PET) is a functional imaging technique that allows to trace physiologic processes, by administration of a radioactive molecule. PET with FDG has been applied to patients with ARDS, thanks to its ability to track the inflammatory cells activity.

SUMMARY

Progresses in lung imaging are key to individualize therapy, diagnosis, and pathophysiological mechanism at play in any patient at any specified time, helping to move toward personalized medicine for ARDS.

Tidal changes on CT and progression of ARDS

Background

Uncertain prediction of outcome in acute respiratory distress syndrome (ARDS) impedes individual patient management and clinical trial design.

Objectives

To develop a radiological metric of injurious inflation derived from matched inspiratory and expiratory CT scans, calibrate it in a model of experimental lung injury, and test it in patients with ARDS.

Methods

73 anaesthetised rats (acid aspiration model) were ventilated (protective or non-protective) for up to 4 hours to generate a spectrum of lung injury. CT was performed (inspiratory and expiratory) at baseline each hour, paired inspiratory and expiratory images were superimposed and voxels tracked in sequential scans. In nine patients with ARDS, paired inspiratory and expiratory CT scans from the first intensive care unit week were analysed.

Results

In experimental studies, regions of lung with unstable inflation (ie, partial or reversible airspace filling reflecting local strain) were the areas in which subsequent progression of injury was greatest in terms of progressive infiltrates (R=0.77) and impaired compliance (R=0.67, p<0.01). In patients with ARDS, a threshold fraction of tissue with unstable inflation was apparent: >28% in all patients who died and ≤28% in all who survived, whereas segregation of survivors versus non-survivors was not possible based on oxygenation or lung mechanics.

Conclusions

A single set of superimposed inspiratory–expiratory CT scans may predict progression of lung injury and outcome in ARDS; if these preliminary results are validated, this could facilitate clinical trial recruitment and individualised care.

In vivo imaging of the progression of acute lung injury using hyperpolarized [1-13 C] pyruvate

PURPOSE

To investigate pulmonary metabolic alterations during progression of acute lung injury.

METHODS

Using hyperpolarized [1-¹³ C] pyruvate imaging, we measured pulmonary lactate and pyruvate in 15 ventilated rats 1, 2, and 4 h after initiation of mechanical ventilation. Lung compliance was used as a marker for injury progression. 5 untreated rats were used as controls; 5 rats (injured-1) received 1 ml/kg and another 5 rats (injured-2) received 2 ml/kg hydrochloric acid (pH 1.25) in the trachea at 70 min.

RESULTS

The mean lactate-to-pyruvate ratio of the injured-1 cohort was 0.15 ± 0.02 and 0.15 ± 0.03 at baseline and 1 h after the injury, and significantly increased from the baseline value 3 h after the injury to 0.23 ± 0.02 (P = 0.002). The mean lactate-to-pyruvate ratio of the injured-2 cohort decreased from 0.14 ± 0.03 at baseline to 0.08 ± 0.02 1 h after the injury and further decreased to 0.07 ± 0.02 (P = 0.08) 3 h after injury. No significant change was observed in the control group. Compliance in both injured groups decreased significantly after the injury (P < 0.01).

CONCLUSIONS

Our findings suggest that in severe cases of lung injury, edema and hyperperfusion in the injured lung tissue may complicate interpretation of the pulmonary lactate-to-pyruvate ratio as a marker of inflammation. However, combining the lactate-to-pyruvate ratio with pulmonary compliance provides more insight into the progression of the injury and its severity. Magn Reson Med 78:2106-2115, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

In vivo pH mapping of injured lungs using hyperpolarized [1-13 C]pyruvate

PURPOSE

To optimize the production of hyperpolarized ¹³ C-bicarbonate from the decarboxylation of hyperpolarized [1-¹³ C]pyruvate and use it to image pH in the lungs and heart of rats with acute lung injury.

METHODS

Two forms of catalysis are compared calorimetrically to maximize the rate of decarboxylation and rapidly produce hyperpolarized bicarbonate from pyruvate while minimizing signal loss. Rats are injured using an acute lung injury model combining ventilator-induced lung injury and acid aspiration. Carbon images are obtained from both healthy (n = 4) and injured (n = 4) rats using a slice-selective chemical shift imaging sequence with low flip angle. pH is calculated from the relative HCO3- and CO₂ signals using the Henderson-Hasselbalch equation.

RESULTS

It is demonstrated that base catalysis is more effective than metal-ion catalysis for this decarboxylation reaction. Bicarbonate polarizations up to 17.2% are achieved using the base-catalyzed reaction. A mean pH difference between lung and heart of 0.14 pH units is measured in the acute lung injury model. A significant pH difference between injured and uninjured lungs is also observed.

CONCLUSION

It is demonstrated that hyperpolarized ¹³ C-bicarbonate can be efficiently produced from the base-catalyzed decarboxylation of pyruvate. This method is used to obtain the first regional pH image of the lungs and heart of an animal. Magn Reson Med 78:1121-1130, 2017. © 2016 International Society for Magnetic Resonance in Medicine.