Airway Closure and Expiratory Flow Limitation in Acute Respiratory Distress Syndrome

Development of an acute lung injury model for drug testing

A challenge that limits our understanding of the underlying pathobiology of pediatric acute respiratory distress syndrome (PARDS) is the lack of a preclinical airway model that can be leveraged for the study of mechanisms and specific molecules for drug testing. We developed a physiologic model system of the small airways for mechanistic application in PARDS using a co-culture of primary human-derived small airway epithelial cells (SAECs) cultured at the air-liquid interface and umbilical vein endothelial cells in a transwell system. The model was validated by exposing the SAECs to a rhinovirus infection, to an inflammatory lung insult using a mixture of cytokines found in ARDS (cytomix), and to airway fluid samples from children with different severity strata of PARDS. We used a combination of transepithelial electrical resistance, immunofluorescence confocal microscopy of tight junctions, targeted gene expression, and cytokine responses to evaluate the model to the aforementioned insults. We then use the model in drug testing and show the reduction in IL-6 expression in conditioned media and STAT3 phosphorylation following co-treatment of SAECs with cytomix and the Janus kinase inhibitor (JAKi) baricitinib. This model enables mechanistic studies of airway pathobiology and may serve as a novel drug testing platform for PARDS.

Distribution of airway pressure opening in the lungs measured with electrical impedance tomography (POET): a prospective physiological study

BACKGROUND

In patients with acute hypoxemic respiratory failure (AHRF) under mechanical ventilation, the change in pressure slope during a low-flow insufflation indicates a global airway opening pressure (AOP) needed to reopen closed airways and may be used for titration of positive end-expiratory pressure.

OBJECTIVES

To understand 1) if airways open homogeneously inside the lungs or significant regional AOP variations exist; 2) whether the pattern of the pressure slope change during low-flow insufflation can indicate the presence of regional AOP variations.

METHODS

Using electrical impedance tomography, we recorded low-flow insufflation maneuvers (< 10 L/min) starting from end-expiratory positive pressure 0-5 cmH2O. We measured global (AOPglobal) and regional AOPs from pressure-impedance curves in the four different lung quadrants, and compared AOPglobal with the highest quadrantal AOP (AOPhighest). We categorized the slope change of the low-flow inflation pressure-time curve into three patterns: no change, progressive change, abrupt change.

RESULTS

Among the 36 patients analyzed, 9 (25%) had AOPglobal ≥ 5 cmH2O whereas 19 (53%) exhibited regional AOPhighest ≥ 5 cmH2O. AOPglobal was on average similar to AOP of the upper right quadrant (P = 0.182) but was lower than AOPs of the other three quadrants (P < 0.01 of each). AOPglobal was significantly lower than AOPhighest: 3.0 [2.0-4.3] vs. 5.0 [2.8-8.3] cmH2O, P < 0.001. AOP was higher in the dependent than the non-dependent ventilated lung (4.0 [2.0-6.3] vs. 3.0 [2.0-5.0] cmH2O, P < 0.001). Seventeen (47%) patients exhibited a 'progressive change' pattern in the pressure-time curve. These patients had a larger difference between AOPhighest and AOPglobal (3.0 [2.0-4.0] cmH2O with a maximum of 8 cmH2O) compared to the other two patterns: 1.0 [0-1.0] cmH2O in 'no change' , P < 0.001 and 1.0 [0-2.0] cmH2O in 'abrupt change' , P = 0.003.

CONCLUSION

AOPglobal mostly reflects the lowest opening pressure in the lung and frequently underestimates the highest regional AOP in mechanically ventilated patients with AHRF. A progressive slope change during the low-flow pressure-time curve indicates the presence of several and higher regional AOPs.

TRIAL REGISTRATION

Clinicaltrials.gov, NCT05825534 (registered, April 24th, 2023), retrospectively registered.

Airway epithelium damage in acute respiratory distress syndrome

BACKGROUND

The airway epithelium (AE) fulfils multiple functions to maintain pulmonary homeostasis, among which ensuring adequate barrier function, cell differentiation and polarization, and actively transporting immunoglobulin A (IgA), the predominant mucosal immunoglobulin in the airway lumen, via the polymeric immunoglobulin receptor (pIgR). Morphological changes of the airways have been reported in ARDS, while their detailed features, impact for mucosal immunity, and causative mechanisms remain unclear. Therefore, this study aimed to assess epithelial alterations in the distal airways of patients with ARDS.

METHODS

We retrospectively analyzed lung tissue samples from ARDS patients and controls to investigate and quantify structural and functional changes in the small airways, using multiplex fluorescence immunostaining and computer-assisted quantification on whole tissue sections. Additionally, we measured markers of mucosal immunity, IgA and pIgR, alongside with other epithelial markers, in the serum and the broncho-alveolar lavage fluid (BALF) prospectively collected from ARDS patients and controls.

RESULTS

Compared to controls, airways of ARDS were characterized by increased epithelial denudation (p = 0.0003) and diffuse epithelial infiltration by neutrophils (p = 0.0005). Quantitative evaluation of multiplex fluorescence immunostaining revealed a loss of ciliated cells (p = 0.0317) a trend towards decreased goblet cells (p = 0.056), and no change regarding cell progenitors (basal and club cells), indicating altered mucociliary differentiation. Increased epithelial permeability was also shown in ARDS with a significant decrease of tight (p < 0.0001) and adherens (p = 0.025) junctional proteins. Additionally, we observed a significant decrease of the expression of pIgR, (p < 0.0001), indicating impaired mucosal IgA immunity. Serum concentrations of secretory component (SC) and S-IgA were increased in ARDS (both p < 0.0001), along other lung-derived proteins (CC16, SP-D, sRAGE). However, their BALF concentrations remained unchanged, suggesting a spillover of airway and alveolar proteins through a damaged AE.

CONCLUSION

The airway epithelium from patients with ARDS exhibits multifaceted alterations leading to altered mucociliary differentiation, compromised defense functions and increased permeability with pneumoproteinemia.

Detecting Intrathoracic Airway Closure during Prehospital Cardiopulmonary Resuscitation Using Quasi-Static Pressure-Volume Curves: A Pilot Study

Background: Intrathoracic airway closure frequently occurs during cardiac arrest, possibly impairing ventilation. Previously, capnogram analysis was used to detect this pathophysiological process. In other populations, quasi-static pressure-volume curves obtained during constant low-flow inflations are routinely used to detect intrathoracic airway closure. This study reports the first use of quasi-static pressure-volume curves to detect intrathoracic airway closure during prehospital cardiopulmonary resuscitation. Methods: Connecting a pressure and flow sensor to the endotracheal tube enabled the performance of low-flow inflations during cardiopulmonary resuscitation using a manual resuscitator. Users connected the device following intubation and performed a low-flow inflation during the next rhythm analysis when chest compressions were interrupted. Determining the lower inflection point on the resulting pressure-volume curves allowed for the detection and quantification of intrathoracic airway closure. Results: The research device was used during the prehospital treatment of ten cardiac arrest patients. A lower inflection point indicating intrathoracic airway closure was detected in all patients. During cardiac arrest, the median pressure at which the lower inflection point occurred was 5.56 cmH20 (IQR 4.80, 8.23 cmH20). This value varied considerably between cases and was lower in patients who achieved return of spontaneous circulation. Conclusions: In this pilot study, quasi-static pressure-volume curves were obtained during prehospital cardiopulmonary resuscitation. Intrathoracic airway closure was detected in all patients. Further research is needed to determine whether the use of ventilation strategies to counter intrathoracic airway closure could lead to improved outcomes and if the degree of airway closure could serve as a prognostic factor.

Airway opening pressure maneuver to detect airway closure in mechanically ventilated pediatric patients

Background

Airway closure, which refers to the complete collapse of the airway, has been described under mechanical ventilation during anesthesia and more recently in adult patients with acute respiratory distress syndrome (ARDS). A ventilator maneuver can be used to identify airway closure and measure the pressure required for the airway to reopen, known as the airway opening pressure (AOP). Without that maneuver, AOP is unknown to clinicians.

Objective

This study aims to demonstrate the technical adaptation of the adult maneuver for children and illustrate its application in two cases of pediatric ARDS (p-ARDS).

Methods

A bench study was performed to adapt the maneuver for 3-50 kg patients. Four maneuvers were performed for each simulated patient, with 1, 2, 3, and 4 s of insufflation time to deliver a tidal volume (Vt) of 6 ml/kg by a continuous flow.

Results

Airway closure was simulated, and AOP was visible at 15 cmH2O with a clear inflection point, except for the 3 kg simulated patient. Regarding insufflation time, a 4 s maneuver exhibited a better performance in 30 and 50 kg simulated patients since shorter insufflation times had excessive flowrates (>10 L/min). Below 20 kg, the difference in resistive pressure between a 3 s and a 4 sec maneuver was negligible; therefore, prolonging the maneuver beyond 3 s was not useful. Airway closure was identified in two p-ARDS patients, with the pediatric maneuver being employed in the 28 kg patient.

Conclusions

We propose a pediatric AOP maneuver delivering 6 ml/kg of Vt at a continuous low-flow inflation for 3 s for patients weighing up to 20 kg and for 4 s for patients weighing beyond 20 kg.

Setting positive end-expiratory pressure: the use of esophageal pressure measurements

PURPOSE OF REVIEW

To summarize the key concepts, physiological rationale and clinical evidence for titrating positive end-expiratory pressure (PEEP) using transpulmonary pressure ( PL ) derived from esophageal manometry, and describe considerations to facilitate bedside implementation.

RECENT FINDINGS

The goal of an esophageal pressure-based PEEP setting is to have sufficient PL at end-expiration to keep (part of) the lung open at the end of expiration. Although randomized studies (EPVent-1 and EPVent-2) have not yet proven a clinical benefit of this approach, a recent posthoc analysis of EPVent-2 revealed a potential benefit in patients with lower APACHE II score and when PEEP setting resulted in end-expiratory PL values close to 0 ± 2 cmH 2 O instead of higher or more negative values. Technological advances have made esophageal pressure monitoring easier to implement at the bedside, but challenges regarding obtaining reliable measurements should be acknowledged.

SUMMARY

Esophageal pressure monitoring has the potential to individualize the PEEP settings. Future studies are needed to evaluate the clinical benefit of such approach.

High Stretch Modulates cAMP/ATP Level in Association with Purine Metabolism via miRNA-mRNA Interactions in Cultured Human Airway Smooth Muscle Cells

High stretch (>10% strain) of airway smooth muscle cells (ASMCs) due to mechanical ventilation (MV) is postulated to contribute to ventilator-induced lung injury (VILI), but the underlying mechanisms remain largely unknown. We hypothesized that ASMCs may respond to high stretch via regulatory miRNA-mRNA interactions, and thus we aimed to identify high stretch-responsive cellular events and related regulating miRNA-mRNA interactions in cultured human ASMCs with/without high stretch. RNA-Seq analysis of whole genome-wide miRNAs revealed 12 miRNAs differentially expressed (DE) in response to high stretch (7 up and 5 down, fold change >2), which target 283 DE-mRNAs as identified by a parallel mRNA sequencing and bioinformatics analysis. The KEGG and GO analysis further indicated that purine metabolism was the first enriched event in the cells during high stretch, which was linked to miR-370-5p-PDE4D/AK7. Since PDE4D/AK7 have been previously linked to cAMP/ATP metabolism in lung diseases and now to miR-370-5p in ASMCs, we thus evaluated the effect of high stretch on the cAMP/ATP level inside ASMCs. The results demonstrated that high stretch modulated the cAMP/ATP levels inside ASMCs, which could be largely abolished by miR-370-5p mimics. Together, these findings indicate that miR-370-5p-PDE4D/AK7 mediated high stretch-induced modulation of cAMP and ATP synthesis inside ASMCs. Furthermore, such interactive miRNA-mRNA pairs may provide new insights for the discovery of effective biomarkers/therapeutic targets for the diagnosis and treatment of VILI and other MV-associated respiratory diseases.

Respiratory Mechanics: Revisiting the Appraisement of Lung Recruitment

Mechanical ventilation has long been recognized as the most vital therapy for patients with ARDS. Compared with lung-protective ventilation, debates that involve the open lung strategy, which consists primarily of the lung recruitment maneuver and higher PEEP, have never been resolved. In terms of the beneficial and detrimental effects of this aggressive maneuver, appraisal of lung recruitment is essential for intensivists to make clinical decisions. This review aimed to clarify how to assess the potential for lung recruitment based on respiratory mechanics when using the pressure-volume curve or loop method and end-expiratory lung volume-static compliance of the respiratory system method. However, their limitations related to excessive generalization, accuracy, and identification of cutoff values cannot be omitted. Finally, future studies are warranted to combine these classic methods with newly invented techniques to achieve safer and more effective lung recruitment.

Attenuation of the Severity of Acute Respiratory Distress Syndrome by Pomiferin through Blocking Inflammation and Oxidative Stress in an AKT/Foxo1 Pathway-Dependent Manner

Acute respiratory distress syndrome (ARDS) gives rise to uncontrolled inflammatory response and oxidative stress, causing very high mortality globally. Pomiferin is a kind of prenylated isoflavonoid extracted from Maclura pomifera, owning anti-inflammatory and antioxidant properties. However, the functions and possible mechanisms of pomiferin in lipopolysaccharide- (LPS-) induced ARDS remain unknown. C57BL/6 mice were injected with LPS (5 mg/kg) intratracheally to induce an in vivo ARDS model while RAW264.7 macrophages were stimulated with LPS (100 ng/ml) to induce an in vitro model. Our data demonstrated that pomiferin (20 mg/kg) significantly improved pulmonary function and lung pathological injury in mice with ARDS, apart from increasing survival rate. Meanwhile, pomiferin treatment also inhibited LPS-induced inflammation as well as oxidative stress in lung tissues. LPS stimulation significantly activated AKT/Foxo1 signal pathway in lung tissues, which could be reversed after pomiferin treatment. In vitro experiments further showed that 10, 20, and 50 μM of pomiferin could enhance cell viability of RAW264.7 macrophages stimulated with LPS. What is more, 3-deoxysappanchalcone (3-DE), one AKT agonist, was used to active AKT in RAW264.7 macrophages. The results further showed that 3-DE could abolish pomiferin-elicited protection in LPS-treated RAW264.7 macrophages, evidenced by activated inflammation and oxidative stress. Taken together, our study showed that pomiferin could exert an ARDS-protective effect by blocking the AKT/Foxo1 signal pathway to inhibit LPS-induced inflammatory response and oxidative injury, which may serve as a potential candidate for the treatment of ARDS in the future.

MircroRNA Let-7a-5p in Airway Smooth Muscle Cells is Most Responsive to High Stretch in Association With Cell Mechanics Modulation

Objective: High stretch (strain >10%) can alter the biomechanical behaviors of airway smooth muscle cells which may play important roles in diverse lung diseases such as asthma and ventilator-induced lung injury. However, the underlying modulation mechanisms for high stretch-induced mechanobiological responses in ASMCs are not fully understood. Here, we hypothesize that ASMCs respond to high stretch with increased expression of specific microRNAs (miRNAs) that may in turn modulate the biomechanical behaviors of the cells. Thus, this study aimed to identify the miRNA in cultured ASMCs that is most responsive to high stretch, and subsequently investigate in these cells whether the miRNA expression level is associated with the modulation of cell biomechanics.

Methods: MiRNAs related to inflammatory airway diseases were obtained via bioinformatics data mining, and then tested with cultured ASMCs for their expression variations in response to a cyclic high stretch (13% strain) simulating in vivo ventilator-imposed strain on airways. Subsequently, we transfected cultured ASMCs with mimics and inhibitors of the miRNA that is most responsive to the high stretch, followed by evaluation of the cells in terms of morphology, stiffness, traction force, and mRNA expression of cytoskeleton/focal adhesion-related molecules.

Results: 29 miRNAs were identified to be related to inflammatory airway diseases, among which let-7a-5p was the most responsive to high stretch. Transfection of cultured human ASMCs with let-7a-5p mimics or inhibitors led to an increase or decrease in aspect ratio, stiffness, traction force, migration, stress fiber distribution, mRNA expression of α-smooth muscle actin (SMA), myosin light chain kinase, some subfamily members of integrin and talin. Direct binding between let-7a-5p and ItgαV was also verified in classical model cell line by using dual-luciferase assays.

Conclusion: We demonstrated that high stretch indeed enhanced the expression of let-7a-5p in ASMCs, which in turn led to changes in the cells’ morphology and biomechanical behaviors together with modulation of molecules associated with cytoskeletal structure and focal adhesion. These findings suggest that let-7a-5p regulation is an alternative mechanism for high stretch-induced effect on mechanobiology of ASMCs, which may contribute to understanding the pathogenesis of high stretch-related lung diseases.