Fifty Years of Research in ARDS. Respiratory Mechanics in Acute Respiratory Distress Syndrome

Accuracy of tidal volume delivery by paediatric intensive care ventilators: A bench-model study

BACKGROUND

Tidal volume (Vt) delivery during mechanical ventilation is influenced by gas compression, humidity, and temperature.

OBJECTIVES

This bench study aimed at assessing the accuracy of Vt delivery by paediatric intensive care ventilators according to the humidification system. Secondary objectives were to assess the following: (i) the accuracy of Vt delivery in ventilators with an integrated Y-piece pneumotachograph and (ii) the ability of ventilators to deliver and maintain a preset positive end-expiratory pressure.

METHODS

Six latest-generation intensive care ventilators equipped with a paediatric mode were tested on the ASL5000 test lung in four simulated paediatric bench models (full-term neonate, infant, preschool-age chile, and school-age child), under volume-controlled mode with a heated humidifier (HH) or a heat moisture exchanger, with various loading conditions. Three ventilators equipped with a Y-piece pneumotachograph were tested with or without the pneumotachograph in the neonatal and infant models. "Accurate Vt" delivery was defined as a volume error (percentage of the preset Vt under body temperature and pressure and saturated water vapour conditions) being ≤10 % of the absolute preset value.

RESULTS

Vt accuracy varied significantly across ventilators but was acceptable in almost all the ventilators and all the models, except the neonatal model. The humidification system had an impact on Vt delivery in the majority of the tested conditions (p < 0.05). The use of an HH was associated with a better Vt accuracy in four ventilators (V500, V800, R860, and ServoU) and allowed to achieve an acceptable level of volume error in the neonatal model as compared to the use of heat moisture exchanger. The use of an integrated pneumotachograph was associated with lower volume error in only one ventilator (p < 0.01). All the tested ventilators were able to maintain adequate positive end-expiratory pressure levels.

CONCLUSION

The humidification system affects Vt accuracy of paediatric intensive care ventilators, especially in the youngest patients for whom the HH should be preferred.

Limiting Overdistention or Collapse When Mechanically Ventilating Injured Lungs: A Randomized Study in a Porcine Model

Rationale: It is unknown whether preventing overdistention or collapse is more important when titrating positive end-expiratory pressure (PEEP) in acute respiratory distress syndrome (ARDS). Objectives: To compare PEEP targeting minimal overdistention or minimal collapse or using a compromise between collapse and overdistention in a randomized trial and to assess the impact on respiratory mechanics, gas exchange, inflammation, and hemodynamics. Methods: In a porcine model of ARDS, lung collapse and overdistention were estimated using electrical impedance tomography during a decremental PEEP titration. Pigs were randomized to three groups and ventilated for 12 hours: PEEP set at ⩽3% of overdistention (low overdistention), ⩽3% of collapse (low collapse), and the crossing point of collapse and overdistention. Measurements and Main Results: Thirty-six pigs (12 per group) were included. Median (interquartile range) values of PEEP were 7 (6-8), 11 (10-11), and 15 (12-16) cm H2O in the three groups (P < 0.001). With low overdistension, 6 (50%) pigs died, whereas survival was 100% in both other groups. Cause of death was hemodynamic in nature, with high transpulmonary vascular gradient and high epinephrine requirements. Compared with the other groups, pigs surviving with low overdistension had worse respiratory mechanics and gas exchange during the entire protocol. Minimal differences existed between crossing-point and low-collapse animals in physiological parameters, but postmortem alveolar density was more homogeneous in the crossing-point group. Inflammatory markers were not significantly different. Conclusions: PEEP to minimize overdistention resulted in high mortality in an animal model of ARDS. Minimizing collapse or choosing a compromise between collapse and overdistention may result in less lung injury, with potential benefits of the compromise approach.

Electrical Impedance Tomography-based Ventilation Patterns in Patients after Major Surgery

Rationale: General anesthesia and mechanical ventilation have negative impacts on the respiratory system, causing heterogeneous distribution of lung aeration, but little is known about the ventilation patterns of postoperative patients and their association with clinical outcomes. Objectives: To clarify the phenotypes of ventilation patterns along a gravitational direction after surgery by using electrical impedance tomography (EIT) and to evaluate their association with postoperative pulmonary complications (PPCs) and other relevant clinical outcomes. Methods: Adult postoperative patients at high risk for PPCs, receiving mechanical ventilation on ICU admission (N = 128), were prospectively enrolled between November 18, 2021 and July 18, 2022. PPCs were prospectively scored until hospital discharge, and their association with phenotypes of ventilation patterns was studied. The secondary outcomes were the times to wean from mechanical ventilation and oxygen use and the length of ICU stay. Measurements and Main Results: Three phenotypes of ventilation patterns were revealed by EIT: phenotype 1 (32% [n = 41], a predominance of ventral ventilation), phenotype 2 (41% [n = 52], homogeneous ventilation), and phenotype 3 (27% [n = 35], a predominance of dorsal ventilation). The median PPC score was higher in phenotype 1 and phenotype 3 than in phenotype 2. The median time to wean from mechanical ventilation was longer in phenotype 1 versus phenotype 2. The median duration of ICU stay was longer in phenotype 1 versus phenotype 2. The median time to wean from oxygen use was longer in phenotype 1 and phenotype 3 than in phenotype 2. Conclusions: Inhomogeneous ventilation patterns revealed by EIT on ICU admission were associated with PPCs, delayed weaning from mechanical ventilation and oxygen use, and a longer ICU stay.

High vs Low PEEP in Patients With ARDS Exhibiting Intense Inspiratory Effort During Assisted Ventilation: A Randomized Crossover Trial

BACKGROUND

Positive end-expiratory pressure (PEEP) can potentially modulate inspiratory effort (ΔPes), which is the major determinant of self-inflicted lung injury.

RESEARCH QUESTION

Does high PEEP reduce ΔPes in patients with moderate-to-severe ARDS on assisted ventilation?

STUDY DESIGN AND METHODS

Sixteen patients with Pao2/Fio2 ≤ 200 mm Hg and ΔPes ≥ 10 cm H2O underwent a randomized sequence of four ventilator settings: PEEP = 5 cm H2O or PEEP = 15 cm H2O + synchronous (pressure support ventilation [PSV]) or asynchronous (pressure-controlled intermittent mandatory ventilation [PC-IMV]) inspiratory assistance. ΔPes and respiratory system, lung, and chest wall mechanics were assessed with esophageal manometry and occlusions. PEEP-induced alveolar recruitment and overinflation, lung dynamic strain, and tidal volume distribution were assessed with electrical impedance tomography.

RESULTS

ΔPes was not systematically different at high vs low PEEP (pressure support ventilation: median, 20 cm H2O; interquartile range (IQR), 15-24 cm H2O vs median, 15 cm H2O; IQR, 13-23 cm H2O; P = .24; pressure-controlled intermittent mandatory ventilation: median, 20; IQR, 18-23 vs median, 19; IQR, 17-25; P = .67, respectively). Similarly, respiratory system and transpulmonary driving pressures, tidal volume, lung/chest wall mechanics, and pendelluft extent were not different between study phases. High PEEP resulted in lower or higher ΔPes, respiratory system driving pressure, and transpulmonary driving pressure according to whether this increased or decreased respiratory system compliance (r = -0.85, P < .001; r = -0.75, P < .001; r = -0.80, P < .001, respectively). PEEP-induced changes in respiratory system compliance were driven by its lung component and were dependent on the extent of PEEP-induced alveolar overinflation (r = -0.66, P = .006). High PEEP caused variable recruitment and systematic redistribution of tidal volume toward dorsal lung regions, thereby reducing dynamic strain in ventral areas (pressure support ventilation: median, 0.49; IQR, 0.37-0.83 vs median, 0.96; IQR, 0.62-1.56; P = .003; pressure-controlled intermittent mandatory ventilation: median, 0.65; IQR, 0.42-1.31 vs median, 1.14; IQR, 0.79-1.52; P = .002). All results were consistent during synchronous and asynchronous inspiratory assistance.

INTERPRETATION

The impact of high PEEP on ΔPes and lung stress is interindividually variable according to different effects on the respiratory system and lung compliance resulting from alveolar overinflation. High PEEP may help mitigate the risk of self-inflicted lung injury solely if it increases lung/respiratory system compliance.

TRIAL REGISTRATION

ClinicalTrials.gov; No.: NCT04241874; URL: www.

CLINICALTRIALS

gov.

Comparison of lung aeration loss in open abdominal oncologic surgeries after ventilation with electrical impedance tomography-guided PEEP versus conventional PEEP: a pilot feasibility study

BACKGROUND

Existing literature lacks high-quality evidence regarding the ideal intraoperative positive end-expiratory pressure (PEEP) to minimize postoperative pulmonary complications (PPCs). We hypothesized that applying individualized PEEP derived from electrical impedance tomography would reduce the severity of postoperative lung aeration loss, deterioration in oxygenation, and PPC incidence.

METHODS

A pilot feasibility study was conducted on 36 patients who underwent open abdominal oncologic surgery. The patients were randomized to receive individualized PEEP or conventional PEEP at 4 cmH2O. The primary outcome was the impact of individualized PEEP on changes in the modified lung ultrasound score (MLUS) derived from preoperative and postoperative lung ultrasonography. A higher MLUS indicated greater lung aeration loss. The secondary outcomes were the PaO2/FiO2 ratio and PPC incidence.

RESULTS

A significant increase in the postoperative MLUS (12.0 ± 3.6 vs 7.9 ± 2.1, P < 0.001) and a significant difference between the postoperative and preoperative MLUS values (7.0 ± 3.3 vs 3.0 ± 1.6, P < 0.001) were found in the conventional PEEP group, indicating increased lung aeration loss. In the conventional PEEP group, the intraoperative PaO2/FiO2 ratios were significantly lower but not the postoperative ratios. The PPC incidence was not significantly different between the groups. Post-hoc analysis showed the increase in lung aeration loss and deterioration of intraoperative oxygenation correlated with the deviation from the individualized PEEP.

CONCLUSIONS

Individualized PEEP appears to protect against lung aeration loss and intraoperative oxygenation deterioration. The advantage was greater in patients whose individualized PEEP deviated more from the conventional PEEP.

Repurposing Disulfiram to Combat Acute Respiratory Distress Syndrome with Targeted Delivery by LET-Functionalized Nanoplatforms

Acute respiratory distress syndrome (ARDS) is a serious respiratory condition characterized by a damaged pulmonary endothelial barrier that causes protein-rich lung edema, an influx of proinflammatory cells, and treatment-resistant hypoxemia. Damage to pulmonary endothelial cells and inflammation are pivotal in ARDS development with a key role played by endothelial cell pyroptosis. Disulfiram (DSF), a drug that has long been used to treat alcohol addiction, has recently been identified as a potent inhibitor of gasdermin D (GSDMD)-induced pore formation and can thus prevent pyroptosis and inflammatory cytokine release. These findings indicate that DSF is a promising treatment for inflammatory disorders. However, addressing the challenge posed by its intrinsic physicochemical properties, which hinder intravenous administration, and effective delivery to pulmonary vascular endothelial cells are crucial. Herein, we used biocompatible liposomes incorporating a lung endothelial cell-targeted peptide (CGSPGWVRC) to produce DSF-loaded nanoparticles (DTP-LET@DSF NPs) for targeted delivery and reactive oxygen species-responsive release facilitated by the inclusion of thioketal (TK) within the liposomal structure. After intravenous administration, DTP-LET@DSF NPs exhibited excellent cytocompatibility and minor systemic toxicity, effectively inhibited pyroptosis, mitigated lipopolysaccharide (LPS)-induced ARDS, and prevented cytokine storms resulting from excessive immune reactions in ARDS mice. This study presents a straightforward nanoplatform for ARDS treatment that potentially paves the way for the clinical use of this nanomedicine.

An enhanced respiratory mechanics model based on double-exponential and fractional calculus

We address mathematical modelling of respiratory mechanics and put forward a model based on double-exponential and fractional calculus for parameter estimation, model simulation, and evaluation based on actual data. Our model has been implemented on a publicly available executable code with adjustable parameters, making it suitable for different applications. Our analysis represents the first application of fractional calculus and double-exponential modelling to respiratory mechanics, and allows us to propose a hybrid model fitting experimental data in different ventilation modes. Furthermore, our model can be used to study the mechanical features of the respiratory system, improve the safety of ventilation techniques, reduce ventilation damages, and provide strong support for fast and adaptive determination of ventilation parameters.

Surfactant status assessment and personalized therapy for surfactant deficiency or dysfunction

Surfactant is a pivotal neonatal drug used both for respiratory distress syndrome due to surfactant deficiency and for more complex surfactant dysfunctions (such as in case of neonatal acute respiratory distress syndrome). Despite its importance, indications for surfactant therapy are often based on oversimplified criteria. Lung biology and modern monitoring provide several diagnostic tools to assess the patient surfactant status and they can be used for a personalized surfactant therapy. This is desirable to improve the efficacy of surfactant treatment and reduce associated costs and side effects. In this review we will discuss these diagnostic tools from a pathophysiological and multi-disciplinary perspective, focusing on the quantitative or qualitative surfactant assays, lung mechanics or aeration measurements, and gas exchange metrics. Their biological and technical characteristics are described with practical information for clinicians. Finally, available evidence-based data are reviewed, and the diagnostic accuracy of the different tools is compared. Lung ultrasound seems the most suitable tool for assessing the surfactant status, while some other promising tests require further research and/or development.

Hyperoxia but not high tidal volume contributes to ventilator-induced lung injury in healthy mice

BACKGROUND

Mechanical ventilation is a supportive therapy used to maintain respiratory function in several clinical and surgical cases but is always accompanied by lung injury risk due to improper treatment. We investigated how tidal volume and oxygen delivery would contribute independently or synergistically to ventilator-induced lung injury (VILI).

METHODS

Under general anesthesia and tracheal intubation, healthy female C57BL/6 N mice (9 weeks old) were randomly ventilated for 2 h by standard (7 ml/kg) or high (14 ml/kg) tidal volume at positive end-expiratory pressure (PEEP) of 2 cmH2O, with room air, 50% O2 (moderate hyperoxia), or 100% O2 (severe hyperoxia); respectively. Mice were sacrificed 4 h after mechanical ventilation, and lung tissues were collected for experimental assessments on lung injury.

RESULTS

Compared with the healthy control, severe hyperoxia ventilation by either standard or high tidal volume resulted in significantly higher wet-to-dry lung weight ratio and higher levels of IL-1β and 8-OHdG in the lungs. However, moderate hyperoxia ventilation, even by high tidal volume did not significantly increase the levels of IL-1β and 8-OHdG in the lungs. Western blot analysis showed that the expression of RhoA, ROCK1, MLC2, and p-MLC2 was not significantly induced in the ventilated lungs, even by high tidal volume at 2 cmH2O PEEP.

CONCLUSION

Severe hyperoxia ventilation causes inflammatory response and oxidative damage in mechanically ventilated lungs, while high tidal volume ventilation at a reasonable PEEP possibly does not cause VILI.

Standardized approach for extubation during extracorporeal membrane oxygenation in severe acute respiratory distress syndrome: a prospective observational study

BACKGROUND

Extubation during extracorporeal oxygenation (ECMO) in severe acute respiratory distress syndrome (ARDS) has not been well studied. Despite the potential benefits of this strategy, weaning from ECMO before liberation from invasive mechanical ventilation remains the most frequent approach. Our aim was to evaluate the safety and feasibility of a standardized approach for extubation during ECMO in patients with severe ARDS.

RESULTS

We conducted a prospective observational study to assess the safety and feasibility of a standardized approach for extubation during ECMO in severe ARDS among 254 adult patients across 4 intensive care units (ICU) from 2 tertiary ECMO centers over 6 years. This consisted of a daily assessment of clinical and gas exchange criteria based on an Extracorporeal Life Support Organization guideline, with extubation during ECMO after validation by a dedicated intensive care medicine specialist. Fifty-four (21%) patients were extubated during ECMO, 167 (66%) did not reach the clinical criteria, and in 33 (13%) patients, gas exchange precluded extubation during ECMO. At ECMO initiation, there were fewer extrapulmonary organ dysfunctions (lower SOFA score [OR, 0.88; 95% CI, 0.79-0.98; P = .02] with similar PaO2/FiO2) when compared with patients not extubated during ECMO. Extubation during ECMO associated with shorter duration of invasive mechanical ventilation (7 (4-18) vs. 32 (18-54) days; P < .01) and of ECMO (12 (7-25) vs. 19 (10-41) days; P = .01). This was accompanied by a lower incidence of hemorrhagic shock (2 vs. 11%; P = .05), but more cannula-associated deep vein thrombosis (49 vs. 31%; P = .02) and failed extubation (20 vs. 6%; P < .01). There were no increased major adverse events. Extubation during ECMO is associated with a lower risk of all-cause death, independently of measured confounding (adjusted logistic regression OR 0.23; 95% confidence interval 0.08-0.69, P = .008).

CONCLUSIONS

A standardized approach was safe and feasible allowing extubation during ECMO in 21% of patients with severe ARDS, selecting patients who will have a shorter duration of invasive mechanical ventilation, ECMO course, and ICU stay, as well as fewer infectious complications, and high hospital survival.

A novel method for assessment of airway opening pressure without the need for low-flow insufflation

BACKGROUND

Airway opening pressure (AOP) detection and measurement are essential for assessing respiratory mechanics and adapting ventilation. We propose a novel approach for AOP assessment during volume assist control ventilation at a usual constant-flow rate of 60 L/min.

OBJECTIVES

To validate the conductive pressure (Pcond) method, which compare the Pcond-defined on the airway pressure waveform as the difference between the airway pressure level at which an abrupt change in slope occurs at the beginning of insufflation and PEEP-to resistive pressure for AOP detection and measurement, and to compare its respiratory and hemodynamic tolerance to the standard low-flow insufflation method.

METHODS

The proof-of-concept of the Pcond method was assessed on mechanical (lung simulator) and physiological (cadavers) bench models. Its diagnostic performance was evaluated in 213 patients, using the standard low-flow insufflation method as a reference. In 45 patients, the respiratory and hemodynamic tolerance of the Pcond method was compared with the standard low-flow method.

MEASUREMENTS AND MAIN RESULTS

Bench assessments validated the Pcond method proof-of-concept. Sensitivity and specificity of the Pcond method for AOP detection were 93% and 91%, respectively. AOP obtained by Pcond and standard low-flow methods strongly correlated (r = 0.84, p < 0.001). Changes in SpO2 were significantly lower during Pcond than during standard method (p < 0.001).

CONCLUSION

Determination of Pcond during constant-flow assist control ventilation may permit to easily and safely detect and measure AOP.

Mechanical ventilation in patients with acute respiratory distress syndrome: current status and future perspectives

INTRODUCTION

Although there has been extensive research on mechanical ventilation for acute respiratory distress syndrome (ARDS), treatment remains mainly supportive. Recent studies and new ventilatory modes have been proposed to manage patients with ARDS; however, the clinical impact of these strategies remains uncertain and not clearly supported by guidelines. The aim of this narrative review is to provide an overview and update on ventilatory management for patients with ARDS.

AREAS COVERED

This article reviews the literature regarding mechanical ventilation in ARDS. A comprehensive overview of the principal settings for the ventilator parameters involved is provided as well as a report on the differences between controlled and assisted ventilation. Additionally, new modes of assisted ventilation are presented and discussed. The evidence concerning rescue strategies, including recruitment maneuvers and extracorporeal membrane oxygenation support, is analyzed. PubMed, EBSCO, and the Cochrane Library were searched up until June 2023, for relevant literature.

EXPERT OPINION

Available evidence for mechanical ventilation in cases of ARDS suggests the use of a personalized mechanical ventilation strategy. Although promising, new modes of assisted mechanical ventilation are still under investigation and guidelines do not recommend rescue strategies as the standard of care. Further research on this topic is required.

Individualised flow-controlled versus pressure-controlled ventilation in a porcine oleic acid-induced acute respiratory distress syndrome model

BACKGROUND

A continuous gas flow provided by flow-controlled ventilation (FCV) facilitates accurate dynamic compliance measurement and allows the clinician to individually optimise positive end-expiratory and peak pressure settings accordingly.

OBJECTIVE

The aim of this study was to compare the efficiency of gas exchange and impact on haemodynamics between individualised FCV and pressure-controlled ventilation (PCV) in a porcine model of oleic acid-induced acute respiratory distress syndrome (ARDS).

DESIGN

Randomised controlled interventional trial conducted on 16 pigs.

SETTING

Animal operating facility at the Medical University Innsbruck.

INTERVENTIONS

ARDS was induced in lung healthy pigs by intravenous infusion of oleic acid until moderate-to-severe ARDS at a stable Horowitz quotient (PaO 2 FiO 2-1 ) of 80 to 120 over a period of 30 min was obtained. Ventilation was then either performed with individualised FCV ( n  = 8) established by compliance-guided pressure titration or PCV ( n  = 8) with compliance-guided titration of the positive end-expiratory pressure and peak pressure set to achieve a tidal volume of 6 ml kg -1 over a period of 2 h.

MAIN OUTCOME MEASURES

Gas exchange parameters were assessed by the PaO 2 FiO 2-1 quotient and CO 2 removal by the PaCO 2 value in relation to required respiratory minute volume. Required catecholamine support for haemodynamic stabilisation was measured.

RESULTS

The FCV group showed significantly improved oxygenation [149.2 vs. 110.4, median difference (MD) 38.7 (8.0 to 69.5) PaO 2 FiO 2-1 ; P  = 0.027] and CO 2 removal [PaCO 2 7.25 vs. 9.05, MD -1.8 (-2.87 to -0.72) kPa; P  = 0.006] at a significantly lower respiratory minute volume [8.4 vs. 11.9, MD -3.6 (-5.6 to -1.5) l min -1 ; P  = 0.005] compared with PCV. In addition, in FCV-pigs, haemodynamic stabilisation occurred with a significant reduction of required catecholamine support [norepinephrine 0.26 vs. 0.86, MD -0.61 (-1.12 to -0.09) μg kg -1  min -1 ; P  = 0.037] during 2 ventilation hours.

CONCLUSION

In this oleic acid-induced porcine ARDS model, individualised FCV significantly improved gas exchange and haemodynamic stability compared with PCV.

TRIAL REGISTRATION

Protocol no.: BMBWF-66.011/0105-V/3b/2019).

Accuracy of the estimations of respiratory mechanics using an expiratory time constant in passive and active breathing conditions: a bench study

BACKGROUND

Respiratory mechanics monitoring provides useful information for guiding mechanical ventilation, but many measuring methods are inappropriate for awake patients. This study aimed to evaluate the accuracy of dynamic mechanics estimation using expiratory time constant (RC_exp) calculation during noninvasive pressure support ventilation (PSV) with air leak in different lung models.

METHODS

A Respironics V60 ventilator was connected to an active breathing simulator for modeling five profiles: normal adult, restrictive, mildly and severely obstructive, and mixed obstructive/restrictive. Inspiratory pressure support was adjusted to maintain tidal volumes (V_T), achieving 5.0, 7.0, and 10.0 ml/kg body weight. PEEP was set at 5 cmH₂O, and the back-up rate was 10 bpm. Measurements were conducted at system leaks of 25-28 L/min. RC_exp was estimated from the ratio at 75% exhaled V_T and flow rate, which was then used to determine respiratory system compliance (C_rs) and airway resistance (R_aw).

RESULTS

In non-obstructive conditions (R_aw ≤ 10 cmH₂O/L/s), the C_rs was overestimated in the PSV mode. Peak inspiratory and expiratory flow and V_T increased with PS levels, as calculated C_rs decreased. In passive breathing, the difference of C_rs between different V_T was no significant. Underestimations of inspiratory resistance and expiratory resistance were observed at V_T of 5.0 ml/kg. The difference was minimal at V_T of 7.0 ml/kg. During non-invasive PSV, the estimation of airway resistance with the RC_exp method was accurately at V_T of 7.0 ml/kg.

CONCLUSIONS

The difference between the calculated C_rs and the preset value was influenced by the volume, status and inspiratory effort in spontaneously breathing.

SOLVe: a closed-loop system focused on protective mechanical ventilation

BACKGROUND

Mechanical ventilation is an essential component in the treatment of patients with acute respiratory distress syndrome. Prompt adaptation of the settings of a ventilator to the variable needs of patients is essential to ensure personalised and protective ventilation. Still, it is challenging and time-consuming for the therapist at the bedside. In addition, general implementation barriers hinder the timely incorporation of new evidence from clinical studies into routine clinical practice.

RESULTS

We present a system combing clinical evidence and expert knowledge within a physiological closed-loop control structure for mechanical ventilation. The system includes multiple controllers to support adequate gas exchange while adhering to multiple evidence-based components of lung protective ventilation. We performed a pilot study on three animals with an induced ARDS. The system achieved a time-in-target of over 75 % for all targets and avoided any critical phases of low oxygen saturation, despite provoked disturbances such as disconnections from the ventilator and positional changes of the subject.

CONCLUSIONS

The presented system can provide personalised and lung-protective ventilation and reduce clinician workload in clinical practice.

Is COVID-19 different from other causes of acute respiratory distress syndrome?

Coronavirus disease 2019 (COVID-19) pneumonia can lead to acute hypoxemic respiratory failure. When mechanical ventilation is needed, almost all patients with COVID-19 pneumonia meet the criteria for acute respiratory distress syndrome (ARDS). The question of the specificities of COVID-19-associated ARDS compared to other causes of ARDS is of utmost importance, as it may justify changes in ventilatory strategies. This review aims to describe the pathophysiology of COVID-19-associated ARDS and discusses whether specific ventilatory strategies are required in these patients.

Diseased and healthy murine local lung strains evaluated using digital image correlation

Tissue remodeling in pulmonary disease irreversibly alters lung functionality and impacts quality of life. Mechanical ventilation is amongst the few pulmonary interventions to aid respiration, but can be harmful or fatal, inducing excessive regional (i.e., local) lung strains. Previous studies have advanced understanding of diseased global-level lung response under ventilation, but do not adequately capture the critical local-level response. Here, we pair a custom-designed pressure-volume ventilator with new applications of digital image correlation, to directly assess regional strains in the fibrosis-induced ex-vivo mouse lung, analyzed via regions of interest. We discuss differences between diseased and healthy lung mechanics, such as distensibility, heterogeneity, anisotropy, alveolar recruitment, and rate dependencies. Notably, we compare local and global compliance between diseased and healthy states by assessing the evolution of pressure-strain and pressure-volume curves resulting from various ventilation volumes and rates. We find fibrotic lungs are less-distensible, with altered recruitment behaviors and regional strains, and exhibit disparate behaviors between local and global compliance. Moreover, these diseased characteristics show volume-dependence and rate trends. Ultimately, we demonstrate how fibrotic lungs may be particularly susceptible to damage when contrasted to the strain patterns of healthy counterparts, helping to advance understanding of how ventilator induced lung injury develops.

Nonlinear N - Compartments model of respiratory mechanics considering viscoelasticity, inertia and surface tension properties

Respiratory biomechanics constitutes an important topic in clinical practice. Different strategies like mathematical models have been implemented to understand and replicate scenarios allowing deeper analysis. In this paper, a nonlinear N - compartments model is presented, allowing to represent the lung in a heterogeneous way. It considers the resistance of each generation of the airway and each alveolar compartment characterized independently. Includes properties of nonlinear elastance, viscoelasticity, inertia, and surface tension. In this work, to show the functionality of the model, a simulation of four alveolar units coupled to the airway model is presented using pressure as input signal simulating mechanical ventilation. However, the model can be used to simulate any desired number of alveolar units. Values at airway output were compared to the linear model, obtaining a correlation close to 1. Also, was compared to a physical test lung using Hamilton - S1 mechanical ventilator obtaining a positive correlation. The model makes it possible to evaluate the effects of different properties during spontaneous respiration or mechanical ventilation, both at the airway opening and alveolar. These properties include viscoelasticity, surface tension, inertia, among others.

Prolonged Prone Position Ventilation Is Associated With Reduced Mortality in Intubated COVID-19 Patients

BACKGROUND

Prone position ventilation (PPV) is resource-intensive, yet the optimal strategy for PPV in intubated patients with COVID-19 is unclear.

RESEARCH QUESTION

Does a prolonged (24 or more h) PPV strategy improve mortality in intubated COVID-19 patients compared with intermittent (∼16 h with daily supination) PPV?

STUDY DESIGN AND METHODS

Multicenter, retrospective cohort study of consecutively admitted intubated COVID-19 patients treated with PPV between March 11 and May 31, 2020. The primary outcome was 30-day all-cause mortality. Secondary outcomes included 90-day all-cause mortality and prone-related complications. Inverse probability treatment weights (IPTW) were used to control for potential treatment selection bias.

RESULTS

Of the COVID-19 patients who received PPV, 157 underwent prolonged and 110 underwent intermittent PPV. Patients undergoing prolonged PPV had reduced 30-day (adjusted hazard ratio [aHR], 0.475; 95% CI, 0.336-0.670; P < .001) and 90-day (aHR, 0.638; 95% CI, 0.461-0.883; P = .006) mortality compared with intermittent PPV. In patients with Pao2/Fio2 ≤ 150 at the time of pronation, prolonged PPV was associated with reduced 30-day (aHR, 0.357; 95% CI, 0.213-0.597; P < .001) and 90-day mortality (aHR, 0.562; 95% CI, 0.357-0.884; P = .008). Patients treated with prolonged PPV underwent fewer pronation and supination events (median, 1; 95% CI, 1-2 vs 3; 95% CI, 1-4; P < .001). PPV strategy was not associated with overall PPV-related complications, although patients receiving prolonged PPV had increased rates of facial edema and lower rates of peri-proning hypotension.

INTERPRETATION

Among intubated COVID-19 patients who received PPV, prolonged PPV was associated with reduced mortality. Prolonged PPV was associated with fewer pronation and supination events and a small increase in rates of facial edema. These findings suggest that prolonged PPV is a safe, effective strategy for mortality reduction in intubated COVID-19 patients.

Pleural and transpulmonary pressures to tailor protective ventilation in children

This review aims to: (1) describe the rationale of pleural (P_PL) and transpulmonary (P_L) pressure measurements in children during mechanical ventilation (MV); (2) discuss its usefulness and limitations as a guide for protective MV; (3) propose future directions for paediatric research. We conducted a scoping review on P_L in critically ill children using PubMed and Embase search engines. We included peer-reviewed studies using oesophageal (P_ES) and P_L measurements in the paediatric intensive care unit (PICU) published until September 2021, and excluded studies in neonates and patients treated with non-invasive ventilation. P_L corresponds to the difference between airway pressure and P_PL Oesophageal manometry allows measurement of P_ES, a good surrogate of P_PL, to estimate P_L directly at the bedside. Lung stress is the P_L, while strain corresponds to the lung deformation induced by the changing volume during insufflation. Lung stress and strain are the main determinants of MV-related injuries with P_L and P_PL being key components. P_L-targeted therapies allow tailoring of MV: (1) Positive end-expiratory pressure (PEEP) titration based on end-expiratory P_L (direct measurement) may be used to avoid lung collapse in the lung surrounding the oesophagus. The clinical benefit of such strategy has not been demonstrated yet. This approach should consider the degree of recruitable lung, and may be limited to patients in which PEEP is set to achieve an end-expiratory P_L value close to zero; (2) Protective ventilation based on end-inspiratory P_L (derived from the ratio of lung and respiratory system elastances), might be used to limit overdistention and volutrauma by targeting lung stress values < 20-25 cmH₂O; (3) P_PL may be set to target a physiological respiratory effort in order to avoid both self-induced lung injury and ventilator-induced diaphragm dysfunction; (4) P_PL or P_L measurements may contribute to a better understanding of cardiopulmonary interactions. The growing cardiorespiratory system makes children theoretically more susceptible to atelectrauma, myotrauma and right ventricle failure. In children with acute respiratory distress, P_PL and P_L measurements may help to characterise how changes in PEEP affect P_PL and potentially haemodynamics. In the PICU, P_PL measurement to estimate respiratory effort is useful during weaning and ventilator liberation. Finally, the use of P_PL tracings may improve the detection of patient ventilator asynchronies, which are frequent in children. Despite these numerous theoritcal benefits in children, P_ES measurement is rarely performed in routine paediatric practice. While the lack of robust clincal data partially explains this observation, important limitations of the existing methods to estimate P_PL in children, such as their invasiveness and technical limitations, associated with the lack of reference values for lung and chest wall elastances may also play a role. P_PL and P_L monitoring have numerous potential clinical applications in the PICU to tailor protective MV, but its usefulness is counterbalanced by technical limitations. Paediatric evidence seems currently too weak to consider oesophageal manometry as a routine respiratory monitoring. The development and validation of a noninvasive estimation of P_L and multimodal respiratory monitoring may be worth to be evaluated in the future.

The complex lipid, SPPCT-800, reduces lung damage, improves pulmonary function and decreases pro-inflammatory cytokines in the murine LPS-induced acute respiratory distress syndrome (ARDS) model

CONTEXT

Acute respiratory distress syndrome (ARDS) is a highly fatal, inflammatory condition of lungs with multiple causes. There is no adequate treatment.

OBJECTIVE

Using the murine LPS-induced ARDS model, we investigate SPPCT-800 (a complex lipid) as treatment for ARDS.

MATERIALS AND METHODS

C57B16/N mice received 50 μg of Escherichia coli O111:B4 lipopolysaccharide (LPS). SPPCT-800 was given as either: (1) 20 or 200 mg/kg dose 3 h after LPS; (2) 200 mg/kg (prophylactically) 30 min before LPS; or (3) eight 200 mg/kg treatments over 72 h. Controls received saline installations.

RESULTS

At 48 and 72 h, SpO₂ was 94% and 90% in controls compared to 97% and 94% in treated animals. Expiration times, at 24 and 48 h, were 160 and 137 msec for controls, but 139 and 107 msec with SPPCT-800. In BALF (24 h), cell counts were 4.7 × 10⁶ (controls) and 2.9 × 10⁶ (treated); protein levels were 1.5 mg (controls) and 0.4 mg (treated); and IL-6 was 942 ± 194 pg/mL (controls) versus 850 ± 212 pg/mL (treated) [at 72 h, 4664 ± 2591 pg/mL (controls) versus 276 ± 151 pg/mL (treated)]. Weight losses, at 48 and 72 h, were 20% and 18% (controls), but 14% and 8% (treated). Lung injury scores, at 24 and 72 h, were 1.4 and 3.0 (controls) and 0.3 and 2.2 (treated).

DISCUSSION AND CONCLUSIONS

SPPCT-800 was effective in reducing manifestations of ARDS. SPPCT-800 should be further investigated as therapy for ARDS, especially in longer duration or higher cumulative dose studies.

Driving pressure-guided ventilation decreases the mechanical power compared to predicted body weight-guided ventilation in the Acute Respiratory Distress Syndrome

Background

Whether targeting the driving pressure (∆P) when adjusting the tidal volume in mechanically ventilated patients with the acute respiratory distress syndrome (ARDS) may decrease the risk of ventilator-induced lung injury remains a matter of research. In this study, we assessed the effect of a ∆P-guided ventilation on the mechanical power.

Methods

We prospectively included adult patients with moderate-to-severe ARDS. Positive end expiratory pressure was set by the attending physician and kept constant during the study. Tidal volume was first adjusted to target 6 ml/kg of predicted body weight (PBW-guided ventilation) and subsequently modified within a range from 4 to 10 ml/kg PBW to target a ∆P between 12 and 14 cm H₂O. The respiratory rate was then re-adjusted within a range from 12 to 40 breaths/min until EtCO₂ returned to its baseline value (∆P-guided ventilation). Mechanical power was computed at each step.

Results

Fifty-one patients were included between December 2019 and May 2021. ∆P-guided ventilation was feasible in all but one patient. The ∆P during PBW-guided ventilation was already within the target range of ∆P-guided ventilation in five (10%) patients, above in nine (18%) and below in 36 (72%). The change from PBW- to ∆P-guided ventilation was thus accompanied by an overall increase in tidal volume from 6.1 mL/kg PBW [5.9–6.2] to 7.7 ml/kg PBW [6.2–8.7], while respiratory rate was decreased from 29 breaths/min [26–32] to 21 breaths/min [16–28] (p < 0.001 for all comparisons). ∆P-guided ventilation was accompanied by a significant decrease in mechanical power from 31.5 J/min [28–35.7] to 28.8 J/min [24.6–32.6] (p < 0.001), representing a relative decrease of 7% [0–16]. With ∆P-guided ventilation, the PaO₂/FiO₂ ratio increased and the ventilatory ratio decreased.

Conclusion

As compared to a conventional PBW-guided ventilation, a ∆P-guided ventilation strategy targeting a ∆P between 12 and 14 cm H₂O required to change the tidal volume in 90% of the patients. Such ∆P-guided ventilation significantly reduced the mechanical power. Whether this physiological observation could be associated with clinical benefit should be assessed in clinical trials.

Whole Transcriptomic Analysis of Key Genes and Signaling Pathways in Endogenous ARDS

Objective

To analyze the differentially expressed genes (DEGs) in rats with endogenous acute respiratory distress syndrome (ARDS) lung injury and explore the pathogenesis and early diagnostic molecular markers using whole transcriptomic data.

Methods

Twelve 8-week-old male Sprague Dawley rats were selected and randomly and equally divided into ARDS lung injury group and normal control group. RNA was extracted from the left lung tissues of both the groups and sequenced using the paired-end sequencing mode of the Illumina Hiseq sequencing platform. The DEGs of miRNA, cirRNA, lncRNA, and mRNA were screened using DESeq2 software, and the ceRNA regulatory network was constructed using Cytoscape. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis were performed using the mRNA DEGs. STRING and Cytoscape software were used to construct the protein interaction network and identify the 15 key genes, which were verified using quantitative real-time polymerase chain reaction (qRT-PCR).

Results

Based on different screening conditions, and compared with the control group, the ARDS lung injury group showed 836 mRNA DEGs (386 upregulated and 450 downregulated), 110 lncRNA DEGs (53 upregulated and 57 downregulated), 19 circRNA DEGs (3 upregulated and 16 downregulated), and 6 miRNA DEGs (5 upregulated and 1 downregulated gene). GO showed that the DEGs of mRNA were mainly involved in biological processes, such as defense response to lipopolysaccharide and other organisms, leukocyte chemotaxis, neutrophil chemotaxis, and cytokine-mediated signaling. KEGG enrichment analysis showed that the DEGs played their biological roles mainly by participating in IL-17, TNF, and chemokine signaling pathways. The PPI analysis showed a total of 281 node proteins and 634 interaction edges. The top 15 key genes, which were screened, included Cxcl10, Mx1, Irf7, Isg15, Ifit3, Ifit2, Rsad2, Ifi47, Oasl, Dhx58, Usp18, Cmpk2, Herc6, Ifit1, and Gbp4. The ceRNA network analysis showed 69 nodes and 73 correlation pairs, where the key gene nodes were miR-21-3p, Camk2g, and Stx2.

Conclusions

The chemotaxis, migration, and degranulation of inflammatory cells, cytokine immune response, autophagy, and apoptosis have significant biological functions in the occurrence and development of endogenous acute lung injury during ARDS. Thus, the camk2g/miR-21-3p/lncRNA/circRNA network, CXCL10/CXCR3, and IL-17 signaling pathways might provide novel insights and targets for further studying the lung injury mechanism and clinical treatment.

Prognostic value of neutrophils to lymphocytes and platelets ratio for 28-day mortality in patients with acute respiratory distress syndrome: a retrospective study

Background

Acute respiratory distress syndrome (ARDS) is a rapidly progressive and fatal respiratory failure disease that often occurs in critically ill patients. Since ARDS is associated with immune dysregulation and coagulation abnormalities, it is necessary to identify an appropriate predictor that can accurately predict ARDS mortality based on its pathophysiology. Therefore, this study aimed to evaluate the clinical value of neutrophils to lymphocytes and platelets ratio (N/LPR) in predicting 28-day mortality in ARDS patients.

Methods

From July 2018 to October 2021, the medical records of ARDS patients were retrospective reviewed. Neutrophil count, lymphocyte count, and platelet count were collected, and the neutrophil-to-lymphocyte ratio (NLR) and N/LPR were calculated. Multivariate logistic regression analyses were performed to identify independent predictors of 28-day mortality in ARDS. Receiver operating characteristic (ROC) curve with the area under curve (AUC) was used to evaluate optimal cut-off values for 28-day mortality in ARDS. Kaplan–Meier analysis was used to estimate the 28-day survival probabilities stratified by optimal cut-off values of N/LPR and NLR.

Results

A total of 136 ARDS patients were included in this study and were further divided into survivors (n = 69) and non-survivors (n = 67) groups according to their survival status on day 28. There were no significant differences between the two groups in age, sex, history of smoking and drinking, comorbidities, and reasons of admission (P > 0.05). Non-survivors had significantly higher neutrophil counts, NLR and N/LPR and had significantly lower platelet counts than survivors (P < 0.05). Multivariate regression analysis revealed that N/LPR, NLR and platelet counts were independent predictors for 28-day mortality in ARDS (P < 0.05). The ROC analyses showed that N/LPR with optimal cut-off value of 10.57 (sensitivity: 74.6%; specificity: 72.5%) is a more reliable predictor for 28-day mortality in ARDS than NLR and platelet count (AUC: 0.785 vs. 0.679 vs. 0.326). Further subgroup analysis confirmed that ARDS patients with N/LPR < 10.57 had significantly lower 28-day mortality than patients with N/LPR ≥ 10.57 (P < 0.001). Kaplan–Meier analysis also confirmed that ARDS patients with N/LPR < 10.57 had significantly longer survival.

Conclusion

N/LPR is an independent risk factor associated with 28-day mortality in ARDS patients and shows better performance in predicting mortality rate than NLR.

Lung Mechanics Over the Century: From Bench to Bedside and Back to Bench

Lung physiology research advanced significantly over the last 100 years. Respiratory mechanics applied to animal models of lung disease extended the knowledge of the workings of respiratory system. In human research, a better understanding of respiratory mechanics has contributed to development of mechanical ventilators. In this review, we explore the use of respiratory mechanics in basic science to investigate asthma and chronic obstructive pulmonary disease (COPD). We also discuss the use of lung mechanics in clinical care and its role on the development of modern mechanical ventilators. Additionally, we analyse some bench-developed technologies that are not in widespread use in the present but can become part of the clinical arsenal in the future. Finally, we explore some of the difficult questions that intensive care doctors still face when managing respiratory failure. Bringing back these questions to bench can help to solve them. Interaction between basic and translational science and human subject investigation can be very rewarding, as in the conceptualization of “Lung Protective Ventilation” principles. We expect this interaction to expand further generating new treatments and managing strategies for patients with respiratory disease.

Comprehensive Analysis of Potential ceRNA Network and Different Degrees of Immune Cell Infiltration in Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is a leading cause of death in critically ill patients due to hypoxemic respiratory failure. The specific pathogenesis underlying ARDS has not been fully elucidated. In this study, we constructed a triple regulatory network involving competing endogenous RNA (ceRNA) to investigate the potential mechanism of ARDS and evaluated the immune cell infiltration patterns in ARDS patients. Overall, we downloaded three microarray datasets that included 60 patients with sepsis-induced ARDS and 79 patients with sepsis alone from the public Gene Expression Omnibus (GEO) database and identified differentially expressed genes (DEGs, including 9 DElncRNAs, 9 DEmiRNAs, and 269 DEmRNAs) by R software. The DEGs were subjected to the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) for functional enrichment analysis, and a protein–protein interaction (PPI) network was generated for uncovering interactive relationships among DEmRNAs. Then, a ceRNA network that contained 5 DElncRNAs, 7 DEmiRNAs, and 71 DEmRNAs was established according to the overlapping genes in both DEGs and predicted genes by public databases. Finally, we identified the TUG1/miR-140-5p/NFE2L2 pathway as the hub pathway in the whole network through Cytoscape. In addition, we evaluated the distribution of 22 subtypes of immune cells and recognized three differentially expressed immune cells in patients with sepsis-induced ARDS by “Cell Type Identification by Estimating Relative Subsets of Known RNA Transcripts (CIBERSORT)” algorithm, namely, naive B cells, regulatory T cells, and eosinophils. Correlations between differentially expressed immune cells and hub genes in the ceRNA network were also performed. In conclusion, we demonstrated a new potential regulatory mechanism underlying ARDS (the TUG1/miR-140-5p/NFE2L2 ceRNA regulatory pathway), which may help in further exploring the pathogenesis of ARDS.

Long term feasibility of ultraprotective lung ventilation with low-flow extracorporeal carbon dioxide removal in ARDS patients

PURPOSE

To explore the feasibility of long-term application of ultraprotective ventilation with low flow ECCO₂R support in moderate-severe ARDS patients and the reduction of mechanical power (MP) compared to lung protective ventilation.

MATERIAL AND METHODS

ARDS patients with PaO₂/FiO₂ < 200, PEEP of 10 cmH₂O, tidal volume 6 ml/Kg of predicted body weight (PBW), plateau pressure > 24 cmH₂O, MP > 17 J/min were prospectively enrolled. After 2 h tidal volume was reduced to 4-5 ml/kg, respiratory rate (RR) and PEEP were changed to maintain similar minute ventilation and mean airway pressure (MAP) to those obtained at baseline. After 2 h, ECCO₂R support was started, RR was decreased and PEEP was increased to maintain similar PaCO₂ and MAP, respectively.

RESULTS

The only reduction of tidal volume with the increase in RR did not decrease MP. The application of low flow ECCO₂R support allowed a reduction of RR from 25 [24-30] to 11 [9-14] bpm and MP from 18 [13-23] to 8 [7-11] J/min. During the following 5 days no changes in mechanics variables and gas exchange occurred.

CONCLUSIONS

The application of low flow ECCO₂R support with ultraprotective ventilation was feasible minimizing the MP without deterioration in oxygenation in ARDS patients.

Monitoring Lung Injury Severity and Ventilation Intensity during Mechanical Ventilation

Acute respiratory distress syndrome (ARDS) is a severe form of respiratory failure burden by high hospital mortality. No specific pharmacologic treatment is currently available and its ventilatory management is a key strategy to allow reparative and regenerative lung tissue processes. Unfortunately, a poor management of mechanical ventilation can induce ventilation induced lung injury (VILI) caused by physical and biological forces which are at play. Different parameters have been described over the years to assess lung injury severity and facilitate optimization of mechanical ventilation. Indices of lung injury severity include variables related to gas exchange abnormalities, ventilatory setting and respiratory mechanics, ventilation intensity, and the presence of lung hyperinflation versus derecruitment. Recently, specific indexes have been proposed to quantify the stress and the strain released over time using more comprehensive algorithms of calculation such as the mechanical power, and the interaction between driving pressure (DP) and respiratory rate (RR) in the novel DP multiplied by four plus RR [(4 × DP) + RR] index. These new parameters introduce the concept of ventilation intensity as contributing factor of VILI. Ventilation intensity should be taken into account to optimize protective mechanical ventilation strategies, with the aim to reduce intensity to the lowest level required to maintain gas exchange to reduce the potential for VILI. This is further gaining relevance in the current era of phenotyping and enrichment strategies in ARDS.

Inhaled CO2 vs. Hypercapnia Obtained by Low Tidal Volume or Instrumental Dead Space in Unilateral Pulmonary Artery Ligation: Any Difference for Lung Protection?

Background

Unilateral ligation of the pulmonary artery (UPAL) induces bilateral lung injury in pigs undergoing controlled mechanical ventilation. Possible mechanisms include redistribution of ventilation toward the non-ligated lung and hypoperfusion of the ligated lung. The addition of 5% CO₂ to the inspiratory gas (FiCO₂) prevents the injury, but it is not clear whether lung protection is a direct effect of CO₂ inhalation or it is mediated by plasmatic hypercapnia. This study aims to compare the effects and mechanisms of FiCO₂vs. hypercapnia induced by low tidal volume ventilation or instrumental dead space.

Methods

Healthy pigs underwent left UPAL and were allocated for 48 h to the following: Volume-controlled ventilation (VCV) with V_T 10 ml/kg (injury, n = 6); VCV plus 5% FiCO₂ (FiCO₂, n = 7); VCV with V_T 6 ml/kg (low V_T, n = 6); VCV plus additional circuit dead space (instrumental V_D, n = 6). Histological score, regional compliance, wet-to-dry ratio, and inflammatory infiltrate were assessed to evaluate lung injury at the end of the study. To investigate the mechanisms of protection, we quantified the redistribution of ventilation to the non-ligated lung, as the ratio between the percentage of tidal volume to the right and to the left lung (V_TRIGHT/LEFT), and the hypoperfusion of the ligated lung as the percentage of blood flow reaching the left lung (Perfusion_LEFT).

Results

In the left ligated lung, injury was prevented only in the FiCO₂ group, as indicated by lower histological score, higher regional compliance, lower wet-to-dry ratio and lower density of inflammatory cells compared to other groups. For the right lung, the histological score was lower both in the FiCO₂ and in the low V_T groups, but the other measures of injury showed lower intensity only in the FiCO₂ group. V_TRIGHT/LEFT was lower and Perfusion_LEFT was higher in the FiCO₂ group compared to other groups.

Conclusion

In a model of UPAL, inhaled CO₂ but not hypercapnia grants bilateral lung protection. Mechanisms of protection include reduced overdistension of the non-ligated and increased perfusion of the ligated lung.

Therapeutic hypothermia attenuates physiologic, histologic, and metabolomic markers of injury in a porcine model of acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a lung injury characterized by noncardiogenic pulmonary edema and hypoxic respiratory failure. The purpose of this study was to investigate the effects of therapeutic hypothermia on short-term experimental ARDS. Twenty adult female Yorkshire pigs were divided into four groups (n = 5 each): normothermic control (C), normothermic injured (I), hypothermic control (HC), and hypothermic injured (HI). Acute respiratory distress syndrome was induced experimentally via intrapulmonary injection of oleic acid. Target core temperature was achieved in the HI group within 1 h of injury induction. Cardiorespiratory, histologic, cytokine, and metabolomic data were collected on all animals prior to and following injury/sham. All data were collected for approximately 12 h from the beginning of the study until euthanasia. Therapeutic hypothermia reduced injury in the HI compared to the I group (histological injury score = 0.51 ± 0.18 vs. 0.76 ± 0.06; p = 0.02) with no change in gas exchange. All groups expressed distinct phenotypes, with a reduction in pro-inflammatory metabolites, an increase in anti-inflammatory metabolites, and a reduction in inflammatory cytokines observed in the HI group compared to the I group. Changes to respiratory system mechanics in the injured groups were due to increases in lung elastance (E) and resistance (R) (ΔE from pre-injury = 46 ± 14 cmH₂ O L^-1 , p < 0.0001; ΔR from pre-injury: 3 ± 2 cmH₂ O L^-1  s^- , p = 0.30) rather than changes to the chest wall (ΔE from pre-injury: 0.7 ± 1.6 cmH₂ O L^-1 , p = 0.99; ΔR from pre-injury: 0.6 ± 0.1 cmH₂ O L^-1  s^- , p = 0.01). Both control groups had no change in respiratory mechanics. In conclusion, therapeutic hypothermia can reduce markers of injury and inflammation associated with experimentally induced short-term ARDS.

BMI1 Silencing Induces Mitochondrial Dysfunction in Lung Epithelial Cells Exposed to Hyperoxia

Acute Lung Injury (ALI), characterized by bilateral pulmonary infiltrates that restrict gas exchange, leads to respiratory failure. It is caused by an innate immune response with white blood cell infiltration of the lungs, release of cytokines, an increase in reactive oxygen species (ROS), oxidative stress, and changes in mitochondrial function. Mitochondrial alterations, changes in respiration, ATP production and the unbalancing fusion and fission processes are key events in ALI pathogenesis and increase mitophagy. Research indicates that BMI1 (B cell-specific Moloney murine leukemia virus integration site 1), a protein of the Polycomb repressive complex 1, is a cell cycle and survival regulator that plays a role in mitochondrial function. BMI1-silenced cultured lung epithelial cells were exposed to hyperoxia to determine the role of BMI1 in mitochondrial metabolism. Its expression significantly decreases in human lung epithelial cells (H441) following hyperoxic insult, as determined by western blot, Qrt-PCR, and functional analysis. This decrease correlates with an increase in mitophagy proteins, PINK1, Parkin, and DJ1; an increase in the expression of tumor suppressor PTEN; changes in the expression of mitochondrial biomarkers; and decreases in the oxygen consumption rate (OCR) and tricarboxylic acid enzyme activity. Our bioinformatics analysis suggested that the BMI1 multifunctionality is determined by its high level of intrinsic disorder that defines the ability of this protein to bind to numerous cellular partners. These results demonstrate a close relationship between BMI1 expression and mitochondrial health in hyperoxia-induced acute lung injury (HALI) and indicate that BMI1 is a potential therapeutic target to treat ALI and Acute Respiratory Distress Syndrome.

Right Ventricular Loading by Lung Inflation During Controlled Mechanical Ventilation

RATIONALE

The inspiratory rise in transpulmonary pressure during mechanical ventilation increases right ventricular (RV) afterload. One mechanism is that when alveolar pressure (Palv) exceeds left atrial pressure, West zone 1 or 2 (non-zone 3) conditions develop and Palv becomes the downstream pressure opposing RV ejection. The tidal volume (VT) at which this impact on the RV becomes hemodynamically evident is not well established.

OBJECTIVES

To determine the magnitude of RV afterload and prevalence of significant non-zone 3 conditions during inspiration across the range of VT currently prescribed in clinical practice.

METHODS

In post-operative passively ventilated cardiac surgery patients, we measured right atrial, RV, pulmonary artery, pulmonary artery occlusion (Ppao), plateau (Pplat), and esophageal (Peso) pressures during short periods of controlled ventilation with VT increments ranging between 2-12 ml/kg PBW. The inspiratory increase in RV afterload was evaluated hemodynamically and echocardiographically. The prevalence of non-zone 3 conditions was determined using 2 definitions based on changes in Peso, Ppao and Pplat.

RESULTS

Fifty-one patients were studied. There was a linear relationship between VT, driving pressure and transpulmonary pressure and the inspiratory increase in the RV isovolumetric contraction pressure. Echocardiographically, increasing VT was associated with a greater inspiratory increase in markers of afterload and a decrease in stroke volume. Non-zone 3 conditions were present in >50% of subjects at a VT ≥ 6 ml/kg PBW.

CONCLUSIONS

In the range of VT currently prescribed, RV afterload increases with increasing VT. A mechanical ventilation strategy that limits VT and driving pressure is cardio-protective.

Sequential lateral positioning as a new lung recruitment maneuver: an exploratory study in early mechanically ventilated Covid-19 ARDS patients

Background

A sequential change in body position from supine-to-both lateral positions under constant ventilatory settings could be used as a postural recruitment maneuver in case of acute respiratory distress syndrome (ARDS), provided that sufficient positive end-expiratory pressure (PEEP) prevents derecruitment. This study aims to evaluate the feasibility and physiological effects of a sequential postural recruitment maneuver in early mechanically ventilated COVID-19 ARDS patients.

Methods

A cohort of 15 patients receiving lung-protective mechanical ventilation in volume-controlled with PEEP based on recruitability were prospectively enrolled and evaluated in five sequentially applied positions for 30 min each: Supine-baseline; Lateral-1st side; 2nd Supine; Lateral-2nd side; Supine-final. PEEP level was selected using the recruitment-to-inflation ratio (R/I ratio) based on which patients received PEEP 12 cmH₂O for R/I ratio ≤ 0.5 or PEEP 15 cmH₂O for R/I ratio > 0.5. At the end of each period, we measured respiratory mechanics, arterial blood gases, lung ultrasound aeration, end-expiratory lung impedance (EELI), and regional distribution of ventilation and perfusion using electric impedance tomography (EIT).

Results

Comparing supine baseline and final, respiratory compliance (29 ± 9 vs 32 ± 8 mL/cmH₂O; p < 0.01) and PaO₂/FIO₂ ratio (138 ± 36 vs 164 ± 46 mmHg; p < 0.01) increased, while driving pressure (13 ± 2 vs 11 ± 2 cmH₂O; p < 0.01) and lung ultrasound consolidation score decreased [5 (4–5) vs 2 (1–4); p < 0.01]. EELI decreased ventrally (218 ± 205 mL; p < 0.01) and increased dorsally (192 ± 475 mL; p = 0.02), while regional compliance increased in both ventral (11.5 ± 0.7 vs 12.9 ± 0.8 mL/cmH₂O; p < 0.01) and dorsal regions (17.1 ± 1.8 vs 18.8 ± 1.8 mL/cmH₂O; p < 0.01). Dorsal distribution of perfusion increased (64.8 ± 7.3% vs 66.3 ± 7.2%; p = 0.01).

Conclusions

Without increasing airway pressure, a sequential postural recruitment maneuver improves global and regional respiratory mechanics and gas exchange along with a redistribution of EELI from ventral to dorsal lung areas and less consolidation.

Trial registration ClinicalTrials.gov, NCT04475068. Registered 17 July 2020, https://clinicaltrials.gov/ct2/show/NCT04475068

Supplementary Information

The online version contains supplementary material available at 10.1186/s13613-022-00988-9.

High Expression of CXCL10/CXCR3 in Ventilator-Induced Lung Injury Caused by High Mechanical Power

BACKGROUND

The energy delivered by a ventilator to the respiratory system in one minute is defined as mechanical power (MP). However, the effect of ventilator-induced lung injury (VILI) in patients suffering from acute respiratory distress syndrome (ARDS) is still unknown. Our previous studies revealed that CXCL10 may be a potential biomarker of lung injury in ARDS. Therefore, the aim of this study was to compare the lung injury of rats and patients under different MP conditions to explore the involvement of CXCL10 and its receptor CXCR3 in VILI.

METHODS

Patients were divided into the high mechanical power group (HMPp group) and low mechanical power group (LMPp group), while rats were assigned to the high mechanical power group (HMPr group), medium mechanical power group (MMPr group), and low mechanical power group (LMPr group). CXCL10 and CXCR3 plasma content in ARDS patients and rats under ventilation at different MP was measured, as well as their protein and mRNA expression in rat lungs.

RESULTS

CXCL10 and CXCR3 content in the plasma of ARDS patients in the HMPp was significantly higher than that in the LMPp. The increase of MP during mechanical ventilation in the rats gradually increased lung damage, and CXCL10 and CXCR3 levels in rat plasma gradually increased with the increase of MP. CXCL10 and CXCR3 protein and mRNA expression in the HMPr group and MMPr group was significantly higher than that in the LMPr group (P < 0.05). More mast cells were present in the trachea, bronchus, blood vessels, and lymphatic system in the rat lungs of the HMPr group, and the number of mast cells in the HMPr group (13.32 ± 3.27) was significantly higher than that in the LMPr group (3.25 ± 0.29) (P < 0.05).

CONCLUSION

The higher the MP, the more severe the lung injury, and the higher the CXCL10/CXCR3 expression. Therefore, CXCL10/CXCR3 might participate in VILI by mediating mast cell chemotaxis.

Reliability of plateau pressure during patient-triggered assisted ventilation. Analysis of a multicentre database

PURPOSE

An inspiratory hold during patient-triggered assisted ventilation potentially allows to measure driving pressure and inspiratory effort. However, muscular activity can make this measurement unreliable. We aim to define the criteria for inspiratory holds reliability during patient-triggered breaths.

MATERIAL AND METHODS

Flow, airway and esophageal pressure recordings during patient-triggered breaths from a multicentre observational study (BEARDS, NCT03447288) were evaluated by six independent raters, to determine plateau pressure readability. Features of "readable" and "unreadable" holds were compared. Muscle pressure estimate from the hold was validated against other measures of inspiratory effort.

RESULTS

Ninety-two percent of the recordings were consistently judged as readable or unreadable by at least four raters. Plateau measurement showed a high consistency among raters. A short time from airway peak to plateau pressure and a stable and longer plateau characterized readable holds. Unreadable plateaus were associated with higher indexes of inspiratory effort. Muscular pressure computed from the hold showed a strong correlation with independent indexes of inspiratory effort.

CONCLUSION

The definition of objective parameters of plateau reliability during assisted-breath provides the clinician with a tool to target a safer assisted-ventilation and to detect the presence of high inspiratory effort.

Mechanical Ventilation during Extracorporeal Membrane Oxygenation in Acute Respiratory Distress Syndrome: A Narrative Review

Acute respiratory distress syndrome (ARDS) is a life-threatening condition involving acute hypoxemic respiratory failure. Mechanical ventilation remains the cornerstone of management for ARDS; however, potentially injurious mechanical forces introduce the risk of ventilator-induced lung injury, multiple organ failure, and death. Extracorporeal membrane oxygenation (ECMO) is a salvage therapy aimed at ensuring adequate gas exchange for patients suffering from severe ARDS with profound hypoxemia where conventional mechanical ventilation has failed. ECMO allows for lower tidal volumes and airway pressures, which can reduce the risk of further lung injury, and allow the lungs to rest. However, the collateral effect of ECMO should be considered. Recent studies have reported correlations between mechanical ventilator settings during ECMO and mortality. In many cases, mechanical ventilation settings should be tailored to the individual; however, researchers have yet to establish optimal ventilator settings or determine the degree to which ventilation load can be decreased. This paper presents an overview of previous studies and clinical trials pertaining to the management of mechanical ventilation during ECMO for patients with severe ARDS, with a focus on clinical findings, suggestions, protocols, guidelines, and expert opinions. We also identified a number of issues that have yet to be adequately addressed.

Massive aspiration syndrome: a possible indication for "emergent" veno-venous extracorporeal membrane oxygenation?: a case report

Background

Veno-venous extracorporeal membrane oxygenation (VV-ECMO) is usually performed in cases of severe respiratory failure in which conventional and advanced mechanical ventilation strategies are ineffective in achieving true lung-protective ventilation, thus triggering ventilatory-induced lung injury. If circulatory failure coexists, veno-arterial ECMO (VA-ECMO) may be preferred over VV-ECMO because of its potential for circulatory support. In VA-ECMO, the respiratory contribution is less effective and the complication rate is higher than in the VV configuration.

Case presentation

The authors present a case in which VV-ECMO was performed in an emergency setting to treat a 68-year-old White male patient who experienced acute respiratory failure after massive aspiration. Despite intubation and intensive care unit admission, multiple organ failure occurred suddenly, thus prompting referral to a level-1 trauma center with an ECMO facility. The patient’s condition slowly improved with VV-ECMO support along with standard treatment for hemodynamic impairment. VV-ECMO was discontinued on day 8. The patient was extubated on day 14 and discharged home fully recovered 34 days after the event.

Conclusions

Attention was focused on the decision to initiate VV-ECMO support even in the presence of severe hemodynamic derangement, although VA-ECMO could have provided better hemodynamic support but less effective respiratory support.

Pathophysiology of Acute Respiratory Distress Syndrome and COVID-19 Lung Injury

The pathophysiology of acute respiratory distress syndrome (ARDS) is marked by inflammation-mediated disruptions in alveolar-capillary permeability, edema formation, reduced alveolar clearance and collapse/derecruitment, reduced compliance, increased pulmonary vascular resistance, and resulting gas exchange abnormalities due to shunting and ventilation-perfusion mismatch. Mechanical ventilation, especially in the setting of regional disease heterogeneity, can propagate ventilator-associated injury patterns including barotrauma/volutrauma and atelectrauma. Lung injury due to the novel coronavirus SARS-CoV-2 resembles other causes of ARDS, though its initial clinical characteristics may include more profound hypoxemia and loss of dyspnea perception with less radiologically-evident lung injury, a pattern not described previously in ARDS.

The Risk Factors for Weaning Failure of Mechanically Ventilated Patients With COVID-19: A Retrospective Study in National Medical Team Work

Purpose: This study aimed to describe the clinical and laboratory characteristics and the parameters of the respiratory mechanics of mechanically ventilated patients with confirmed COVID-19 pneumonia and to clarify the risk or protective factors for weaning failure. Methods: Patients diagnosed with COVID-19 pneumonia were selected from the special intensive care unit (ICU) of the Sino-French New City Branch of Tong Ji Hospital, Wuhan, and treated by the National Medical Team Work. They were divided into successful weaning (SW) group (N = 15) and unsuccessful weaning (USW) group (N = 18) according to the prognosis. Information of these patients was analyzed. Results: There were 33 patients included in this study. Patients in the USW group were associated with a poor outcome; the 28-day mortality rate was higher than in the SW group (86.7 vs. 16.7% p < 0.001). By comparison, we found that the initial plateau pressure (Pplat) and driving pressure (DP) of the USW group were higher and that compliance was lower than that of the SW group, but there was no difference between positive end-expiratory pressure (PEEP), partial pressure of carbon dioxide (PCO₂), and the ratio of partial pressure arterial oxygen and fraction of inspired oxygen (P/F ratio). Comparing the worst respiratory mechanics parameters of the two groups, the results of the Pplat, DP, compliance, and PEEP were the same as the initial data. The PCO₂ of the USW group was higher, while the P/F ratio was lower. A logistic regression analysis suggested that higher Pplat might be an independent risk factor and that higher compliance and lower DP might be protective factors for weaning failure of invasive mechanically ventilated patients with COVID-19 pneumonia. Conclusions: Patients with USW were associated with a poor outcome, higher Pplat might be a risk factor, and a higher compliance and a lower DP might be protective factors for the weaning failure of ventilated COVID-19 patients. Mechanical ventilation settings will affect the patient's prognosis.

Static compliance and driving pressure are associated with ICU mortality in intubated COVID-19 ARDS

Background

Pathophysiological features of coronavirus disease 2019-associated acute respiratory distress syndrome (COVID-19 ARDS) were indicated to be somewhat different from those described in nonCOVID-19 ARDS, because of relatively preserved compliance of the respiratory system despite marked hypoxemia. We aim ascertaining whether respiratory system static compliance (Crs), driving pressure (DP), and tidal volume normalized for ideal body weight (VT/kg IBW) at the 1st day of controlled mechanical ventilation are associated with intensive care unit (ICU) mortality in COVID-19 ARDS.

Methods

Observational multicenter cohort study. All consecutive COVID-19 adult patients admitted to 25 ICUs belonging to the COVID-19 VENETO ICU network (February 28th–April 28th, 2020), who received controlled mechanical ventilation, were screened. Only patients fulfilling ARDS criteria and with complete records of Crs, DP and VT/kg IBW within the 1st day of controlled mechanical ventilation were included. Crs, DP and VT/kg IBW were collected in sedated, paralyzed and supine patients.

Results

A total of 704 COVID-19 patients were screened and 241 enrolled. Seventy-one patients (29%) died in ICU. The logistic regression analysis showed that: (1) Crs was not linearly associated with ICU mortality (p value for nonlinearity = 0.01), with a greater risk of death for values < 48 ml/cmH₂O; (2) the association between DP and ICU mortality was linear (p value for nonlinearity = 0.68), and increasing DP from 10 to 14 cmH₂O caused significant higher odds of in-ICU death (OR 1.45, 95% CI 1.06–1.99); (3) VT/kg IBW was not associated with a significant increase of the risk of death (OR 0.92, 95% CI 0.55–1.52). Multivariable analysis confirmed these findings.

Conclusions

Crs < 48 ml/cmH₂O was associated with ICU mortality, while DP was linearly associated with mortality. DP should be kept as low as possible, even in the case of relatively preserved Crs, irrespective of VT/kg IBW, to reduce the risk of death.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-021-03667-6.

Pneumothorax due to COVID-19: Analysis of case reports

Cases of pneumothorax/pneumomediastinum have been reported in patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); however, the time to onset and hospital stay have rarely been studied. Coronavirus disease 2019 (COVID-19) patients with these complications are described to determine the time to onset, associated comorbidities, and location and duration of pneumothorax. A search in PubMed yielded simple frequencies and a bivariate analysis of deaths. There were 113 confirmed cases in 67 articles. The median time from the date of hospital admission to the presence of pneumothorax was 8 days. Right hemithorax was the most frequent form of pneumothorax. Almost half of the patients required intubation for invasive mechanical ventilation. Although the frequency of this phenomenon was not high among hospitalized patients with confirmed SARS-CoV-2, it was high among those who developed acute respiratory distress syndrome (ARDS). This study contributes to the literature because it presents a large number of patients who developed pneumothorax after admission, which was characterized by clinical deterioration (dyspnea, tachypnea, pleuritic chest pain, and subcutaneous emphysema) and low oxygen saturation levels. Pneumothorax/pneumomediastinum is recommended as a differential diagnosis, even without considering the presence of chronic pulmonary comorbidities or invasive mechanical ventilation.

COVID-19 versus Non-COVID-19 Acute Respiratory Distress Syndrome: Comparison of Demographics, Physiologic Parameters, Inflammatory Biomarkers, and Clinical Outcomes

Rationale: There is an urgent need for improved understanding of the mechanisms and clinical characteristics of acute respiratory distress syndrome (ARDS) due to coronavirus disease (COVID-19).Objectives: To compare key demographic and physiologic parameters, biomarkers, and clinical outcomes of COVID-19 ARDS and ARDS secondary to direct lung injury from other etiologies of pneumonia.Methods: We enrolled 27 patients with COVID-19 ARDS in a prospective, observational cohort study and compared them with a historical, pre-COVID-19 cohort of patients with viral ARDS (n = 14), bacterial ARDS (n = 21), and ARDS due to culture-negative pneumonia (n = 30). We recorded clinical demographics; measured respiratory mechanical parameters; collected serial peripheral blood specimens for measurement of plasma interleukin (IL)-6, IL-8, and IL-10; and followed patients prospectively for patient-centered outcomes. We conducted between-group comparisons with nonparametric tests and analyzed time-to-event outcomes with Kaplan-Meier and Cox proportional hazards models.Results: Patients with COVID-19 ARDS had higher body mass index and were more likely to be Black, or residents of skilled nursing facilities, compared with those with non-COVID-19 ARDS (P < 0.05). Patients with COVID-19 had lower delivered minute ventilation compared with bacterial and culture-negative ARDS (post hoc P < 0.01) but not compared with viral ARDS. We found no differences in static compliance, hypoxemic indices, or carbon dioxide clearance between groups. Patients with COVID-19 had lower IL-6 levels compared with bacterial and culture-negative ARDS at early time points after intubation but no differences in IL-6 levels compared with viral ARDS. Patients with COVID-19 had longer duration of mechanical ventilation but similar 60-day mortality in both unadjusted and adjusted analyses.Conclusions: COVID-19 ARDS bears several similarities to viral ARDS but demonstrates lower minute ventilation and lower systemic levels of IL-6 compared with bacterial and culture-negative ARDS. COVID-19 ARDS was associated with longer dependence on mechanical ventilation compared with non-COVID-19 ARDS. Such detectable differences of COVID-19 do not merit deviation from evidence-based management of ARDS but suggest priorities for clinical research to better characterize and treat this new clinical entity.

Outcome of acute hypoxaemic respiratory failure: insights from the LUNG SAFE Study

BACKGROUND

Current incidence and outcome of patients with acute hypoxaemic respiratory failure requiring mechanical ventilation in the intensive care unit (ICU) are unknown, especially for patients not meeting criteria for acute respiratory distress syndrome (ARDS).

METHODS

An international, multicentre, prospective cohort study of patients presenting with hypoxaemia early in the course of mechanical ventilation, conducted during four consecutive weeks in the winter of 2014 in 459 ICUs from 50 countries (LUNG SAFE). Patients were enrolled with arterial oxygen tension/inspiratory oxygen fraction ratio ≤300 mmHg, new pulmonary infiltrates and need for mechanical ventilation with a positive end-expiratory pressure of ≥5 cmH₂O. ICU prevalence, causes of hypoxaemia, hospital survival and factors associated with hospital mortality were measured. Patients with unilateral versus bilateral opacities were compared.

FINDINGS

12 906 critically ill patients received mechanical ventilation and 34.9% with hypoxaemia and new infiltrates were enrolled, separated into ARDS (69.0%), unilateral infiltrate (22.7%) and congestive heart failure (CHF; 8.2%). The global hospital mortality was 38.6%. CHF patients had a mortality comparable to ARDS (44.1% versus 40.4%). Patients with unilateral-infiltrate had lower unadjusted mortality, but similar adjusted mortality compared to those with ARDS. The number of quadrants on chest imaging was associated with an increased risk of death. There was no difference in mortality comparing patients with unilateral-infiltrate and ARDS with only two quadrants involved.

INTERPRETATION

More than one-third of patients receiving mechanical ventilation have hypoxaemia and new infiltrates with a hospital mortality of 38.6%. Survival is dependent on the degree of pulmonary involvement whether or not ARDS criteria are reached.

Accuracy of the dynamic signal analysis approach in respiratory mechanics during noninvasive pressure support ventilation: a bench study

OBJECTIVE

To evaluate the accuracy of respiratory mechanics using dynamic signal analysis during noninvasive pressure support ventilation (PSV).

METHODS

A Respironics V60 ventilator was connected to an active lung simulator to model normal, restrictive, obstructive, and mixed obstructive and restrictive profiles. The PSV was adjusted to maintain tidal volumes (V_T) that achieved 5.0, 7.0, and 10.0 mL/kg body weight, and the positive end-expiration pressure (PEEP) was set to 5 cmH₂O. Ventilator performance was evaluated by measuring the flow, airway pressure, and volume. The system compliance (C_rs) and airway resistance (inspiratory and expiratory resistance, R_insp and R_exp, respectively) were calculated.

RESULTS

Under active breathing conditions, the C_rs was overestimated in the normal and restrictive models, and it decreased with an increasing pressure support (PS) level. The R_insp calculated error was approximately 10% at 10.0 mL/kg of V_T, and similar results were obtained for the calculated R_exp at 7.0 mL/kg of V_T.

CONCLUSION

Using dynamic signal analysis, appropriate tidal volume was beneficial for R_rs, especially for estimating R_exp during assisted ventilation. The C_rs measurement was also relatively accurate in obstructive conditions.