Positive End-Expiratory Pressure, Pleural Pressure, and Regional Compliance during Pronation: An Experimental Study

Extended versus Standard Proning Duration for COVID-19-associated Acute Respiratory Distress Syndrome: A Target Trial Emulation Study

Rationale: Prone positioning for ⩾16 hours in moderate-to-severe acute respiratory distress syndrome (ARDS) improves survival. However, the optimal duration of proning is unknown. Objectives: To estimate the effect of extended versus standard proning duration on patients with moderate-to-severe coronavirus disease (COVID-19) ARDS. Methods: Data were extracted from a five-hospital electronic medical record registry. Patients who were proned within 72 hours of mechanical ventilation were categorized as receiving extended (⩾24 h) versus standard (16-24 h) proning based on the first proning session length. We used a target trial emulation design to estimate the effect of extended versus standard proning on the primary outcome of 90-day mortality and secondary outcomes of ventilator liberation and intensive care unit (ICU) discharge. Analytically, we used inverse probability of treatment weighted (IPTW) Cox or Fine-Gray regression models. Results: A total of 314 patients were included; 234 received extended proning, and 80 received standard-duration proning. Patients who received extended proning were older, had greater comorbidity, were more often at an academic hospital, and had shorter time from admission to mechanical ventilation. After IPTW, characteristics were well balanced. Unadjusted 90-day mortality in the extended versus standard proning groups was 39% versus 58%. In doubly robust IPTW analyses, we found no significant effects of extended versus standard proning duration on mortality (hazard ratio [95% confidence interval], 0.95 [0.51-1.77]), ventilator liberation (subdistribution hazard, 1.60 [0.97-2.64], or ICU discharge (subdistribution hazard, 1.31 [0.82-2.10]). Conclusions: Using target trial emulation, we found no significant effect of extended versus standard proning duration on mortality, ventilator liberation, or ICU discharge. However, given the imprecision of estimates, further study is justified.

Impact of airway closure and lung collapse on inhaled nitric oxide effect in acute lung injury: an experimental study

BACKGROUND

Efficacy of inhaled therapy such as Nitric Oxide (iNO) during mechanical ventilation may depend on airway patency. We hypothesized that airway closure and lung collapse, countered by positive end-expiratory pressure (PEEP), influence iNO efficacy. This could support the role of an adequate PEEP titration for inhalation therapy. The main aim of this study was to assess the effect of iNO with PEEP set above or below the airway opening pressure (AOP) generated by airway closure, on hemodynamics and gas exchange in swine models of acute respiratory distress syndrome. Fourteen pigs randomly underwent either bilateral or asymmetrical two-hit model of lung injury. Airway closure and lung collapse were measured with electrical impedance tomography as well as ventilation/perfusion ratio (V/Q). After AOP detection, the effect of iNO (10ppm) was studied with PEEP set randomly above or below regional AOP. Respiratory mechanics, hemodynamics, and gas-exchange were recorded.

RESULTS

All pigs presented airway closure (AOP > 0.5cmH2O) after injury. In bilateral injury, iNO was associated with an improved mean pulmonary pressure from 49 ± 8 to 42 ± 7mmHg; (p = 0.003), and ventilation/perfusion matching, caused by a reduction in pixels with low V/Q and shunt from 16%[IQR:13-19] to 9%[IQR:4-12] (p = 0.03) only at PEEP set above AOP. iNO had no effect on hemodynamics or gas exchange for PEEP below AOP (low V/Q 25%[IQR:16-30] to 23%[IQR:14-27]; p = 0.68). In asymmetrical injury, iNO improved pulmonary hemodynamics and ventilation/perfusion matching independently from the PEEP set. iNO was associated with improved oxygenation in all cases.

CONCLUSIONS

In an animal model of bilateral lung injury, PEEP level relative to AOP markedly influences iNO efficacy on pulmonary hemodynamics and ventilation/perfusion match, independently of oxygenation.

Positive end-expiratory pressure management in patients with severe ARDS: implications of prone positioning and extracorporeal membrane oxygenation

The optimal strategy for positive end-expiratory pressure (PEEP) titration in the management of severe acute respiratory distress syndrome (ARDS) patients remains unclear. Current guidelines emphasize the importance of a careful risk-benefit assessment for PEEP titration in terms of cardiopulmonary function in these patients. Over the last few decades, the primary goal of PEEP usage has shifted from merely improving oxygenation to emphasizing lung protection, with a growing focus on the individual pattern of lung injury, lung and chest wall mechanics, and the hemodynamic consequences of PEEP. In moderate-to-severe ARDS patients, prone positioning (PP) is recommended as part of a lung protective ventilation strategy to reduce mortality. However, the physiologic changes in respiratory mechanics and hemodynamics during PP may require careful re-assessment of the ventilation strategy, including PEEP. For the most severe ARDS patients with refractory gas exchange impairment, where lung protective ventilation is not possible, veno-venous extracorporeal membrane oxygenation (V-V ECMO) facilitates gas exchange and allows for a "lung rest" strategy using "ultraprotective" ventilation. Consequently, the importance of lung recruitment to improve oxygenation and homogenize ventilation with adequate PEEP may differ in severe ARDS patients treated with V-V ECMO compared to those managed conservatively. This review discusses PEEP management in severe ARDS patients and the implications of management with PP or V-V ECMO with respect to respiratory mechanics and hemodynamic function.

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.

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.

Dynamic inflation prevents and standardized lung recruitment reverts volume loss associated with percutaneous tracheostomy during volume control ventilation: results from a Neuro-ICU population

To determine how percutaneous tracheostomy (PT) impacts on respiratory system compliance (Crs) and end-expiratory lung volume (EELV) during volume control ventilation and to test whether a recruitment maneuver (RM) at the end of PT may reverse lung derecruitment. This is a single center, prospective, applied physiology study. 25 patients with acute brain injury who underwent PT were studied. Patients were ventilated in volume control ventilation. Electrical impedance tomography (EIT) monitoring and respiratory mechanics measurements were performed in three steps: (a) baseline, (b) after PT, and (c) after a standardized RM (10 sighs of 30 cmH2O lasting 3 s each within 1 min). End-expiratory lung impedance (EELI) was used as a surrogate of EELV. PT determined a significant EELI loss (mean reduction of 432 arbitrary units p = 0.049) leading to a reduction in Crs (55 ± 13 vs. 62 ± 13 mL/cmH2O; p < 0.001) as compared to baseline. RM was able to revert EELI loss and restore Crs (68 ± 15 vs. 55 ± 13 mL/cmH2O; p < 0.001). In a subgroup of patients (N = 8, 31%), we observed a gradual but progressive increase in EELI. In this subgroup, patients did not experience a decrease of Crs after PT as compared to patients without dynamic inflation. Dynamic inflation did not cause hemodynamic impairment nor raising of intracranial pressure. We propose a novel and explorative hyperinflation risk index (HRI) formula. Volume control ventilation did not prevent the PT-induced lung derecruitment. RM could restore the baseline lung volume and mechanics. Dynamic inflation is common during PT, it can be monitored real-time by EIT and anticipated by HRI. The presence of dynamic inflation during PT may prevent lung derecruitment.

Study on risk factors and treatment strategies of hypoxemia after acute type a aortic dissection surgery

Acute type A aortic dissection is a life-threatening cardiovascular disease characterized by rapid onset and high mortality. Emergency surgery is the preferred and reliable treatment option. However, postoperative complications significantly impact patient prognosis. Hypoxemia, a common complication, poses challenges in clinical treatment, negatively affecting patient outcomes and increasing the risk of mortality. Therefore, it is crucial to study and comprehend the risk factors and treatment strategies for hypoxemia following acute type A aortic dissection to facilitate early intervention.

Effects of prone positioning on lung mechanical power components in patients with acute respiratory distress syndrome: a physiologic study

BACKGROUND

Prone positioning (PP) homogenizes ventilation distribution and may limit ventilator-induced lung injury (VILI) in patients with moderate to severe acute respiratory distress syndrome (ARDS). The static and dynamic components of ventilation that may cause VILI have been aggregated in mechanical power, considered a unifying driver of VILI. PP may affect mechanical power components differently due to changes in respiratory mechanics; however, the effects of PP on lung mechanical power components are unclear. This study aimed to compare the following parameters during supine positioning (SP) and PP: lung total elastic power and its components (elastic static power and elastic dynamic power) and these variables normalized to end-expiratory lung volume (EELV).

METHODS

This prospective physiologic study included 55 patients with moderate to severe ARDS. Lung total elastic power and its static and dynamic components were compared during SP and PP using an esophageal pressure-guided ventilation strategy. In SP, the esophageal pressure-guided ventilation strategy was further compared with an oxygenation-guided ventilation strategy defined as baseline SP. The primary endpoint was the effect of PP on lung total elastic power non-normalized and normalized to EELV. Secondary endpoints were the effects of PP and ventilation strategies on lung elastic static and dynamic power components non-normalized and normalized to EELV, respiratory mechanics, gas exchange, and hemodynamic parameters.

RESULTS

Lung total elastic power (median [interquartile range]) was lower during PP compared with SP (6.7 [4.9-10.6] versus 11.0 [6.6-14.8] J/min; P < 0.001) non-normalized and normalized to EELV (3.2 [2.1-5.0] versus 5.3 [3.3-7.5] J/min/L; P < 0.001). Comparing PP with SP, transpulmonary pressures and EELV did not significantly differ despite lower positive end-expiratory pressure and plateau airway pressure, thereby reducing non-normalized and normalized lung elastic static power in PP. PP improved gas exchange, cardiac output, and increased oxygen delivery compared with SP.

CONCLUSIONS

In patients with moderate to severe ARDS, PP reduced lung total elastic and elastic static power compared with SP regardless of EELV normalization because comparable transpulmonary pressures and EELV were achieved at lower airway pressures. This resulted in improved gas exchange, hemodynamics, and oxygen delivery.

TRIAL REGISTRATION

German Clinical Trials Register (DRKS00017449). Registered June 27, 2019. https://drks.de/search/en/trial/DRKS00017449.

Prone position in the mechanical ventilation of acute respiratory distress syndrome children: a systematic review and meta-analysis

Background

Prone position has been well recognized for the treatment of adult acute respiratory distress syndrome (ARDS). We aimed to evaluate the role of prone position in the mechanical ventilation in children with ARDS, to provide evidence to the treatment and care of children with ARDS.

Methods

We searched the Pubmed et al. databases by computer until January 23, 2024 for randomized controlled trials (RCTs) on the role of prone position in the mechanical ventilation in children with ARDS. We evaluated the quality of included studies according to the quality evaluation criteria recommended by the Cochrane library. RevMan 5.3 software was used for meta-analysis.

Results

7 RCTs involving 433 children with ARDS were included. Meta-analysis indicated that prone position is beneficial to improve the arterial oxygenation pressure [MD = 4.27 mmHg, 95% CI (3.49, 5.06)], PaO2/FiO2 [MD = 26.97, 95% CI (19.17, 34.77)], reduced the oxygenation index [MD = -3.52, 95% CI (-5.41, -1.64)], mean airway pressure [MD = -1.91 cmH2O, 95% CI (-2.27, -1.55)] and mortality [OR = 0.33, 95% CI (0.15, 0.73), all P < 0.05]. There were no statistical differences in the duration of mechanical ventilation between the prone position group and control group [MD = -17.01, 97.27, 95% CI (-38.28, 4.26), P = 0.12]. Egger test results showed that no significant publication bias was found (all P > 0.05).

Conclusions

Prone position ventilation has obvious advantages in improving oxygenation, but there is no significant improvement in the time of mechanical ventilation in the treatment of children with ARDS. In the future, more large-sample, high-quality RCTs are still needed to further analyze the role of prone position in the mechanical ventilation in children with ARDS.

The effects of prone position ventilation on patients with acute respiratory distress syndrome after cardiac surgery

BACKGROUND

This study aimed to investigate the effects of prone position ventilation treatment on patients with acute respiratory distress syndrome (ARDS) after cardiac surgery.

METHODS

Clinical data were collected from 93 hospitalized patients with acute respiratory distress syndrome (ARDS) in the intensive care unit (ICU) of cardiology from February 2021 to February 2023. Patients were divided into supine position group (n = 45) and prone position group (n = 48). The difference in 28-days survival rates, blood gas indicators, respiratory mechanics indicators, and adverse events before and after treatment was analyzed.

RESULTS

We found that within 28 days of admission, 8.33% of ARDS patients in prone position group and 11.11% in supine position group died of all causes (p > .05). After treatment, the levels of arterial PaO2 (103.25 ± 9.44 in prone position group and 91.62 ± 9.18 in supine position group), PaCO2 (30.26 ± 5.54 and 36.56 ± 6.37), blood LAC (1.35 ± 0.37 and 1.68 ± 0.42), oxygenation (232.23 ± 28.56 and 205.13 ± 31.34) and diffusion index (453.48 ± 63.30 and 395.18 ± 58.54) in both groups were improved (p < .001). Moreover, the increase in prone position group was more remarkable. After treatment, the respiratory mechanics indexes of the lung compliance as well as respiratory resistance were improved (p < .05). Moreover, the increase in supine position group was more remarkable (p < .05). The incidence of atelectasis in prone position group was lower than that in supine position group (p < .05). Additionally, the alteration in other adverse events showed no significant difference between the two groups (p > .05).

CONCLUSIONS

Taken together, prone position ventilation in patients with ARDS after cardiac surgery improved blood gas indexes, hypoxemia, and respiratory mechanics indexes, as well as reduced the incidence of atelectasis.

Setting positive end-expiratory pressure: does the 'best compliance' concept really work?

PURPOSE OF REVIEW

Determining the optimal positive end-expiratory pressure (PEEP) setting remains a central yet debated issue in the management of acute respiratory distress syndrome (ARDS).The 'best compliance' strategy set the PEEP to coincide with the peak respiratory system compliance (or 2 cmH 2 O higher) during a decremental PEEP trial, but evidence is conflicting.

RECENT FINDINGS

The physiological rationale that best compliance is always representative of functional residual capacity and recruitment has raised serious concerns about its efficacy and safety, due to its association with increased 28-day all-cause mortality in a randomized clinical trial in ARDS patients.Moreover, compliance measurement was shown to underestimate the effects of overdistension, and neglect intra-tidal recruitment, airway closure, and the interaction between lung and chest wall mechanics, especially in obese patients. In response to these concerns, alternative approaches such as recruitment-to-inflation ratio, the nitrogen wash-in/wash-out technique, and electrical impedance tomography (EIT) are gaining attention to assess recruitment and overdistention more reliably and precisely.

SUMMARY

The traditional 'best compliance' strategy for determining optimal PEEP settings in ARDS carries risks and overlooks some key physiological aspects. The advent of new technologies and methods presents more reliable strategies to assess recruitment and overdistention, facilitating personalized approaches to PEEP optimization.

Estimation of normal lung weight index in healthy female domestic pigs

INTRODUCTION

Lung weight is an important study endpoint to assess lung edema in porcine experiments on acute respiratory distress syndrome and ventilatory induced lung injury. Evidence on the relationship between lung-body weight relationship is lacking in the literature. The aim of this work is to provide a reference equation between normal lung and body weight in female domestic piglets.

MATERIALS AND METHODS

177 healthy female domestic piglets from previous studies were included in the analysis. Lung weight was assessed either via a CT-scan before any experimental injury or with a scale after autopsy. The animals were randomly divided in a training (n = 141) and a validation population (n = 36). The relation between body weight and lung weight index (lung weight/body weight, g/kg) was described by an exponential function on the training population. The equation was tested on the validation population. A Bland-Altman analysis was performed to compare the lung weight index in the validation population and its theoretical value calculated with the reference equation.

RESULTS

A good fit was found between the validation population and the exponential equation extracted from the training population (RMSE = 0.060). The equation to determine lung weight index from body weight was: [Formula: see text] At the Bland and Altman analyses, the mean bias between the real and the expected lung weight index was - 0.26 g/kg (95% CI - 0.96-0.43), upper LOA 3.80 g/kg [95% CI 2.59-5.01], lower LOA - 4.33 g/kg [95% CI = - 5.54-(- 3.12)].

CONCLUSIONS

This exponential function might be a valuable tool to assess lung edema in experiments involving 16-50 kg female domestic piglets. The error that can be made due to the 95% confidence intervals of the formula is smaller than the one made considering the lung to body weight as a linear relationship.

Future directions of lung-protective ventilation strategies in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is characterized by the heterogeneous distribution of lung aeration along a gravitational direction due to increased lung density. Therefore, the lung available for ventilation is usually limited to ventral, nondependent lung regions and has been called the "baby" lung. In ARDS, ventilator-induced lung injury is known to occur in nondependent "baby" lungs, as ventilation is shifted to ventral, nondependent lung regions, increasing stress and strain. To protect this nondependent "baby" lung, the clinician targets and limits global parameters such as tidal volume and plateau pressure. In addition, positive end-expiratory pressure (PEEP) is used to prevent dorsal, dependent atelectasis and, if successful, increases the size of the baby lung and lessens its susceptibility to injury from inspiratory stretch. Although many clinical trials have been performed in patients with ARDS over the last two decades, there are few successfully showing benefits on mortality (ie, prone positioning and neuromuscular blocking agents). These disappointing results contrast with other medical disciplines, especially in oncology, where the heterogeneity of diseases is recognized widely and precision medicine has been promoted. Thus, lung-protective ventilation strategies need to take an innovative approach that accounts for the heterogeneity of injured lungs. This article summarizes ventilator-induced lung injury and ARDS and discusses how to implement precision medicine in the field of ARDS. Potentially useful methods to individualize PEEP with esophageal balloon manometry, lung recruitability, and electrical impedance tomography were discussed.

Impact of Supine Versus Semirecumbent Body Posture on the Distribution of Ventilation in Acute Respiratory Distress Syndrome

In some patients with acute respiratory distress syndrome (ARDS), a paradoxical improvement in respiratory system compliance (CRS) has been observed when assuming a supine (head of bed [HOB] 0°) compared with semirecumbent (HOB 35-40°) posture. We sought to test the hypothesis that mechanically ventilated patients with ARDS would have improved CRS, due to changes in ventilation distribution, when moving from the semirecumbent to supine position. We conducted a prospective, observational ICU study including 14 mechanically ventilated patients with ARDS. For each patient, ventilation distribution (assessed by electrical impedance tomography) and pulmonary mechanics were compared in supine versus semirecumbent postures. Compared with semirecumbent, in the supine posture CRS increased (33 ± 21 vs. 26 ± 14 mL/cm H2O, p = 0.005), driving pressure was reduced (14 ± 6 vs. 17 ± 7 cm H2O, p < 0.001), and dorsal fraction of ventilation was decreased (48.5 ± 14.1% vs. 54.5 ± 12.0%, p = 0.003). Posture change from semirecumbent to supine resulted in a favorable physiologic response in terms of improved CRS and reduced driving pressure-with a corresponding increase in ventral ventilation, possibly related to reduced ventral overdistension.

Clinical Applicability of Electrical Impedance Tomography in Patient-Tailored Ventilation: A Narrative Review

Electrical Impedance Tomography (EIT) is a non-invasive bedside imaging technique that provides real-time lung ventilation information on critically ill patients. EIT can potentially become a valuable tool for optimising mechanical ventilation, especially in patients with acute respiratory distress syndrome (ARDS). In addition, EIT has been shown to improve the understanding of ventilation distribution and lung aeration, which can help tailor ventilatory strategies according to patient needs. Evidence from critically ill patients shows that EIT can reduce the duration of mechanical ventilation and prevent lung injury due to overdistension or collapse. EIT can also identify the presence of lung collapse or recruitment during a recruitment manoeuvre, which may guide further therapy. Despite its potential benefits, EIT has not yet been widely used in clinical practice. This may, in part, be due to the challenges associated with its implementation, including the need for specialised equipment and trained personnel and further validation of its usefulness in clinical settings. Nevertheless, ongoing research focuses on improving mechanical ventilation and clinical outcomes in critically ill patients.

Pronation Reveals a Heterogeneous Response of Global and Regional Respiratory Mechanics in Patients With Acute Hypoxemic Respiratory Failure

OBJECTIVES

Experimental models suggest that prone position and positive end-expiratory pressure (PEEP) homogenize ventral-dorsal ventilation distribution and regional respiratory compliance. However, this response still needs confirmation on humans. Therefore, this study aimed to assess the changes in global and regional respiratory mechanics in supine and prone positions over a range of PEEP levels in acute respiratory distress syndrome (ARDS) patients.

DESIGN

A prospective cohort study.

PATIENTS

Twenty-two intubated patients with ARDS caused by COVID-19 pneumonia.

INTERVENTIONS

Electrical impedance tomography and esophageal manometry were applied during PEEP titrations from 20 cm H2O to 6 cm H2O in supine and prone positions.

MEASUREMENTS

Global respiratory system compliance (Crs), chest wall compliance, regional lung compliance, ventilation distribution in supine and prone positions.

MAIN RESULTS

Compared with supine position, the maximum level of Crs changed after prone position in 59% of ARDS patients (n = 13), of which the Crs decreased in 32% (n = 7) and increased in 27% (n = 6). To reach maximum Crs after pronation, PEEP was changed in 45% of the patients by at least 4 cm H2O. After pronation, the ventilation and compliance of the dorsal region did not consistently change in the entire sample of patients, increasing specifically in a subgroup of patients who showed a positive change in Crs when transitioning from supine to prone position. These combined changes in ventilation and compliance suggest dorsal recruitment postpronation. In addition, the subgroup with increased Crs postpronation demonstrated the most pronounced difference between dorsal and ventral ventilation distribution from supine to prone position (p = 0.01), indicating heterogeneous ventilation distribution in prone position.

CONCLUSIONS

Prone position modifies global respiratory compliance in most patients with ARDS. Only a subgroup of patients with a positive change in Crs postpronation presented a consistent improvement in dorsal ventilation and compliance. These data suggest that the response to pronation on global and regional mechanics can vary among ARDS patients, with some patients presenting more dorsal lung recruitment than others.

Management of severe acute respiratory distress syndrome: a primer

This narrative review explores the physiology and evidence-based management of patients with severe acute respiratory distress syndrome (ARDS) and refractory hypoxemia, with a focus on mechanical ventilation, adjunctive therapies, and veno-venous extracorporeal membrane oxygenation (V-V ECMO). Severe ARDS cases increased dramatically worldwide during the Covid-19 pandemic and carry a high mortality. The mainstay of treatment to improve survival and ventilator-free days is proning, conservative fluid management, and lung protective ventilation. Ventilator settings should be individualized when possible to improve patient-ventilator synchrony and reduce ventilator-induced lung injury (VILI). Positive end-expiratory pressure can be individualized by titrating to best respiratory system compliance, or by using advanced methods, such as electrical impedance tomography or esophageal manometry. Adjustments to mitigate high driving pressure and mechanical power, two possible drivers of VILI, may be further beneficial. In patients with refractory hypoxemia, salvage modes of ventilation such as high frequency oscillatory ventilation and airway pressure release ventilation are additional options that may be appropriate in select patients. Adjunctive therapies also may be applied judiciously, such as recruitment maneuvers, inhaled pulmonary vasodilators, neuromuscular blockers, or glucocorticoids, and may improve oxygenation, but do not clearly reduce mortality. In select, refractory cases, the addition of V-V ECMO improves gas exchange and modestly improves survival by allowing for lung rest. In addition to VILI, patients with severe ARDS are at risk for complications including acute cor pulmonale, physical debility, and neurocognitive deficits. Even among the most severe cases, ARDS is a heterogeneous disease, and future studies are needed to identify ARDS subgroups to individualize therapies and advance care.

Lung Recruitment Assessed by Electrical Impedance Tomography (RECRUIT): A Multicenter Study of COVID-19 Acute Respiratory Distress Syndrome

Rationale: Defining lung recruitability is needed for safe positive end-expiratory pressure (PEEP) selection in mechanically ventilated patients. However, there is no simple bedside method including both assessment of recruitability and risks of overdistension as well as personalized PEEP titration. Objectives: To describe the range of recruitability using electrical impedance tomography (EIT), effects of PEEP on recruitability, respiratory mechanics and gas exchange, and a method to select optimal EIT-based PEEP. Methods: This is the analysis of patients with coronavirus disease (COVID-19) from an ongoing multicenter prospective physiological study including patients with moderate-severe acute respiratory distress syndrome of different causes. EIT, ventilator data, hemodynamics, and arterial blood gases were obtained during PEEP titration maneuvers. EIT-based optimal PEEP was defined as the crossing point of the overdistension and collapse curves during a decremental PEEP trial. Recruitability was defined as the amount of modifiable collapse when increasing PEEP from 6 to 24 cm H2O (ΔCollapse24-6). Patients were classified as low, medium, or high recruiters on the basis of tertiles of ΔCollapse24-6. Measurements and Main Results: In 108 patients with COVID-19, recruitability varied from 0.3% to 66.9% and was unrelated to acute respiratory distress syndrome severity. Median EIT-based PEEP differed between groups: 10 versus 13.5 versus 15.5 cm H2O for low versus medium versus high recruitability (P < 0.05). This approach assigned a different PEEP level from the highest compliance approach in 81% of patients. The protocol was well tolerated; in four patients, the PEEP level did not reach 24 cm H2O because of hemodynamic instability. Conclusions: Recruitability varies widely among patients with COVID-19. EIT allows personalizing PEEP setting as a compromise between recruitability and overdistension. Clinical trial registered with www.clinicaltrials.gov (NCT04460859).

Respiratory mechanics and gas exchange in an ovine model of congenital heart disease with increased pulmonary blood flow and pressure

In a model of congenital heart disease (CHD), we evaluated if chronically increased pulmonary blood flow and pressure were associated with altered respiratory mechanics and gas exchange. Respiratory mechanics and gas exchange were evaluated in 6 shunt, 7 SHAM, and 7 control age-matched lambs. Lambs were anesthetized and mechanically ventilated for 15 min with tidal volume of 10 mL/kg, positive end-expiratory pressure of 5 cmH2O, and inspired oxygen fraction of 0.21. Respiratory system, lung and chest wall compliances (Crs, CL and Ccw, respectively) and resistances (Rrs, RL and Rcw, respectively), and the profile of the elastic pressure-volume curve (%E2) were evaluated. Arterial blood gases and volumetric capnography variables were collected. Comparisons between groups were performed by one-way ANOVA followed by Tukey-Kramer test for normally distributed data and with Kruskal-Wallis test followed by Steel-Dwass test for non-normally distributed data. Average Crs and CL in shunt lambs were 30% and 58% lower than in control, and 56% and 68% lower than in SHAM lambs, respectively. Ccw was 52% and 47% higher and Rcw was 53% and 40% lower in shunt lambs compared to controls and SHAMs, respectively. No difference in %E2 was identified between groups. No difference in respiratory mechanics was observed between control and SHAM lambs. In shunt lambs, Rcw, Crs and CL were decreased and Ccw was increased when compared to control and SHAM lambs. Pulmonary gas exchange did not seem to be impaired in shunt lambs when compared to controls and SHAMs.

Temperature Trajectory Subphenotypes in Oncology Patients with Neutropenia and Suspected Infection

Rationale: Despite etiologic and severity heterogeneity in neutropenic sepsis, management is often uniform. Understanding host response clinical subphenotypes might inform treatment strategies for neutropenic sepsis. Objectives: In this retrospective two-hospital study, we analyzed whether temperature trajectory modeling could identify distinct, clinically relevant subphenotypes among oncology patients with neutropenia and suspected infection. Methods: Among adult oncologic admissions with neutropenia and blood cultures within 24 hours, a previously validated model classified patients' initial 72-hour temperature trajectories into one of four subphenotypes. We analyzed subphenotypes' independent relationships with hospital mortality and bloodstream infection using multivariable models. Measurements and Main Results: Patients (primary cohort n = 1,145, validation cohort n = 6,564) fit into one of four temperature subphenotypes. "Hyperthermic slow resolvers" (pooled n = 1,140 [14.8%], mortality n = 104 [9.1%]) and "hypothermic" encounters (n = 1,612 [20.9%], mortality n = 138 [8.6%]) had higher mortality than "hyperthermic fast resolvers" (n = 1,314 [17.0%], mortality n = 47 [3.6%]) and "normothermic" (n = 3,643 [47.3%], mortality n = 196 [5.4%]) encounters (P < 0.001). Bloodstream infections were more common among hyperthermic slow resolvers (n = 248 [21.8%]) and hyperthermic fast resolvers (n = 240 [18.3%]) than among hypothermic (n = 188 [11.7%]) or normothermic (n = 418 [11.5%]) encounters (P < 0.001). Adjusted for confounders, hyperthermic slow resolvers had increased adjusted odds for mortality (primary cohort odds ratio, 1.91 [P = 0.03]; validation cohort odds ratio, 2.19 [P < 0.001]) and bloodstream infection (primary odds ratio, 1.54 [P = 0.04]; validation cohort odds ratio, 2.15 [P < 0.001]). Conclusions: Temperature trajectory subphenotypes were independently associated with important outcomes among hospitalized patients with neutropenia in two independent cohorts.

Personalized ventilatory strategy based on lung recruitablity in COVID-19-associated acute respiratory distress syndrome: a prospective clinical study

BACKGROUND

Heterogeneity is an inherent nature of ARDS. Recruitment-to-inflation ratio has been developed to identify the patients who has lung recruitablity. This technique might be useful to identify the patients that match specific interventions, such as higher positive end-expiratory pressure (PEEP) or prone position or both. We aimed to evaluate the physiological effects of PEEP and body position on lung mechanics and regional lung inflation in COVID-19-associated ARDS and to propose the optimal ventilatory strategy based on recruitment-to-inflation ratio.

METHODS

Patients with COVID-19-associated ARDS were consecutively enrolled. Lung recruitablity (recruitment-to-inflation ratio) and regional lung inflation (electrical impedance tomography [EIT]) were measured with a combination of body position (supine or prone) and PEEP (low 5 cmH₂O or high 15 cmH₂O). The utility of recruitment-to-inflation ratio to predict responses to PEEP were examined with EIT.

RESULTS

Forty-three patients were included. Recruitment-to-inflation ratio was 0.68 (IQR 0.52-0.84), separating high recruiter versus low recruiter. Oxygenation was the same between two groups. In high recruiter, a combination of high PEEP with prone position achieved the highest oxygenation and less dependent silent spaces in EIT (vs. low PEEP in both positions) without increasing non-dependent silent spaces in EIT. In low recruiter, low PEEP in prone position resulted in better oxygenation (vs. both PEEPs in supine position), less dependent silent spaces (vs. low PEEP in supine position) and less non-dependent silent spaces (vs. high PEEP in both positions). Recruitment-to-inflation ratio was positively correlated with the improvement in oxygenation and respiratory system compliance, the decrease in dependent silent spaces, and was inversely correlated with the increase in non-dependent silent spaces, when applying high PEEP.

CONCLUSIONS

Recruitment-to-inflation ratio may be useful to personalize PEEP in COVID-19-associated ARDS. Higher PEEP in prone position and lower PEEP in prone position decreased the amount of dependent silent spaces (suggesting lung collapse) without increasing the amount of non-dependent silent spaces (suggesting overinflation) in high recruiter and in low recruiter, respectively.

Real-time effects of lateral positioning on regional ventilation and perfusion in an experimental model of acute respiratory distress syndrome

Low-volume lung injury encompasses local concentration of stresses in the vicinity of collapsed regions in heterogeneously ventilated lungs. We aimed to study the effects on ventilation and perfusion distributions of a sequential lateral positioning (30°) strategy using electrical impedance tomography imaging in a porcine experimental model of early acute respiratory distress syndrome (ARDS). We hypothesized that such strategy, including a real-time individualization of positive end-expiratory pressure (PEEP) whenever in lateral positioning, would provide attenuation of collapse in the dependent lung regions. A two-hit injury acute respiratory distress syndrome experimental model was established by lung lavages followed by injurious mechanical ventilation. Then, all animals were studied in five body positions in a sequential order, 15 min each: Supine 1; Lateral Left; Supine 2; Lateral Right; Supine 3. The following functional images were analyzed by electrical impedance tomography: ventilation distributions and regional lung volumes, and perfusion distributions. The induction of the acute respiratory distress syndrome model resulted in a marked fall in oxygenation along with low regional ventilation and compliance of the dorsal half of the lung (gravitational-dependent in supine position). Both the regional ventilation and compliance of the dorsal half of the lung greatly increased along of the sequential lateral positioning strategy, and maximally at its end. In addition, a corresponding improvement of oxygenation occurred. In conclusion, our sequential lateral positioning strategy, with sufficient positive end-expiratory pressure to prevent collapse of the dependent lung units during lateral positioning, provided a relevant diminution of collapse in the dorsal lung in a porcine experimental model of early acute respiratory distress syndrome.

Positive- and Negative-Pressure Ventilation Characterized by Local and Global Pulmonary Mechanics

Rationale: There is continued debate regarding the equivalency of positive-pressure ventilation (PPV) and negative-pressure ventilation (NPV). Resolving this question is important because of the different practical ramifications of the two paradigms. Objectives: We sought to investigate the parallel between PPV and NPV and determine whether or not these two paradigms cause identical ventilation profiles by analyzing the local strain mechanics when the global tidal volume (Vt) and inflation pressure was matched. Methods: A custom-designed electromechanical apparatus was used to impose equal global loads and displacements on the same ex vivo healthy porcine lung using PPV and NPV. High-speed high-resolution cameras recorded local lung surface deformations and strains in real time, and differences between PPV and NPV global energetics, viscoelasticity, as well as local tissue distortion were assessed. Measurements and Main Results: During initial inflation, NPV exhibited significantly more bulk pressure-volume compliance than PPV, suggestive of earlier lung recruitment. NPV settings also showed reduced relaxation, hysteresis, and energy loss compared with PPV. Local strain trends were also decreased in NPV, with reduced tissue distortion trends compared with PPV, as revealed through analysis of tissue anisotropy. Conclusions: Apparently, contradictory previous studies are not mutually exclusive. Equivalent changes in transpulmonary pressures in PPV and NPV lead to the same changes in lung volume and pressures, yet local tissue strains differ between PPV and NPV. Although limited to healthy specimens and ex vivo experiments in the absence of a chest cavity, these results may explain previous reports of better oxygenation and less lung injury in NPV.

Understanding clinical and biological heterogeneity to advance precision medicine in paediatric acute respiratory distress syndrome

Paediatric acute respiratory distress syndrome (PARDS) is a heterogeneous clinical syndrome that is associated with high rates of mortality and long-term morbidity. Factors that distinguish PARDS from adult acute respiratory distress syndrome (ARDS) include changes in developmental stage and lung maturation with age, precipitating factors, and comorbidities. No specific treatment is available for PARDS and management is largely supportive, but methods to identify patients who would benefit from specific ventilation strategies or ancillary treatments, such as prone positioning, are needed. Understanding of the clinical and biological heterogeneity of PARDS, and of differences in clinical features and clinical course, pathobiology, response to treatment, and outcomes between PARDS and adult ARDS, will be key to the development of novel preventive and therapeutic strategies and a precision medicine approach to care. Studies in which clinical, biomarker, and transcriptomic data, as well as informatics, are used to unpack the biological and phenotypic heterogeneity of PARDS, and implementation of methods to better identify patients with PARDS, including methods to rapidly identify subphenotypes and endotypes at the point of care, will drive progress on the path to precision medicine.

Electric impedance tomography-guided PEEP titration reduces mechanical power in ARDS: a randomized crossover pilot trial

BACKGROUND

In patients with acute respiratory distress syndrome undergoing mechanical ventilation, positive end-expiratory pressure (PEEP) can lead to recruitment or overdistension. Current strategies utilized for PEEP titration do not permit the distinction. Electric impedance tomography (EIT) detects and quantifies the presence of both collapse and overdistension. We investigated whether using EIT-guided PEEP titration leads to decreased mechanical power compared to high-PEEP/FiO2 tables.

METHODS

A single-center, randomized crossover pilot trial comparing EIT-guided PEEP selection versus PEEP selection using the High-PEEP/FiO2 table in patients with moderate-severe acute respiratory distress syndrome. The primary outcome was the change in mechanical power after each PEEP selection strategy. Secondary outcomes included changes in the 4 × driving pressure + respiratory rate (4 ΔP, + RR index) index, driving pressure, plateau pressure, PaO2/FiO2 ratio, and static compliance.

RESULTS

EIT was consistently associated with a decrease in mechanical power compared to PEEP/FiO2 tables (mean difference - 4.36 J/min, 95% CI - 6.7, - 1.95, p = 0.002) and led to lower values in the 4ΔP + RR index (- 11.42 J/min, 95% CI - 19.01, - 3.82, p = 0.007) mainly driven by a decrease in the elastic-dynamic power (- 1.61 J/min, - 2.99, - 0.22, p = 0.027). The elastic-static and resistive powers were unchanged. Similarly, EIT led to a statistically significant change in set PEEP (- 2 cmH2O, p = 0.046), driving pressure, (- 2.92 cmH2O, p = 0.003), peak pressure (- 6.25 cmH2O, p = 0.003), plateau pressure (- 4.53 cmH2O, p = 0.006), and static respiratory system compliance (+ 7.93 ml/cmH2O, p = 0.008).

CONCLUSIONS

In patients with moderate-severe acute respiratory distress syndrome, EIT-guided PEEP titration reduces mechanical power mainly through a reduction in elastic-dynamic power. Trial registration This trial was prospectively registered on Clinicaltrials.gov (NCT03793842) on January 4th, 2019.

Unilateral acute lung injury in pig: a promising animal model

Background

Acute lung injury (ALI) occurs in 23% unilateral. Models of unilateral ALI were developed and used previously without clearly demonstrating the strictly unilateral nature and severity of lung injury by the key parameters characterizing ALI as defined by the American Thoracic Society (ATS). Thus, the use of unilateral ALI remained rare despite the innovative approach. Therefore, we developed a unilateral model of ALI and focused on the crucial parameters characterizing ALI. This model can serve for direct comparisons between the injured and intact lungs within single animals, thus, reducing the number of animals required for valid experimental conclusions.

Methods

We established the model in nine pigs, followed by an evaluation of key parameters in six pigs (main study). Pigs were ventilated using an adapted left double-lumen tube for lung separation and two ventilators. ALI was induced in the left lung with cyclic rinsing (NaCl 0.9% + Triton® X-100), after which pigs were ventilated for different time spans to test for the timing of ALI onset. Ventilatory and metabolic parameters were evaluated, and bronchoalveolar lavage (BAL) was performed for measurements of inflammatory mediators. Finally, histopathological specimens were collected and examined in respect of characteristics defining the lung injury score (LIS) as suggested by the ATS.

Results

After adjustments of the model (n = 9) we were able to induce strictly left unilateral ALI in all six pigs of the evaluation study. The median lung injury score was 0.72 (IQR 0.62–0.79) in the left lung vs 0.14 (IQR 0.14–0.16; p < 0.05) in the right lung, confirming unilateral ALI. A significant and sustained drop in pulmonary compliance (Cdyn) of the left lung occurred immediately, whereas Cdyn of the right lung remained unchanged (p < 0.05). BAL fluid concentrations of interleukin-6 and -8 were increased in both lungs.

Conclusions

We established a model of unilateral ALI in pigs, confirmed by histopathology, and typical changes in respiratory mechanics and an inflammatory response. This thoroughly evaluated model could serve as a basis for future studies and for comparing pathophysiological and pharmacological changes in the uninjured and injured lung within the same animal.

Prone positioning improves ventilation-perfusion matching assessed by electrical impedance tomography in patients with ARDS: a prospective physiological study

BACKGROUND

The physiological effects of prone ventilation in ARDS patients have been discussed for a long time but have not been fully elucidated. Electrical impedance tomography (EIT) has emerged as a tool for bedside monitoring of pulmonary ventilation and perfusion, allowing the opportunity to obtain data. This study aimed to investigate the effect of prone positioning (PP) on ventilation-perfusion matching by contrast-enhanced EIT in patients with ARDS.

DESIGN

Monocenter prospective physiologic study.

SETTING

University medical ICU.

PATIENTS

Ten mechanically ventilated ARDS patients who underwent PP.

INTERVENTIONS

We performed EIT evaluation at the initiation of PP, 3 h after PP initiation and the end of PP during the first PP session.

MEASUREMENTS AND MAIN RESULTS

The regional distribution of ventilation and perfusion was analyzed based on EIT images and compared to the clinical variables regarding respiratory and hemodynamic status. Prolonged prone ventilation improved oxygenation in the ARDS patients. Based on EIT measurements, the distribution of ventilation was homogenized and dorsal lung ventilation was significantly improved by PP administration, while the effect of PP on lung perfusion was relatively mild, with increased dorsal lung perfusion observed. The ventilation-perfusion matched region was found to increase and correlate with the increased PaO₂/FiO₂ by PP, which was attributed mainly to reduced shunt in the lung.

CONCLUSIONS

Prolonged prone ventilation increased dorsal ventilation and perfusion, which resulted in improved ventilation-perfusion matching and oxygenation.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT04725227. Registered on 25 January 2021.

Electrical Impedance Tomography in Acute Respiratory Distress Syndrome Management

OBJECTIVE

To describe, through a narrative review, the physiologic principles underlying electrical impedance tomography, and its potential applications in managing acute respiratory distress syndrome (ARDS). To address the current evidence supporting its use in different clinical scenarios along the ARDS management continuum.

DATA SOURCES

We performed an online search in Pubmed to review articles. We searched MEDLINE, Cochrane Central Register, and clinicaltrials.gov for controlled trials databases.

STUDY SELECTION

Selected publications included case series, pilot-physiologic studies, observational cohorts, and randomized controlled trials. To describe the rationale underlying physiologic principles, we included experimental studies.

DATA EXTRACTION

Data from relevant publications were reviewed, analyzed, and its content summarized.

DATA SYNTHESIS

Electrical impedance tomography is an imaging technique that has aided in understanding the mechanisms underlying multiple interventions used in ARDS management. It has the potential to monitor and predict the response to prone positioning, aid in the dosage of flow rate in high-flow nasal cannula, and guide the titration of positive-end expiratory pressure during invasive mechanical ventilation. The latter has been demonstrated to improve physiologic and mechanical parameters correlating with lung recruitment. Similarly, its use in detecting pneumothorax and harmful patient-ventilator interactions such as pendelluft has been proven effective. Nonetheless, its impact on clinically meaningful outcomes remains to be determined.

CONCLUSIONS

Electrical impedance tomography is a potential tool for the individualized management of ARDS throughout its different stages. Clinical trials should aim to determine whether a specific approach can improve clinical outcomes in ARDS management.

External chest-wall compression in prolonged COVID-19 ARDS with low-compliance: a physiological study

Background

External chest-wall compression (ECC) is sometimes used in ARDS patients despite lack of evidence. It is currently unknown whether this practice has any clinical benefit in patients with COVID-19 ARDS (C-ARDS) characterized by a respiratory system compliance (C_rs) < 35 mL/cmH₂O.

Objectives

To test if an ECC with a 5 L-bag in low-compliance C-ARDS can lead to a reduction in driving pressure (DP) and improve gas exchange, and to understand the underlying mechanisms.

Methods

Eleven patients with low-compliance C-ARDS were enrolled and underwent 4 steps: baseline, ECC for 60 min, ECC discontinuation and PEEP reduction. Respiratory mechanics, gas exchange, hemodynamics and electrical impedance tomography were recorded. Four pigs with acute ARDS were studied with ECC to understand the effect of ECC on pleural pressure gradient using pleural pressure transducers in both non-dependent and dependent lung regions.

Results

Five minutes of ECC reduced DP from baseline 14.2 ± 1.3 to 12.3 ± 1.3 cmH₂O (P < 0.001), explained by an improved lung compliance. Changes in DP by ECC were strongly correlated with changes in DP obtained with PEEP reduction (R² = 0.82, P < 0.001). The initial benefit of ECC decreased over time (DP = 13.3 ± 1.5 cmH₂O at 60 min, P = 0.03 vs. baseline). Gas exchange and hemodynamics were unaffected by ECC. In four pigs with lung injury, ECC led to a decrease in the pleural pressure gradient at end-inspiration [2.2 (1.1–3) vs. 3.0 (2.2–4.1) cmH₂O, P = 0.035].

Conclusions

In C-ARDS patients with C_rs < 35 mL/cmH₂O, ECC acutely reduces DP. ECC does not improve oxygenation but it can be used as a simple tool to detect hyperinflation as it improves C_rs and reduces P_pl gradient. ECC benefits seem to partially fade over time. ECC produces similar changes compared to PEEP reduction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13613-022-01008-6.

Lung and diaphragm protective ventilation: a synthesis of recent data

INTRODUCTION

: To adhere to the Hippocratic Oath, to "first, do no harm", we need to make every effort to minimize the adverse effects of mechanical ventilation. Our understanding of the mechanisms of ventilator-induced lung injury (VILI) and ventilator-induced diaphragm dysfunction (VIDD) has increased in recent years. Research focuses now on methods to monitor lung stress and inhomogeneity and targets we should aim for when setting the ventilator. In parallel, efforts to promote early assisted ventilation to prevent VIDD have revealed new challenges, such as titrating inspiratory effort and synchronizing the mechanical with the patients' spontaneous breaths, while at the same time adhering to lung-protective targets.

AREAS COVERED

This is a narrative review of the key mechanisms contributing to VILI and VIDD and the methods currently available to evaluate and mitigate the risk of lung and diaphragm injury.

EXPERT OPINION

Implementing lung and diaphragm protective ventilation requires individualizing the ventilator settings, and this can only be accomplished by exploiting in everyday clinical practice the tools available to monitor lung stress and inhomogeneity, inspiratory effort, and patient-ventilator interaction.

Effects of different positive end-expiratory pressure titration strategies during prone positioning in patients with acute respiratory distress syndrome: a prospective interventional study

Background

Prone positioning in combination with the application of low tidal volume and adequate positive end-expiratory pressure (PEEP) improves survival in patients with moderate to severe acute respiratory distress syndrome (ARDS). However, the effects of PEEP on end-expiratory transpulmonary pressure (Ptp_exp) during prone positioning require clarification. For this purpose, the effects of three different PEEP titration strategies on Ptp_exp, respiratory mechanics, mechanical power, gas exchange, and hemodynamics were evaluated comparing supine and prone positioning.

Methods

In forty consecutive patients with moderate to severe ARDS protective ventilation with PEEP titrated according to three different titration strategies was evaluated during supine and prone positioning: (A) ARDS Network recommendations (PEEP_ARDSNetwork), (B) the lowest static elastance of the respiratory system (PEEP_Estat,RS), and (C) targeting a positive Ptp_exp (PEEP_Ptpexp). The primary endpoint was to analyze whether Ptp_exp differed significantly according to PEEP titration strategy during supine and prone positioning.

Results

Ptp_exp increased progressively with prone positioning compared with supine positioning as well as with PEEP_Estat,RS and PEEP_Ptpexp compared with PEEP_ARDSNetwork (positioning effect p < 0.001, PEEP strategy effect p < 0.001). PEEP was lower during prone positioning with PEEP_Estat,RS and PEEP_Ptpexp (positioning effect p < 0.001, PEEP strategy effect p < 0.001). During supine positioning, mechanical power increased progressively with PEEP_Estat,RS and PEEP_Ptpexp compared with PEEP_ARDSNetwork, and prone positioning attenuated this effect (positioning effect p < 0.001, PEEP strategy effect p < 0.001). Prone compared with supine positioning significantly improved oxygenation (positioning effect p < 0.001, PEEP strategy effect p < 0.001) while hemodynamics remained stable in both positions.

Conclusions

Prone positioning increased transpulmonary pressures while improving oxygenation and hemodynamics in patients with moderate to severe ARDS when PEEP was titrated according to the ARDS Network lower PEEP table. This PEEP titration strategy minimized parameters associated with ventilator-induced lung injury induction, such as transpulmonary driving pressure and mechanical power. We propose that a lower PEEP strategy (PEEP_ARDSNetwork) in combination with prone positioning may be part of a lung protective ventilation strategy in patients with moderate to severe ARDS.

Trial registration

German Clinical Trials Register (DRKS00017449). Registered June 27, 2019. https://www.drks.de/drks_web/navigate.do?navigationId=trial.HTML&TRIAL_ID=DRKS00017449

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-022-03956-8.

Repeated endo-tracheal tube disconnection generates pulmonary edema in a model of volume overload: an experimental study

BACKGROUND

An abrupt lung deflation in rodents results in lung injury through vascular mechanisms. Ventilator disconnections during endo-tracheal suctioning in humans often cause cardio-respiratory instability. Whether repeated disconnections or lung deflations cause lung injury or oedema is not known and was tested here in a porcine large animal model.

METHODS

Yorkshire pigs (~ 12 weeks) were studied in three series. First, we compared PEEP abruptly deflated from 26 cmH2O or from PEEP 5 cmH2O to zero. Second, pigs were randomly crossed over to receive rapid versus gradual PEEP removal from 20 cmH2O. Third, pigs with relative volume overload, were ventilated with PEEP 15 cmH2O and randomized to repeated ETT disconnections (15 s every 15 min) or no disconnection for 3 h. Hemodynamics, pulmonary variables were monitored, and lung histology and bronchoalveolar lavage studied.

RESULTS

As compared to PEEP 5 cmH2O, abrupt deflation from PEEP 26 cmH2O increased PVR, lowered oxygenation, and increased lung wet-to-dry ratio. From PEEP 20 cmH2O, gradual versus abrupt deflation mitigated the changes in oxygenation and vascular resistance. From PEEP 15, repeated disconnections in presence of fluid loading led to reduced compliance, lower oxygenation, higher pulmonary artery pressure, higher lung wet-to-dry ratio, higher lung injury score and increased oedema on morphometry, compared to no disconnects.

CONCLUSION

Single abrupt deflation from high PEEP, and repeated short deflations from moderate PEEP cause pulmonary oedema, impaired oxygenation, and increased PVR, in this large animal model, thus replicating our previous finding from rodents. Rapid deflation may thus be a clinically relevant cause of impaired lung function, which may be attenuated by gradual pressure release.

Association of PEEP and Lung Recruitment Selection Strategies with Mortality in Acute Respiratory Distress Syndrome: A Systematic Review and Network Meta-Analysis

RATIONALE

The most beneficial positive end-expiratory pressure (PEEP) selection strategy in patients with acute respiratory distress syndrome (ARDS) is unknown and current practice is variable.

OBJECTIVES

To compare the relative effects of different PEEP selection strategies on mortality in adults with moderate to severe ARDS.

METHODS

We conducted a network meta-analysis using a Bayesian framework. Certainty of evidence was evaluated using GRADE methodology.

RESULTS

We included 18 randomized trials (4646 participants). In comparison to a lower PEEP strategy, the posterior probability of mortality benefit from a higher PEEP without lung recruitment maneuver (LRM) strategy was 99% (RR 0.77, 95% Crl 0.60-0.96, high certainty), the posterior probability of benefit of the Pes-guided strategy was 87% (RR 0.77, 95% CrI 0.48-1.22, moderate certainty), the posterior probability of benefit of a higher PEEP with brief LRM strategy was 96% (RR 0.83, 95% CrI 0.67-1.02, moderate certainty), and the posterior probability of increased mortality from a higher PEEP with prolonged LRM strategy was 77% (RR 1.06, 95% Crl 0.89-1.22, low certainty). In comparison to a higher PEEP without LRM strategy, the posterior probability of increased mortality from a higher PEEP with prolonged LRM strategy was 99% (RR 1.37, 95% CrI 1.04-1.81, moderate certainty).

CONCLUSIONS AND RELEVANCE

In patients with moderate to severe ARDS, higher PEEP without LRM is associated with a lower risk of death as compared to lower PEEP. A higher PEEP with prolonged LRM strategy is associated with increased risk of death when compared to higher PEEP without LRM.

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.

Prone Positioning in Acute Respiratory Distress Syndrome

Prone Positioning in ARDSCovid-19 has greatly expanded the use of prone positioning for patients with respiratory failure. Pearce and colleagues review the physiology of prone positioning and the evidence for its use, including in nonintubated patients.

Homogenizing effect of PEEP on tidal volume distribution during neurally adjusted ventilatory assist: study of an animal model of acute respiratory distress syndrome

BACKGROUND

The physiological response and the potentially beneficial effects of positive end-expiratory pressure (PEEP) for lung protection and optimization of ventilation during spontaneous breathing in patients with acute respiratory distress syndrome (ARDS) are not fully understood. The aim of the study was to compare the effect of different PEEP levels on tidal volume distribution and on the ventilation of dependent lung region during neurally adjusted ventilatory assist (NAVA).

METHODS

ARDS-like lung injury was induced by using saline lavage in 10 anesthetized and spontaneously breathing farm-bred pigs. The animals were ventilated in NAVA modality and tidal volume distribution as well as dependent lung ventilation were assessed using electric impedance tomography during the application of PEEP levels from 0 to 15 cmH₂0, in steps of 3 cmH₂0. Tidal volume distribution and dependent fraction of ventilation were analysed using Wilcoxon signed rank test. Furthermore, airway, esophageal and transpulmonary pressure, as well as airway flow and delivered volume, were continuously measured during the assisted spontaneous breathing.

RESULTS

Increasing PEEP improved oxygenation and re-distributed tidal volume. Specifically, ventilation distribution changed from a predominant non-dependent to a more even distribution between non-dependent and dependent areas of the lung. Dependent fraction of ventilation reached 47 ± 9% at PEEP 9 cmH₂0. Further increasing PEEP led to a predominant dependent ventilation.

CONCLUSION

During assisted spontaneous breathing in this model of induced ARDS, PEEP modifies the distribution of ventilation and can achieve a homogenizing effect on its spatial arrangement. The study indicates that PEEP is an important factor during assisted spontaneous breathing and that EIT can be of valuable interest when titrating PEEP level during spontaneous breathing, by indicating the most homogeneous distribution of gas volumes throughout the PEEP spectrum.

COVID-19 ARDS: Points to Be Considered in Mechanical Ventilation and Weaning

The COVID-19 disease can cause hypoxemic respiratory failure due to ARDS, requiring invasive mechanical ventilation. Although early studies reported that COVID-19-associated ARDS has distinctive features from ARDS of other causes, recent observational studies have demonstrated that ARDS related to COVID-19 shares common clinical characteristics and respiratory system mechanics with ARDS of other origins. Therefore, mechanical ventilation in these patients should be based on strategies aiming to mitigate ventilator-induced lung injury. Assisted mechanical ventilation should be applied early in the course of mechanical ventilation by considering evaluation and minimizing factors associated with patient-inflicted lung injury. Extracorporeal membrane oxygenation should be considered in selected patients with refractory hypoxia not responding to conventional ventilation strategies. This review highlights the current and evolving practice in managing mechanically ventilated patients with ARDS related to COVID-19.

A rational approach on the use of extracorporeal membrane oxygenation in severe hypoxemia: advanced technology is not a panacea

Veno-venous extracorporeal membrane oxygenation (ECMO) is a helpful intervention in patients with severe refractory hypoxemia either because mechanical ventilation cannot ensure adequate oxygenation or because lung protective ventilation is not feasible. Since ECMO is a highly invasive procedure with several, potentially devastating complications and its implementation is complex and expensive, simpler and less invasive therapeutic options should be first exploited. Low tidal volume and driving pressure ventilation, prone position, neuromuscular blocking agents and individualized ventilation based on transpulmonary pressure measurements have been demonstrated to successfully treat the vast majority of mechanically ventilated patients with severe hypoxemia. Veno-venous ECMO has a place in the small portion of severely hypoxemic patients in whom these strategies fail. A combined analysis of recent ARDS trials revealed that ECMO was used in only 2.15% of patients (n = 145/6736). Nevertheless, ECMO use has sharply increased in the last decade, raising questions regarding its thoughtful use. Such a policy could be harmful both for patients as well as for the ECMO technique itself. This narrative review attempts to describe together the practical approaches that can be offered to the sickest patients before going to ECMO, as well as the rationale and the limitations of ECMO. The benefit and the drawbacks associated with ECMO use along with a direct comparison with less invasive therapeutic strategies will be analyzed.

Respiratory Physiology of Prone Positioning With and Without Inhaled Nitric Oxide Across the Coronavirus Disease 2019 Acute Respiratory Distress Syndrome Severity Spectrum

Supplemental Digital Content is available in the text.

IMPORTANCE:

Prone positioning improves clinical outcomes in moderate-to-severe acute respiratory distress syndrome and has been widely adopted for the treatment of patients with acute respiratory distress syndrome due to coronavirus disease 2019. Little is known about the effects of prone positioning among patients with less severe acute respiratory distress syndrome, obesity, or those treated with pulmonary vasodilators.

OBJECTIVES:

We characterize the change in oxygenation, respiratory system compliance, and dead-space-to-tidal-volume ratio in response to prone positioning in patients with coronavirus disease 2019 acute respiratory distress syndrome with a range of severities. A subset analysis of patients treated with inhaled nitric oxide and subsequent prone positioning explored the influence of pulmonary vasodilation on the physiology of prone positioning.

DESIGN, SETTING, AND PARTICIPANTS:

Retrospective cohort study of all consecutively admitted adult patients with acute respiratory distress syndrome due to coronavirus disease 2019 treated with mechanical ventilation and prone positioning in the ICUs of an academic hospital between March 11, 2020, and May 1, 2020.

MAIN OUTCOMES AND MEASURES:

Respiratory system mechanics and gas exchange during the first episode of prone positioning.

RESULTS:

Among 122 patients, median (interquartile range) age was 60 years (51–71 yr), median body mass index was 31.5 kg/m² (27–35 kg/m²), and 50 patients (41%) were female. The ratio of Pao₂ to Fio₂ improved with prone positioning in 90% of patients. Prone positioning was associated with a significant increase in the ratio of Pao₂ to Fio₂ (from median 149 [123–170] to 226 [169–268], p < 0.001) but no change in dead-space-to-tidal-volume ratio or respiratory system compliance. Supine ratio of Pao₂ to Fio₂, respiratory system compliance, positive end-expiratory pressure, and body mass index did not correlate with absolute change in the ratio of Pao₂ to Fio₂ with prone positioning. However, patients with ratio of Pao₂ to Fio₂ less than 150 experienced a greater relative improvement in oxygenation with prone positioning than patients with ratio of Pao₂ to Fio₂ greater than or equal to 150 (median percent change in ratio of Pao₂ to Fio₂ 62 [29–107] vs 30 [10–70], p = 0.002). Among 12 patients, inhaled nitric oxide prior to prone positioning was associated with a significant increase in the ratio of Pao₂ to Fio₂ (from median 136 [77–168] to 170 [138–213], p = 0.003) and decrease in dead-space-to-tidal-volume ratio (0.54 [0.49–0.58] to 0.46 [0.44–0.53], p = 0.001). Subsequent prone positioning in this subgroup further improved the ratio of Pao₂ to Fio₂ (from 145 [122–183] to 205 [150–232], p = 0.017) but did not change dead-space-to-tidal-volume ratio.

CONCLUSIONS AND RELEVANCE:

Prone positioning improves oxygenation across the acute respiratory distress syndrome severity spectrum, irrespective of supine respiratory system compliance, positive end-expiratory pressure, or body mass index. There was a greater relative benefit among patients with more severe disease. Prone positioning confers an additive benefit in oxygenation among patients treated with inhaled nitric oxide.

Sustained oxygenation improvement after first prone positioning is associated with liberation from mechanical ventilation and mortality in critically ill COVID-19 patients: a cohort study

Background

Prone positioning (PP) has been used to improve oxygenation in patients affected by the SARS-CoV-2 disease (COVID-19). Several mechanisms, including lung recruitment and better lung ventilation/perfusion matching, make a relevant rational for using PP. However, not all patients maintain the oxygenation improvement after returning to supine position. Nevertheless, no evidence exists that a sustained oxygenation response after PP is associated to outcome in mechanically ventilated COVID-19 patients. We analyzed data from 191 patients affected by COVID-19-related acute respiratory distress syndrome undergoing PP for clinical reasons. Clinical history, severity scores and respiratory mechanics were analyzed. Patients were classified as responders (≥ median PaO₂/FiO₂ variation) or non-responders (< median PaO₂/FiO₂ variation) based on the PaO₂/FiO₂ percentage change between pre-proning and 1 to 3 h after re-supination in the first prone positioning session. Differences among the groups in physiological variables, complication rates and outcome were evaluated. A competing risk regression analysis was conducted to evaluate if PaO₂/FiO₂ response after the first pronation cycle was associated to liberation from mechanical ventilation.

Results

The median PaO₂/FiO₂ variation after the first PP cycle was 49 [19–100%] and no differences were found in demographics, comorbidities, ventilatory treatment and PaO₂/FiO₂ before PP between responders (96/191) and non-responders (95/191). Despite no differences in ICU length of stay, non-responders had a higher rate of tracheostomy (70.5% vs 47.9, P = 0.008) and mortality (53.7% vs 33.3%, P = 0.006), as compared to responders. Moreover, oxygenation response after the first PP was independently associated to liberation from mechanical ventilation at 28 days and was increasingly higher being higher the oxygenation response to PP.

Conclusions

Sustained oxygenation improvement after first PP session is independently associated to improved survival and reduced duration of mechanical ventilation in critically ill COVID-19 patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13613-021-00853-1.