Severity of Hypoxemia and Effect of High-Frequency Oscillatory Ventilation in Acute Respiratory Distress Syndrome

Protocol for the Prone and Oscillation Pediatric Clinical Trial (PROSpect)

OBJECTIVES

Respiratory management for pediatric acute respiratory distress syndrome (PARDS) remains largely supportive without data to support one approach over another, including supine versus prone positioning (PP) and conventional mechanical ventilation (CMV) versus high-frequency oscillatory ventilation (HFOV).

DESIGN

We present the research methodology of a global, multicenter, two-by-two factorial, response-adaptive, randomized controlled trial of supine versus PP and CMV versus HFOV in high moderate-severe PARDS, the Prone and Oscillation Pediatric Clinical Trial (PROSpect, www.ClinicalTrials.gov, NCT03896763).

SETTING

Approximately 60 PICUs with on-site extracorporeal membrane oxygenation support in North and South America, Europe, Asia, and Oceania with experience using PP and HFOV in the care of patients with PARDS.

PATIENTS

Eligible pediatric patients (2 wk old or older and younger than 21 yr) are randomized within 48 h of meeting eligibility criteria occurring within 96 h of endotracheal intubation.

INTERVENTIONS

One of four arms, including supine/CMV, prone/CMV, supine/HFOV, or prone/HFOV. We hypothesize that children with high moderate-severe PARDS treated with PP or HFOV will demonstrate greater than or equal to 2 additional ventilator-free days (VFD).

MEASUREMENTS AND MAIN RESULTS

The primary outcome is VFD through day 28; nonsurvivors receive zero VFD. Secondary and exploratory outcomes include nonpulmonary organ failure-free days, interaction effects of PP with HFOV on VFD, 90-day in-hospital mortality, and among survivors, duration of mechanical ventilation, PICU and hospital length of stay, and post-PICU functional status and health-related quality of life. Up to 600 patients will be randomized, stratified by age group and direct/indirect lung injury. Adaptive randomization will first occur 28 days after 300 patients are randomized and every 100 patients thereafter. At these randomization updates, new allocation probabilities will be computed based on intention-to-treat trial results, increasing allocation to well-performing arms and decreasing allocation to poorly performing arms. Data will be analyzed per intention-to-treat for the primary analyses and per-protocol for primary, secondary, and exploratory analyses.

CONCLUSIONS

PROSpect will provide clinicians with data to inform the practice of PP and HFOV in PARDS.

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.

Pediatric adenovirus pneumonia: clinical practice and current treatment

Adenovirus pneumonia is common in pediatric upper respiratory tract infection, which is comparatively easy to develop into severe cases and has a high mortality rate with many influential sequelae. As for pathogenesis, adenoviruses can directly damage target cells and activate the immune response to varying degrees. Early clinical recognition depends on patients' symptoms and laboratory tests, including those under 2 years old, dyspnea with systemic toxic symptoms, atelectasis or emphysema in CT image, decreased leukocytes, and significantly increased C-reaction protein (CRP) and procalcitonin (PCT), indicating the possibility of severe cases. Until now, there is no specific drug for adenovirus pneumonia, so in clinical practice, current treatment comprises antiviral drugs, respiratory support and bronchoscopy, immunomodulatory therapy, and blood purification. Additionally, post-infectious bronchiolitis obliterans (PIBO), hemophagocytic syndrome, and death should be carefully noted. Independent risk factors associated with the development of PIBO are invasive mechanical ventilation, intravenous steroid use, duration of fever, and male gender. Meanwhile, hypoxemia, hypercapnia, invasive mechanical ventilation, and low serum albumin levels are related to death. Among these, viral load and serological identification are not only "gold standard" for adenovirus pneumonia, but are also related to the severity and prognosis. Here, we discuss the progress of pathogenesis, early recognition, therapy, and risk factors for poor outcomes regarding severe pediatric adenovirus pneumonia.

Acute Respiratory Distress Syndrome; A Review of Recent Updates and a Glance into the Future

Acute respiratory distress syndrome (ARDS) is a rapidly progressive form of respiratory failure that accounts for 10% of admissions to the ICU and is associated with approximately 40% mortality in severe cases. Despite significant mortality and healthcare burden, the mainstay of management remains supportive care. The recent pandemic of SARS-CoV-2 has re-ignited a worldwide interest in exploring the pathophysiology of ARDS, looking for innovative ideas to treat this disease. Recently, many trials have been published utilizing different pharmacotherapy targets; however, the long-term benefits of these agents remain unknown. Metabolomics profiling and stem cell transplantation offer strong enthusiasm and may completely change the outlook of ARDS management in the near future.

A review of the utility of high-frequency oscillatory ventilation in burn and trauma ICU patients

PURPOSE OF REVIEW

The purpose was to examine the utility of high-frequency oscillatory ventilation (HFOV) in trauma and burn ICU patients who require mechanical ventilation, and provide recommendations on its use.

RECENT FINDINGS

HFOV may be beneficial in burn patients with smoke inhalation injury with or without acute lung injury/acute respiratory distress syndrome (ARDS), as it improves oxygenation and minimizes ventilator-induced lung injury. It also may have a role in improving oxygenation in trauma patients with blast lung injury, pulmonary contusions, pneumothorax with massive air leak, and ARDS; however, the mortality benefit is unknown.

SUMMARY

Although some studies have shown promise and improved outcomes associated with HFOV, we recommend its use as a rescue modality for patients who have failed conventional ventilation.

Challenges in ARDS Definition, Management, and Identification of Effective Personalized Therapies

Over the last decade, the management of acute respiratory distress syndrome (ARDS) has made considerable progress both regarding supportive and pharmacologic therapies. Lung protective mechanical ventilation is the cornerstone of ARDS management. Current recommendations on mechanical ventilation in ARDS include the use of low tidal volume (V_T) 4-6 mL/kg of predicted body weight, plateau pressure (P_PLAT) < 30 cmH₂O, and driving pressure (∆P) < 14 cmH₂O. Moreover, positive end-expiratory pressure should be individualized. Recently, variables such as mechanical power and transpulmonary pressure seem promising for limiting ventilator-induced lung injury and optimizing ventilator settings. Rescue therapies such as recruitment maneuvers, vasodilators, prone positioning, extracorporeal membrane oxygenation, and extracorporeal carbon dioxide removal have been considered for patients with severe ARDS. Regarding pharmacotherapies, despite more than 50 years of research, no effective treatment has yet been found. However, the identification of ARDS sub-phenotypes has revealed that some pharmacologic therapies that have failed to provide benefits when considering all patients with ARDS can show beneficial effects when these patients were stratified into specific sub-populations; for example, those with hyperinflammation/hypoinflammation. The aim of this narrative review is to provide an overview on current advances in the management of ARDS from mechanical ventilation to pharmacological treatments, including personalized therapy.

Advances in Ventilator Management for Patients with Acute Respiratory Distress Syndrome

The ventilatory care of patients with acute respiratory distress syndrome (ARDS) is evolving as our understanding of physiologic mechanisms of respiratory failure improves. Despite several decades of research, the mortality rate for ARDS remains high. Over the years, we continue to expand strategies to identify and mitigate ventilator-induced lung injury. This now includes a greater understanding of the benefits and harms associated with spontaneous breathing.

Effect of high-frequency oscillatory ventilation combined with prone positioning in infants with acute respiratory distress syndrome after congenital heart surgery: A prospective randomized controlled trial

Objectives

This study aimed to evaluate the effect of high-frequency oscillatory ventilation, (HFOV) combined with prone positioning, on oxygenation and pulmonary ventilation in infants with acute respiratory distress syndrome (ARDS) after congenital heart surgery.

Design

A randomized controlled trial.

Setting

A single-center study at a tertiary teaching hospital.

Participants

Patients with postoperative ARDS after congenital heart disease were divided randomly into the following 2 groups: HFOV combined with prone position (HFOV-PP), and HFOV combined with supine position (HFOV-SP).

Interventions

The primary outcomes were the PaO₂/F_IO₂ ratio and the oxygenation index after the intervention, and the secondary outcomes were respiratory variables, hemodynamics, complications, and other short-term outcomes.

Results

Sixty-five eligible infants with ARDS were randomized to either the HFOV-PP (n = 32) or HFOV-SP (n = 33) group. No significant difference in baseline data was found between the 2 groups (p > 0.05). Oxygenation was improved in both groups after HFOV intervention. Compared with the HFOV-SP group, the HFOV-PP group had significantly increased PaO₂/F_IO₂ and oxygenation index and a shorter duration of invasive ventilation and length of cardiac intensive care unit stay. No serious complications occurred in the 2 groups.

Conclusion

HFOV-PP significantly improved oxygenation in infants with ARDS after cardiac surgery and had no serious complications.

The Physiological Basis of High-Frequency Oscillatory Ventilation and Current Evidence in Adults and Children: A Narrative Review

High-frequency oscillatory ventilation (HFOV) is a type of invasive mechanical ventilation that employs supra-physiologic respiratory rates and low tidal volumes (V_T) that approximate the anatomic deadspace. During HFOV, mean airway pressure is set and gas is then displaced towards and away from the patient through a piston. Carbon dioxide (CO₂) is cleared based on the power (amplitude) setting and frequency, with lower frequencies resulting in higher V_T and CO₂ clearance. Airway pressure amplitude is significantly attenuated throughout the respiratory system and mechanical strain and stress on the alveoli are theoretically minimized. HFOV has been purported as a form of lung protective ventilation that minimizes volutrauma, atelectrauma, and biotrauma. Following two large randomized controlled trials showing no benefit and harm, respectively, HFOV has largely been abandoned in adults with ARDS. A multi-center clinical trial in children is ongoing. This article aims to review the physiologic rationale for the use of HFOV in patients with acute respiratory failure, summarize relevant bench and animal models, and discuss the potential use of HFOV as a primary and rescue mode in adults and children with severe respiratory failure.

Acute Respiratory Distress Syndrome in the Perioperative Period of Cardiac Surgery: Predictors, Diagnosis, Prognosis, Management Options, and Future Directions

Acute respiratory distress syndrome (ARDS) after cardiac surgery is reported with a widely variable incidence (from 0.4%-8.1%). Cardiac surgery patients usually are affected by several comorbidities, and the development of ARDS significantly affects their prognosis. Herein, evidence regarding the current knowledge in the field of ARDS in cardiac surgery is summarized and is followed by a discussion on therapeutic strategies, with consideration of the peculiar aspects of ARDS after cardiac surgery. Prevention of lung injury during and after cardiac surgery remains pivotal. Blood product transfusions should be limited to minimize the risk, among others, of lung injury. Open lung ventilation strategy (ventilation during cardiopulmonary bypass, recruitment maneuvers, and the use of moderate positive end-expiratory pressure) has not shown clear benefits on clinical outcomes. Clinicians in the intraoperative and postoperative ventilatory settings carefully should consider the effect of mechanical ventilation on cardiac function (in particular the right ventricle). Driving pressure should be kept as low as possible, with low tidal volumes (on predicted body weight) and optimal positive end-expiratory pressure. Regarding the therapeutic options, management of ARDS after cardiac surgery challenges the common approach. For instance, prone positioning may not be easily applicable after cardiac surgery. In patients who develop ARDS after cardiac surgery, extracorporeal techniques may be a valid choice in experienced hands. The use of neuromuscular blockade and inhaled nitric oxide can be considered on a case-by-case basis, whereas the use of aggressive lung recruitment and oscillatory ventilation should be discouraged.

Unsuccessful and Successful Clinical Trials in Acute Respiratory Distress Syndrome: Addressing Physiology-Based Gaps

The acute respiratory distress syndrome (ARDS) is a severe form of acute hypoxemic respiratory failure caused by an insult to the alveolar-capillary membrane, resulting in a marked reduction of aerated alveoli, increased vascular permeability and subsequent interstitial and alveolar pulmonary edema, reduced lung compliance, increase of physiological dead space, and hypoxemia. Most ARDS patients improve their systemic oxygenation, as assessed by the ratio between arterial partial pressure of oxygen and inspired oxygen fraction, with conventional intensive care and the application of moderate-to-high levels of positive end-expiratory pressure. However, in some patients hypoxemia persisted because the lungs are markedly injured, remaining unresponsive to increasing the inspiratory fraction of oxygen and positive end-expiratory pressure. For decades, mechanical ventilation was the only standard support technique to provide acceptable oxygenation and carbon dioxide removal. Mechanical ventilation provides time for the specific therapy to reverse the disease-causing lung injury and for the recovery of the respiratory function. The adverse effects of mechanical ventilation are direct consequences of the changes in pulmonary airway pressures and intrathoracic volume changes induced by the repetitive mechanical cycles in a diseased lung. In this article, we review 14 major successful and unsuccessful randomized controlled trials conducted in patients with ARDS on a series of techniques to improve oxygenation and ventilation published since 2010. Those trials tested the effects of adjunctive therapies (neuromuscular blocking agents, prone positioning), methods for selecting the optimum positive end-expiratory pressure (after recruitment maneuvers, or guided by esophageal pressure), high-frequency oscillatory ventilation, extracorporeal oxygenation, and pharmacologic immune modulators of the pulmonary and systemic inflammatory responses in patients affected by ARDS. We will briefly comment physiology-based gaps of negative trials and highlight the possible needs to address in future clinical trials in ARDS.

Management of Severe Influenza

Influenza infection causes severe illness in 3 to 5 million people annually, with up to an estimated 650,000 deaths per annum. As such, it represents an ongoing burden to health care systems and human health. Severe acute respiratory infection can occur, resulting in respiratory failure requiring intensive care support. Herein we discuss diagnostic approaches, including development of CLIA-waived point of care tests that allow rapid diagnosis and treatment of influenza. Bacterial and fungal coinfections in severe influenza pneumonia are associated with worse outcomes, and we summarize the approach and treatment options for diagnosis and treatment of bacterial and Aspergillus coinfection. We discuss the available drug options for the treatment of severe influenza, and treatments which are no longer supported by the evidence base. Finally, we describe the supportive management and ventilatory approach to patients with respiratory failure as a result of severe influenza in the intensive care unit.

OLA strategy for ARDS: Its effect on mortality depends on achieved recruitment (PaO2/FiO2) and mechanical power. Systematic review and meta-analysis with meta-regression

OBJECTIVE

The "Open Lung Approach" (OLA), that includes high levels of positive end-expiratory pressure coupled with limited tidal volumes, is considered optimal for adult patients with ARDS. However, many previous meta-analyses have shown only marginal benefits of OLA on mortality but with statistical heterogeneity. It is crucial to identify the most likely moderators of this effect. To determine the effect of OLA strategy on mortality of ventilated ARDS patients. We hypothesized that the degree of recruitment achieved in the control group (PaO₂/FiO₂ ratio on day 3 of ventilation), and the difference in Mechanical Power (MP) or Driving Pressure (DP) between experimental and control groups will be the most likely sources of heterogeneity.

DESIGN

A Systematic Review and Meta-analysis was performed according to PRISMA statement and registered in PROSPERO database. We searched only for randomized controlled trials (RCTs). GRADE guidelines were used for rating the quality of evidence. Publication bias was assessed. For the Meta-analysis, we used a Random Effects Model. Sources of heterogeneity were explored with Meta-Regression, using a priori proposed set of possible moderators. For model comparison, Akaike's Information Criterion with the finite sample correction (AICc) was used.

SETTING

Not applicable.

PATIENTS

Fourteen RCTs were included in the study.

INTERVENTIONS

Not applicable.

MAIN VARIABLES OF INTEREST

Not applicable.

RESULTS

Evidence of publication bias was detected, and quality of evidence was downgraded. Pooled analysis did not show a significant difference in the 28-day mortality between OLA strategy and control groups. Overall risk of bias was low. The analysis detected statistical heterogeneity. The two "best" explicative meta-regression models were those that used control PaO₂/FiO₂ on day 3 and difference in MP between experimental and control groups. The DP and MP models were highly correlated.

CONCLUSIONS

There is no clear benefit of OLA strategy on mortality of ARDS patients, with significant heterogeneity among RCTs. Mortality effect of OLA is mediated by lung recruitment and mechanical power.

Precision Medicine and Heterogeneity of Treatment Effect in Therapies for Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is a clinically heterogenous syndrome, rather than a distinct disease. This heterogeneity at least partially explains the difficulty in studying treatments for these patients and contributes to the numerous trials of therapies for the syndrome that have not shown benefit. Recent studies have identified different subphenotypes within the heterogenous patient population. These different subphenotypes likely have variable clinical responses to specific therapies, a concept known as heterogeneity of treatment effect (HTE). Recognizing different subphenotypes and HTE has important implications for the clinical management of patients with ARDS. In this review, we will present studies that have identified different subphenotypes and discuss how they can modify the effects of therapies evaluated in trials that are commonly considered to have demonstrated no overall benefit in patients with ARDS.

OLA strategy for ARDS: Its effect on mortality depends on achieved recruitment (PaO2/FiO2) and mechanical power. Systematic review and meta-analysis with meta-regression

OBJECTIVE

The "Open Lung Approach" (OLA), that includes high levels of positive end-expiratory pressure coupled with limited tidal volumes, is considered optimal for adult patients with ARDS. However, many previous meta-analyses have shown only marginal benefits of OLA on mortality but with statistical heterogeneity. It is crucial to identify the most likely moderators of this effect. To determine the effect of OLA strategy on mortality of ventilated ARDS patients. We hypothesized that the degree of recruitment achieved in the control group (PaO2/FiO2 ratio on day 3 of ventilation), and the difference in Mechanical Power (MP) or Driving Pressure (DP) between experimental and control groups will be the most likely sources of heterogeneity.

DESIGN

A Systematic Review and Meta-analysis was performed according to PRISMA statement and registered in PROSPERO database. We searched only for randomized controlled trials (RCTs). GRADE guidelines were used for rating the quality of evidence. Publication bias was assessed. For the Meta-analysis, we used a Random Effects Model. Sources of heterogeneity were explored with Meta-Regression, using a priori proposed set of possible moderators. For model comparison, Akaike's Information Criterion with the finite sample correction (AICc) was used.

SETTING

Not applicable.

PATIENTS

Fourteen RCTs were included in the study.

INTERVENTIONS

Not applicable.

MAIN VARIABLES OF INTEREST

Not applicable.

RESULTS

Evidence of publication bias was detected, and quality of evidence was downgraded. Pooled analysis did not show a significant difference in the 28-day mortality between OLA strategy and control groups. Overall risk of bias was low. The analysis detected statistical heterogeneity. The two "best" explicative meta-regression models were those that used control PaO2/FiO2 on day 3 and difference in MP between experimental and control groups. The DP and MP models were highly correlated.

CONCLUSIONS

There is no clear benefit of OLA strategy on mortality of ARDS patients, with significant heterogeneity among RCTs. Mortality effect of OLA is mediated by lung recruitment and mechanical power.

Oscillatory ventilation redux: alternative perspectives on ventilator-induced lung injury in the acute respiratory distress syndrome

For patients with the acute respiratory distress syndrome (ARDS), ventilation strategies that limit end-expiratory derecruitment and end-inspiratory overdistension are the only interventions to have significantly reduced the morbidity and mortality. For this reason, the use of high-frequency oscillatory ventilation (HFOV) was considered to be an ideal protective strategy, given its reliance on very low tidal volumes cycled at very high rates. However, results from clinical trials in adults with ARDS have demonstrated that HFOV does not improve clinical outcomes. Recent experimental and computational studies have shown that oscillation of a mechanically heterogeneous lung with multiple simultaneous frequencies can reduce parenchymal strain, improve gas exchange, and maintain lung recruitment at lower distending pressures compared to traditional ‘single-frequency’ HFOV. This review will discuss the theoretical rationale for the use of multiple oscillatory frequencies in ARDS, as well as the mechanisms by which it may reduce the risk for ventilator-induced lung injury.

Treatment with 7% and 10% CO2 enhanced expression of IL-1β, TNF-α, and IL-6 in hypoxic cultures of human whole blood

Objective

This study investigated whether hypercapnia influenced the inflammatory response of hypoxic blood.

Methods

Human whole blood was cultured with 0.2% oxygen (O₂) and treated with 5%, 7%, or 10% carbon dioxide (CO₂). Interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and IL-6 were evaluated in whole blood cultures. Reactive oxygen species (ROS) production and expression levels of caspase-1 and IL-1β were evaluated in THP-1 monocytic cells.

Results

IL-1β, TNF-α, and IL-6 levels were higher in the hypoxia + 7% CO₂ group than in the hypoxia + 5% CO₂ group. The hypoxia + 10% CO₂ group had the highest IL-1β, TNF-α, and IL-6 levels, compared with the hypoxia + 7% CO₂ and hypoxia + 5% CO₂ groups. Expression levels of IL-1β, TNF-α, and IL-6 were significantly negatively correlated with pH levels in the cell culture medium. Treatment with 7% and 10% CO₂ increased the production of ROS and the expression of caspase-1 and IL-1β in hypoxia-activated THP-1 cells.

Conclusions

High levels of CO₂ treatment increased expression levels of IL-1β, TNF-α, and IL-6 in hypoxic whole blood cultures. High levels of CO₂-induced ROS overproduction and NLRP3 inflammasome activation in monocytes may comprise a target to mitigate the inflammatory response of hypoxic blood.

Management of ARDS - What Works and What Does Not

Acute respiratory distress syndrome (ARDS) is a clinically and biologically heterogeneous disorder associated with a variety of disease processes that lead to acute lung injury with increased non-hydrostatic extravascular lung water, reduced compliance, and severe hypoxemia. Despite significant advances, mortality associated with this syndrome remains high. Mechanical ventilation remains the most important aspect of managing patients with ARDS. An in-depth knowledge of lung protective ventilation, optimal PEEP strategies, modes of ventilation and recruitment maneuvers are essential for ventilatory management of ARDS. Although, the management of ARDS is constantly evolving as new studies are published and guidelines being updated; we present a detailed review of the literature including the most up-to-date studies and guidelines in the management of ARDS. We believe this review is particularly helpful in the current times where more than half of the acute care hospitals lack in-house intensivists and the burden of ARDS is at large.

Management of hypercapnia in critically ill mechanically ventilated patients-A narrative review of literature

The use of lower tidal volume ventilation was shown to improve survival in mechanically ventilated patients with acute lung injury. In some patients this strategy may cause hypercapnic acidosis. A significant body of recent clinical data suggest that hypercapnic acidosis is associated with adverse clinical outcomes including increased hospital mortality. We aimed to review the available treatment options that may be used to manage acute hypercapnic acidosis that may be seen with low tidal volume ventilation. The databases of MEDLINE and EMBASE were searched. Studies including animals or tissues were excluded. We also searched bibliographic references of relevant studies, irrespective of study design with the intention of finding relevant studies to be included in this review. The possible options to treat hypercapnia included optimising the use of low tidal volume mechanical ventilation to enhance carbon dioxide elimination. These include techniques to reduce dead space ventilation, and physiological dead space, use of buffers, airway pressure release ventilation and prone positon ventilation. In patients where hypercapnic acidosis could not be managed with lung protective mechanical ventilation, extracorporeal techniques may be used. Newer, minimally invasive low volume venovenous extracorporeal devices are currently being investigated for managing hypercapnia associated with low and ultra-low volume mechanical ventilation.

Hypercapnia promotes microglial pyroptosis via inhibiting mitophagy in hypoxemic adult rats

Background

Hypoxemia is a typical symptom of acute respiratory distress syndrome. To avoid pulmonary morbidity, low tidal volume ventilation is often applied. The ventilation strategy will certainly cause hypercapnia. This study aimed to explore whether hypercapnia would promote microglial pyroptosis via inhibiting mitophagy in adult rats with hypoxemia.

Methods

The cerebral oxygen extraction ratio (CERO₂) and partial pressure of brain tissue oxygen (PbtO₂) in a rat model of hypercapnia/hypoxemia were assessed. The reactive oxygen species (ROS) production and the expression of LC3‐II/I, p62, caspase‐1, gasdermin D‐N domains (GSDMD‐N), IL‐1β, and IL‐18 in microglial cells were detected.

Results

Hypercapnia decreased the PbtO₂ levels of the hypoxic rats, which was further evidenced by the increased levels of CERO₂. Expression levels of LC3‐II were reduced, while p62 expression was increased by hypercapnia in hypoxic microglia. Hypercapnia increased the production of ROS and the expression of caspase‐1, GSDMD‐N, IL‐1β, and IL‐18 in hypoxia‐activated microglia. Scavenging ROS inhibited microglial pyroptosis and expression of IL‐1β and IL‐18.

Conclusions

These results suggest that hypercapnia‐induced mitophagy inhibition may promote pyroptosis and enhance IL‐1β and IL‐18 release in hypoxia‐activated microglia.

High-Frequency Oscillatory Ventilation and Ventilator-Induced Lung Injury: Size Does Matter

OBJECTIVES

The theoretical basis for minimizing tidal volume during high-frequency oscillatory ventilation may not be appropriate when lung tissue stretch occurs heterogeneously and/or rapidly. The objective of this study was to assess the extent to which increased ventilation heterogeneity may contribute to ventilator-induced lung injury during high-frequency oscillatory ventilation in adults compared with neonates on the basis of lung size, using a computational model of human lungs.

DESIGN

Computational modeling study.

SETTING

Research laboratory.

SUBJECTS

High-fidelity, 3D computational models of human lungs, scaled to various sizes representative of neonates, children, and adults, with varying injury severity. All models were generated from one thoracic CT image of a healthy adult male.

INTERVENTIONS

Oscillatory ventilation was simulated in each lung model at frequencies ranging from 0.2 to 40 Hz. Sinusoidal flow oscillations were delivered at the airway opening of each model and distributed through the lungs according to regional parenchymal mechanics.

MEASUREMENTS AND MAIN RESULTS

Acinar flow heterogeneity was assessed by the coefficient of variation in flow magnitudes across all acini in each model. High-frequency oscillatory ventilation simulations demonstrated increasing heterogeneity of regional parenchymal flow with increasing lung size, with decreasing ratio of deadspace to total acinar volume, and with increasing frequency above lung corner frequency and resonant frequency. Potential for resonant amplification was greatest in injured adult-sized lungs with higher regional quality factors indicating the presence of underdamped lung regions.

CONCLUSIONS

The potential for ventilator-induced lung injury during high-frequency oscillatory ventilation is enhanced at frequencies above lung corner frequency or resonant frequency despite reduced tidal volumes, especially in adults, due to regional amplification of heterogeneous flow. Measurements of corner frequency and resonant frequency should be considered during high-frequency oscillatory ventilation management.

Quantifying Regional Lung Deformation Using Four-Dimensional Computed Tomography: A Comparison of Conventional and Oscillatory Ventilation

Mechanical ventilation strategies that reduce the heterogeneity of regional lung stress and strain may reduce the risk of ventilator-induced lung injury (VILI). In this study, we used registration of four-dimensional computed tomographic (4DCT) images to assess regional lung aeration and deformation in 10 pigs under baseline conditions and following acute lung injury induced with oleic acid. CT images were obtained via dynamic axial imaging (Siemens SOMATOM Force) during conventional pressure-controlled mechanical ventilation (CMV), as well as high-frequency and multi-frequency oscillatory ventilation modalities (HFOV and MFOV, respectively). Our results demonstrate that oscillatory modalities reduce intratidal strain throughout the lung in comparison to conventional ventilation, as well as the spatial gradients of dynamic strain along the dorsal-ventral axis. Harmonic distortion of parenchymal deformation was observed during HFOV with a single discrete sinusoid delivered at the airway opening, suggesting inherent mechanical nonlinearity of the lung tissues. MFOV may therefore provide improved lung-protective ventilation by reducing strain magnitudes and spatial gradients of strain compared to either CMV or HFOV.

Critical care management of adults with community-acquired severe respiratory viral infection

With the expanding use of molecular assays, viral pathogens are increasingly recognized among critically ill adult patients with community-acquired severe respiratory illness; studies have detected respiratory viral infections (RVIs) in 17-53% of such patients. In addition, novel pathogens including zoonotic coronaviruses like the agents causing Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS) and the 2019 novel coronavirus (2019 nCoV) are still being identified. Patients with severe RVIs requiring ICU care present typically with hypoxemic respiratory failure. Oseltamivir is the most widely used neuraminidase inhibitor for treatment of influenza; data suggest that early use is associated with reduced mortality in critically ill patients with influenza. At present, there are no antiviral therapies of proven efficacy for other severe RVIs. Several adjunctive pharmacologic interventions have been studied for their immunomodulatory effects, including macrolides, corticosteroids, cyclooxygenase-2 inhibitors, sirolimus, statins, anti-influenza immune plasma, and vitamin C, but none is recommended at present in severe RVIs. Evidence-based supportive care is the mainstay for management of severe respiratory viral infection. Non-invasive ventilation in patients with severe RVI causing acute hypoxemic respiratory failure and pneumonia is associated with a high likelihood of transition to invasive ventilation. Limited existing knowledge highlights the need for data regarding supportive care and adjunctive pharmacologic therapy that is specific for critically ill patients with severe RVI. There is a need for more pragmatic and efficient designs to test different therapeutics both individually and in combination.

The impact of high frequency oscillatory ventilation on mortality in paediatric acute respiratory distress syndrome

BACKGROUND

High-frequency oscillatory ventilation (HFOV) use was associated with greater mortality in adult acute respiratory distress syndrome (ARDS). Nevertheless, HFOV is still frequently used as rescue therapy in paediatric acute respiratory distress syndrome (PARDS). In view of the limited evidence for HFOV in PARDS and evidence demonstrating harm in adult patients with ARDS, we hypothesized that HFOV use compared to other modes of mechanical ventilation is associated with increased mortality in PARDS.

METHODS

Patients with PARDS from 10 paediatric intensive care units across Asia from 2009 to 2015 were identified. Data on epidemiology and clinical outcomes were collected. Patients on HFOV were compared to patients on other modes of ventilation. The primary outcome was 28-day mortality and secondary outcomes were 28-day ventilator- (VFD) and intensive care unit- (IFD) free days. Genetic matching (GM) method was used to analyse the association between HFOV treatment with the primary outcome. Additionally, we performed a sensitivity analysis, including propensity score (PS) matching, inverse probability of treatment weighting (IPTW) and marginal structural modelling (MSM) to estimate the treatment effect.

RESULTS

A total of 328 patients were included. In the first 7 days of PARDS, 122/328 (37.2%) patients were supported with HFOV. There were significant differences in baseline oxygenation index (OI) between the HFOV and non-HFOV groups (18.8 [12.0, 30.2] vs. 7.7 [5.1, 13.1] respectively; p < 0.001). A total of 118 pairs were matched in the GM method which found a significant association between HFOV with 28-day mortality in PARDS [odds ratio 2.3, 95% confidence interval (CI) 1.3, 4.4, p value 0.01]. VFD was indifferent between the HFOV and non-HFOV group [mean difference - 1.3 (95%CI - 3.4, 0.9); p = 0.29] but IFD was significantly lower in the HFOV group [- 2.5 (95%CI - 4.9, - 0.5); p = 0.03]. From the sensitivity analysis, PS matching, IPTW and MSM all showed consistent direction of HFOV treatment effect in PARDS.

CONCLUSION

The use of HFOV was associated with increased 28-day mortality in PARDS. This study suggests caution but does not eliminate equivocality and a randomized controlled trial is justified to examine the true association.

Acute Respiratory Distress Syndrome in Cancer Patients

Acute respiratory distress syndrome (ARDS) is a heterogeneous form of acute, diffuse lung injury that is characterized by dysregulated inflammation, increased alveolar-capillary interface permeability, and non-cardiogenic pulmonary edema. In the general population, the incidence and mortality associated with ARDS over the last two decades have steadily declined in parallel with optimized approaches to pneumonia and other underlying causes of ARDS as well as increased utilization of multimodal treatment strategies that include lung-protective ventilation. In the cancer settings, significant declines in the incidence and mortality of ARDS over the past two decades have also been reported, although these rates remain significantly higher than those in the general population. Epidemiologic studies identify infection, including disseminated fungal pneumonias, as a major underlying cause of ARDS in the cancer setting. More than half of cancer patients who develop ARDS will not survive to hospital discharge. Those who do survive often face a protracted and often incomplete recovery, resulting in significant long-term physical, psychological, and cognitive sequelae. The residual organ dysfunction and poor functional status after ARDS may delay or preclude subsequent cancer treatments. As such, close collaboration between the critical care physicians and oncology team is essential in identifying and reversing the underlying causes and optimizing treatments for cancer patients with ARDS. This chapter reviews the diagnosis and common causes of ARDS in cancer and gives an update on the general management principles for cancer patients with ARDS in the ICU.

Formal guidelines: management of acute respiratory distress syndrome

Fifteen recommendations and a therapeutic algorithm regarding the management of acute respiratory distress syndrome (ARDS) at the early phase in adults are proposed. The Grade of Recommendation Assessment, Development and Evaluation (GRADE) methodology has been followed. Four recommendations (low tidal volume, plateau pressure limitation, no oscillatory ventilation, and prone position) had a high level of proof (GRADE 1 + or 1 -); four (high positive end-expiratory pressure [PEEP] in moderate and severe ARDS, muscle relaxants, recruitment maneuvers, and venovenous extracorporeal membrane oxygenation [ECMO]) a low level of proof (GRADE 2 + or 2 -); seven (surveillance, tidal volume for non ARDS mechanically ventilated patients, tidal volume limitation in the presence of low plateau pressure, PEEP > 5 cmH2O, high PEEP in the absence of deleterious effect, pressure mode allowing spontaneous ventilation after the acute phase, and nitric oxide) corresponded to a level of proof that did not allow use of the GRADE classification and were expert opinions. Lastly, for three aspects of ARDS management (driving pressure, early spontaneous ventilation, and extracorporeal carbon dioxide removal), the experts concluded that no sound recommendation was possible given current knowledge. The recommendations and the therapeutic algorithm were approved by the experts with strong agreement.

Formal guidelines: management of acute respiratory distress syndrome

Fifteen recommendations and a therapeutic algorithm regarding the management of acute respiratory distress syndrome (ARDS) at the early phase in adults are proposed. The Grade of Recommendation Assessment, Development and Evaluation (GRADE) methodology has been followed. Four recommendations (low tidal volume, plateau pressure limitation, no oscillatory ventilation, and prone position) had a high level of proof (GRADE 1 + or 1 −); four (high positive end-expiratory pressure [PEEP] in moderate and severe ARDS, muscle relaxants, recruitment maneuvers, and venovenous extracorporeal membrane oxygenation [ECMO]) a low level of proof (GRADE 2 + or 2 −); seven (surveillance, tidal volume for non ARDS mechanically ventilated patients, tidal volume limitation in the presence of low plateau pressure, PEEP > 5 cmH2O, high PEEP in the absence of deleterious effect, pressure mode allowing spontaneous ventilation after the acute phase, and nitric oxide) corresponded to a level of proof that did not allow use of the GRADE classification and were expert opinions. Lastly, for three aspects of ARDS management (driving pressure, early spontaneous ventilation, and extracorporeal carbon dioxide removal), the experts concluded that no sound recommendation was possible given current knowledge. The recommendations and the therapeutic algorithm were approved by the experts with strong agreement.

Optimal Ventilator Strategies in Acute Respiratory Distress Syndrome

Mechanical ventilation practices in patients with acute respiratory distress syndrome (ARDS) have progressed with a growing understanding of the disease pathophysiology. Paramount to the care of affected patients is the delivery of lung-protective mechanical ventilation which prioritizes tidal volume and plateau pressure limitation. Lung protection can probably be further enhanced by scaling target tidal volumes to the specific respiratory mechanics of individual patients. The best procedure for selecting optimal positive end-expiratory pressure (PEEP) in ARDS remains uncertain; several relevant issues must be considered when selecting PEEP, particularly lung recruitability. Noninvasive ventilation must be used with caution in ARDS as excessively high respiratory drive can further exacerbate lung injury; newer modes of delivery offer promising approaches in hypoxemic respiratory failure. Airway pressure release ventilation offers an alternative approach to maximize lung recruitment and oxygenation, but clinical trials have not demonstrated a survival benefit of this mode over conventional ventilation strategies. Rescue therapy with high-frequency oscillatory ventilation is an important option in refractory hypoxemia. Despite a disappointing lack of benefit (and possible harm) in patients with moderate or severe ARDS, possibly due to lung hyperdistention and right ventricular dysfunction, high-frequency oscillation may improve outcome in patients with very severe hypoxemia.

High-frequency oscillatory ventilation: A narrative review

High-frequency oscillatory ventilation (HFOV) is a lung-protective strategy that can be utilized in the full spectrum of patient populations ranging from neonatal to adults with acute lung injury. HFOV is often utilized as a rescue strategy when conventional mechanical ventilation (CV) has failed. HFOV uses low tidal volumes and constant mean airway pressures in conjunction with high respiratory rates to provide beneficial effects on oxygenation and ventilation, while eliminating the traumatic “inflate–deflate” cycle imposed by CV. Although statistical evidence supporting HFOV is particularly low, potential benefits for its application in many clinical manifestations still remain. High-frequency oscillation is a safe and effective rescue mode of ventilation for the treatment of acute respiratory distress syndrome (ARDS). All patients who have ventilator-induced lung injury (VILI) or are at risk of developing VILI or ARDS would be suitable candidates for HFOV, especially those who have failed conventional mechanical ventilation. This narrative aims to provide a review of HFOV vis-à-vis its indications, contraindications, hazards, parameters to monitoring, patient selection, clinical goals, mechanisms of action, controls for optimizing ventilation and oxygenation, clinical application in ARDS, and a comparison with other modes of mechanical ventilation.

Explorative Frequency Analysis of Leaf Temperature Behavior of Maize (Zea mays subsp. mays) at Water Deficit

In this study, different standard frequency analysis (FA) methods are applied to measured leaf temperature data of maize plants (developmental stages EC13-15). These FA methods are used to identify specific behaviors, regularities, and sudden changes in frequencies/amplitudes of data, e.g., in control engineering. The thorough application of different FA methods in plant studies is novel. The aim of this paper is to analyze features of the measured data and to explore the explanatory power of different methods for the detection of plant dynamic behavioral changes. The basic assumption is an expected relation between plant water stress and resulting changes in leaf temperature oscillations caused by stress-induced changes in stomatal behavior. Therefore, an irrigation experiment (laboratory; controlled environmental conditions) was implemented to compare leaf temperature behavior of stressed and unstressed plants. Leaf temperature time series are processed and the results are compared as functions of time showing the behavioral changes in terms of the different methods applied. The analysis of results is explained; conclusions, which can be made based on different methods, are given. The study confirms the applicability of FA methods and provides new insights into leaf temperature behavioral patterns. Results are discussed regarding the hypothesized incipience of leaf temperature oscillations due to water stress.

Inflammatory lung injury in rabbits: effects of high-frequency oscillatory ventilation in the prone position

OBJECTIVE

To compare the effects that prone and supine positioning during high-frequency oscillatory ventilation (HFOV) have on oxygenation and lung inflammation, histological injury, and oxidative stress in a rabbit model of acute lung injury (ALI).

METHODS

Thirty male Norfolk white rabbits were induced to ALI by tracheal saline lavage (30 mL/kg, 38°C). The injury was induced during conventional mechanical ventilation, and ALI was considered confirmed when a PaO2/FiO2 ratio < 100 mmHg was reached. Rabbits were randomly divided into two groups: HFOV in the supine position (SP group, n = 15); and HFOV with prone positioning (PP group, n = 15). For HFOV, the mean airway pressure was initially set at 16 cmH2O. At 30, 60, and 90 min after the start of the HFOV protocol, the mean airway pressure was reduced to 14, 12, and 10 cmH2O, respectively. At 120 min, the animals were returned to or remained in the supine position for an extra 30 min. We evaluated oxygenation indices and histological lung injury scores, as well as TNF-α levels in BAL fluid and lung tissue.

RESULTS

After ALI induction, all of the animals showed significant hypoxemia, decreased respiratory system compliance, decreased oxygenation, and increased mean airway pressure in comparison with the baseline values. There were no statistically significant differences between the two groups, at any of the time points evaluated, in terms of the PaO2 or oxygenation index. However, TNF-α levels in BAL fluid were significantly lower in the PP group than in the SP group, as were histological lung injury scores.

CONCLUSIONS

Prone positioning appears to attenuate inflammatory and histological lung injury during HFOV in rabbits with ALI.

Recruitment Maneuvers and Higher PEEP, the So-Called Open Lung Concept, in Patients with ARDS

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency Medicine 2019. Other selected articles can be found online at https://www.biomedcentral.com/collections/annualupdate2019. Further information about the Annual Update in Intensive Care and Emergency Medicine is available from http://www.springer.com/series/8901.

Beyond Low Tidal Volume Ventilation: Treatment Adjuncts for Severe Respiratory Failure in Acute Respiratory Distress Syndrome

OBJECTIVES

Despite decades of research, the acute respiratory distress syndrome remains associated with significant morbidity and mortality. This Concise Definitive Review provides a practical and evidence-based summary of treatments in addition to low tidal volume ventilation and their role in the management of severe respiratory failure in acute respiratory distress syndrome.

DATA SOURCES

We searched the PubMed database for clinical trials, observational studies, and review articles describing treatment adjuncts in acute respiratory distress syndrome patients, including high positive end-expiratory pressure strategies, recruitment maneuvers, high-frequency oscillatory ventilation, neuromuscular blockade, prone positioning, inhaled pulmonary vasodilators, extracorporeal membrane oxygenation, glucocorticoids, and renal replacement therapy.

STUDY SELECTION AND DATA EXTRACTION

Results were reviewed by the primary author in depth. Disputed findings and conclusions were then reviewed with the other authors until consensus was achieved.

DATA SYNTHESIS

Severe respiratory failure in acute respiratory distress syndrome may present with refractory hypoxemia, severe respiratory acidosis, or elevated plateau airway pressures despite lung-protective ventilation according to acute respiratory distress syndrome Network protocol. For severe hypoxemia, first-line treatment adjuncts include high positive end-expiratory pressure strategies, recruitment maneuvers, neuromuscular blockade, and prone positioning. For refractory acidosis, we recommend initial modest liberalization of tidal volumes, followed by neuromuscular blockade and prone positioning. For elevated plateau airway pressures, we suggest first decreasing tidal volumes, followed by neuromuscular blockade, modification of positive end-expiratory pressure, and prone positioning. Therapies such as inhaled pulmonary vasodilators, glucocorticoids, and renal replacement therapy have significantly less evidence in favor of their use and should be considered second line. Extracorporeal membrane oxygenation may be life-saving in selected patients with severe acute respiratory distress syndrome but should be used only when other alternatives have been applied.

CONCLUSIONS

Severe respiratory failure in acute respiratory distress syndrome often necessitates the use of treatment adjuncts. Evidence-based application of these therapies in acute respiratory distress syndrome remains a significant challenge. However, a rational stepwise approach with frequent monitoring for improvement or harm can be achieved.

"Low-" versus "high"-frequency oscillation and right ventricular function in ARDS. A randomized crossover study

Background

Recent, large trials of high-frequency oscillation (HFO) versus conventional ventilation (CV) in acute respiratory distress syndrome (ARDS) reported negative results. This could be explained by an HFO-induced right ventricular (RV) dysfunction/failure due to high intrathoracic pressures and hypercapnia. We hypothesized that HFO strategies aimed at averting/attenuating hypercapnia, such as "low-frequency" (i.e., 4 Hz) HFO and 4-Hz HFO with tracheal-gas insufflation (HFO-TGI), may result in an improved RV function relative to "high-frequency" (i.e., 7 Hz) HFO (which may promote hypercapnia) and similar RV function relative to lung protective CV.

Methods

We studied 17 patients with moderate-to-severe ARDS [PaO₂-to-inspiratory O₂ fraction ratio (PaO₂/FiO₂) < 150]. RV function was assessed by transesophageal echocardiography (TEE). Patients received 60 min of CV for TEE-guided, positive end-expiratory pressure (PEEP) "optimization" and subsequent stabilization; 60 min of 4-Hz HFO for "study mean airway pressure (mPaw)" titration to peripheral oxygen saturation ≥ 95%, without worsening RV function as assessed by TEE; 60 min of each tested HFO strategy in random order; and another 60 min of CV using the pre-HFO, TEE-guided PEEP setting. Study measurements (i.e., gas exchange, hemodynamics, and TEE data) were obtained over the last 10 min of pre-HFO CV, of each one of the three tested HFO strategies, and of post-HFO CV.

Results

The mean "study HFO mPaw" was 8-10 cmH₂O higher relative to pre-HFO CV. Seven-Hz HFO versus 4-Hz HFO and 4-Hz HFO-TGI resulted in higher mean ± SD right-to-left ventricular end-diastolic area ratio (RVEDA/LVEDA) (0.64 ± 0.15 versus 0.56 ± 0.14 and 0.52 ± 0.10, respectively, both p < 0.05). Higher diastolic/systolic eccentricity indexes (1.33 ± 0.19/1.42 ± 0.17 versus 1.21 ± 0.10/1.26 ± 0.10 and 1.17 ± 0.11/1.17 ± 0.13, respectively, all p < 0.05). Seven-Hz HFO resulted in 18-28% higher PaCO₂ relative to all other ventilatory strategies (all p < 0.05). Four-Hz HFO-TGI versus pre-HFO CV resulted in 15% lower RVEDA/LVEDA, and 7%/10% lower diastolic/systolic eccentricity indexes (all p < 0.05). Mean PaO₂/FiO₂ improved by 77-80% during HFO strategies versus CV (all p < 0.05). Mean cardiac index varied by ≤ 10% among strategies. Percent changes in PaCO₂ among strategies were predictive of concurrent percent changes in measures of RV function (R² = 0.21-0.43).

Conclusions

In moderate-to-severe ARDS, "short-term" 4-Hz HFO strategies resulted in better RV function versus 7-Hz HFO, partly attributable to improved PaCO₂ control, and similar or improved RV function versus CV.

Trial registration

This study was registered 40 days prior to the enrollment of the first patient at ClinicalTrials.gov, ID no. NCT02027129, Principal Investigator Spyros D. Mentzelopoulos, date of registration January 3, 2014.

Recent advances in understanding and treating acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a clinically and biologically heterogeneous disorder associated with many disease processes that injure the lung, culminating in increased non-hydrostatic extravascular lung water, reduced compliance, and severe hypoxemia. Despite enhanced understanding of molecular mechanisms, advances in ventilatory strategies, and general care of the critically ill patient, mortality remains unacceptably high. The Berlin definition of ARDS has now replaced the American-European Consensus Conference definition. The recently concluded Large Observational Study to Understand the Global Impact of Severe Acute Respiratory Failure (LUNG-SAFE) provided worldwide epidemiological data of ARDS including prevalence, geographic variability, mortality, and patterns of mechanical ventilation use. Failure of clinical therapeutic trials prompted the investigation and subsequent discovery of two distinct phenotypes of ARDS (hyper-inflammatory and hypo-inflammatory) that have different biomarker profiles and clinical courses and respond differently to the random application of positive end expiratory pressure (PEEP) and fluid management strategies. Low tidal volume ventilation remains the predominant mainstay of the ventilatory strategy in ARDS. High-frequency oscillatory ventilation, application of recruitment maneuvers, higher PEEP, extracorporeal membrane oxygenation, and alternate modes of mechanical ventilation have failed to show benefit. Similarly, most pharmacological therapies including keratinocyte growth factor, beta-2 agonists, and aspirin did not improve outcomes. Prone positioning and early neuromuscular blockade have demonstrated mortality benefit, and clinical guidelines now recommend their use. Current ongoing trials include the use of mesenchymal stem cells, vitamin C, re-evaluation of neuromuscular blockade, and extracorporeal carbon dioxide removal. In this article, we describe advances in the diagnosis, epidemiology, and treatment of ARDS over the past decade.

Salvage therapies for refractory hypoxemia in ARDS

Acute Respiratory Distress Syndrome (ARDS) is a condition of varied etiology characterized by the acute onset (within 1 week of the inciting event) of hypoxemia, reduced lung compliance, diffuse lung inflammation and bilateral opacities on chest imaging attributable to noncardiogenic (increased permeability) pulmonary edema. Although multi-organ failure is the most common cause of death in ARDS, an estimated 10-15% of the deaths in ARDS are caused due to refractory hypoxemia, i.e.- hypoxemia despite lung protective conventional ventilator modes. In these cases, clinicians may resort to other measures with less robust evidence -referred to as "salvage therapies". These include proning, 48 h of paralysis early in the course of ARDS, various recruitment maneuvers, unconventional ventilator modes, inhaled pulmonary vasodilators, and Extracorporeal membrane oxygenation (ECMO). All the salvage therapies described have been associated with improved oxygenation, but with the exception of proning and 48 h of paralysis early in the course of ARDS, none of them have a proven mortality benefit. Based on the current evidence, no salvage therapy has been shown to be superior to the others and each of them is associated with its own risks and benefits. Hence, the order of application of these therapies varies in different institutions and should be applied following a risk-benefit analysis specific to the patient and local experience. This review explores the rationale, evidence, advantages and risks behind each of these strategies.

Acute life-threatening hypoxemia during mechanical ventilation

PURPOSE OF REVIEW

To describe current evidence-based practice in the management of acute life-threatening hypoxemia in mechanically ventilated patients and some of the methods used to individualize the care of the patient.

RECENT FINDINGS

Patients with acute life-threatening hypoxemia will often meet criteria for severe ARDS, for which there are only a few treatment strategies that have been shown to improve survival outcomes. Recent findings have increased our knowledge of the physiological effects of spontaneous breathing and the application of PEEP. Additionally, the use of advanced bedside monitoring has a promising future in the management of hypoxemic patients to fine-tune the ventilator and to evaluate the individual patient response to therapy.

SUMMARY

Treating the patient with acute life-threatening hypoxemia during mechanical ventilation should begin with an evidence-based approach, with the goal of improving oxygenation and minimizing the harmful effects of mechanical ventilation. The use of advanced monitoring and the application of simple maneuvers at the bedside may assist clinicians to better individualize treatment and improve clinical outcomes.

Mechanical Ventilation in Adults with Acute Respiratory Distress Syndrome. Summary of the Experimental Evidence for the Clinical Practice Guideline

RATIONALE

The American Thoracic Society/European Society for Intensive Care Medicine/Society of Critical Care Medicine guidelines on mechanical ventilation in adult patients with acute respiratory distress syndrome (ARDS) provide treatment recommendations derived from a thorough analysis of the clinical evidence on six clinical interventions. However, each of the recommendations contains areas of uncertainty and controversy, which may affect their appropriate clinical application.

OBJECTIVES

To provide a critical review of the experimental evidence surrounding the pathophysiology of ventilator-induced lung injury and to help clinicians apply the clinical recommendations to individual patients.

METHODS

We conducted a literature search and narrative review.

RESULTS

A large number of experimental studies have been performed with the aim of improving understanding of the pathophysiological effects of mechanical ventilation. These studies have formed the basis for the design of many clinical trials. Translational research has fundamentally advanced understanding of the mechanisms of ventilator-induced lung injury, thus informing the design of interventions that improve survival in patients with ARDS.

CONCLUSIONS

Because daily management of patients with ARDS presents the challenge of competing considerations, clinicians should consider the mechanism of ventilator-induced lung injury, as well as the rationale for interventions designed to mitigate it, when applying evidence-based recommendations at the bedside.

High-Frequency Oscillation for Adult Patients with Acute Respiratory Distress Syndrome. A Systematic Review and Meta-Analysis

RATIONALE

By minimizing tidal lung strain and maintaining alveolar recruitment, high-frequency oscillatory ventilation (HFOV) may protect against ventilator-induced lung injury.

OBJECTIVES

To summarize the current evidence in support of the use of HFOV in adult patients with acute respiratory distress syndrome.

METHODS

We conducted a systematic review and meta-analysis of randomized trials comparing mortality rates with the use of HFOV versus conventional mechanical ventilation for adult patients with acute respiratory distress syndrome. Eligible trials were identified from previously published systematic reviews and an updated literature search. Data on 28-day mortality, oxygenation, adverse events, and use of rescue therapies were collected; effects were pooled using random effects models weighted by inverse variance. Strength of evidence was assessed using Grading of Recommendations Assessment, Development, and Evaluation methodology.

RESULTS

Six trials were eligible for inclusion (total n = 1,715 patients). Four trials mandated lung-protective ventilation in the control group and one trial applied a higher positive end-expiratory pressure (PEEP) ventilation strategy in the control group. None of the trials were judged to be at high risk of bias, though all were unblinded. In trials that did not systematically employ any cointerventions with HFOV and that targeted low tidal volumes in the patients randomized to conventional ventilation (primary analysis), HFOV had no significant effect on mortality (three trials; risk ratio [RR], 1.14; 95% confidence interval [CI], 0.88 to 1.48; evidence grade = high). Pooled analysis of all six trials also did not suggest a significant mortality reduction (RR, 0.94; 95% CI, 0.71 to 1.24; evidence grade = low). The single trial that employed a conventional ventilation strategy with both lower tidal volumes and higher PEEP as control reported higher mortality in patients receiving HFOV (RR, 1.41; 95% CI, 1.12 to 1.79). HFOV was not associated with improved oxygenation after 24 hours (five trials; mean increase of 10 mm Hg; 95% CI, -16 to 37 mm Hg). Rates of barotrauma were not different between HFOV and conventional ventilation, although significant benefit or harm could not be excluded (RR, 1.15; 95% CI, 0.61 to 2.17).

CONCLUSIONS

Published randomized trials suggest that HFOV is not associated with a mortality benefit, and may even be harmful in comparison to ventilation with low tidal volumes and higher levels of PEEP.

Ventilator-induced lung injury during controlled ventilation in patients with acute respiratory distress syndrome: less is probably better

INTRODUCTION

Mechanical ventilation is required to support respiratory function in the acute respiratory distress syndrome (ARDS), but it may promote lung damage, a phenomenon known as ventilator-induced lung injury (VILI). Areas covered: Several mechanisms of VILI have been described, such as: inspiratory and/or expiratory stress inducing overdistension (volutrauma); interfaces between collapsed or edema-filled alveoli with surrounding open alveoli, acting as stress raisers; alveoli that repetitively open and close during tidal breathing (atelectrauma); and peripheral airway dynamics. In this review, we discuss: the definition and classification of ARDS; ventilatory parameters that act as VILI determinants (tidal volume, respiratory rate, positive end-expiratory pressure, peak, plateau, driving and transpulmonary pressures, energy, mechanical power, and intensity); and the roles of prone positioning and muscle paralysis. We seek to provide an up-to-date overview of the evidence in the field from a clinical perspective. Expert commentary: To prevent VILI, mechanical ventilation strategies should minimize inspiratory/expiratory stress, dynamic/static strain, energy, mechanical power, and intensity, as well as mitigate the hemodynamic consequences of positive-pressure ventilation. In patients with moderate to severe ARDS, prone positioning can reduce lung damage and improve survival. Overall, volutrauma seems to be more harmful than atelectrauma. Extracorporeal support should be considered in selected cases.

Acute Respiratory Distress Syndrome: Advances in Diagnosis and Treatment

IMPORTANCE

Acute respiratory distress syndrome (ARDS) is a life-threatening form of respiratory failure that affects approximately 200 000 patients each year in the United States, resulting in nearly 75 000 deaths annually. Globally, ARDS accounts for 10% of intensive care unit admissions, representing more than 3 million patients with ARDS annually.

OBJECTIVE

To review advances in diagnosis and treatment of ARDS over the last 5 years.

EVIDENCE REVIEW

We searched MEDLINE, EMBASE, and the Cochrane Database of Systematic Reviews from 2012 to 2017 focusing on randomized clinical trials, meta-analyses, systematic reviews, and clinical practice guidelines. Articles were identified for full text review with manual review of bibliographies generating additional references.

FINDINGS

After screening 1662 citations, 31 articles detailing major advances in the diagnosis or treatment of ARDS were selected. The Berlin definition proposed 3 categories of ARDS based on the severity of hypoxemia: mild (200 mm Hg<Pao2/Fio2≤300 mm Hg), moderate (100 mm Hg<Pao2/Fio2≤200 mm Hg), and severe (Pao2/Fio2 ≤100 mm Hg), along with explicit criteria related to timing of the syndrome's onset, origin of edema, and the chest radiograph findings. The Berlin definition has significantly greater predictive validity for mortality than the prior American-European Consensus Conference definition. Clinician interpretation of the origin of edema and chest radiograph criteria may be less reliable in making a diagnosis of ARDS. The cornerstone of management remains mechanical ventilation, with a goal to minimize ventilator-induced lung injury (VILI). Aspirin was not effective in preventing ARDS in patients at high-risk for the syndrome. Adjunctive interventions to further minimize VILI, such as prone positioning in patients with a Pao2/Fio2 ratio less than 150 mm Hg, were associated with a significant mortality benefit whereas others (eg, extracorporeal carbon dioxide removal) remain experimental. Pharmacologic therapies such as β2 agonists, statins, and keratinocyte growth factor, which targeted pathophysiologic alterations in ARDS, were not beneficial and demonstrated possible harm. Recent guidelines on mechanical ventilation in ARDS provide evidence-based recommendations related to 6 interventions, including low tidal volume and inspiratory pressure ventilation, prone positioning, high-frequency oscillatory ventilation, higher vs lower positive end-expiratory pressure, lung recruitment maneuvers, and extracorporeal membrane oxygenation.

CONCLUSIONS AND RELEVANCE

The Berlin definition of acute respiratory distress syndrome addressed limitations of the American-European Consensus Conference definition, but poor reliability of some criteria may contribute to underrecognition by clinicians. No pharmacologic treatments aimed at the underlying pathology have been shown to be effective, and management remains supportive with lung-protective mechanical ventilation. Guidelines on mechanical ventilation in patients with acute respiratory distress syndrome can assist clinicians in delivering evidence-based interventions that may lead to improved outcomes.