Airway Pressure Release Ventilation in Adult Patients With Acute Hypoxemic Respiratory Failure: A Systematic Review and Meta-Analysis*

Improving oxygenation in a patient with respiratory failure due to morbid obesity by applying airway pressure release ventilation: a case report

INTRODUCTION

Morbidly obese patients occasionally have respiratory problems owing to hypoventilation. Airway pressure release ventilation is one of the ventilation settings often used for respiratory management of acute respiratory distress syndrome. However, previous reports indicating that airway pressure release ventilation may become a therapeutic measure as ventilator management in morbid obesity with respiratory failure is limited. We report a case of markedly improved oxygenation in a morbidly obese patient after airway pressure release ventilation application.

CASE REPORT

A 50s-year-old Asian man (body mass index 41 kg/m2) presented with breathing difficulties. The patient had respiratory failure with a PaO2/FIO2 ratio of approximately 100 and severe atelectasis in the left lung, and ventilator management was initiated. Although the patient was managed on a conventional ventilate mode, oxygenation did not improve. On day 11, we changed the ventilation setting to airway pressure release ventilation, which showed marked improvement in oxygenation with a PaO2/FIO2 ratio of approximately 300. We could reduce sedative medication and apply respiratory rehabilitation. The patient was weaned from the ventilator on day 29 and transferred to another hospital for further rehabilitation on day 31.

CONCLUSION

Airway pressure release ventilation ventilator management in morbidly obese patients may contribute to improving oxygenation and become one of the direct therapeutic measures in the early stage of critical care.

Phenotypes and Lung Microbiota Signatures of Immunocompromised Patients with Pneumonia-Related Acute Respiratory Distress Syndrome

Objective

We aim to identify the clinical phenotypes of immunocompromised patients with pneumonia-related ARDS, to investigate the lung microbiota signatures and the outcomes of different phenotypes, and finally, to develop a machine learning classifier for a specified phenotype.

Methods

This prospective study included immunocompromised patients with pneumonia-related ARDS. We identified phenotypes using hierarchical clustering to analyze clinical variables and serum cytokine levels. We then compared outcomes and lung microbiota signatures between phenotypes. Based on lung microbiota markers, we developed a random forest classifier for a specified phenotype with worse outcomes.

Results

This study included 92 patients, who were divided into three phenotypes, namely "type α" (N = 33), "type β" (N = 12), and "type γ" (N = 47). Compared to type α or type β, patients with type γ had no obvious inflammatory presentation and had significantly lower IL-6 levels and more severe oxygenation failure. Type γ was also related to higher 30-day mortality and lower ventilator free days. The microbiota signatures of type γ were characterized by lower alpha diversity and distinct compositions than those of other patients. We developed a lung microbiota-derived random forest model to differentiate patients with type γ from other phenotypes.

Conclusion

Immunocompromised patients with pneumonia-related ARDS can be clustered into three clinical phenotypes, namely type α, type β, and type γ. Phenotypes were distinguished from each other with different outcomes and lung microbiota signatures. Type γ, which was characterized by insufficient inflammation response and worse outcomes, can be detected with a random forest model based on lung microbiota markers.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

American Association for the Surgery of Trauma/American College of Surgeons Committee on Trauma clinical protocol for management of acute respiratory distress syndrome and severe hypoxemia

LEVEL OF EVIDENCE

Therapeutic/Care Management: Level V.

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.

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

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

Airway Pressure Release Ventilation for Acute Respiratory Failure Due to Coronavirus Disease 2019: A Systematic Review and Meta-Analysis

Objective: To explore the evidence surrounding the use of Airway Pressure Release Ventilation (APRV) in patients with coronavirus disease 2019 (COVID-19). Methods: A Systematic electronic search of PUBMED, EMBASE, and the WHO COVID-19 database. We also searched the grey literature via Google and preprint servers (medRxive and research square). Eligible studies included randomised controlled trials and observational studies comparing APRV to conventional mechanical ventilation (CMV) in adults with acute hypoxemic respiratory failure due to COVID-19 and reporting at least one of the following outcomes; in-hospital mortality, ventilator free days (VFDs), ICU length of stay (LOS), changes in gas exchange parameters, and barotrauma. Two authors independently screened and selected articles for inclusion and extracted data in a pre-specified form. Results: Of 181 articles screened, seven studies (one randomised controlled trial, two cohort studies, and four before-after studies) were included comprising 354 patients. APRV was initiated at a mean of 1.2-13 days after intubation. APRV wasn't associated with improved mortality compared to CMV (relative risk [RR], 1.20; 95% CI 0.70-2.05; I², 61%) neither better VFDs (ratio of means [RoM], 0.80; 95% CI, 0.52-1.24; I², 0%) nor ICU LOS (RoM, 1.10; 95% CI, 0.79-1.51; I², 57%). Compared to CMV, APRV was associated with a 33% increase in PaO₂/FiO₂ ratio (RoM, 1.33; 95% CI, 1.21-1.48; I², 29%) and a 9% decrease in PaCO₂ (RoM, 1.09; 95% CI, 1.02-1.15; I², 0%). There was no significant increased risk of barotrauma compared to CMV (RR, 1.55; 95% CI, 0.60-4.00; I², 0%). Conclusions: In adult patients with COVID-19 requiring mechanical ventilation, APRV is associated with improved gas exchange but not mortality nor VFDs when compared with CMV. The results were limited by high uncertainty given the low quality of the available studies and limited number of patients. Adequately powered and well-designed clinical trials to define the role of APRV in COVID-19 patients are still needed. Registration: PROSPERO; CRD42021291234.

Techniques for Oxygenation and Ventilation in Coronavirus Disease 2019

This paper discusses mechanisms of hypoxemia and interventions to oxygenate critically ill patients with COVID-19 which range from nasal cannula to noninvasive and mechanical ventilation. Noninvasive ventilation includes continuous positive airway pressure ventilation (CPAP) and high-flow nasal cannula (HFNC) with or without proning. The evidence for each of these modalities is discussed and thereafter, when to transition to mechanical ventilation (MV). Various techniques of MV, again with and without proning, and rescue strategies which would include extra corporeal membrane oxygenation (ECMO) when it is available and permissive hypoxemia where it is not, are discussed.

Inflammatory biomarkers and pendelluft magnitude in ards patients transitioning from controlled to partial support ventilation

The transition from controlled to partial support ventilation is a challenge in acute respiratory distress syndrome (ARDS) patients due to the risks of patient-self-inflicted lung injury. The magnitude of tidal volume (V_T) and intrapulmonary dyssynchrony (pendelluft) are suggested mechanisms of lung injury. We conducted a prospective, observational, physiological study in a tertiary academic intensive care unit. ARDS patients transitioning from controlled to partial support ventilation were included. On these, we evaluated the association between changes in inflammatory biomarkers and esophageal pressure swing (ΔP_es), transpulmonary driving pressure (ΔP_L), V_T, and pendelluft. Pendelluft was defined as the percentage of the tidal volume that moves from the non-dependent to the dependent lung region during inspiration, and its frequency at different thresholds (- 15, - 20 and - 25%) was also registered. Blood concentrations of inflammatory biomarkers (IL-6, IL-8, TNF-α, ANGPT2, RAGE, IL-18, Caspase-1) were measured before (T₀) and after 4-h (T₄) of partial support ventilation. Pendelluft, ΔP_es, ΔP_L and V_T were recorded. Nine out of twenty-four patients (37.5%) showed a pendelluft mean ≥ 10%. The mean values of ΔP_es, ΔP_L, and V_T were - 8.4 [- 6.7; - 10.2] cmH₂O, 15.2 [12.3-16.5] cmH₂O and 8.1 [7.3-8.9] m/kg PBW, respectively. Significant associations were observed between the frequency of high-magnitude pendelluft and IL-8, IL-18, and Caspase-1 changes (T₀/T₄ ratio). These results suggest that the frequency of high magnitude pendelluft may be a potential determinant of inflammatory response related to inspiratory efforts in ARDS patients transitioning to partial support ventilation. Future studies are needed to confirm these results.

Myths and Misconceptions of Airway Pressure Release Ventilation: Getting Past the Noise and on to the Signal

In the pursuit of science, competitive ideas and debate are necessary means to attain knowledge and expose our ignorance. To quote Murray Gell-Mann (1969 Nobel Prize laureate in Physics): “Scientific orthodoxy kills truth”. In mechanical ventilation, the goal is to provide the best approach to support patients with respiratory failure until the underlying disease resolves, while minimizing iatrogenic damage. This compromise characterizes the philosophy behind the concept of “lung protective” ventilation. Unfortunately, inadequacies of the current conceptual model–that focuses exclusively on a nominal value of low tidal volume and promotes shrinking of the “baby lung” - is reflected in the high mortality rate of patients with moderate and severe acute respiratory distress syndrome. These data call for exploration and investigation of competitive models evaluated thoroughly through a scientific process. Airway Pressure Release Ventilation (APRV) is one of the most studied yet controversial modes of mechanical ventilation that shows promise in experimental and clinical data. Over the last 3 decades APRV has evolved from a rescue strategy to a preemptive lung injury prevention approach with potential to stabilize the lung and restore alveolar homogeneity. However, several obstacles have so far impeded the evaluation of APRV’s clinical efficacy in large, randomized trials. For instance, there is no universally accepted standardized method of setting APRV and thus, it is not established whether its effects on clinical outcomes are due to the ventilator mode per se or the method applied. In addition, one distinctive issue that hinders proper scientific evaluation of APRV is the ubiquitous presence of myths and misconceptions repeatedly presented in the literature. In this review we discuss some of these misleading notions and present data to advance scientific discourse around the uses and misuses of APRV in the current literature.

Managing Severe Hypoxic Respiratory Failure in COVID-19

Purpose of Review

Adult respiratory distress syndrome is a life-threatening complication from severe COVID-19 infection resulting in severe hypoxic respiratory failure. Strategies at improving oxygenation have evolved over the course of the pandemic.

Recent Findings

Although non-invasive respiratory support reduces the need for intubation, a significant number of patients with COVID-19 progress to invasive mechanical ventilation. Once intubated, a lung protective ventilation strategy should be employed that limits tidal volumes to 6 ml/kg of predicted body weight and employs sufficient positive end-expiratory pressure to maximize oxygen delivery while minimizing the fraction of inspired oxygen. Intermittent prone positioning is effective at improving survival, and there is a growing body of evidence that it can be safely performed in spontaneously breathing patients to reduce the need for invasive mechanical ventilation. Inhaled pulmonary vasodilators have not been shown to improve survival or cost-effectiveness in COVID-19 and should be used selectively.

Summary

Finally, the best outcomes are likely achieved at centers with experience at severe ARDS management and protocols for escalation of care.

Validation of at-the-bedside formulae for estimating ventilator driving pressure during airway pressure release ventilation using computer simulation

BACKGROUND

Airway pressure release ventilation (APRV) is widely available on mechanical ventilators and has been proposed as an early intervention to prevent lung injury or as a rescue therapy in the management of refractory hypoxemia. Driving pressure ([Formula: see text]) has been identified in numerous studies as a key indicator of ventilator-induced-lung-injury that needs to be carefully controlled. [Formula: see text] delivered by the ventilator in APRV is not directly measurable in dynamic conditions, and there is no "gold standard" method for its estimation.

METHODS

We used a computational simulator matched to data from 90 patients with acute respiratory distress syndrome (ARDS) to evaluate the accuracy of three "at-the-bedside" methods for estimating ventilator [Formula: see text] during APRV.

RESULTS

Levels of [Formula: see text] delivered by the ventilator in APRV were generally within safe limits, but in some cases exceeded levels specified by protective ventilation strategies. A formula based on estimating the intrinsic positive end expiratory pressure present at the end of the APRV release provided the most accurate estimates of [Formula: see text]. A second formula based on assuming that expiratory flow, volume and pressure decay mono-exponentially, and a third method that requires temporarily switching to volume-controlled ventilation, also provided accurate estimates of true [Formula: see text].

CONCLUSIONS

Levels of [Formula: see text] delivered by the ventilator during APRV can potentially exceed levels specified by standard protective ventilation strategies, highlighting the need for careful monitoring. Our results show that [Formula: see text] delivered by the ventilator during APRV can be accurately estimated at the bedside using simple formulae that are based on readily available measurements.

The efficacy of airway pressure release ventilation in acute respiratory distress syndrome adult patients: A meta-analysis of clinical trials

BACKGROUND

To recruit poorly ventilated lung areas by providing active and adequate oxygenation is a core aspect of treating patients with acute respiratory distress syndrome (ARDS). The airway pressure release ventilation (APRV) mode is increasingly accepted as a means of supporting patients with ARDS. This study aimed to determine whether the APRV mode is effective in improving oxygenation, compared to conventional ventilation, in adult ARDS patients.

METHODS

We conducted the study according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched for clinical trials in PubMed, Embase, Web of Science, and the Cochrane Library until April 2019. We included all studies comparing APRV and other conventional mechanical ventilation modes for adult ARDS patients. Our primary outcome was oxygenation status (defined as the day 3 PaO2/FiO2 ratio). The secondary outcomes were the length of stay (LOS) in the intensive care unit (ICU) and mortality. Sensitivity analyses were performed including studies with conventional low-tidal volume ventilation as a comparator ventilation strategy.

RESULTS

We included six clinical trials enrolling a total of 375 patients. The day 3 PaO2/FiO2 was reported in all the studies, and it was significantly higher in patients receiving APRV (mean difference [MD] 51.9 mmHg, 95% confidence intervals (CI) 8.2-95.5, P = 0.02, I 2= 92%). There was no significant difference in mortality between APRV and the other conventional ventilator modes (risk difference 0.07, 95% CI: -0.01-0.15, P = 0.08, I 20%). The point estimate for the effect of APRV on the LOS in ICU indicated a significant reduction in the ICU LOS for the APRV group compared to the counter group (MD 3.1 days, 95% CI 0.4-5.9, P = 0.02, I 2= 53%).

CONCLUSION

In this study, using the APRV mode may improve oxygenation on day 3 and contribute to reducing the LOS in ICU. However, it is difficult to draw a clinical message about APRV, and well-designed clinical trials are required to investigate this issue.

Airway pressure release ventilation in mechanically ventilated patients with COVID-19: a multicenter observational study

BACKGROUND

Evidence prior to the coronavirus disease 2019 (COVID-19) pandemic suggested that, compared with conventional ventilation strategies, airway pressure release ventilation (APRV) can improve oxygenation and reduce mortality in patients with acute respiratory distress syndrome. We aimed to assess the association between APRV use and clinical outcomes among adult patients receiving mechanical ventilation for COVID-19 and hypothesized that APRV use would be associated with improved survival compared with conventional ventilation.

METHODS

A total of 25 patients with COVID-19 pneumonitis was admitted to intensive care units (ICUs) for invasive ventilation in Perth, Western Australia, between February and May 2020. Eleven of these patients received APRV. The primary outcome was survival to day 90. Secondary outcomes were ventilation-free survival days to day 90, mechanical complications from ventilation, and number of days ventilated.

RESULTS

Patients who received APRV had a lower probability of survival than did those on other forms of ventilation (hazard ratio, 0.17; 95% confidence interval, 0.03-0.89; P=0.036). This finding was independent of indices of severity of illness to predict the use of APRV. Patients who received APRV also had fewer ventilator-free survival days up to 90 days after initiation of ventilation compared to patients who did not receive APRV, and survivors who received APRV had fewer ventilator-free days than survivors who received other forms of ventilation. There were no differences in mechanical complications according to mode of ventilation.

CONCLUSIONS

Based on the findings of this study, we urge caution with the use of APRV in COVID-19.

Non-specialist therapeutic strategies in acute respiratory distress syndrome

INTRODUCTION

Acute respiratory distress syndrome (ARDS) is associated with significant morbidity and mortality. We undertook a meta-analysis of randomized controlled trials (RCTs) to determine the mortality benefit of non-specialist therapeutic interventions for ARDS available to general critical care units.

EVIDENCE ACQUISITION

A systematic search of MEDLINE, Embase, and the Cochrane Central Register for RCTs investigating therapeutic interventions in ARDS including corticosteroids, fluid management strategy, high PEEP, low tidal volume ventilation, neuromuscular blockade, prone position ventilation, or recruitment maneuvers. Data was collected on demographic information, treatment strategy, duration and dose of treatment, and primary (28 or 30-day mortality) and secondary (P<inf>a</inf>O<inf>2</inf>:FiO<inf>2</inf> ratio at 24-48 hours) outcomes.

EVIDENCE SYNTHESIS

No improvement in 28-day mortality could be demonstrated in three RCTs investigating high PEEP (28.0% vs. 30.2% control; risk ratio [confidence interval] 0.93 [0.82-1.06]; eight assessing prone position ventilation (39.3% vs. 44.5%; RR 0.83 [0.68-1.01]; seven investigating neuromuscular blockade (37.8% vs. 42.0%; RR 0.91 [0.81-1.03]); ten investigating recruitment maneuvers (42.4% vs. 42.1%; RR 1.01 [0.91-1.12]); eight investigating steroids (34.8% vs. 41.1%; RR 0.81 [0.59-1.12]); and one investigating conservative fluid strategies (25.4% vs. 28.4%; RR 0.90 [0.73-1.10]). Three studies assessing low tidal volume ventilation (33.1% vs. 41.9%; RR 0.79 (0.68-0.91); P=0.001), and subgroup analyses within studies investigating prone position ventilation greater than 12 hours (33.1% vs. 44.4%; RR 0.75 [0.59-0.95), P=0.02) did reveal outcome benefit.

CONCLUSIONS

Among non-specialist therapeutic strategies available to general critical care units, low tidal volumes and prone position ventilation for greater than 12 hours improve mortality in ARDS.

Management of Respiratory Distress Syndrome due to COVID-19 infection

The management of Acute Respiratory Distress Syndrome (ARDS) secondary to the novel Coronavirus Disease 2019 (COVID-19) proves to be challenging and controversial. Multiple studies have suggested the likelihood of an atypical pathophysiology to explain the spectrum of pulmonary and systemic manifestations caused by the virus. The principal paradox of COVID-19 pneumonia is the presence of severe hypoxemia with preserved pulmonary mechanics. Data derived from the experience of multiple centers around the world have demonstrated that initial clinical efforts should be focused into avoid intubation and mechanical ventilation in hypoxemic COVID-19 patients. On the other hand, COVID-19 patients progressing or presenting into frank ARDS with typical decreased pulmonary compliance, represents another clinical enigma to many clinicians, since routine therapeutic interventions for ARDS are still a subject of debate.

Airway Pressure Release Ventilation Combined With Prone Positioning in Acute Respiratory Distress Syndrome: Old Tricks New Synergy: A Case Series

Airway pressure release ventilation (APRV) shares several overlapping mechanisms with prone positioning in improving ventilation-perfusion mismatch in patients with acute respiratory distress syndrome (ARDS). However, the combination of APRV and prone positioning is seldom performed because assist/controlled ventilation remains the mainstay ventilatory mode. We describe 5 cases of severe ARDS where APRV and prone positioning were applied. All patients' partial pressure of arterial oxygen (PaO2):inspired oxygen concentration (FiO2) ratios improved after treatment, and 3 patients were extubated within 72 hours of turning supine. In our experience, APRV can be safely used in the prone position in a select subgroup of ARDS patients with resulting significant oxygenation improvement.

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

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

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

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