Airway pressure release ventilation prevents ventilator-induced lung injury in normal lungs

Time-Controlled Adaptive Ventilation (TCAV): a personalized strategy for lung protection

Acute respiratory distress syndrome (ARDS) alters the dynamics of lung inflation during mechanical ventilation. Repetitive alveolar collapse and expansion (RACE) predisposes the lung to ventilator-induced lung injury (VILI). Two broad approaches are currently used to minimize VILI: (1) low tidal volume (LVT) with low-moderate positive end-expiratory pressure (PEEP); and (2) open lung approach (OLA). The LVT approach attempts to protect already open lung tissue from overdistension, while simultaneously resting collapsed tissue by excluding it from the cycle of mechanical ventilation. By contrast, the OLA attempts to reinflate potentially recruitable lung, usually over a period of seconds to minutes using higher PEEP used to prevent progressive loss of end-expiratory lung volume (EELV) and RACE. However, even with these protective strategies, clinical studies have shown that ARDS-related mortality remains unacceptably high with a scarcity of effective interventions over the last two decades. One of the main limitations these varied interventions demonstrate to benefit is the observed clinical and pathologic heterogeneity in ARDS. We have developed an alternative ventilation strategy known as the Time Controlled Adaptive Ventilation (TCAV) method of applying the Airway Pressure Release Ventilation (APRV) mode, which takes advantage of the heterogeneous time- and pressure-dependent collapse and reopening of lung units. The TCAV method is a closed-loop system where the expiratory duration personalizes VT and EELV. Personalization of TCAV is informed and tuned with changes in respiratory system compliance (CRS) measured by the slope of the expiratory flow curve during passive exhalation. Two potentially beneficial features of TCAV are: (i) the expiratory duration is personalized to a given patient's lung physiology, which promotes alveolar stabilization by halting the progressive collapse of alveoli, thereby minimizing the time for the reopened lung to collapse again in the next expiration, and (ii) an extended inspiratory phase at a fixed inflation pressure after alveolar stabilization gradually reopens a small amount of tissue with each breath. Subsequently, densely collapsed regions are slowly ratcheted open over a period of hours, or even days. Thus, TCAV has the potential to minimize VILI, reducing ARDS-related morbidity and mortality.

Airway Pressure Release Ventilation in COVID-19-Associated Acute Respiratory Distress Syndrome-A Multicenter Propensity Score-Matched Analysis

Background: There are limited and partially contradictory data on the effects of airway pressure release ventilation (APRV) in COVID-19-associated acute respiratory distress syndrome (CARDS). Therefore, we analyzed the clinical outcome, complications, and longitudinal course of ventilation parameters and laboratory values in patients with CARDS, who were mechanically ventilated using APRV. Methods: Respective data from 4 intensive care units (ICUs) were collected and compared to a matched cohort of patients receiving conventional low tidal volume ventilation (LTV). Propensity score matching was performed based on age, sex, blood gas analysis, and APACHE II score at admission, as well as the implementation of prone positioning. Findings: Forty patients with CARDS, who were mechanically ventilated using APRV, and 40 patients receiving LTV were matched. No significant differences were detected for tidal volumes per predicted body weight, peak pressure values, and blood gas analyses on admission, 6 h post admission as well as on day 3 and day 7. Regarding ICU survival, no significant difference was identified between APRV patients (40%) and LTV patients (42%). Median duration of mechanical ventilation and duration of ICU treatment were comparable in both groups. Similar complication rates with respect to ventilator-associated pneumonia, septic shock, thromboembolic events, barotrauma, as well as the necessity for hemodialysis were detected for both groups. Clinical characteristics that were associated with increased mortality in a Cox proportional hazards regression analysis included age (hazard ratio [HR] 1.08, 95% confidence interval [CI] 1.04-1.1; P < .001), severe acute respiratory distress syndrome (HR 2.62, 95% CI 1.02-6.7; P = .046) and the occurrence of septic shock (HR 17.18, 95% CI 2.06-143.2; P = .009), but not the ventilation mode. Interpretation: Intensive care unit survival, duration of mechanical ventilation, and ICU treatment as well as ventilation-associated complication rates were equivalent using APRV compared to conventional LTV in patients with CARDS.

Future directions in ventilator-induced lung injury associated cognitive impairment: a new sight

Mechanical ventilation is a widely used short-term life support technique, but an accompanying adverse consequence can be pulmonary damage which is called ventilator-induced lung injury (VILI). Mechanical ventilation can potentially affect the central nervous system and lead to long-term cognitive impairment. In recent years, many studies revealed that VILI, as a common lung injury, may be involved in the central pathogenesis of cognitive impairment by inducing hypoxia, inflammation, and changes in neural pathways. In addition, VILI has received attention in affecting the treatment of cognitive impairment and provides new insights into individualized therapy. The combination of lung protective ventilation and drug therapy can overcome the inevitable problems of poor prognosis from a new perspective. In this review, we summarized VILI and non-VILI factors as risk factors for cognitive impairment and concluded the latest mechanisms. Moreover, we retrospectively explored the role of improving VILI in cognitive impairment treatment. This work contributes to a better understanding of the pathogenesis of VILI-induced cognitive impairment and may provide future direction for the treatment and prognosis of cognitive impairment.

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.

A structured narrative review of clinical and experimental studies of the use of different positive end-expiratory pressure levels during thoracic surgery

OBJECTIVES

This study aimed to present a review on the general effects of different positive end-expiratory pressure (PEEP) levels during thoracic surgery by qualitatively categorizing the effects into detrimental, beneficial, and inconclusive.

DATA SOURCE

Literature search of Pubmed, CNKI, and Wanfang was made to find relative articles about PEEP levels during thoracic surgery. We used the following keywords as one-lung ventilation, PEEP, and thoracic surgery.

RESULTS

We divide the non-individualized PEEP value into five grades, that is, less than 5, 5, 5-10, 10, and more than 10 cmH₂ O, among which 5 cmH₂ O is the most commonly used in clinic at present to maintain alveolar dilatation and reduce the shunt fraction and the occurrence of atelectasis, whereas individualized PEEP, adjusted by test titration or imaging method to adapt to patients' personal characteristics, can effectively ameliorate intraoperative oxygenation and obtain optimal pulmonary compliance and better indexes relating to respiratory mechanics.

CONCLUSIONS

Available data suggest that PEEP might play an important role in one-lung ventilation, the understanding of which will help in exploring a simple and economical method to set the appropriate PEEP level.

Unshrinking the baby lung to calm the VILI vortex

A hallmark of ARDS is progressive shrinking of the ‘baby lung,’ now referred to as the ventilator-induced lung injury (VILI) ‘vortex.’ Reducing the risk of the VILI vortex is the goal of current ventilation strategies; unfortunately, this goal has not been achieved nor has mortality been reduced. However, the temporal aspects of a mechanical breath have not been considered. A brief expiration prevents alveolar collapse, and an extended inspiration can recruit the atelectatic lung over hours. Time-controlled adaptive ventilation (TCAV) is a novel ventilator approach to achieve these goals, since it considers many of the temporal aspects of dynamic lung mechanics.

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.

The Effects of Airway Pressure Release Ventilation on Pulmonary Permeability in Severe Acute Respiratory Distress Syndrome Pig Models

Objective: The aim of the study was to compare the effects of APRV and LTV ventilation on pulmonary permeability in severe ARDS.

Methods: Mini Bama adult pigs were randomized into the APRV group (n = 5) and LTV group (n = 5). A severe ARDS animal model was induced by the whole lung saline lavage. Pigs were ventilated and monitored continuously for 48 h.

Results: Compared with the LTV group, CStat was significantly better (p &lt; 0.05), and the PaO2/FiO2 ratio showed a trend to be higher throughout the period of the experiment in the APRV group. The extravascular lung water index and pulmonary vascular permeability index showed a trend to be lower in the APRV group. APRV also significantly mitigates lung histopathologic injury determined by the lung histopathological injury score (p &lt; 0.05) and gross pathological changes of lung tissues. The protein contents of occludin (p &lt; 0.05), claudin-5 (p &lt; 0.05), E-cadherin (p &lt; 0.05), and VE-cadherin (p &lt; 0.05) in the middle lobe of the right lung were higher in the APRV group than in the LTV group; among them, the contents of occludin (p &lt; 0.05) and E-cadherin (p &lt; 0.05) of the whole lung were higher in the APRV group. Transmission electron microscopy showed that alveolar–capillary barrier damage was more severe in the middle lobe of lungs in the LTV group.

Conclusion: In comparison with LTV, APRV could preserve the alveolar–capillary barrier architecture, mitigate lung histopathologic injury, increase the expression of cell junction protein, improve respiratory system compliance, and showed a trend to reduce extravascular lung water and improve oxygenation. These findings indicated that APRV might lead to more profound beneficial effects on the integrity of the alveolar–capillary barrier architecture and on the expression of biomarkers related to pulmonary permeability.

Does airway pressure release ventilation offer new hope for treating acute respiratory distress syndrome?

Mechanical ventilation (MV) is an essential life support method for patients with acute respiratory distress syndrome (ARDS), which is one of the most common critical illnesses with high mortality in the intensive care unit (ICU). A lung-protective ventilation strategy based on low tidal volume (LTV) has been recommended since a few years; however, as this did not result in a significant decrease of ARDS-related mortality, a more optimal ventilation mode was required. Airway pressure release ventilation (APRV) is an old method defined as a continuous positive airway pressure (CPAP) with a brief intermittent release phase based on the open lung concept; it also perfectly fits the ARDS treatment principle. Despite this, APRV has not been widely used in the past, rather only as a rescue measure for ARDS patients who are difficult to oxygenate. Over recent years, with an increased understanding of the pathophysiology of ARDS, APRV has been reproposed to improve patient prognosis. Nevertheless, this mode is still not routinely used in ARDS patients given its vague definition and complexity. Consequently, in this paper, we summarize the studies that used APRV in ARDS, including adults, children, and animals, to illustrate the settings of parameters, effectiveness in the population, safety (especially in children), incidence, and mechanism of ventilator-induced lung injury (VILI) and effects on extrapulmonary organs. Finally, we found that APRV is likely associated with improvement in ARDS outcomes, and does not increase injury to the lungs and other organs, thereby indicating that personalized APRV settings may be the new hope for ARDS treatment.

A narrative review of advanced ventilator modes in the pediatric intensive care unit

Respiratory failure is a common reason for pediatric intensive care unit admission. The vast majority of children requiring mechanical ventilation can be supported with conventional mechanical ventilation (CMV) but certain cases with refractory hypoxemia or hypercapnia may require more advanced modes of ventilation. This paper discusses what we have learned about the use of advanced ventilator modes [e.g., high-frequency oscillatory ventilation (HFOV), high-frequency percussive ventilation (HFPV), high-frequency jet ventilation (HFJV) airway pressure release ventilation (APRV), and neurally adjusted ventilatory assist (NAVA)] from clinical, animal, and bench studies. The evidence supporting advanced ventilator modes is weak and consists of largely of single center case series, although a few RCTs have been performed. Animal and bench models illustrate the complexities of different modes and the challenges of applying these clinically. Some modes are proprietary to certain ventilators, are expensive, or may only be available at well-resourced centers. Future efforts should include large, multicenter observational, interventional, or adaptive design trials of different rescue modes (e.g., PROSpect trial), evaluate their use during ECMO, and should incorporate assessments through volumetric capnography, electric impedance tomography, and transpulmonary pressure measurements, along with precise reporting of ventilator parameters and physiologic variables.

The effect of preemptive airway pressure release ventilation on patients with high risk for acute respiratory distress syndrome: a randomized controlled trial

Background and objectives

The objective of this study was to investigate the use of early APRV mode as a lung protective strategy compared to conventional methods with regard to ARDS development.

Methods

The study was designed as a randomized, non-blinded, single-center, superiority trial with two parallel groups and a primary endpoint of ARDS development. Patients under invasive mechanical ventilation who were not diagnosed with ARDS and had Lung Injury Prediction Score greater than 7 were included in the study. The patients were assigned to APRV and P-SIMV + PS mode groups.

Results

Patients were treated with P-SIMV+PS or APRV mode; 33 (50.8%) and 32 (49.2%), respectively. The P/F ratio values were higher in the APRV group on day 3 (p = 0.032). The fraction of inspired oxygen value was lower in the APRV group at day 7 (p = 0.011).While 5 of the 33 patients (15.2%) in the P-SIMV+PS group developed ARDS, one out of the 32 patients (3.1%) in the APRV group developed ARDS during follow-up (p = 0.197). The groups didn’t differ in terms of vasopressor/inotrope requirement, successful extubation rates, and/or mortality rates (p = 1.000, p = 0.911, p = 0.705, respectively). Duration of intensive care unit stay was 8 (2–11) days in the APRV group and 13 (8–81) days in the P-SIMV+PS group (p = 0.019).

Conclusions

The APRV mode can be used safely in selected groups of surgical and medical patients while preserving spontaneous respiration to a make benefit of its lung-protective effects. In comparison to the conventional mode, it is associated with improved oxygenation, higher mean airway pressures, and shorter intensive care unit stay. However, it does not reduce the sedation requirement, ARDS development, or mortality.

Lung protective ventilation strategy to reduce postoperative pulmonary complications (PPCs) in patients undergoing robot-assisted laparoscopic radical cystectomy for bladder cancer: A randomized double blinded clinical trial

STUDY OBJECTIVE

To evaluate the effects of ventilation with low tidal volume and positive end-expiratory pressure (PEEP) on postoperative pulmonary complications in patients undergoing robot-assisted laparoscopic radical cystectomy (RARC) for bladder cancer.

DESIGN

A prospective randomized double-blinded study.

SETTING

A single center trial in a comprehensive tertiary hospital from January 2017 to January 2019.

PATIENTS

A total of 258 patients undergoing RARC for bladder cancer.

INTERVENTIONS

Patients were randomly assigned to receive either lung-protective ventilation (LPV group) [tidal volume 6 ml/ kg predicated body weight (PBW) + PEEP 7 cmH₂O] or nonprotective ventilation (control group) (tidal volume 9 ml/ kg PBW without PEEP) during anesthesia.

MEASUREMENTS

The primary outcome was the occurrence of postoperative pulmonary complications (PPCs) during the first 90 days after surgery. The secondary outcomes were extubation time, oxygenation index (OI) after extubation and at postoperative day 1 in blood gas.

MAIN RESULTS

The incidence of PPCs at postoperative day1, 2 and 3 were lower in LPV group [26.8% vs. 47.2%, odds ratio (OR) 0.41, 95% confidence interval (CI), 0.24-0.69, P = 0.0007, 21.3% vs. 43.3%, OR 0.36, 95% CI, 0.20-0.61, P = 0.0002, 14.2% vs. 27.5%, OR0.43, 95%CI, 0.23-0.82, P = 0.0087, respectively], while no differences were observed at day 7 and 28 (3.9% vs. 9.4%, P = 0.0788, 0% vs. 1.6%, P = 0.4980, respectively). No PPCs were observed at postoperative day 90 in both groups. Furthermore, immediately after extubating and at postoperative day 1, OI was significantly higher in LPV group compared with control group [390(337-467) vs. 343(303-420), P = 0.0005, 406.7(73.0) vs. 425.5(74.7), P = 0.0440, respectively]. Patients in LPV group had a significant shorter extubation time after operation compared with control group [38(33-54) vs. 35(25-46), P = 0.0012].

CONCLUSION

LPV combining low tidal volume and PEEP during anesthesia for RARC may decrease the incidence of postoperative pulmonary complications.

Activation of the mechanosensitive Ca2+ channel TRPV4 induces endothelial barrier permeability via the disruption of mitochondrial bioenergetics

Mechanical ventilation is a life-saving intervention in critically ill patients with respiratory failure due to acute respiratory distress syndrome (ARDS), a refractory lung disease with an unacceptable high mortality rate. Paradoxically, mechanical ventilation also creates excessive mechanical stress that directly augments lung injury, a syndrome known as ventilator-induced lung injury (VILI). The specific mechanisms involved in VILI-induced pulmonary capillary leakage, a key pathologic feature of VILI are still far from resolved. The mechanoreceptor, transient receptor potential cation channel subfamily V member 4, TRPV4 plays a key role in the development of VILI through unresolved mechanism. Endothelial nitric oxide synthase (eNOS) uncoupling plays an important role in sepsis-mediated ARDS so in this study we investigated whether there is a role for eNOS uncoupling in the barrier disruption associated with TRPV4 activation during VILI. Our data indicate that the TRPV4 agonist, 4α-Phorbol 12,13-didecanoate (4αPDD) induces pulmonary arterial endothelial cell (EC) barrier disruption through the disruption of mitochondrial bioenergetics. Mechanistically, this occurs via the mitochondrial redistribution of uncoupled eNOS secondary to a PKC-dependent phosphorylation of eNOS at Threonine 495 (T495). A specific decoy peptide to prevent T495 phosphorylation reduced eNOS uncoupling and mitochondrial redistribution and preserved PAEC barrier function under 4αPDD challenge. Further, our eNOS decoy peptide was able to preserve lung vascular integrity in a mouse model of VILI. Thus, we have revealed a functional link between TRPV4 activation, PKC-dependent eNOS phosphorylation at T495, and EC barrier permeability. Reducing pT495-eNOS could be a new therapeutic approach for the prevention of VILI.

Airway pressure release ventilation versus low tidal volume ventilation for patients with acute respiratory distress syndrome/acute lung injury: a meta-analysis of randomized clinical trials

Background

It is uncertain whether airway pressure release ventilation (APRV) is better than low tidal volume ventilation (LTVV) for patients with acute respiratory distress syndrome (ARDS). The purpose of this meta-analysis was to compare APRV and LTVV on patients with ARDS.

Methods

Randomized controlled trials (RCTs) comparing outcomes in ARDS ventilator therapy with APRV or LTVV were identified using Medical Literature Analysis and Retrieval System Online (MEDLINE), Excerpta Medica Database (EMBASE), Cumulative Index to Nursing and Allied Health Literature (CINAHL), Web of Science, the Cochrane Library, and The Chinese Biomedicine Literature Database (SinoMed) from inception to March 2019.

Results

A total of 7 RCTs with a 405 patients were eligible for our meta-analysis. The results revealed that APRV was associated with lower hospital mortality [405 patients; odds ratio (OR), 0.57; 95% confidence interval (CI), 0.37-0.88; P=0.01], a shorter time of ventilator therapy [373 patients; mean difference (MD), 5.36; 95% CI, 1.99-8.73; P=0.002], and intensive care unit (ICU) stay (315 patients; MD, -4.50; 95% CI, -6.56 to -2.44; P<0.0001), better respiratory system compliance on day 3 (202 patients; MD, 8.19; 95% CI, 0.84-15.54; P=0.03), arterial partial pressure of oxygen/fraction of inspired oxygen (PaO2/FiO2) on day 3 (294 patients; MD, 44.40; 95% CI, 16.05-72.76; P=0.002), and higher mean arterial pressure (MAP) on day 3 (285 patients; MD, 4.18; 95% CI, 3.10-5.25; P<0.00001). There was no statistical difference in the incidence of pneumothorax (170 patients; OR, 0.40; 95% CI, 0.12-1.34; P=0.14).

Conclusions

The meta-analysis showed that APRV could reduce hospital mortality, duration of ventilation and ICU stay, improve lung compliance, oxygenation index, and MAP compared with LTVV for patients with ARDS. We found APRV to be a safe and effective ventilation mode for patients with ARDS.

Airway Pressure Release Ventilation as a Rescue Therapy in Pediatric Acute Respiratory Distress Syndrome

OBJECTIVES

To describe experience with airway pressure release ventilation (APRV) in children with severe acute respiratory distress syndrome (ARDS) refractory to conventional low tidal volume ventilation.

METHODS

This retrospective observational study was performed in an 11-bed, level 3 pediatric intensive care unit. Evaluation was made of 30 pediatric patients receiving airway pressure release ventilation as rescue therapy for severe ARDS.

RESULTS

Patients were switched to APRV on an average 3.2 ± 2.6 d following intubation. When changed from conventional mechanical ventilation (CMV) to APRV, there was an expected increase in the SpO₂/FiO₂ ratio (165.1 ± 13.6 vs. 131.7 ± 10.2; p = 0.035). Mean peak inspiratory pressure was significantly lower during APRV (25.4 ± 1.26 vs. 29.8 ± 0.60, p < 0.001) compared to CMV prior to APRV but mean airway pressure (P_aw) was significantly higher during APRV (19.1 ± 0.9 vs. 15.3 ± 1.3, p < 0.001). Hospital mortality in this study group was 16.6%.

CONCLUSIONS

The results of this study support the hypothesis that APRV may offer potential clinical advantages for ventilatory management and may be considered as an alternative rescue mechanical ventilation mode in pediatric ARDS patients refractory to conventional ventilation.

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.

A comprehensive review of the use and understanding of airway pressure release ventilation

Introduction: Airway pressure release ventilation (APRV) is a mode of ventilation typically utilized as a rescue or alternative mode for patients with acute respiratory distress syndrome (ARDS) and hypoxemia that is refractory to conventional mechanical ventilation. APRV's indication and efficacy continue to remain unclear given lack of consensus amongst practitioners, inconsistent methodology for its use, and scarcity of convincing evidence.Areas covered: This review discusses the history of APRV, how APRV works, rationales for its use, and its theoretical advantages and disadvantages. This is followed by a review of current available literature examining APRV's use in the intensive care unit, with further focus on its use in the pediatric intensive care unit.Expert opinion: APRV is a ventilation mode with theoretical risks and benefits. Appropriate study of APRV's clinical efficacy is difficult given a heterogeneous patient population and widely variable use of APRV between centers. Despite a paucity of definitive evidence in support of either mode, it is possible that the use of APRV will begin to outpace the use of high-frequency oscillatory ventilation (HFOV) for the management of refractory hypoxemia as more attention is paid to benefits of spontaneous breathing and minimizing sedation. Furthermore, APRV's role during ECMO deserves further investigation.

The safety and efficacy of airway pressure release ventilation in acute respiratory distress syndrome patients: A PRISMA-compliant systematic review and meta-analysis

BACKGROUND

The acute respiratory distress syndrome (ARDS) is a critical illness with high mortality and a worse prognosis. Mechanical ventilation (MV) is currently considered to be one of the most effective methods of treating ARDS. In this meta-analysis, we discussed the efficacy of airway pressure release ventilation (APRV) in treating ARDS.

METHODS

Following the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA), Ovid Medline, Embase, and PubMed were systematically searched with the keywords of "ARDS" and "APRV". The studies containing the treatment of APRV in ARDS were included. According to the MV protocol used in the studies, the comparison was undertaken between the APRV group vs low tidal volume (LTV) group and synchronized intermittent mandatory ventilation (SIMV) group. The relative risk (RR) and the standard mean difference with 95% confidence intervals (CI) were used for the comparison between groups.

RESULTS

Fourteen studies with 2096 patients were included in the meta-analysis. The average increasing rate of PaO2/FiO2 was 75.4% in the APRV group vs 44.1% in the non-APRV group. No significant differences were found in mortality and duration of ICU stay between APRV vs LTV (P = .073 and P = .404) and APRV vs SIMV (P = .370 and P = .894).

CONCLUSION

The APRV protocol would have a higher increase in the PaO2/FiO2 ratio, which was a safe protocol with a compatible effect comparing to LTV and SIMV.

Randomized Feasibility Trial of a Low Tidal Volume-Airway Pressure Release Ventilation Protocol Compared With Traditional Airway Pressure Release Ventilation and Volume Control Ventilation Protocols

Supplemental Digital Content is available in the text.

Objectives:

Low tidal volume (= tidal volume ≤ 6 mL/kg, predicted body weight) ventilation using volume control benefits patients with acute respiratory distress syndrome. Airway pressure release ventilation is an alternative to low tidal volume-volume control ventilation, but the release breaths generated are variable and can exceed tidal volume breaths of low tidal volume-volume control. We evaluate the application of a low tidal volume-compatible airway pressure release ventilation protocol that manages release volumes on both clinical and feasibility endpoints.

Design:

We designed a prospective randomized trial in patients with acute hypoxemic respiratory failure. We randomized patients to low tidal volume-volume control, low tidal volume-airway pressure release ventilation, and traditional airway pressure release ventilation with a planned enrollment of 246 patients. The study was stopped early because of low enrollment and inability to consistently achieve tidal volumes less than 6.5 mL/kg in the low tidal volume-airway pressure release ventilation arm. Although the primary clinical study endpoint was Pao₂/Fio₂ on study day 3, we highlight the feasibility outcomes related to tidal volumes in both arms.

Setting:

Four Intermountain Healthcare tertiary ICUs.

Patients:

Adult ICU patients with hypoxemic respiratory failure anticipated to require prolonged mechanical ventilation.

Interventions:

Low tidal volume-volume control, airway pressure release ventilation, and low tidal volume-airway pressure release ventilation.

Measurements and Main Results:

We observed wide variability and higher tidal (release for airway pressure release ventilation) volumes in both airway pressure release ventilation (8.6 mL/kg; 95% CI, 7.8–9.6) and low tidal volume-airway pressure release ventilation (8.0; 95% CI, 7.3–8.9) than volume control (6.8; 95% CI, 6.2–7.5; p = 0.005) with no difference between airway pressure release ventilation and low tidal volume-airway pressure release ventilation (p = 0.58). Recognizing the limitations of small sample size, we observed no difference in 52 patients in day 3 Pao₂/ Fio₂ (p = 0.92). We also observed no significant difference between arms in sedation, vasoactive medications, or occurrence of pneumothorax.

Conclusions:

Airway pressure release ventilation resulted in release volumes often exceeding 12 mL/kg despite a protocol designed to target low tidal volume ventilation. Current airway pressure release ventilation protocols are unable to achieve consistent and reproducible delivery of low tidal volume ventilation goals. A large-scale efficacy trial of low tidal volume-airway pressure release ventilation is not feasible at this time in the absence of an explicit, generalizable, and reproducible low tidal volume-airway pressure release ventilation protocol.

Airway Pressure Release Ventilation in Pediatric Acute Respiratory Distress Syndrome. A Randomized Controlled Trial

RATIONALE

Although case series describe benefits of airway pressure release ventilation (APRV), this mode of ventilation has not been evaluated against the conventional low-tidal volume ventilation (LoTV) in children with acute respiratory distress syndrome (ARDS).

OBJECTIVES

To compare the effect of APRV and conventional LoTV on ventilator-free days in children with ARDS.

METHODS

This open-label, parallel-design randomized controlled trial was conducted in a 15-bed ICU. Children aged 1 month to 12 years satisfying the modified Berlin definition were included. We excluded children with air leaks, increased intracranial pressure, poor spontaneous breathing efforts, chronic lung disease, and beyond 24 hours of ARDS diagnosis or 72 hours of ventilation. Children were randomized using unstratified, variable-sized block technique. A priori interim analysis was planned at 50% enrollment. All enrolled children were followed up until 180 days after enrollment or death, whichever was earlier.

MEASUREMENTS AND MAIN RESULTS

The trial was terminated after 50% enrollment (52 children) when analysis revealed higher mortality in the intervention arm. Ventilator-free days were statistically similar in both arms (P = 0.23). The 28-day all-cause mortality was 53.8% in APRV as compared with 26.9% among control subjects (risk ratio, 2.0; 95% confidence interval, 0.97-4.1; Fisher exact P = 0.089). The multivariate-adjusted risk ratio of death for APRV compared with LoTV was 2.02 (95% confidence interval, 0.99-4.12; P = 0.05). Higher mean airway pressures, greater spontaneous breathing, and early improvement in oxygenation were seen in the intervention arm.

CONCLUSIONS

APRV, as a primary ventilation strategy in children with ARDS, was associated with a trend toward higher mortality compared with the conventional LoTV. Limitations should be considered while interpreting these results. Clinical trial registered with www.clinicaltrials.gov (NCT02167698) and Clinical Trials Registry of India (CTRI/2014/06/004677).

Acute Respiratory Distress Syndrome: Etiology, Pathogenesis, and Summary on Management

The acute respiratory distress syndrome (ARDS) has multiple causes and is characterized by acute lung inflammation and increased pulmonary vascular permeability, leading to hypoxemic respiratory failure and bilateral pulmonary radiographic opacities. The acute respiratory distress syndrome is associated with substantial morbidity and mortality, and effective treatment strategies are limited. This review presents the current state of the literature regarding the etiology, pathogenesis, and management strategies for ARDS.

The time-controlled adaptive ventilation protocol: mechanistic approach to reducing ventilator-induced lung injury

Airway pressure release ventilation (APRV) is a ventilator mode that has previously been considered a rescue mode, but has gained acceptance as a primary mode of ventilation. In clinical series and experimental animal models of extrapulmonary acute respiratory distress syndrome (ARDS), the early application of APRV was able to prevent the development of ARDS. Recent experimental evidence has suggested mechanisms by which APRV, using the time-controlled adaptive ventilation (TCAV) protocol, may reduce lung injury, including: 1) an improvement in alveolar recruitment and homogeneity; 2) reduction in alveolar and alveolar duct micro-strain and stress-risers; 3) reduction in alveolar tidal volumes; and 4) recruitment of the chest wall by combating increased intra-abdominal pressure. This review examines these studies and discusses our current understanding of the pleiotropic mechanisms by which TCAV protects the lung. APRV set according to the TCAV protocol has been misunderstood and this review serves to highlight the various protective physiological and mechanical effects it has on the lung, so that its clinical application may be broadened.

Ventilator-induced lung injury and lung mechanics

Mechanical ventilation applies physical stresses to the tissues of the lung and thus may give rise to ventilator-induced lung injury (VILI), particular in patients with acute respiratory distress syndrome (ARDS). The most dire consequences of VILI result from injury to the blood-gas barrier. This allows plasma-derived fluid and proteins to leak into the airspaces where they flood some alveolar regions, while interfering with the functioning of pulmonary surfactant in those regions that remain open. These effects are reflected in commensurately increased values of dynamic lung elastance (EL ), a quantity that in principle is readily measured at the bedside. Recent mathematical/computational modeling studies have shown that the way in which EL varies as a function of both time and positive end-expiratory pressure (PEEP) reflects the nature and degree of lung injury, and can even be used to infer the separate contributions of volutrauma and atelectrauma to VILI. Interrogating such models for minimally injurious regimens of mechanical ventilation that apply to a particular lung may thus lead to personalized approaches to the ventilatory management of ARDS.

Effects of Preoxygenation with Tidal Volume Breathing Followed by Apneic Oxygenation with and without Continuous Positive Airway Pressure on Duration of Safe Apnea Time and Arterial Blood Gases

Background:

Application of continuous positive airway pressure (CPAP) helps to recruit collapsed areas of the lung, which improves the oxygen reserve.

Aim of the Study:

To compare the time to desaturate to 90% during apnea following preoxygenation and apneic ventilation with tidal volume breathing for 3 min with and without the application of CPAP.

Settings and Design:

This prospective randomized study was conducted in a tertiary care institution.

Subjects and Methods:

Twenty adult surgical patients were allocated into two groups. Group C patients were preoxygenated with 100% oxygen with CPAP of 20 cm H₂O for 3 min. Group P patients were preoxygenated for 3 min without CPAP. In Group C, apneic oxygenation was initiated following induction and neuromuscular blockade with CPAP of 20 cm H₂O. In Group P, no CPAP was applied. The study was terminated when the patient desaturated to 90%.

Statistical Analysis Used:

Chi-square test and Mann–Whitney test.

Results:

Group C had a significantly longer apnea time as compared to Group P (816.00 ± 30.98 vs. 348.00 ± 122.64 s). Three patients in Group P desaturated to <90% by 3 min and the remaining soon after 6 min. No patient in Group C desaturated till 12 min of apnea. PaO₂ was significantly higher in Group C at 3 and 6 min of apnea. At 3 and 6 min, Group P had significantly lower saturation as compared to Group C.

Conclusion:

Preoxygenation with CPAP significantly delayed desaturation during apnea with significantly higher arterial partial pressure of oxygen as compared to preoxygenation without CPAP.

High-Frequency Ventilation Modalities as Salvage Therapy for Smoke Inhalation-Associated Acute Lung Injury: A Systematic Review

BACKGROUND

Smoke inhalation-associated acute lung injury (SI-ALI) is a major cause of morbidity and mortality in victims of fire tragedies. To date, there are no evidence-based guidelines on ventilation strategies in acute respiratory distress syndrome (ARDS) after smoke inhalation. We reviewed the existing literature for clinical studies of salvage mechanical ventilation (MV) strategies in patients with SI-ALI, focusing on mortality and pneumonia as outcomes.

METHODS

A systematic search was designed in accordance with preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. Risk of bias assessment was performed using the Newcastle-Ottawa Quality Assessment Scale (NOS; 0 to 9 stars), with a score ≥7 being the threshold for inclusion in the meta-analysis. A systematic search strategy was used to search 10 databases. Clinical studies were included in which patients: (1) experienced smoke inhalation, (2) treated with MV, and (3) described a concurrent or historical control group.

RESULTS

A total of 226 potentially relevant studies were identified, of which 7 studies on high-frequency percussive ventilation (HFPV) met inclusion criteria. No studies met inclusion for meta-analysis (NOS ≥ 7). In studies comparing HFPV to conventional mechanical ventilation (CMV), mortality and pneumonia incidence improved in 3 studies and remained unchanged in 3 others. No change in ventilator days or ICU length of stay was observed; however, oxygenation and work of breathing improved with HFPV.

CONCLUSIONS

Mechanical ventilation in patients with SI-ALI has not been well studied. High-frequency percussive ventilation may decrease in-hospital mortality and pneumonia incidence when compared to CMV. The absence of "good" quality evidence precluded meta-analysis. Based upon low-quality evidence, there was a very weak recommendation that HFPV use may be associated with lower mortality and pneumonia rates in patients with SI-ALI. Given SI-ALI's unique underlying pathophysiology, and its potential implications on therapy, randomized controlled studies are required to ensure that patients receive the safest and most effective care.

TRIAL REGISTRATION

The study was registered with PROSPERO International prospective register of systematic reviews (#47015).

Purinergic signalling links mechanical breath profile and alveolar mechanics with the pro-inflammatory innate immune response causing ventilation-induced lung injury

Severe pulmonary infection or vigorous cyclic deformation of the alveolar epithelial type I (AT I) cells by mechanical ventilation leads to massive extracellular ATP release. High levels of extracellular ATP saturate the ATP hydrolysis enzymes CD39 and CD73 resulting in persistent high ATP levels despite the conversion to adenosine. Above a certain level, extracellular ATP molecules act as danger-associated molecular patterns (DAMPs) and activate the pro-inflammatory response of the innate immunity through purinergic receptors on the surface of the immune cells. This results in lung tissue inflammation, capillary leakage, interstitial and alveolar oedema and lung injury reducing the production of surfactant by the damaged AT II cells and deactivating the surfactant function by the concomitant extravasated serum proteins through capillary leakage followed by a substantial increase in alveolar surface tension and alveolar collapse. The resulting inhomogeneous ventilation of the lungs is an important mechanism in the development of ventilation-induced lung injury. The high levels of extracellular ATP and the upregulation of ecto-enzymes and soluble enzymes that hydrolyse ATP to adenosine (CD39 and CD73) increase the extracellular adenosine levels that inhibit the innate and adaptive immune responses rendering the host susceptible to infection by invading microorganisms. Moreover, high levels of extracellular adenosine increase the expression, the production and the activation of pro-fibrotic proteins (such as TGF-β, α-SMA, etc.) followed by the establishment of lung fibrosis.

Severe Pediatric Acute Respiratory Distress Syndrome Due to Scrub Typhus: Successful Ventilation with Airway Pressure Release Ventilation Mode after Becoming Refractory to Protective Ventilation

Scrub typhus can affect lungs from mild illness like pneumonitis to a severe illness like acute respiratory distress syndrome (ARDS). Such patients may be very challenging to treat when their hypoxemia becomes severe and refractory to treatment. Main treatment is supportive in terms of mechanical ventilation. In adult ARDS, low tidal volume (TV) ventilation has been recommended, but there is no consensus on most effective ventilation mode in children. We present a case of a 12-year-old girl who developed severe ARDS (PO₂/FiO₂ ratio – 58), refractory to low TV ventilation. There was a rapid improvement in oxygenation on the application of airway pressure release ventilation (APRV) mode within ½ h. She was successfully ventilated and weaned off the ventilator over 5 days. This case highlights the utility of APRV mode of ventilation as a rescue therapy for severe refractory ARDS in children.

Blunt thoracic trauma: recent advances and outstanding questions

PURPOSE OF REVIEW

The treatment of blunt thoracic injuries is complex and evolving. The aim of this review is to focus on what is new with ventilation for blunt chest trauma as well as an update on the current management strategies for blunt aortic injury and rib fractures.

RECENT FINDINGS

Early use of noninvasive ventilation appears to be well tolerated in select hemodynamically stable blunt trauma patients. For those patients requiring intubation, airway pressure release ventilation is an excellent mode to decrease the risk of posttraumatic acute lung injury. Endovascular repair of blunt thoracic aortic injuries provides benefit over open repair and, if possible, delayed repair confers a mortality advantage. Despite its increasing use, there continue to be conflicting results about the role of surgical rib fixation for the treatment of flail chest.

SUMMARY

Blunt thoracic injuries are commonly treated in the ICU and a solid knowledge of mechanical ventilation strategies (both noninvasive and invasive) is essential. Blunt thoracic aortic injuries require early diagnosis and aggressive blood pressure management. Not all such injuries need operative repair but those that do benefit from an endovascular approach. The management of flail chest includes early aggressive multimodal analgesia, adequate oxygen, and ventilatory support. Surgical rib fixation should be considered in select patients.

Zero expiratory pressure and low oxygen concentration promote heterogeneity of regional ventilation and lung densities

Background

It is not well known what is the main mechanism causing lung heterogeneity in healthy lungs under mechanical ventilation. We aimed to investigate the mechanisms causing heterogeneity of regional ventilation and parenchymal densities in healthy lungs under anesthesia and mechanical ventilation.

Methods

In a small animal model, synchrotron imaging was used to measure lung aeration and regional‐specific ventilation (sV̇). Heterogeneity of ventilation was calculated as the coefficient of variation in sV̇ (CV_sV̇). The coefficient of variation in lung densities (CV_D) was calculated for all lung tissue, and within hyperinflated, normally and poorly aerated areas. Three conditions were studied: zero end‐expiratory pressure (ZEEP) and F_IO₂ 0.21; ZEEP and F_IO₂ 1.0; PEEP 12 cmH₂O and F_IO₂1.0 (Open Lung‐PEEP = OLP).

Results

The mean tissue density at OLP was lower than ZEEP‐1.0 and ZEEP‐0.21. There were larger subregions with low sV̇ and poor aeration at ZEEP‐0.21 than at OLP: 12.9 ± 9.0 vs. 0.6 ± 0.4% in the non‐dependent level, and 17.5 ± 8.2 vs. 0.4 ± 0.1% in the dependent one (P = 0.041). The CV_sV̇ of the total imaged lung at PEEP 12 cmH₂O was significantly lower than on ZEEP, regardless of F_IO₂, indicating more heterogeneity of ventilation during ZEEP (0.23 ± 0.03 vs. 0.54 ± 0.37, P = 0.049). CV_D changed over the different mechanical ventilation settings (P = 0.011); predominantly, CV_D increased during ZEEP. The spatial distribution of the CV_D calculated for the poorly aerated density category changed with the mechanical ventilation settings, increasing in the dependent level during ZEEP.

Conclusion

ZEEP together with low F_IO₂ promoted heterogeneity of ventilation and lung tissue densities, fostering a greater amount of airway closure and ventilation inhomogeneities in poorly aerated regions.

The 30-year evolution of airway pressure release ventilation (APRV)

Airway pressure release ventilation (APRV) was first described in 1987 and defined as continuous positive airway pressure (CPAP) with a brief release while allowing the patient to spontaneously breathe throughout the respiratory cycle. The current understanding of the optimal strategy to minimize ventilator-induced lung injury is to "open the lung and keep it open". APRV should be ideal for this strategy with the prolonged CPAP duration recruiting the lung and the minimal release duration preventing lung collapse. However, APRV is inconsistently defined with significant variation in the settings used in experimental studies and in clinical practice. The goal of this review was to analyze the published literature and determine APRV efficacy as a lung-protective strategy. We reviewed all original articles in which the authors stated that APRV was used. The primary analysis was to correlate APRV settings with physiologic and clinical outcomes. Results showed that there was tremendous variation in settings that were all defined as APRV, particularly CPAP and release phase duration and the parameters used to guide these settings. Thus, it was impossible to assess efficacy of a single strategy since almost none of the APRV settings were identical. Therefore, we divided all APRV studies divided into two basic categories: (1) fixed-setting APRV (F-APRV) in which the release phase is set and left constant; and (2) personalized-APRV (P-APRV) in which the release phase is set based on changes in lung mechanics using the slope of the expiratory flow curve. Results showed that in no study was there a statistically significant worse outcome with APRV, regardless of the settings (F-ARPV or P-APRV). Multiple studies demonstrated that P-APRV stabilizes alveoli and reduces the incidence of acute respiratory distress syndrome (ARDS) in clinically relevant animal models and in trauma patients. In conclusion, over the 30 years since the mode's inception there have been no strict criteria in defining a mechanical breath as being APRV. P-APRV has shown great promise as a highly lung-protective ventilation strategy.

Mechanical Ventilation as a Therapeutic Tool to Reduce ARDS Incidence

Trauma, hemorrhagic shock, or sepsis can incite systemic inflammatory response syndrome, which can result in early acute lung injury (EALI). As EALI advances, improperly set mechanical ventilation (MV) can amplify early injury into a secondary ventilator-induced lung injury that invariably develops into overt ARDS. Once established, ARDS is refractory to most therapeutic strategies, which have not been able to lower ARDS mortality below the current unacceptably high 40%. Low tidal volume ventilation is one of the few treatments shown to have a moderate positive impact on ARDS survival, presumably by reducing ventilator-induced lung injury. Thus, there is a compelling case to be made that the focus of ARDS management should switch from treatment once this syndrome has become established to the application of preventative measures while patients are still in the EALI stage. Indeed, studies have shown that ARDS incidence is markedly reduced when conventional MV is applied preemptively using a combination of low tidal volume and positive end-expiratory pressure in both patients in the ICU and in surgical patients at high risk for developing ARDS. Furthermore, there is evidence from animal models and high-risk trauma patients that superior prevention of ARDS can be achieved using preemptive airway pressure release ventilation with a very brief duration of pressure release. Preventing rather than treating ARDS may be the way forward in dealing with this recalcitrant condition and would represent a paradigm shift in the way that MV is currently practiced.

Open Lung in Lateral Decubitus With Differential Selective Positive End-Expiratory Pressure in an Experimental Model of Early Acute Respiratory Distress Syndrome

OBJECTIVE

After lung recruitment, lateral decubitus and differential lung ventilation may enable the titration and application of optimum-selective positive end-expiratory pressure values for the dependent and nondependent lungs. We aimed at compare the effects of optimum-selective positive end-expiratory pressure with optimum global positive end-expiratory pressure on regional collapse and aeration distribution in an experimental model of acute respiratory distress syndrome.

DESIGN

Prospective laboratory investigation.

SETTING

University animal research laboratory.

SUBJECTS

Seven piglets.

INTERVENTIONS

A one-hit injury acute respiratory distress syndrome model was established by repeated lung lavages. After replacing the tracheal tube by a double-lumen one, we initiated lateral decubitus and differential ventilation. After maximum-recruitment maneuver, decremental positive end-expiratory pressure titration was performed. The positive end-expiratory pressure corresponding to maximum dynamic compliance was defined globally (optimum global positive end-expiratory pressure) and for each individual lung (optimum-selective positive end-expiratory pressure). After new maximum-recruitment maneuver, two steps were performed in randomized order (15 min each): ventilation applying the optimum global positive end-expiratory pressure and the optimum-selective positive end-expiratory pressure. CT scans were acquired at end expiration and end inspiration.

MEASUREMENTS AND MAIN RESULTS

Aeration homogeneity was evaluated as a nondependent/dependent ratio (percent of total gas content in upper lung/percent of total gas content in lower lung) and tidal recruitment as the difference in the percent mass of nonaerated tissue between expiration and inspiration. At the end of the 15-minute optimum-selective positive end-expiratory pressure, compared with the optimum global positive end-expiratory pressure, resulted in 1) decrease in the percent mass of collapse in the lower lung at expiratory CT (19% ± 15% vs 4% ± 5%; p = 0.03); 2) decrease in the nondependent/dependent ratio between the optimum global positive end-expiratory pressure-expiratory-CT and optimum-selective positive end-expiratory pressure-expiratory-CT (3.7 ± 1.2 vs 0.8 ± 0.5; p = 0.01); 3) decrease in the nondependent/dependent ratio between the optimum global positive end-expiratory pressure-inspiratory-CT and optimum-selective positive end-expiratory pressure-inspiratory-CT (2.8 ± 1.1 vs 0.6 ± 0.3; p = 0.01); and 4) less tidal recruitment (p = 0.049).

CONCLUSIONS

After maximum lung recruitment, lateral decubitus and differential lung ventilation enabled the titration of optimum-selective positive end-expiratory pressure values for the dependent and the nondependent lungs, made possible the application of an optimized regional open lung approach, promoted better aeration distribution, and minimized lung tissue inhomogeneities.

Reducing acute respiratory distress syndrome occurrence using mechanical ventilation

The standard treatment for acute respiratory distress syndrome (ARDS) is supportive in the form of low tidal volume ventilation applied after significant lung injury has already developed. Nevertheless, ARDS mortality remains unacceptably high (> 40%). Indeed, once ARDS is established it becomes refractory to treatment, and therefore avoidance is key. However, preventive techniques and therapeutics to reduce the incidence of ARDS in patients at high-risk have not been validated clinically. This review discusses the current data suggesting that preemptive application of the properly adjusted mechanical breath can block progressive acute lung injury and significantly reduce the occurrence of ARDS.

Impact of mechanical ventilation on the pathophysiology of progressive acute lung injury

The earliest description of what is now known as the acute respiratory distress syndrome (ARDS) was a highly lethal double pneumonia. Ashbaugh and colleagues (Ashbaugh DG, Bigelow DB, Petty TL, Levine BE Lancet 2: 319-323, 1967) correctly identified the disease as ARDS in 1967. Their initial study showing the positive effect of mechanical ventilation with positive end-expiratory pressure (PEEP) on ARDS mortality was dampened when it was discovered that improperly used mechanical ventilation can cause a secondary ventilator-induced lung injury (VILI), thereby greatly exacerbating ARDS mortality. This Synthesis Report will review the pathophysiology of ARDS and VILI from a mechanical stress-strain perspective. Although inflammation is also an important component of VILI pathology, it is secondary to the mechanical damage caused by excessive strain. The mechanical breath will be deconstructed to show that multiple parameters that comprise the breath-airway pressure, flows, volumes, and the duration during which they are applied to each breath-are critical to lung injury and protection. Specifically, the mechanisms by which a properly set mechanical breath can reduce the development of excessive fluid flux and pulmonary edema, which are a hallmark of ARDS pathology, are reviewed. Using our knowledge of how multiple parameters in the mechanical breath affect lung physiology, the optimal combination of pressures, volumes, flows, and durations that should offer maximum lung protection are postulated.

Lung Injury Prediction Score Is Useful in Predicting Acute Respiratory Distress Syndrome and Mortality in Surgical Critical Care Patients

Background. Lung injury prediction score (LIPS) is valuable for early recognition of ventilated patients at high risk for developing acute respiratory distress syndrome (ARDS). This study analyzes the value of LIPS in predicting ARDS and mortality among ventilated surgical patients. Methods. IRB approved, prospective observational study including all ventilated patients admitted to the surgical intensive care unit at a single tertiary center over 6 months. ARDS was defined using the Berlin criteria. LIPS were calculated for all patients and analyzed. Logistic regression models evaluated the ability of LIPS to predict development of ARDS and mortality. A receiver operator characteristic (ROC) curve demonstrated the optimal LIPS value to statistically predict development of ARDS. Results. 268 ventilated patients were observed; 141 developed ARDS and 127 did not. The average LIPS for patients who developed ARDS was 8.8 ± 2.8 versus 5.4 ± 2.8 for those who did not (p < 0.001). An ROC area under the curve of 0.79 demonstrates LIPS is statistically powerful for predicting ARDS development. Furthermore, for every 1-unit increase in LIPS, the odds of developing ARDS increase by 1.50 (p < 0.001) and odds of ICU mortality increase by 1.22 (p < 0.001). Conclusion. LIPS is reliable for predicting development of ARDS and predicting mortality in critically ill surgical patients.

Alveolar instability (atelectrauma) is not identified by arterial oxygenation predisposing the development of an occult ventilator-induced lung injury

BACKGROUND

Improperly set mechanical ventilation (MV) with normal lungs can advance lung injury and increase the incidence of acute respiratory distress syndrome (ARDS). A key mechanism of ventilator-induced lung injury (VILI) is an alteration in alveolar mechanics including alveolar instability or recruitment/derecruitment (R/D). We hypothesize that R/D cannot be identified by PaO2 (masking occult VILI), and if protective ventilation is not applied, ARDS incidence will increase.

METHODS

Sprague-Dawley rats (n = 8) were anesthetized, surgically instrumented, and placed on MV. A thoracotomy was performed and an in vivo microscope attached to the pleural surface of the lung with baseline dynamic changes in alveolar size during MV recorded. Alveolar instability was induced by intra-tracheal instillation of Tween and alveolar R/D identified as a marked change in alveolar size from inspiration to expiration with increases in positive end-expiratory pressure (PEEP) levels.

RESULTS

Despite maintaining a clinically acceptable PaO2 (55-80 mmHg), the alveoli remained unstable with significant R/D at low PEEP levels. Although PaO2 consistently increased with an increase in PEEP, R/D did not plateau until PEEP was >9 cmH2O.

CONCLUSIONS

PaO2 remained clinically acceptable while alveolar instability persisted at all levels of PEEP (especially PEEP <9 cmH2O). Therefore, PaO2 levels cannot be used reliably to guide protective MV strategies or infer that VILI is not occurring. Using PaO2 to set a PEEP level necessary to stabilize the alveoli could underestimate the potential for VILI. These findings highlight the need for more accurate marker(s) of alveolar stability to guide protective MV necessary to prevent VILI.

Ventilator Strategies and Rescue Therapies for Management of Acute Respiratory Failure in the Emergency Department

Acute respiratory failure is commonly encountered in the emergency department (ED), and early treatment can have effects on long-term outcome. Noninvasive ventilation is commonly used for patients with respiratory failure and has been demonstrated to improve outcomes in acute exacerbations of chronic obstructive lung disease and congestive heart failure, but should be used carefully, if at all, in the management of asthma, pneumonia, and acute respiratory distress syndrome. Lung-protective tidal volumes should be used for all patients receiving mechanical ventilation, and FiO2 should be reduced after intubation to achieve a goal of less than 60%. For refractory hypoxemia, new rescue therapies have emerged to help improve the oxygenation, and in some cases mortality, and should be considered in ED patients when necessary, as deferring until ICU admission may be deleterious. This review article summarizes the pathophysiology of acute respiratory failure, management options, and rescue therapies including airway pressure release ventilation, continuous neuromuscular blockade, inhaled nitric oxide, and extracorporeal membrane oxygenation.

Airway pressure release ventilation reduces conducting airway micro-strain in lung injury

BACKGROUND

Improper mechanical ventilation can exacerbate acute lung damage, causing a secondary ventilator-induced lung injury (VILI). We hypothesized that VILI can be reduced by modifying specific components of the ventilation waveform (mechanical breath), and we studied the impact of airway pressure release ventilation (APRV) and controlled mandatory ventilation (CMV) on the lung micro-anatomy (alveoli and conducting airways). The distribution of gas during inspiration and expiration and the strain generated during mechanical ventilation in the micro-anatomy (micro-strain) were calculated.

STUDY DESIGN

Rats were anesthetized, surgically prepared, and randomized into 1 uninjured control group (n = 2) and 4 groups with lung injury: APRV 75% (n = 2), time at expiration (TLow) set to terminate appropriately at 75% of peak expiratory flow rate (PEFR); APRV 10% (n = 2), TLow set to terminate inappropriately at 10% of PEFR; CMV with PEEP 5 cm H2O (PEEP 5; n = 2); or PEEP 16 cm H2O (PEEP 16; n = 2). Lung injury was induced in the experimental groups by Tween lavage and ventilated with their respective settings. Lungs were fixed at peak inspiration and end expiration for standard histology. Conducting airway and alveolar air space areas were quantified and conducting airway micro-strain was calculated.

RESULTS

All lung injury groups redistributed inspired gas away from alveoli into the conducting airways. The APRV 75% minimized gas redistribution and micro-strain in the conducting airways and provided the alveolar air space occupancy most similar to control at both inspiration and expiration.

CONCLUSIONS

In an injured lung, APRV 75% maintained micro-anatomic gas distribution similar to that of the normal lung. The lung protection demonstrated in previous studies using APRV 75% may be due to a more homogeneous distribution of gas at the micro-anatomic level as well as a reduction in conducting airway micro-strain.

Mechanical ventilation in acute hypoxemic respiratory failure: a review of new strategies for the practicing hospitalist

BACKGROUND

The goal of mechanical ventilation in acute hypoxemic respiratory failure is to support adequate gas exchange without harming the lungs. How patients are mechanically ventilated can significantly impact their ultimate outcomes.

METHODS

This review focuses on emerging evidence regarding strategies for mechanical ventilation in patients with acute hypoxemic respiratory failure including: low tidal volume ventilation in the acute respiratory distress syndrome (ARDS), novel ventilator modes as alternatives to low tidal volume ventilation, adjunctive strategies that may enhance recovery in ARDS, the use of lung-protective strategies in patients without ARDS, rescue therapies in refractory hypoxemia, and an evidence-based approach to weaning from mechanical ventilation.

RESULTS

Once a patient is intubated and mechanically ventilated, low tidal volume ventilation remains the best strategy in ARDS. Adjunctive therapies in ARDS include a conservative fluid management strategy, as well as neuromuscular blockade and prone positioning in moderate-to-severe disease. There is also emerging evidence that a lung-protective strategy may benefit non-ARDS patients. For patients with refractory hypoxemia, extracorporeal membrane oxygenation should be considered. Once the patient demonstrates signs of recovery, the best approach to liberation from mechanical ventilation involves daily spontaneous breathing trials and protocolized assessment of readiness for extubation.

CONCLUSIONS

Prompt recognition of ARDS and use of lung-protective ventilation, as well as evidence-based adjunctive therapies, remain the cornerstones of caring for patients with acute hypoxemic respiratory failure. In the absence of contraindications, it is reasonable to consider lung-protective ventilation in non-ARDS patients as well, though the evidence supporting this practice is less conclusive.

[Ventilator-induced immune dysfunction]

Although mechanical ventilation is an essential support in patients admitted to the intensive care unit, clinical and experimental studies have shown that it could be harmful and could induce lung injury. Pulmonary and immune cells can convert mechanical stimuli into biological signals that will lead to inflammation. This sterile inflammation both locally and systemically will cause immunosuppression.