Recruitment and derecruitment during acute respiratory failure: an experimental study

Regional Behavior of Airspaces During Positive Pressure Reduction Assessed by Synchrotron Radiation Computed Tomography

INTRODUCTION

The mechanisms of lung inflation and deflation are only partially known. Ventilatory strategies to support lung function rely upon the idea that lung alveoli are isotropic balloons that progressively inflate or deflate and that lung pressure/volume curves derive only by the interplay of critical opening pressures, critical closing pressures, lung history, and position of alveoli inside the lung. This notion has been recently challenged by subpleural microscopy, magnetic resonance, and computed tomography (CT). Phase-contrast synchrotron radiation CT (PC-SRCT) can yield in vivo images at resolutions higher than conventional CT.

OBJECTIVES

We aimed to assess the numerosity (ASden) and the extension of the surface of airspaces (ASext) in healthy conditions at different volumes, during stepwise lung deflation, in concentric regions of the lung.

METHODS

The study was conducted in seven anesthetized New Zealand rabbits. They underwent PC-SRCT scans (resolution of 47.7 μm) of the lung at five decreasing positive end expiratory pressure (PEEP) levels of 12, 9, 6, 3, and 0 cmH2O during end-expiratory holds. Three concentric regions of interest (ROIs) of the lung were studied: subpleural, mantellar, and core. The images were enhanced by phase contrast algorithms. ASden and ASext were computed by using the Image Processing Toolbox for MatLab. Statistical tests were used to assess any significant difference determined by PEEP or ROI on ASden and ASext.

RESULTS

When reducing PEEP, in each ROI the ASden significantly decreased. Conversely, ASext variation was not significant except for the core ROI. In the latter, the angular coefficient of the regression line was significantly low.

CONCLUSION

The main mechanism behind the decrease in lung volume at PEEP reduction is derecruitment. In our study involving lung regions laying on isogravitational planes and thus equally influenced by gravitational forces, airspace numerosity and extension of surface depend on the local mechanical properties of the lung.

EFFECTS OF ALVEOLAR MORPHOLOGY ON ALVEOLAR MECHANICS: AN EXPERIMENTAL STUDY OF MOUSE LUNG BASED ON TWO- AND THREE-DIMENSIONAL IMAGING METHODS

Understanding alveolar mechanics is important for preventing the possible lung injuries during mechanical ventilation. Alveolar clusters with smaller size are found having lower compliance in two-dimensional studies. But the influence of alveolar shape on compliance is unclear. In order to investigate how alveolar morphology affects their behavior, we tracked subpleural alveoli of isolated mouse lungs during quasi-static ventilation using two- and three-dimensional imaging techniques. Results showed that alveolar clusters with smaller size and more spherical shape had lower compliance. There was a better correlation of sphericity rather than circularity with alveolar compliance. The compliance of clusters with great shape change was larger than that with relatively slight shape change. These findings suggest the contribution of lung heterogeneous expansion to lung injuries associated with mechanical ventilation.

Optimising mechanical ventilation through model-based methods and automation

Mechanical ventilation (MV) is a core life-support therapy for patients suffering from respiratory failure or acute respiratory distress syndrome (ARDS). Respiratory failure is a secondary outcome of a range of injuries and diseases, and results in almost half of all intensive care unit (ICU) patients receiving some form of MV. Funding the increasing demand for ICU is a major issue and MV, in particular, can double the cost per day due to significant patient variability, over-sedation, and the large amount of clinician time required for patient management. Reducing cost in this area requires both a decrease in the average duration of MV by improving care, and a reduction in clinical workload. Both could be achieved by safely automating all or part of MV care via model-based dynamic systems modelling and control methods are ideally suited to address these problems. This paper presents common lung models, and provides a vision for a more automated future and explores predictive capacity of some current models. This vision includes the use of model-based methods to gain real-time insight to patient condition, improve safety through the forward prediction of outcomes to changes in MV, and develop virtual patients for in-silico design and testing of clinical protocols. Finally, the use of dynamic systems models and system identification to guide therapy for improved personalised control of oxygenation and MV therapy in the ICU will be considered. Such methods are a major part of the future of medicine, which includes greater personalisation and predictive capacity to both optimise care and reduce costs. This review thus presents the state of the art in how dynamic systems and control methods can be applied to transform this core area of ICU medicine.

Esophageal pressure monitoring: why, when and how?

PURPOSE OF REVIEW

Esophageal manometry has shown its usefulness to estimate transpulmonary pressure, that is lung stress, and the intensity of spontaneous effort in patients with acute respiratory distress syndrome. However, clinical uptake of esophageal manometry in ICU is still low. Thus, the purpose of review is to describe technical tips to adequately measure esophageal pressure at the bedside, and then update the most important clinical applications of esophageal manometry in ICU.

RECENT FINDINGS

Each esophageal balloon has its own nonstressed volume and it should be calibrated properly to measure pleural pressure accurately: transpulmonary pressure calculated on absolute esophageal pressure reflects values in the lung regions adjacent to the esophageal balloon (i.e. dependent to middle lung). Inspiratory transpulmonary pressure calculated from airway plateau pressure and the chest wall to respiratory system elastance ratio reasonably reflects lung stress in the nondependent 'baby' lung, at highest risk of hyperinflation. Also esophageal pressure can be used to detect and minimize patient self-inflicted lung injury.

SUMMARY

Esophageal manometry is not a complicated technique. There is a large potential to improve clinical outcome in patients with acute respiratory distress syndrome, acting as an early detector of risk of lung injury from mechanical ventilation and vigorous spontaneous effort.

Effect of transpulmonary pressure-guided positive end-expiratory pressure titration on lung injury in pigs with acute respiratory distress syndrome

To investigate the effect of positive end-expiratory pressure (PEEP) guided by transpulmonary pressure or with maximum oxygenation-directed PEEP on lung injury in a porcine model of acute respiratory distress syndrome (ARDS). The porcine model of ARDS was induced in 12 standard pigs by intratracheal infusion with normal saline. The pigs were then randomly divided into two groups who were ventilated with the lung-protective strategy of low tidal volume (VT) (6 ml/kg), using different methods to titrate PEEP level: transpulmonary pressure (TP group; n = 6) or maximum oxygenation (MO group; n = 6). Gas exchange, pulmonary mechanics, and hemodynamics were determined and pulmonary inflammatory response indices were measured after 4 h of ventilation. The titrated PEEP level in the TP group (6.12 ± 0.89 cmH₂O) was significantly lower than that in the MO group (11.33 ± 2.07 cmH2O) (P < 0.05). The PaO₂/FiO₂ (P/F) after PEEP titration both improved in the TP and MO groups as compared with that at T0 (when the criteria for ARDS were obtained). The P/F in the TP group did not differ significantly from that in the MO group during the 4 h of ventilation (P > 0.05). Respiratory system compliance and lung compliance were significantly improved in the TP group compared to the MO group (P < 0.05). The VD/VT in the TP group was significantly lower than that in the MO group after 4 h of ventilation (P < 0.05). Central venous pressure increased and the cardiac index decreased significantly in the MO group as compared with the TP group (P < 0.05), whereas oxygen delivery did not differ significantly between the groups (P > 0.05). The pulmonary vascular permeability index and the extravascular lung water index in the TP group were significantly lower than those in the MO group (P < 0.05). The TP group had a lower lung wet to dry weight ratio, lung injury score, and MPO, TNF-, and IL-8 concentrations than the MO group (P < 0.05). In summary, in a pig model of ARDS, ventilation with low VT and transpulmonary pressure-guided PEEP adjustment was associated with improved compliance, reduced dead space ventilation, increased cardiac output, and relieved lung injury, as compared to maximum oxygenation-guide PEEP adjustment.

Effect of Titrating Positive End-Expiratory Pressure (PEEP) With an Esophageal Pressure-Guided Strategy vs an Empirical High PEEP-Fio2 Strategy on Death and Days Free From Mechanical Ventilation Among Patients With Acute Respiratory Distress Syndrome: A Randomized Clinical Trial

IMPORTANCE

Adjusting positive end-expiratory pressure (PEEP) to offset pleural pressure might attenuate lung injury and improve patient outcomes in acute respiratory distress syndrome (ARDS).

OBJECTIVE

To determine whether PEEP titration guided by esophageal pressure (PES), an estimate of pleural pressure, was more effective than empirical high PEEP-fraction of inspired oxygen (Fio2) in moderate to severe ARDS.

DESIGN, SETTING, AND PARTICIPANTS

Phase 2 randomized clinical trial conducted at 14 hospitals in North America. Two hundred mechanically ventilated patients aged 16 years and older with moderate to severe ARDS (Pao2:Fio2 ≤200 mm Hg) were enrolled between October 31, 2012, and September 14, 2017; long-term follow-up was completed July 30, 2018.

INTERVENTIONS

Participants were randomized to PES-guided PEEP (n = 102) or empirical high PEEP-Fio2 (n = 98). All participants received low tidal volumes.

MAIN OUTCOMES AND MEASURES

The primary outcome was a ranked composite score incorporating death and days free from mechanical ventilation among survivors through day 28. Prespecified secondary outcomes included 28-day mortality, days free from mechanical ventilation among survivors, and need for rescue therapy.

RESULTS

Two hundred patients were enrolled (mean [SD] age, 56 [16] years; 46% female) and completed 28-day follow-up. The primary composite end point was not significantly different between treatment groups (probability of more favorable outcome with PES-guided PEEP: 49.6% [95% CI, 41.7% to 57.5%]; P = .92). At 28 days, 33 of 102 patients (32.4%) assigned to PES-guided PEEP and 30 of 98 patients (30.6%) assigned to empirical PEEP-Fio2 died (risk difference, 1.7% [95% CI, -11.1% to 14.6%]; P = .88). Days free from mechanical ventilation among survivors was not significantly different (median [interquartile range]: 22 [15-24] vs 21 [16.5-24] days; median difference, 0 [95% CI, -1 to 2] days; P = .85). Patients assigned to PES-guided PEEP were significantly less likely to receive rescue therapy (4/102 [3.9%] vs 12/98 [12.2%]; risk difference, -8.3% [95% CI, -15.8% to -0.8%]; P = .04). None of the 7 other prespecified secondary clinical end points were significantly different. Adverse events included gross barotrauma, which occurred in 6 patients with PES-guided PEEP and 5 patients with empirical PEEP-Fio2.

CONCLUSIONS AND RELEVANCE

Among patients with moderate to severe ARDS, PES-guided PEEP, compared with empirical high PEEP-Fio2, resulted in no significant difference in death and days free from mechanical ventilation. These findings do not support PES-guided PEEP titration in ARDS.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier NCT01681225.

Correlation between Extravascular Lung Water and Oxygenation in ALI/ARDS Patients in Septic Shock: Possible Role in the Development of Atelectasis?

This study aimed to evaluate the relationship between PaO2/FiO2 ratio and extravascular lung water in septic shock-induced acute respiratory distress syndrome in a prospective observational clinical trial.

Twenty-three patients suffering from sepsis induced acute respiratory distress syndrome were recruited. All patients were ventilated in pressure control/support mode. Haemodynamic parameters were determined by arterial thermodilution (PiCCO) eight hourly for 72 hours. At the same time blood gas analyses were done and respiratory parameters were also recorded. Data are presented as mean±SD. For statistical analysis Pearson's correlation test, and analysis of variance (ANOVA) was used respectively.

Significant negative correlation was found between extravascular lung water and PaO2/FiO2 (r= −0.355, P<0.001), and significant positive correlation was shown between extravascular lung water and PEEP (r=0.557, P<0.001). A post-hoc analysis was performed when “low” PEEP: <10 cmH2O and “high” PEEP: (10 cmH2O PEEP was applied, and neither the oxygenation, nor the driving pressure or the PaCO2 differed significantly, but the extravascular lung water showed significant difference when “high” or “low” PEEP was applied (13±5 vs 9±2 ml/kg respectively, P=0.001).

This study found significant negative correlation between extravascular lung water and PaO2/FiO2. The mechanism by which extravascular lung water affects oxygenation is unknown but the significant positive correlation between PEEP and extravascular lung water shown in this trial suggests that the latter may have a role in the development of alveolar atelectasis.

The Acute Respiratory Distress Syndrome: Diagnosis and Management

Acute respiratory distress syndrome (ARDS) is characterized by a new acute onset of hypoxemia secondary to a pulmonary edema of non-cardiogenic origin, bilateral lung opacities and reduction in respiratory system compliance after an insult direct or indirect to lungs. Its first description was in 1970s, and then several shared definitions tried to describe this clinical entity; the last one, known as Berlin definition, brought an improvement in predictive ability for mortality.

In the present chapter, the diagnostic workup of the syndrome will be presented with particular attention to microbiological investigations which represent a milestone in the diagnostic process and to imaging techniques such as CT scan and lung ultrasound.

Despite the treatment is mainly based on supportive strategies, attention should be applied to assure adequate respiratory gas exchange while minimizing the risk of ventilator-induced lung injury (VILI) onset. Therefore will be described several therapeutic approaches to ARDS, including noninvasive mechanical ventilation (NIMV), high-flow nasal cannulas (HFNC) and invasive ventilation with particular emphasis to risks and benefits of mechanical ventilation, PEEP optimization and lung protective ventilation strategies. Rescue techniques, such as permissive hypercapnia, prone positioning, neuromuscular blockade, inhaled vasodilators, corticosteroids, recruitment maneuvers and extracorporeal life support, will also be reviewed.

Finally, the chapter will deal with the mechanical ventilation weaning process with particular emphasis on extrapulmonary factors such as neurologic, diaphragmatic or cardiovascular alterations which can lead to weaning failure.

Positive end-expiratory pressure titrated according to respiratory system mechanics or to ARDSNetwork table did not guarantee positive end-expiratory transpulmonary pressure in acute respiratory distress syndrome

PURPOSE

Pulmonary recruitment and positive end-expiratory pressure (PEEP) titrated according to minimal static elastance of the respiratory system (PEEP_Estat,RS) compared to PEEP set according to the ARDSNetwork table (PEEP_ARDSNetwork) as a strategy to prevent ventilator-associated lung injury (VALI) in patients with acute respiratory distress syndrome (ARDS) increases mortality. Alternatively, avoiding negative end-expiratory transpulmonary pressure has been discussed as superior PEEP titration strategy. Therefore, we tested whether PEEP_Estat,RS or PEEP_ARDSNetwork prevent negative end-expiratory transpulmonary pressure in ARDS patients.

MATERIAL AND METHODS

Thirteen patients with moderate to severe ARDS were studied at PEEP_ARDSNetwork versus PEEP_Estat,RS. Patients were then grouped post hoc according to the end-expiratory transpulmonary pressure (positive or negative).

RESULTS

7 out of 13 patients showed negative end-expiratory transpulmonary pressures (Ptp-) with both strategies (PEEP_ARDSNetwork: - 5.4 ± 3.5 vs. 2.2 ± 3.7 cm H₂O, p = .005; PEEP_Estat,RS: - 3.6 ± 1.5 vs. 3.5 ± 3.3 cm H₂O, p < .001). Ptp- was associated with higher intra-abdominal pressure and lower end-expiratory lung volume with both PEEP strategies.

CONCLUSIONS

In patients with moderate-to-severe ARDS, PEEP titrated according to the minimal static elastance of the respiratory system or according to the ARDSNetwork table did not prevent negative end-expiratory transpulmonary pressure.

Should we titrate peep based on end-expiratory transpulmonary pressure?-yes

Ventilator management of patients with acute respiratory distress syndrome (ARDS) has been characterized by implementation of basic physiology principles by minimizing harmful distending pressures and preventing lung derecruitment. Such strategies have led to significant improvements in outcomes. Positive end expiratory pressure (PEEP) is an important part of a lung protective strategy but there is no standardized method to set PEEP level. With widely varying types of lung injury, body habitus and pulmonary mechanics, the use of esophageal manometry has become important for personalization and optimization of mechanical ventilation in patients with ARDS. Esophageal manometry estimates pleural pressures, and can be used to differentiate the chest wall and lung (transpulmonary) contributions to the total respiratory system mechanics. Elevated pleural pressures may result in negative transpulmonary pressures at end expiration, leading to lung collapse. Measuring the esophageal pressures and adjusting PEEP to make transpulmonary pressures positive can decrease atelectasis, derecruitment of lung, and cyclical opening and closing of airways and alveoli, thus optimizing lung mechanics and oxygenation. Although there is some spatial and positional artifact, esophageal pressures in numerous animal and human studies in healthy, obese and critically ill patients appear to be a good estimate for the "effective" pleural pressure. Multiple studies have illustrated the benefit of using esophageal pressures to titrate PEEP in patients with obesity and with ARDS. Esophageal pressure monitoring provides a window into the unique physiology of a patient and helps improve clinical decision making at the bedside.

Should we titrate ventilation based on driving pressure? Maybe not in the way we would expect

Mechanical ventilation maintains adequate gas exchange in patients during general anaesthesia, as well as in critically ill patients without and with acute respiratory distress syndrome (ARDS). Optimization of mechanical ventilation is important to minimize ventilator induced lung injury and improve outcome. Tidal volume (V_T), positive end-expiratory pressure (PEEP), respiratory rate (RR), plateau pressures as well as inspiratory oxygen are the main parameters to set mechanical ventilation. Recently, the driving pressure (∆P), i.e., the difference of the plateau pressure and end-expiratory pressure of the respiratory system or of the lung, has been proposed as a key role parameter to optimize mechanical ventilation parameters. The ∆P depends on the V_T as well as on the relative balance between the amount of aerated and/or overinflated lung at end-expiration and end-inspiration at different levels of PEEP. During surgery, higher ∆P, mainly due to V_T, was progressively associated with an increased risk to develop post-operative pulmonary complications; in two large randomized controlled trials the reduction in ∆P by PEEP did not result in better outcome. In non-ARDS patients, ∆P was not found even associated with morbidity and mortality. In ARDS patients, an association between ∆P (higher than 13-15 cmH₂O) and mortality has been reported. In several randomized controlled trials, when ∆P was minimized by the use of higher PEEP with or without recruitment manoeuvres, this strategy resulted in equal or even higher mortality. No clear data are currently available about the interpretation and clinical use of ∆P during assisted ventilation. In conclusion, ∆P is an indicator of severity of the lung disease, is related to V_T size and associated with complications and mortality. We advocate the use of ∆P to optimize individually V_T but not PEEP in mechanically ventilated patients with and without ARDS.

Interpretation of the transpulmonary pressure in the critically ill patient

Mechanical ventilation is a life-saving procedure, which takes over the function of the respiratory muscles while buying time for healing to take place. However, it can also promote or worsen lung injury, so that careful monitoring of respiratory mechanics is suggested to titrate the level of support and avoid injurious pressures and volumes to develop. Standard monitoring includes flow, volume and airway pressure (Paw). However, Paw represents the pressure acting on the respiratory system as a whole, and does not allow to differentiate the part of pressure that is spent di distend the chest wall. Moreover, if spontaneous breathing efforts are allowed, the Paw is the sum of that applied by the ventilator and that generated by the patient. As a consequence, monitoring of Paw has significant shortcomings. Assessment of esophageal pressure (Pes), as a surrogate for pleural pressure (Ppl), may allow the clinicians to discriminate between the elastic behaviour of the lung and the chest wall, and to calculate the degree of spontaneous respiratory effort. In the present review, the characteristics and limitations of airway and transpulmonary pressure monitoring will be presented; we will highlight the different assumptions underlying the various methods for measuring transpulmonary pressure (i.e., the elastance-derived and the release-derived method, and the direct measurement), as well as the potential application of transpulmonary pressure assessment during both controlled and spontaneous/assisted mechanical ventilation in critically ill patients.

Can we estimate transpulmonary pressure without an esophageal balloon?-yes

A protective ventilation strategy is based on separation of lung and chest wall mechanics and determination of transpulmonary pressure. So far, this has required esophageal pressure measurement, which is cumbersome, rarely used clinically and associated with lack of consensus on the interpretation of measurements. We have developed an alternative method based on a positive end expiratory pressure (PEEP) step procedure where the PEEP-induced change in end-expiratory lung volume is determined by the ventilator pneumotachograph. In pigs, lung healthy patients and acute lung injury (ALI) patients, it has been verified that the determinants of the change in end-expiratory lung volume following a PEEP change are the size of the PEEP step and the elastic properties of the lung, ∆PEEP × Clung. As a consequence, lung compliance can be calculated as the change in end-expiratory lung volume divided by the change in PEEP and esophageal pressure measurements are not needed. When lung compliance is determined in this way, transpulmonary driving pressure can be calculated on a breath-by-breath basis. As the end-expiratory transpulmonary pressure increases as much as PEEP is increased, it is also possible to determine the end-inspiratory transpulmonary pressure at any PEEP level. Thus, the most crucial factors of ventilator induced lung injury can be determined by a simple PEEP step procedure. The measurement procedure can be repeated with short intervals, which makes it possible to follow the course of the lung disease closely. By the PEEP step procedure we may also obtain information (decision support) on the mechanical consequences of changes in PEEP and tidal volume performed to improve oxygenation and/or carbon dioxide removal.

Should we titrate positive end-expiratory pressure based on an end-expiratory transpulmonary pressure?

Arguments continue to swirl regarding the need for and best method of positive end-expiratory pressure (PEEP) titration. An appropriately conducted decremental method that uses modest peak pressures for the recruiting maneuver (RM), a lung protective tidal excursion, relatively small PEEP increments and appropriate timing intervals is currently the most logical and attractive option, particularly when the esophageal balloon pressure (Pes) is used to calculate transpulmonary driving pressures relevant to the lung. The setting of PEEP by the Pes-guided end-expiratory pressure at the 'polarity transition' point of the transmural end-expiratory pressure is quite relevant to the locale of the esophageal balloon catheter. Its desirability, however, is limited by its tendency to encourage PEEP levels that are higher than most other PEEP titration methods. These Pes-set PEEP values promote higher mean airway pressures and are likely to be unnecessary when small tidal driving pressures are in use. Because high airway pressures increase global lung stress and risk hemodynamic compromise, the Pes-determined PEEP would seem associated with a relatively high hazard to benefit ratio for many patients.

Clinical implementation of electric impedance tomography in the treatment of ARDS: a single centre experience

To report on our clinical experience using EIT in individualized PEEP titration in ARDS. Using EIT assessment, we optimized PEEP settings in 39 ARDS patients. The EIT PEEP settings were compared with the physicians' PEEP settings and the PEEP settings according to the ARDS network. We defined a PEEP difference equal to or greater than 4 cm H₂O as clinically relevant. Changes in lung compliance and PaO₂/FiO₂-ratio were compared in patients with EIT-based PEEP adjustments and in patients with unaltered PEEP. In 28% of the patients, the difference in EIT-based PEEP and physician-PEEP was clinically relevant; in 36%, EIT-based PEEP and physician-PEEP were equal. The EIT-based PEEP disagreed with the PEEP settings according to the ARDS network. Adjusting PEEP based upon EIT led to a rapid increase in lung compliance and PaO₂/FiO₂-ratio. However, this increase was also observed in the group where the PEEP difference was less than 4 cm H₂O. We hypothesize that this can be attributed to the alveolar recruitment during the PEEP trial. EIT based individual PEEP setting appears to be a promising method to optimize PEEP in ARDS patients. The clinical impact, however, remains to be established.

Intraoperative Ventilation of Morbidly Obese Patients Guided by Transpulmonary Pressure

BACKGROUND

Bariatric surgery has proven a successful approach in the treatment of morbid obesity and its concomitant diseases such as diabetes mellitus and arterial hypertension. Aiming for optimal management of this challenging patient cohort, tailored concepts directly guided by individual patient physiology may outperform standardized care. Implying esophageal pressure measurement and electrical impedance tomography-increasingly applied monitoring approaches to individually adjust mechanical ventilation in challenging circumstances like acute respiratory distress syndrome (ARDS) and intraabdominal hypertension-we compared our institutions standard ventilator regimen with an individually adjusted positive end expiratory pressure (PEEP) level aiming for a positive transpulmonary pressure (P _L) throughout the respiratory cycle.

METHODS

After obtaining written informed consent, 37 patients scheduled for elective bariatric surgery were studied during mechanical ventilation in reverse Trendelenburg position. Before and after installation of capnoperitoneum, PEEP levels were gradually raised from a standard value of 10 cm H₂O until a P _L of 0 +/- 1 cm H₂O was reached. Changes in ventilation were monitored by electrical impedance tomography (EIT) and arterial blood gases (ABGs) were obtained at the end of surgery and 5 and 60 min after extubation, respectively.

RESULTS

To achieve the goal of a transpulmonary pressure (P _L) of 0 cm H₂O at end expiration, PEEP levels of 16.7 cm H₂O (95% KI 15.6-18.1) before and 23.8 cm H₂O (95% KI 19.6-40.4) during capnoperitoneum were necessary. EIT measurements confirmed an optimal PEEP level between 10 and 15 cm H₂O before and 20 and 25 cm H₂O during capnoperitoneum, respectively. Intra- and postoperative oxygenation did not change significantly.

CONCLUSION

Patients during laparoscopic bariatric surgery require high levels of PEEP to maintain a positive transpulmonary pressure throughout the respiratory cycle. EIT monitoring allows for non-invasive monitoring of increasing PEEP demand during capnoperitoneum. Individually adjusted PEEP levels did not result in improved postoperative oxygenation.

Optimization of Positive End-Expiratory Pressure Targeting the Best Arterial Oxygen Transport in the Acute Respiratory Distress Syndrome: The OPTIPEP Study

The optimal setting for positive end-expiratory pressure (PEEP) in mechanical ventilation remains controversial in the treatment of acute respiratory distress syndrome (ARDS). The aim of this study was to determine the optimum PEEP level in ARDS, which we defined as the level that allowed the best arterial oxygen delivery (DO2). We conducted a physiologic multicenter prospective study on patients who suffering from ARDS according to standard definition and persistent after 6 hours of ventilation. The PEEP was set to 6 cm H2O at the beginning of the test and then was increased by 2 cm H2O after at least 15 minutes of being stabilized until the plateau pressure achieved 30 cm H2O. At each step, the cardiac output was measured by transesophageal echocardiography and gas blood was sampled. We were able to determine the optimal PEEP for 12 patients. The ratio of PaO2/FiO2 at inclusion was 131 ± 40 with a mean FiO2 of 71 ± 3%. The optimal PEEP level was lower than the higher PEEP despite a constant increase in SaO2. The optimal PEEP levels varied between 8 and 18 cm H2O. Our results show that in patients with ARDS the optimal PEEP differs between each patient and require being determined with monitoring.

Does Pulmonary Artery Systolic Pressure as Estimated by Transthoracic Echocardiography Alter the Effect of Positive End-Expiratory Pressure on Arterial Blood Gases and Hemodynamics in Morbidly Obese Patients?

Background

Positive end-expiratory pressure (PEEP) at the time of induction increases oxygenation by preventing lung atelectasis. However, PEEP may not prove beneficial in all cases. Factors affecting the action of PEEP have not been elucidated well and remain controversial. Pulmonary vasculature has direct bearing on the action of PEEP as has been proven in the previous studies. Thus, this prospective study was planned to evaluate the action of PEEP on the basis of pulmonary artery systolic pressure (PASP) which is noninvasive and easily measured by transthoracic echocardiography.

Materials and Methods

Seventy morbidly obese patients, the American Society of Anesthesiologists Grade II, or III, aged 20-65 years with body mass index >40 kg/m², scheduled for elective laparoscopic bariatric surgery were included. Patients who denied consent, those undergoing emergency and/or open surgery and those requiring >2 attempts for intubation were excluded from the study. Ten patients had to be excluded. Thus, a total of sixty patients participated in the study. Thirty patients received no PEEP at the time of induction while other thirty patients were given a PEEP of 10 cm of H₂O. Serial ABG samples were taken preoperatively, at the time of intubation, 5 min after intubation, and 10 min after intubation. Patients were then divided into four groups on the basis of PASP value of ≤30 mm Hg with and without PEEP or >30 mm Hg with and without PEEP.

Primary Outcome

The primary outcome was the effect of PEEP of 10 cm of H2 O on ABG and hemodynamics in morbidly obese patients.

Secondary Outcome

The secondary outcome was the effect of PASP on the action of PEEP in morbidly obese patients undergoing laparoscopic surgery.

Results

Patients having PASP of >30 mm Hg had significant improvement in oxygenation on PEEP application (270.11 ± 119.26 mm Hg) as compared to those without PEEP (157.57 ± 109.29 mm Hg) just after intubation. The increase in oxygenation remained significant at all time intervals. Patients with PASP ≤30 mm Hg did not show significant improvement in oxygenation with PEEP application (177.09 ± 85.85 mm Hg as compared to 226.27 ± 92.42 mm Hg without PEEP). Hemodynamic parameters did not show statistically significant alterations.

Conclusion

Morbidly obese patients who have PASP >30 mm Hg benefit most from the PEEP. Thus, PASP which is an easily measurable noninvasive parameter can be used as a criterion for selecting patients who benefit from PEEP application.

What is the proper ventilation strategy during laparoscopic surgery?

The main stream of intraabdominal surgery has changed from laparotomy to laparoscopy, but anesthetic care for laparoscopic surgery is challenging for clinicians, because pneumoperitoneum might aggravate respiratory mechanics and arterial oxygenation. The authors reviewed the literature regarding ventilation strategies that reduce deleterious pulmonary physiologic changes during pneumoperitoneum for laparoscopic surgery under general anesthesia and make appropriate recommendations.

Effects of positive end-expiratory pressure and recruitment maneuvers in a ventilator-induced injury mouse model

Background

Positive-pressure mechanical ventilation is an essential therapeutic intervention, yet it causes the clinical syndrome known as ventilator-induced lung injury. Various lung protective mechanical ventilation strategies have attempted to reduce or prevent ventilator-induced lung injury but few modalities have proven effective. A model that isolates the contribution of mechanical ventilation on the development of acute lung injury is needed to better understand biologic mechanisms that lead to ventilator-induced lung injury.

Objectives

To evaluate the effects of positive end-expiratory pressure and recruitment maneuvers in reducing lung injury in a ventilator-induced lung injury murine model in short- and longer-term ventilation.

Methods

5–12 week-old female BALB/c mice (n = 85) were anesthetized, placed on mechanical ventilation for either 2 hrs or 4 hrs with either low tidal volume (8 ml/kg) or high tidal volume (15 ml/kg) with or without positive end-expiratory pressure and recruitment maneuvers.

Results

Alteration of the alveolar-capillary barrier was noted at 2 hrs of high tidal volume ventilation. Standardized histology scores, influx of bronchoalveolar lavage albumin, proinflammatory cytokines, and absolute neutrophils were significantly higher in the high-tidal volume ventilation group at 4 hours of ventilation. Application of positive end-expiratory pressure resulted in significantly decreased standardized histology scores and bronchoalveolar absolute neutrophil counts at low- and high-tidal volume ventilation, respectively. Recruitment maneuvers were essential to maintain pulmonary compliance at both 2 and 4 hrs of ventilation.

Conclusions

Signs of ventilator-induced lung injury are evident soon after high tidal volume ventilation (as early as 2 hours) and lung injury worsens with longer-term ventilation (4 hrs). Application of positive end-expiratory pressure and recruitment maneuvers are protective against worsening VILI across all time points. Dynamic compliance can be used guide the frequency of recruitment maneuvers to help ameloriate ventilator-induced lung injury.

Early application of airway pressure release ventilation may reduce the duration of mechanical ventilation in acute respiratory distress syndrome

PURPOSE

Experimental animal models of acute respiratory distress syndrome (ARDS) have shown that the updated airway pressure release ventilation (APRV) methodologies may significantly improve oxygenation, maximize lung recruitment, and attenuate lung injury, without circulatory depression. This led us to hypothesize that early application of APRV in patients with ARDS would allow pulmonary function to recover faster and would reduce the duration of mechanical ventilation as compared with low tidal volume lung protective ventilation (LTV).

METHODS

A total of 138 patients with ARDS who received mechanical ventilation for <48 h between May 2015 to October 2016 while in the critical care medicine unit (ICU) of the West China Hospital of Sichuan University were enrolled in the study. Patients were randomly assigned to receive APRV (n = 71) or LTV (n = 67). The settings for APRV were: high airway pressure (P_high) set at the last plateau airway pressure (P_plat), not to exceed 30 cmH₂O) and low airway pressure ( P_low) set at 5 cmH₂O; the release phase (T_low) setting adjusted to terminate the peak expiratory flow rate to ≥ 50%; release frequency of 10-14 cycles/min. The settings for LTV were: target tidal volume of 6 mL/kg of predicted body weight; P_plat not exceeding 30 cmH₂O; positive end-expiratory pressure (PEEP) guided by the PEEP-FiO₂ table according to the ARDSnet protocol. The primary outcome was the number of days without mechanical ventilation from enrollment to day 28. The secondary endpoints included oxygenation, P_plat, respiratory system compliance, and patient outcomes.

RESULTS

Compared with the LTV group, patients in the APRV group had a higher median number of ventilator-free days {19 [interquartile range (IQR) 8-22] vs. 2 (IQR 0-15); P < 0.001}. This finding was independent of the coexisting differences in chronic disease. The APRV group had a shorter stay in the ICU (P = 0.003). The ICU mortality rate was 19.7% in the APRV group versus 34.3% in the LTV group (P = 0.053) and was associated with better oxygenation and respiratory system compliance, lower P_plat, and less sedation requirement during the first week following enrollment (P < 0.05, repeated-measures analysis of variance).

CONCLUSIONS

Compared with LTV, early application of APRV in patients with ARDS improved oxygenation and respiratory system compliance, decreased P_plat and reduced the duration of both mechanical ventilation and ICU stay.

Transpulmonary Pressure Describes Lung Morphology During Decremental Positive End-Expiratory Pressure Trials in Obesity

OBJECTIVES

Atelectasis develops in critically ill obese patients when undergoing mechanical ventilation due to increased pleural pressure. The current study aimed to determine the relationship between transpulmonary pressure, lung mechanics, and lung morphology and to quantify the benefits of a decremental positive end-expiratory pressure trial preceded by a recruitment maneuver.

DESIGN

Prospective, crossover, nonrandomized interventional study.

SETTING

Medical and Surgical Intensive Care Units at Massachusetts General Hospital (Boston, MA) and University Animal Research Laboratory (São Paulo, Brazil).

PATIENTS/SUBJECTS

Critically ill obese patients with acute respiratory failure and anesthetized swine.

INTERVENTIONS

Clinical data from 16 mechanically ventilated critically ill obese patients were analyzed. An animal model of obesity with reversible atelectasis was developed by placing fluid filled bags on the abdomen to describe changes of lung mechanics, lung morphology, and pulmonary hemodynamics in 10 swine.

MEASUREMENTS AND MAIN RESULTS

In obese patients (body mass index, 48 ± 11 kg/m), 21.7 ± 3.7 cm H2O of positive end-expiratory pressure resulted in the lowest elastance of the respiratory system (18.6 ± 6.1 cm H2O/L) after a recruitment maneuver and decremental positive end-expiratory pressure and corresponded to a positive (2.1 ± 2.2 cm H2O) end-expiratory transpulmonary pressure. Ventilation at lowest elastance positive end-expiratory pressure preceded by a recruitment maneuver restored end-expiratory lung volume (30.4 ± 9.1 mL/kg ideal body weight) and oxygenation (273.4 ± 72.1 mm Hg). In the swine model, lung collapse and intratidal recruitment/derecruitment occurred when the positive end-expiratory transpulmonary pressure decreased below 2-4 cm H2O. After the development of atelectasis, a decremental positive end-expiratory pressure trial preceded by lung recruitment identified the positive end-expiratory pressure level (17.4 ± 2.1 cm H2O) needed to restore poorly and nonaerated lung tissue, reestablishing lung elastance and oxygenation while avoiding increased pulmonary vascular resistance.

CONCLUSIONS

In obesity, low-to-negative values of transpulmonary pressure predict lung collapse and intratidal recruitment/derecruitment. A decremental positive end-expiratory pressure trial preceded by a recruitment maneuver reverses atelectasis, improves lung mechanics, distribution of ventilation and oxygenation, and does not increase pulmonary vascular resistance.

Bedside selection of positive end-expiratory pressure by electrical impedance tomography in hypoxemic patients: a feasibility study

Background

Positive end-expiratory pressure (PEEP) is a key element of mechanical ventilation. It should optimize recruitment, without causing excessive overdistension, but controversy exists on the best method to set it. The purpose of the study was to test the feasibility of setting PEEP with electrical impedance tomography in order to prevent lung de-recruitment following a recruitment maneuver. We enrolled 16 patients undergoing mechanical ventilation with PaO₂/FiO₂ <300 mmHg. In all patients, under constant tidal volume (6–8 ml/kg) PEEP was set based on the PEEP/FiO₂ table proposed by the ARDS network (PEEP_ARDSnet). We performed a recruitment maneuver and monitored the end-expiratory lung impedance (EELI) over 10 min. If the EELI signal decreased during this period, the recruitment maneuver was repeated and PEEP increased by 2 cmH₂O. This procedure was repeated until the EELI maintained a stability over time (PEEP_EIT).

Results

The procedure was feasible in 87% patients. PEEP_EIT was higher than PEEP_ARDSnet (13 ± 3 vs. 9 ± 2 cmH₂O, p < 0.001). PaO₂/FiO₂ improved during PEEP_EIT and driving pressure decreased. Recruited volume correlated with the decrease in driving pressure but not with oxygenation improvement. Finally, regional alveolar hyperdistention and collapse was reduced in dependent lung layers and increased in non-dependent lung layers.

Conclusions

In hypoxemic patients, a PEEP selection strategy aimed at stabilizing alveolar recruitment guided by EIT at the bedside was feasible and safe. This strategy led, in comparison with the ARDSnet table, to higher PEEP, improved oxygenation and reduced driving pressure, allowing to estimate the relative weight of overdistension and recruitment.

Electronic supplementary material

The online version of this article (doi:10.1186/s13613-017-0299-9) contains supplementary material, which is available to authorized users.

Positive end-expiratory pressure: how to set it at the individual level

The positive end-expiratory pressure (PEEP), since its introduction in the treatment of acute respiratory failure, up to the 1980s was uniquely aimed to provide a viable oxygenation. Since the first application, a large debate about the criteria for selecting the PEEP levels arose within the scientific community. Lung mechanics, oxygen transport, venous admixture thresholds were all proposed, leading to PEEP recommendations from 5 up to 25 cmH₂O. Throughout this period, the main concern was the hemodynamics. This paradigm changed during the 1980s after the wide acceptance of atelectrauma as one of the leading causes of ventilator induced lung injury. Accordingly, the PEEP aim shifted from oxygenation to lung protection. In this framework, the prevention of lung opening and closing became an almost unquestioned dogma. Consequently, as PEEP keeps open the pulmonary units opened during the previous inspiratory phase, new methods were designed to identify the 'optimal' PEEP during the expiratory phase. The open lung approach requires that every collapsed unit potentially openable is opened and maintained open. The methods to assess the recruitment are based on imaging (computed tomography, electric impedance tomography, ultrasound) or mechanically-driven gas exchange modifications. All the latest assume that whatever change in respiratory system compliance is due to changes in lung compliance, which in turn is uniquely function of the recruitment. Comparative studies, however, showed that the only possible approach to measure the amount of collapsed tissue regaining inflation is the CT scan. In fact, all the other methods estimate as recruitment the gas entry in pulmonary units already open at lower PEEP, but increasing their compliance at higher PEEP. Since higher PEEP is usually more indicated (also for oxygenation) when the recruitability is higher, as occurs with increasing severity, a meaningful PEEP selection requires the assessment of recruitment. The Berlin definition may help in this assessment.

Transpulmonary pressure: importance and limits

Transpulmonary pressure (P_L) is computed as the difference between airway pressure and pleural pressure and separates the pressure delivered to the lung from the one acting on chest wall and abdomen. Pleural pressure is measured as esophageal pressure (P_ES) through dedicated catheters provided with esophageal balloons. We discuss the role of P_L in assessing the effects of mechanical ventilation in patients with acute respiratory distress syndrome (ARDS). In the supine position, directly measured P_L represents the pressure acting on the alveoli and airways. Because there is a pressure gradient in the pleural space from the non-dependent to the dependent zones, the pressure in the esophagus probably represents the pressure at a mid-level between sternal and vertebral regions. For this reason, it has been proposed to set the end-expiratory pressure in order to get a positive value of P_L. This improves oxygenation and compliance. P_L can also be estimated from airway pressure plateau and the ratio of lung to respiratory elastance (elastance-derived method). Some data suggest that this latter calculation may better estimate P_L in the nondependent lung zones, at risk for hyperinflation. Elastance-derived P_L at end-inspiration (P_Lend-insp) may be a good surrogate of end-inspiratory lung stress for the "baby lung", at least in non-obese patients. Limiting end-inspiratory P_L to 20-25 cmH₂O appears physiologically sound to mitigate ventilator-induced lung injury (VILI). Last, lung driving pressure (∆P_L) reflects the tidal distending pressure. Changes in P_L may also be assessed during assisted breathing to take into account the additive effects of spontaneous breathing and mechanical breaths on lung distension. In summary, despite limitations, assessment of P_L allows a deeper understanding of the risk of VILI and may potentially help tailor ventilator settings.

Trends in mechanical ventilation: are we ventilating our patients in the best possible way?

This review addresses how the combination of physiology, medicine and engineering principles contributed to the development and advancement of mechanical ventilation, emphasising the most urgent needs for improvement and the most promising directions of future development. Several aspects of mechanical ventilation are introduced, highlighting on one side the importance of interdisciplinary research for further development and, on the other, the importance of training physicians sufficiently on the technological aspects of modern devices to exploit properly the great complexity and potentials of this treatment.

EDUCATIONAL AIMS

To learn how mechanical ventilation developed in recent decades and to provide a better understanding of the actual technology and practice.To learn how and why interdisciplinary research and competences are necessary for providing the best ventilation treatment to patients.To understand which are the most relevant technical limitations in modern mechanical ventilators that can affect their performance in delivery of the treatment.To better understand and classify ventilation modes.To learn the classification, benefits, drawbacks and future perspectives of automatic ventilation tailoring algorithms.

Fifty Years of Research in ARDS. Setting Positive End-Expiratory Pressure in Acute Respiratory Distress Syndrome

Positive end-expiratory pressure (PEEP) has been used during mechanical ventilation since the first description of acute respiratory distress syndrome (ARDS). In the subsequent decades, many different strategies for optimally titrating PEEP have been proposed. Higher PEEP can improve arterial oxygenation, reduce tidal lung stress and strain, and promote more homogenous ventilation by preventing alveolar collapse at end expiration. However, PEEP may also cause circulatory depression and contribute to ventilator-induced lung injury through alveolar overdistention. The overall effect of PEEP is primarily related to the balance between the number of alveoli that are recruited to participate in ventilation and the amount of lung that is overdistended when PEEP is applied. Techniques to assess lung recruitment from PEEP may help to direct safer and more effective PEEP titration. Some PEEP titration strategies attempt to weigh beneficial effects on arterial oxygenation and on prevention of cyclic alveolar collapse with the harmful potential of overdistention. One method for PEEP titration is a PEEP/FiO2 table that prioritizes support for arterial oxygenation. Other methods set PEEP based on mechanical parameters, such as the plateau pressure, respiratory system compliance, or transpulmonary pressure. No single method of PEEP titration has been shown to improve clinical outcomes compared with other approaches of setting PEEP. Future trials should focus on identifying individuals who respond to higher PEEP with recruitment and on clinically important outcomes (e.g., mortality).

Endexpiratory lung volume measurement correlates with the ventilation/perfusion mismatch in lung injured pigs

Background

In acute respiratory respiratory distress syndrome (ARDS) a sustained mismatch of alveolar ventilation and perfusion (V_A/Q) impairs the pulmonary gas exchange. Measurement of endexpiratory lung volume (EELV) by multiple breath-nitrogen washout/washin is a non-invasive, bedside technology to assess pulmonary function in mechanically ventilated patients. The present study examines the association between EELV changes and V_A/Q distribution and the possibility to predict V_A/Q normalization by means of EELV in a porcine model.

Methods

After approval of the state and institutional animal care committee 12 anesthetized pigs were randomized to ARDS either by bronchoalveolar lavage (n = 6) or oleic acid injection (n = 6). EELV, V_A/Q ratios by multiple inert gas elimination and ventilation distribution by electrical impedance tomography were assessed at healthy state and at five different positive endexpiratory pressure (PEEP) steps in ARDS (0, 20, 15, 10, 5 cmH₂O; each maintained for 30 min).

Results

V_A/Q, EELV and tidal volume distribution all displayed the PEEP-induced recruitment in ARDS. We found a close correlation between V_A/Q < 0.1 (representing shunt and low V_A/Q units) and changes in EELV (spearman correlation coefficient −0.79). Logistic regression reveals the potential to predict V_A/Q normalization (V_A/Q < 0.1 less than 5%) from changes in EELV with an area under the curve of 0.89 with a 95%-CI of 0.81–0.96 in the receiver operating characteristic. Different lung injury models and recruitment characteristics did not influence these findings.

Conclusion

In a porcine ARDS model EELV measurement depicts PEEP-induced lung recruitment and is strongly associated with normalization of the V_A/Q distribution in a model-independent fashion. Determination of EELV could be an intriguing addition in the context of lung protection strategies.

Mechanical ventilation in the acute respiratory distress syndrome

The management of the acute respiratory distress syndrome (ARDS) patient is fundamental to the field of intensive care medicine, and it presents unique challenges owing to the specialized mechanical ventilation techniques that such patients require. ARDS is a highly lethal disease, and there is compelling evidence that mechanical ventilation itself, if applied in an injurious fashion, can be a contributor to ARDS mortality. Therefore, it is imperative for any clinician central to the care of ARDS patients to understand the fundamental framework that underpins the approach to mechanical ventilation in this special scenario. The current review summarizes the major components of the mechanical ventilation strategy as it applies to ARDS.

Respiratory monitoring in adult intensive care unit

INTRODUCTION

The mortality of patients with respiratory failure has steadily decreased with the advancements in protective ventilation and treatment options. Although respiratory monitoring per se has not been proven to affect the mortality of critically ill patients, it plays a crucial role in patients' care, as it helps to titrate the ventilatory support. Several new monitoring techniques have recently been made available at the bedside. The goals of monitoring comprise alerting physicians to detect the change in the patients' conditions, to improve the understanding of pathophysiology to guide the diagnosis and provide cost-effective clinical management. Areas covered: We performed a review of the recent scientific literature to provide an overview of the different methods used for respiratory monitoring in adult intensive care units, including bedside imaging techniques such as ultrasound and electrical impedance tomography. Expert commentary: Appropriate respiratory monitoring plays an important role in patients with and without respiratory failure as a guiding tool for the optimization of ventilation support, avoiding further complications and decreasing morbidity and mortality. The physician should tailor the monitoring strategy for each individual patient and know how to correctly interpret the data.

Coupling of EIT with computational lung modeling for predicting patient-specific ventilatory responses

Providing optimal personalized mechanical ventilation for patients with acute or chronic respiratory failure is still a challenge within a clinical setting for each case anew. In this article, we integrate electrical impedance tomography (EIT) monitoring into a powerful patient-specific computational lung model to create an approach for personalizing protective ventilatory treatment. The underlying computational lung model is based on a single computed tomography scan and able to predict global airflow quantities, as well as local tissue aeration and strains for any ventilation maneuver. For validation, a novel “virtual EIT” module is added to our computational lung model, allowing to simulate EIT images based on the patient's thorax geometry and the results of our numerically predicted tissue aeration. Clinically measured EIT images are not used to calibrate the computational model. Thus they provide an independent method to validate the computational predictions at high temporal resolution. The performance of this coupling approach has been tested in an example patient with acute respiratory distress syndrome. The method shows good agreement between computationally predicted and clinically measured airflow data and EIT images. These results imply that the proposed framework can be used for numerical prediction of patient-specific responses to certain therapeutic measures before applying them to an actual patient. In the long run, definition of patient-specific optimal ventilation protocols might be assisted by computational modeling.

NEW & NOTEWORTHY In this work, we present a patient-specific computational lung model that is able to predict global and local ventilatory quantities for a given patient and any selected ventilation protocol. For the first time, such a predictive lung model is equipped with a virtual electrical impedance tomography module allowing real-time validation of the computed results with the patient measurements. First promising results obtained in an acute respiratory distress syndrome patient show the potential of this approach for personalized computationally guided optimization of mechanical ventilation in future.

Global and regional assessment of sustained inflation pressure-volume curves in patients with acute respiratory distress syndrome

OBJECTIVE

Static or quasi-static pressure-volume (P-V ) curves can be used to determine the lung mechanical properties of patients suffering from acute respiratory distress syndrome (ARDS). According to the traditional interpretation, lung recruitment occurs mainly below the lower point of maximum curvature (LPMC) of the inflation P-V curve. Although some studies have questioned this assumption, setting of positive end-expiratory pressure 2 cmH₂O above the LPMC was part of a 'lung-protective' ventilation strategy successfully applied in several clinical trials. The aim of our study was to quantify the amount of unrecruited lung at different clinically relevant points of the P-V curve.

APPROACH

P-V curves and electrical impedance tomography (EIT) data from 30 ARDS patients were analysed. We determined the regional opening pressures for every EIT image pixel and fitted the global P-V curves to five sigmoid model equations to determine the LPMC, inflection point (IP) and upper point of maximal curvature (UPMC). Points of maximal curvature and IP were compared between the models by one-way analysis of variance (ANOVA). The percentages of lung pixels remaining closed ('unrecruited lung') at LPMC, IP and UPMC were calculated from the number of lung pixels exhibiting regional opening pressures higher than LPMC, IP and UPMC and were also compared by one-way ANOVA.

MAIN RESULTS

As results, we found a high variability of LPMC values among the models, a smaller variability of IP and UPMC values. We found a high percentage of unrecruited lung at LPMC, a small percentage of unrecruited lung at IP and no unrecruited lung at UPMC.

SIGNIFICANCE

Our results confirm the notion of ongoing lung recruitment at pressure levels above LPMC for all investigated model equations and highlight the importance of a regional assessment of lung recruitment in patients with ARDS.

An experimental study on the impacts of inspiratory and expiratory muscles activities during mechanical ventilation in ARDS animal model

In spite of intensive investigations, the role of spontaneous breathing (SB) activity in ARDS has not been well defined yet and little has been known about the different contribution of inspiratory or expiratory muscles activities during mechanical ventilation in patients with ARDS. In present study, oleic acid-induced beagle dogs' ARDS models were employed and ventilated with the same level of mean airway pressure. Respiratory mechanics, lung volume, gas exchange and inflammatory cytokines were measured during mechanical ventilation, and lung injury was determined histologically. As a result, for the comparable ventilator setting, preserved inspiratory muscles activity groups resulted in higher end-expiratory lung volume (EELV) and oxygenation index. In addition, less lung damage scores and lower levels of system inflammatory cytokines were revealed after 8 h of ventilation. In comparison, preserved expiratory muscles activity groups resulted in lower EELV and oxygenation index. Moreover, higher lung injury scores and inflammatory cytokines levels were observed after 8 h of ventilation. Our findings suggest that the activity of inspiratory muscles has beneficial effects, whereas that of expiratory muscles exerts adverse effects during mechanical ventilation in ARDS animal model. Therefore, for mechanically ventilated patients with ARDS, the demands for deep sedation or paralysis might be replaced by the strategy of expiratory muscles paralysis through epidural anesthesia.

A comprehensive computational human lung model incorporating inter-acinar dependencies: Application to spontaneous breathing and mechanical ventilation

In this article, we propose a comprehensive computational model of the entire respiratory system, which allows simulating patient-specific lungs under different ventilation scenarios and provides a deeper insight into local straining and stressing of pulmonary acini. We include novel 0D inter-acinar linker elements to respect the interplay between neighboring alveoli, an essential feature especially in heterogeneously distended lungs. The model is applicable to healthy and diseased patient-specific lung geometries. Presented computations in this work are based on a patient-specific lung geometry obtained from computed tomography data and composed of 60,143 conducting airways, 30,072 acini, and 140,135 inter-acinar linkers. The conducting airways start at the trachea and end before the respiratory bronchioles. The acini are connected to the conducting airways via terminal airways and to each other via inter-acinar linkers forming a fully coupled anatomically based respiratory model. Presented numerical examples include simulation of breathing during a spirometry-like test, measurement of a quasi-static pressure-volume curve using a supersyringe maneuver, and volume-controlled mechanical ventilation. The simulations show that our model incorporating inter-acinar dependencies successfully reproduces physiological results in healthy and diseased states. Moreover, within these scenarios, a deeper insight into local pressure, volume, and flow rate distribution in the human lung is investigated and discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Current Concepts of ARDS: A Narrative Review

Acute respiratory distress syndrome (ARDS) is characterized by the acute onset of pulmonary edema of non-cardiogenic origin, along with bilateral pulmonary infiltrates and reduction in respiratory system compliance. The hallmark of the syndrome is refractory hypoxemia. Despite its first description dates back in the late 1970s, a new definition has recently been proposed. However, the definition remains based on clinical characteristic. In the present review, the diagnostic workup and the pathophysiology of the syndrome will be presented. Therapeutic approaches to ARDS, including lung protective ventilation, prone positioning, neuromuscular blockade, inhaled vasodilators, corticosteroids and recruitment manoeuvres will be reviewed. We will underline how a holistic framework of respiratory and hemodynamic support should be provided to patients with ARDS, aiming to ensure adequate gas exchange by promoting lung recruitment while minimizing the risk of ventilator-induced lung injury. To do so, lung recruitability should be considered, as well as the avoidance of lung overstress by monitoring transpulmonary pressure or airway driving pressure. In the most severe cases, neuromuscular blockade, prone positioning, and extra-corporeal life support (alone or in combination) should be taken into account.

Accelerated deflation promotes homogeneous airspace liquid distribution in the edematous lung

Edematous lungs contain regions with heterogeneous alveolar flooding. Liquid is trapped in flooded alveoli by a pressure barrier-higher liquid pressure at the border than in the center of flooded alveoli-that is proportional to surface tension, T Stress is concentrated between aerated and flooded alveoli, to a degree proportional to T Mechanical ventilation, by cyclically increasing T, injuriously exacerbates stress concentrations. Overcoming the pressure barrier to redistribute liquid more homogeneously between alveoli should reduce stress concentration prevalence and ventilation injury. In isolated rat lungs, we test whether accelerated deflation can overcome the pressure barrier and catapult liquid out of flooded alveoli. We generate a local edema model with normal T by microinfusing liquid into surface alveoli. We generate a global edema model with high T by establishing hydrostatic edema, which does not alter T, and then gently ventilating the edematous lungs, which increases T at 15 cmH₂O transpulmonary pressure by 52%. Thus ventilation of globally edematous lungs increases T, which should increase stress concentrations and, with positive feedback, cause escalating ventilation injury. In the local model, when the pressure barrier is moderate, accelerated deflation causes liquid to escape from flooded alveoli and redistribute more equitably. Flooding heterogeneity tends to decrease. In the global model, accelerated deflation causes liquid escape, but-because of elevated T-the liquid jumps to nearby, aerated alveoli. Flooding heterogeneity is unaltered. In pulmonary edema with normal T, early ventilation with accelerated deflation might reduce the positive feedback mechanism through which ventilation injury increases over time.NEW & NOTEWORTHY We introduce, in the isolated rat lung, a new model of pulmonary edema with elevated surface tension. We first generate hydrostatic edema and then ventilate gently to increase surface tension. We investigate the mechanical mechanisms through which 1) ventilation injures edematous lungs and 2) ventilation with accelerated deflation might lessen ventilation injury.

The promises and problems of transpulmonary pressure measurements in acute respiratory distress syndrome

PURPOSE OF REVIEW

The optimal strategy for assessing and preventing ventilator-induced lung injury in the acute respiratory distress syndrome (ARDS) is controversial. Recent investigative efforts have focused on personalizing ventilator settings to individual respiratory mechanics. This review examines the strengths and weaknesses of using transpulmonary pressure measurements to guide ventilator management in ARDS.

RECENT FINDINGS

Recent clinical studies suggest that adjusting ventilator settings based on transpulmonary pressure measurements is feasible, may improve oxygenation, and reduce ventilator-induced lung injury.

SUMMARY

The measurement of transpulmonary pressure relies upon esophageal manometry, which requires the acceptance of several assumptions and potential errors. Notably, this includes the ability of localized esophageal pressures to represent global pleural pressure. Recent investigations demonstrated improved oxygenation in ARDS patients when positive end-expiratory pressure was adjusted to target specific end-inspiratory or end-expiratory transpulmonary pressures. However, there are different methods for estimating transpulmonary pressure and different goals for positive end-expiratory pressure titration among recent studies. More research is needed to refine techniques for the estimation and utilization of transpulmonary pressure to guide ventilator settings in ARDS patients.

Effect of tidal volume and positive end-expiratory pressure on expiratory time constants in experimental lung injury

We utilized a multicompartment model to describe the effects of changes in tidal volume (V_T) and positive end‐expiratory pressure (PEEP) on lung emptying during passive deflation before and after experimental lung injury. Expiratory time constants (τ_E) were determined by partitioning the expiratory flow–volume (V˙_EV) curve into multiple discrete segments and individually calculating τ_E for each segment. Under all conditions of PEEP and V_T, τ_E increased throughout expiration both before and after injury. Segmented τ_E values increased throughout expiration with a slope that was different than zero (P < 0. 01). On average, τ_E increased by 45.08 msec per segment. When an interaction between injury status and τ_E segment was included in the model, it was significant (P < 0.05), indicating that later segments had higher τ_E values post injury than early τ_E segments. Higher PEEP and V_T values were associated with higher τ_E values. No evidence was found for an interaction between injury status and V_T, or PEEP. The current experiment confirms previous observations that τ_E values are smaller in subjects with injured lungs when compared to controls. We are the first to demonstrate changes in the pattern of τ_E before and after injury when examined with a multiple compartment model. Finally, increases in PEEP or V_T increased τ_E throughout expiration, but did not appear to have effects that differed between the uninjured and injured state.

Transpulmonary and pleural pressure in a respiratory system model with an elastic recoiling lung and an expanding chest wall

Background

We have shown in acute lung injury patients that lung elastance can be determined by a positive end-expiratory pressure (PEEP) step procedure and proposed that this is explained by the spring-out force of the rib cage off-loading the chest wall from the lung at end-expiration. The aim of this study was to investigate the effect of the expanding chest wall on pleural pressure during PEEP inflation by building a model with an elastic recoiling lung and an expanding chest wall complex.

Methods

Test lungs with a compliance of 19, 38, or 57 ml/cmH₂O were placed in a box connected to a plastic container, 3/4 filled with water, connected to a water sack of 10 l, representing the abdomen. The space above the water surface and in the lung box constituted the pleural space. The contra-directional forces of the recoiling lung and the expanding chest wall were obtained by evacuating the pleural space to a negative pressure of 5 cmH₂O. Chest wall elastance was increased by strapping the plastic container. Pressure was measured in the airway and pleura. Changes in end-expiratory lung volume (ΔEELV), during PEEP steps of 4, 8, and 12 cmH₂O, were determined in the isolated lung, where airway equals transpulmonary pressure and in the complete model as the cumulative inspiratory-expiratory tidal volume difference. Transpulmonary pressure was calculated as airway minus pleural pressure.

Results

Lung pressure/volume curves of an isolated lung coincided with lung P/V curves in the complete model irrespective of chest wall stiffness. ΔEELV was equal to the size of the PEEP step divided by lung elastance (EL), ΔEELV = ΔPEEP/EL. The end-expiratory “pleural” pressure did not increase after PEEP inflation, and consequently, transpulmonary pressure increased as much as PEEP was increased.

Conclusions

The rib cage spring-out force causes off-loading of the chest wall from the lung and maintains a negative end-expiratory “pleural” pressure after PEEP inflation. The behavior of the respiratory system model confirms that lung elastance can be determined by a simple PEEP step without using esophageal pressure measurements.

Effect of high-frequency oscillatory ventilation on esophageal and transpulmonary pressures in moderate-to-severe acute respiratory distress syndrome

Background

High-frequency oscillatory ventilation (HFOV) has not been shown to be beneficial in the management of moderate-to-severe acute respiratory distress syndrome (ARDS). There is uncertainty about the actual pressure applied into the lung during HFOV. We therefore performed a study to compare the transpulmonary pressure (P_L) during conventional mechanical ventilation (CMV) and different levels of mean airway pressure (mPaw) during HFOV.

Methods

This is a prospective randomized crossover study in a university teaching hospital. An esophageal balloon catheter was used to measure esophageal pressures (Pes) at end inspiration and end expiration and to calculate P_L. Measurements were taken during ventilation with CMV (CMVpre) after which patients were switched to HFOV with three 1-h different levels of mPaw set at +5, +10 and +15 cm H₂O above the mean airway pressure measured during CMV. Patients were thereafter switched back to CMV (CMVpost).

Results

Ten patients with moderate-to-severe ARDS were included. We demonstrated a linear increase in Pes and P_L with the increase in mPaw during HFOV. Contrary to CMV, P_L was always positive during HFOV whatever the level of mPaw applied but not associated with improvement in oxygenation. We found significant correlations between mPaw and Pes.

Conclusion

HFOV with high level of mPaw increases transpulmonary pressures without improvement in oxygenation.