Recruitment and derecruitment during acute respiratory failure: an experimental study

Short-term effects of noisy pressure support ventilation in patients with acute hypoxemic respiratory failure

Introduction

This study aims at comparing the very short-term effects of conventional and noisy (variable) pressure support ventilation (PSV) in mechanically ventilated patients with acute hypoxemic respiratory failure.

Methods

Thirteen mechanically ventilated patients with acute hypoxemic respiratory failure were enrolled in this monocentric, randomized crossover study. Patients were mechanically ventilated with conventional and noisy PSV, for one hour each, in random sequence. Pressure support was titrated to reach tidal volumes approximately 8 mL/kg in both modes. The level of positive end-expiratory pressure and fraction of inspired oxygen were kept unchanged in both modes. The coefficient of variation of pressure support during noisy PSV was set at 30%. Gas exchange, hemodynamics, lung functional parameters, distribution of ventilation by electrical impedance tomography, breathing patterns and patient-ventilator synchrony were analyzed.

Results

Noisy PSV was not associated with any adverse event, and was well tolerated by all patients. Gas exchange, hemodynamics, respiratory mechanics and spatial distribution of ventilation did not differ significantly between conventional and noisy PSV. Noisy PSV increased the variability of tidal volume (24.4 ± 7.8% vs. 13.7 ± 9.1%, P <0.05) and was associated with a reduced number of asynchrony events compared to conventional PSV (5 (0 to 15)/30 min vs. 10 (1 to 37)/30 min, P <0.05).

Conclusions

In the very short term, noisy PSV proved safe and feasible in patients with acute hypoxemic respiratory failure. Compared to conventional PSV, noisy PSV increased the variability of tidal volumes, and was associated with improved patient-ventilator synchrony, at comparable levels of gas exchange.

Trial registration

ClinicialTrials.gov, NCT00786292

Value and limitations of transpulmonary pressure calculations during intra-abdominal hypertension

OBJECTIVE

To clarify the effect of progressively increasing intra-abdominal pressure on esophageal pressure, transpulmonary pressure, and functional residual capacity.

DESIGN

Controlled application of increased intra-abdominal pressure at two positive end-expiratory pressure levels (1 and 10 cm H2O) in an anesthetized porcine model of controlled ventilation.

SETTING

Large animal laboratory of a university-affiliated hospital.

SUBJECTS

Eleven deeply anesthetized swine (weight 46.2 ± 6.2 kg).

INTERVENTIONS

Air-regulated intra-abdominal hypertension (0-25 mm Hg).

MEASUREMENTS

Esophageal pressure, tidal compliance, bladder pressure, and end-expiratory lung aeration by gas dilution.

MAIN RESULTS

Functional residual capacity was significantly reduced by increasing intra-abdominal pressure at both positive end-expiratory pressure levels (p ≤ 0.0001) without corresponding changes of end-expiratory esophageal pressure. Above intra-abdominal pressure 5 mm Hg, plateau airway pressure increased linearly by ~ 50% of the applied intra-abdominal pressure value, associated with commensurate changes of esophageal pressure. With tidal volume held constant, negligible changes occurred in transpulmonary pressure due to intra-abdominal pressure. Driving pressures calculated from airway pressures alone (plateau airway pressure--positive end-expiratory pressure) did not equate to those computed from transpulmonary pressure (tidal changes in transpulmonary pressure). Increasing positive end-expiratory pressure shifted the predominantly negative end-expiratory transpulmonary pressure at positive end-expiratory pressure 1 cm H2O (mean -3.5 ± 0.4 cm H2O) into the positive range at positive end-expiratory pressure 10 cm H2O (mean 0.58 ± 1.2 cm H2O).

CONCLUSIONS

Despite its insensitivity to changes in functional residual capacity, measuring transpulmonary pressure may be helpful in explaining how different levels of positive end-expiratory pressure influence recruitment and collapse during tidal ventilation in the presence of increased intra-abdominal pressure and in calculating true transpulmonary driving pressure (tidal changes of transpulmonary pressure). Traditional interpretations of respiratory mechanics based on unmodified airway pressure were misleading regarding lung behavior in this setting.

Pleural pressure and optimal positive end-expiratory pressure based on esophageal pressure versus chest wall elastance: incompatible results*

OBJECTIVES

1) To compare two published methods for estimating pleural pressure, one based on directly measured esophageal pressure and the other based on chest wall elastance. 2) To evaluate the agreement between two published positive end-expiratory pressure optimization strategies based on these methods, one targeting an end-expiratory esophageal pressure-based transpulmonary pressure of 0 cm H2O and the other targeting an end-inspiratory elastance-based transpulmonary pressure of 26 cm H2O.

DESIGN

Retrospective study using clinical data.

SETTING

Medical and surgical ICUs.

PATIENTS

Sixty-four patients mechanically ventilated for acute respiratory failure with esophageal balloons placed for clinical management.

METHODS

Esophageal pressure and chest wall elastance-based methods for estimating pleural pressure and setting positive end-expiratory pressure were retrospectively applied to each of the 64 patients. In patients who were ventilated at two positive end-expiratory pressure levels, chest wall and respiratory system elastances were calculated at each positive end-expiratory pressure level.

MEASUREMENTS AND MAIN RESULTS

The pleural pressure estimates using both methods were discordant and differed by as much as 10 cm H2O for a given patient. The two positive end-expiratory pressure optimization strategies recommended positive end-expiratory pressure changes in opposite directions in 33% of patients. The ideal positive end-expiratory pressure levels recommended by the two methods for each patient were discordant and uncorrelated (R = 0.05). Chest wall and respiratory system elastances grew with increases in positive end-expiratory pressure in patients with positive end-expiratory esophageal pressure-based transpulmonary pressures (p < 0.05).

CONCLUSIONS

Esophageal pressure and chest wall elastance-based methods for estimating pleural pressure do not yield similar results. The strategies of targeting an end-expiratory esophageal pressure-based transpulmonary pressure of 0 cm H2O and targeting an end-inspiratory elastance-based transpulmonary pressure of 26 cm H2O cannot be considered interchangeable. Finally, chest wall and respiratory system elastances may vary unpredictably with changes in positive end-expiratory pressure.

Assessing the Progression of Ventilator-Induced Lung Injury in Mice

Patients with acute respiratory distress syndrome receiving mechanical ventilation typically experience repetitive closure (derecruitment) and subsequent reopening (recruitment) of airways and alveoli. This can lead, over time, to further ventilator-induced lung injury (VILI). Recruitment and derecruitment (R/D) thus reflect both the current level of lung injury and the risk for sustaining further injury. Accordingly, we investigated how the dynamics of R/D are altered as VILI develops following application of high tidal volume ventilation in initially healthy mice. R/D occurring on subsecond timescales was assessed from the shape of the pressure-volume ( PV) loop measured during a single large breath. R/D occurring on a timescale of minutes was evaluated via a derecruitability test in which we tracked the progressive increases in lung elastance occurring during periods of mechanical ventilation immediately following a recruitment maneuver. The degrees of R/D occurring on these different times scales were strongly correlated. To interpret these findings in quantitative terms, we developed a computational model of the lung in which changes in lung volume occurred both via R/D and distention of already open lung units. Fitting this model to measured PV loops indicates that VILI causes R/D both to increase and to occur at progressively higher pressures, and that the lung tissue that remains open during the breath becomes progressively more overdistended. We conclude that the dynamic PV loop in conjunction with our computational model can be used to assess the current injury state of the lung as well as its likelihood of sustaining further VILI.

Pleural effusion in patients with acute lung injury: a CT scan study

OBJECTIVES

Pleural effusion is a frequent finding in patients with acute respiratory distress syndrome. To assess the effects of pleural effusion in patients with acute lung injury on lung volume, respiratory mechanics, gas exchange, lung recruitability, and response to positive end-expiratory pressure.

DESIGN, SETTING, AND PATIENTS

A total of 129 acute lung injury or acute respiratory distress syndrome patients, 68 analyzed retrospectively and 61 prospectively, studied at two University Hospitals.

INTERVENTIONS

Whole-lung CT was performed during two breath-holding pressures (5 and 45 cm H2O). Two levels of positive end-expiratory pressure (5 and 15 cm H2O) were randomly applied.

MEASUREMENTS

Pleural effusion volume was determined on each CT scan section; respiratory system mechanics, gas exchange, and hemodynamics were measured at 5 and 15 cm H2O positive end-expiratory pressure. In 60 patients, elastances of lung and chest wall were computed, and lung and chest wall displacements were estimated.

RESULTS

Patients were divided into higher and lower pleural effusion groups according to the median value (287 mL). Patients with higher pleural effusion were older (62±16 yr vs. 54±17 yr, p<0.01) with a lower minute ventilation (8.8±2.2 L/min vs. 10.1±2.9 L/min, p<0.01) and respiratory rate (16±5 bpm vs. 19±6 bpm, p<0.01) than those with lower pleural effusion. Both at 5 and 15 cm H2O of positive end-expiratory pressure PaO2/FIO2, respiratory system elastance, lung weight, normally aerated tissue, collapsed tissue, and lung and chest wall elastances were similar between the two groups. The thoracic cage expansion (405±172 mL vs. 80±87 mL, p<0.0001, for higher pleural effusion group vs. lower pleural effusion group) was greater than the estimated lung compression (178±124 mL vs. 23±29 mL, p<0.0001 for higher pleural effusion group vs. lower pleural effusion group, respectively).

CONCLUSIONS

Pleural effusion in acute lung injury or acute respiratory distress syndrome patients is of modest entity and leads to a greater chest wall expansion than lung reduction, without affecting gas exchange or respiratory mechanics.

Interaction of dependent and non-dependent regions of the acutely injured lung during a stepwise recruitment manoeuvre

The benefit of treating acute lung injury with recruitment manoeuvres is controversial. An impediment to settling this debate is the difficulty in visualizing how distinct lung regions respond to the manoeuvre. Here, regional lung mechanics were studied by electrical impedance tomography (EIT) during a stepwise recruitment manoeuvre in a porcine model with acute lung injury. The following interaction between dependent and non-dependent regions consistently occurred: atelectasis in the most dependent region was reversed only after the non-dependent region became overdistended. EIT estimates of overdistension and atelectasis were validated by histological examination of lung tissue, confirming that the dependent region was primarily atelectatic and the non-dependent region was primarily overdistended. The pulmonary pressure-volume equation, originally designed for modelling measurements at the airway opening, was adapted for EIT-based regional estimates of overdistension and atelectasis. The adaptation accurately modelled the regional EIT data from dependent and non-dependent regions (R(2) > 0.93, P < 0.0001) and predicted their interaction during recruitment. In conclusion, EIT imaging of regional lung mechanics reveals that overdistension in the non-dependent region precedes atelectasis reversal in the dependent region during a stepwise recruitment manoeuvre.

Clinical review: Respiratory monitoring in the ICU - a consensus of 16

Monitoring plays an important role in the current management of patients with acute respiratory failure but sometimes lacks definition regarding which 'signals' and 'derived variables' should be prioritized as well as specifics related to timing (continuous versus intermittent) and modality (static versus dynamic). Many new techniques of respiratory monitoring have been made available for clinical use recently, but their place is not always well defined. Appropriate use of available monitoring techniques and correct interpretation of the data provided can help improve our understanding of the disease processes involved and the effects of clinical interventions. In this consensus paper, we provide an overview of the important parameters that can and should be monitored in the critically ill patient with respiratory failure and discuss how the data provided can impact on clinical management.

Comparative effects of proportional assist and variable pressure support ventilation on lung function and damage in experimental lung injury

OBJECTIVE

To investigate the effects of proportional assist ventilation, variable pressure support, and conventional pressure support ventilation on lung function and damage in experimental acute lung injury.

DESIGN

: Randomized experimental study.

SETTING

University hospital research facility.

SUBJECTS

: Twenty-four juvenile pigs.

INTERVENTIONS

Pigs were anesthetized, intubated, and mechanically ventilated. Acute lung injury was induced by saline lung lavage. After resuming of spontaneous breathing, animals were randomly assigned to 6 hrs of assisted ventilation with pressure support ventilation, proportional assist ventilation, or variable pressure support (n = 8 per group). Mean tidal volume was kept at ≈6 mL/kg in all modes.

MEASUREMENTS AND MAIN RESULTS

Lung functional parameters, distribution of ventilation by electrical impedance tomography, and breathing patterns were analyzed. Histological lung damage and pulmonary inflammatory response were determined postmortem. Variable -pressure support and proportional assist ventilation improved oxygenation and venous admixture compared with pressure support ventilation. Proportional assist ventilation led to higher esophageal pressure time product than variable pressure support and pressure support ventilation, and redistributed ventilation from central to dorsal lung regions compared to pressure support ventilation. Variable pressure support and proportional assist ventilation yielded higher tidal volume variability than pressure support ventilation. Such pattern was deterministic (self-organized) during proportional assist ventilation and stochastic (random) during variable pressure support. Subject-ventilator synchrony as well as pulmonary inflammatory response and damage did not differ among groups.

CONCLUSIONS

In a lung lavage model of acute lung injury, both variable pressure support and proportional assist ventilation increased the variability of tidal volume and improved oxygenation and venous admixture, without influencing subject-ventilator synchrony or affecting lung injury compared with pressure support ventilation. However, variable pressure support yielded less inspiratory effort than proportional assist ventilation at comparable mean tidal volumes of 6 mL/kg.

Linking the development of ventilator-induced injury to mechanical function in the lung

Management of ALI/ARDS involves supportive ventilation at low tidal volumes (V t) to minimize the rate at which ventilator induced lung injury (VILI) develops while the lungs heal. However, we currently have few details to guide the minimization of VILI in the ALI/ARDS patient. The goal of the present study was to determine how VILI progresses with time as a function of the manner in which the lung is ventilated in mice. We found that the progression of VILI caused by over-ventilating the lung at a positive end-expiratory pressure of zero is accompanied by progressive increases in lung stiffness as well as the rate at which the lung derecruits over time. We were able to accurately recapitulate these findings in a computational model that attributes changes in the dynamics of recruitment and derecruitment to two populations of lung units. One population closes over a time scale of minutes following a recruitment maneuver and the second closes in a matter of seconds or less, with the relative sizes of the two populations changing as VILI develops. This computational model serves as a basis from which to link the progression of VILI to changes in lung mechanical function.

Relationship between regional lung compliance and ventilation homogeneity in the supine and prone position

BACKGROUND

The prone position (PP) improves ventilation homogeneity in acute respiratory distress syndrome. The aim of this study was to investigate whether the alleviation of ventilation inhomogeneity in PP was due to changes in regional lung compliance.

METHODS

Ten lung-lavaged piglets were mechanically ventilated in supine position (SP) and in PP. In each position, positive end-expiratory pressure (PEEP) was reduced from 20 to 6 cmH(2)O in steps of 2 cmH(2)O every 10 min after full lung recruitment. Respiratory mechanics, blood gas, haemodynamic data and whole-lung computed tomography scans were recorded at each PEEP. The compliances of normally aerated (C(normal)) and newly recruited (C(recruited)) lung regions were calculated. Open lung PEEP (OL-PEEP) was defined as the lowest PEEP to maintain full lung recruitment.

RESULTS

At OL-PEEP, PP significantly increased normally aerated lung regions, decreased poorly aerated and hyperinflated lung regions and decreased tidal recruitment and hyperinflation. C(normal) was significantly reduced in PP compared with SP (12.8 ± 4.2 ml/cmH(2)O vs. 20.1 ± 6.2 ml/cmH(2)O, P < 0.001), whereas C(recruited) was increased in PP (13.9 ± 3.9 ml/cmH(2)O vs. 9.4 ± 2.4 ml/cmH(2)O, P < 0.001). C(normal) was correlated with hyperinflated lung regions at end-expiration (rho = 0.67) and end-inspiration (rho = 0.56) at OL-PEEP. C(recruited) was correlated with normally (r(2) = 0.36) and poorly aerated lung regions (rho = -0.58) at OL-PEEP.

CONCLUSION

This surfactant-depleted model shows that the improvement of ventilation homogeneity in PP is related to an increase in C(recruited) and a decrease in C(normal).

Physiological relevance and performance of a minimal lung model: an experimental study in healthy and acute respiratory distress syndrome model piglets

Background

Mechanical ventilation (MV) is the primary form of support for acute respiratory distress syndrome (ARDS) patients. However, intra- and inter- patient-variability reduce the efficacy of general protocols. Model-based approaches to guide MV can be patient-specific. A physiological relevant minimal model and its patient-specific performance are tested to see if it meets this objective above.

Methods

Healthy anesthetized piglets weighing 24.0 kg [IQR: 21.0-29.6] underwent a step-wise PEEP increase manoeuvre from 5cmH₂O to 20cmH₂O. They were ventilated under volume control using Engström Care Station (Datex, General Electric, Finland), with pressure, flow and volume profiles recorded. ARDS was then induced using oleic acid. The data were analyzed with a Minimal Model that identifies patient-specific mean threshold opening and closing pressure (TOP and TCP), and standard deviation (SD) of these TOP and TCP distributions. The trial and use of data were approved by the Ethics Committee of the Medical Faculty of the University of Liege, Belgium.

Results and discussions

3 of the 9 healthy piglets developed ARDS, and these data sets were included in this study. Model fitting error during inflation and deflation, in healthy or ARDS state is less than 5.0% across all subjects, indicating that the model captures the fundamental lung mechanics during PEEP increase. Mean TOP was 42.4cmH₂O [IQR: 38.2-44.6] at PEEP = 5cmH₂O and decreased with PEEP to 25.0cmH₂O [IQR: 21.5-27.1] at PEEP = 20cmH₂O. In contrast, TCP sees a reverse trend, increasing from 10.2cmH₂O [IQR: 9.0-10.4] to 19.5cmH₂O [IQR: 19.0-19.7]. Mean TOP increased from average 21.2-37.4cmH₂O to 30.4-55.2cmH₂O between healthy and ARDS subjects, reflecting the higher pressure required to recruit collapsed alveoli. Mean TCP was effectively unchanged.

Conclusion

The minimal model is capable of capturing physiologically relevant TOP, TCP and SD of both healthy and ARDS lungs. The model is able to track disease progression and the response to treatment.

Goal-directed mechanical ventilation: are we aiming at the right goals? A proposal for an alternative approach aiming at optimal lung compliance, guided by esophageal pressure in acute respiratory failure

Patients with acute respiratory failure and decreased respiratory system compliance due to ARDS frequently present a formidable challenge. These patients are often subjected to high inspiratory pressure, and in severe cases in order to improve oxygenation and preserve life, we may need to resort to unconventional measures. The currently accepted ARDSNet guidelines are characterized by a generalized approach in which an algorithm for PEEP application and limited plateau pressure are applied to all mechanically ventilated patients. These guidelines do not make any distinction between patients, who may have different chest wall mechanics with diverse pathologies and different mechanical properties of their respiratory system. The ability of assessing pleural pressure by measuring esophageal pressure allows us to partition the respiratory system into its main components of lungs and chest wall. Thus, identifying the dominant factor affecting respiratory system may better direct and optimize mechanical ventilation. Instead of limiting inspiratory pressure by plateau pressure, PEEP and inspiratory pressure adjustment would be individualized specifically for each patient's lung compliance as indicated by transpulmonary pressure. The main goal of this approach is to specifically target transpulmonary pressure instead of plateau pressure, and therefore achieve the best lung compliance with the least transpulmonary pressure possible.

Effect of regional lung inflation on ventilation heterogeneity at different length scales during mechanical ventilation of normal sheep lungs

Heterogeneous, small-airway diameters and alveolar derecruitment in poorly aerated regions of normal lungs could produce ventilation heterogeneity at those anatomic levels. We modeled the washout kinetics of (13)NN with positron emission tomography to examine how specific ventilation (sV) heterogeneity at different length scales is influenced by lung aeration. Three groups of anesthetized, supine sheep were studied: high tidal volume (Vt; 18.4 ± 4.2 ml/kg) and zero end-expiratory pressure (ZEEP) (n = 6); low Vt (9.2 ± 1.0 ml/kg) and ZEEP (n = 6); and low Vt (8.2 ± 0.2 ml/kg) and positive end-expiratory pressure (PEEP; 19 ± 1 cmH(2)O) (n = 4). We quantified fractional gas content with transmission scans, and sV with emission scans of infused (13)NN-saline. Voxel (13)NN-washout curves were fit with one- or two-compartment models to estimate sV. Total heterogeneity, measured as SD[log(10)(sV)], was divided into length-scale ranges by measuring changes in variance of log(10)(sV), resulting from progressive filtering of sV images. High-Vt ZEEP showed higher sV heterogeneity at <12- (P < 0.01), 12- to 36- (P < 0.01), and 36- to 60-mm (P < 0.05) length scales compared with low-Vt PEEP, with low-Vt ZEEP in between. Increased heterogeneity was associated with the emergence of low sV units in poorly aerated regions, with a high correlation (r = 0.95, P < 0.001) between total heterogeneity and the fraction of lung with slow washout. Regional mean fractional gas content was inversely correlated with regional sV heterogeneity at <12- (r = -0.67), 12- to 36- (r = -0.74), and >36-mm (r = -0.72) length scales (P < 0.001). We conclude that sV heterogeneity at length scales <60 mm increases in poorly aerated regions of mechanically ventilated normal lungs, likely due to heterogeneous small-airway narrowing and alveolar derecruitment. PEEP reduces sV heterogeneity by maintaining lung expansion and airway patency at those small length scales.

[Positive end-expiratory pressure : adjustment in acute lung injury]

Treatment of patients suffering from acute lung injury is a challenge for the treating physician. In recent years ventilation of patients with acute hypoxic lung injury has changed fundamentally. Besides the use of low tidal volumes, the most beneficial setting of positive end-expiratory pressure (PEEP) has been in the focus of researchers. The findings allow adaption of treatment to milder forms of acute lung injury and severe forms. Additionally computed tomography techniques to assess the pulmonary situation and recruitment potential as well as bed-side techniques to adjust PEEP on the ward have been modified and improved. This review gives an outline of recent developments in PEEP adjustment for patients suffering from acute hypoxic and hypercapnic lung injury and explains the fundamental pathophysiology necessary as a basis for correct treatment.

Pathophysiology of ventilator-associated lung injury

PURPOSE OF REVIEW

Mechanical ventilation is essential for the support of critically ill patients, but may aggravate lung damage, leading to ventilator-associated lung injury (VALI). VALI results from a succession of events beginning with mechanical alteration of lung parenchyma, because of disproportionate stress and strain. The resulting structural tension initiates a biological inflammatory cascade; however, tension can reach the limits of stress, triggering the destruction of structures. This article reviews and discusses the ongoing research into the mechanisms of VALI and their implications for the management of ventilated patients.

RECENT FINDINGS

Several experimental and clinical studies have been performed to evaluate the contribution of pathogenic mechanical forces to organ and cellular deformation and the implications for guiding ventilator management in patients at risk for VALI. VALI may be attenuated by reducing tidal volume, but the key variable in determining pulmonary overdistension is transpulmonary pressure. Other parameters associated with the induction of VALI include positive end-expiratory pressure, inspiratory airflow and time, and respiratory frequency.

SUMMARY

How ventilation strategy, specific mechanisms of mechanotransduction, and their individual threshold values impact on VALI remains to be elucidated. In addition, clinical studies are required to evaluate the usefulness of individualized ventilator strategies based on lung mechanics.

Lung elastance and transpulmonary pressure can be determined without using oesophageal pressure measurements

INTRODUCTION

The aim of the present study was to demonstrate that lung elastance and transpulmonary pressure can be determined without using oesophageal pressure measurements.

METHODS

Studies were performed on 13 anesthetized and sacrificed ex vivo pigs. Tracheal and oesophageal pressures were measured and changes in end-expiratory lung volume (ΔEELV) determined by spirometry as the cumulative inspiratory-expiratory tidal volume difference. Studies were performed with different end-expiratory pressure steps [change in end-expiratory airway pressure (ΔPEEP)], body positions and with abdominal load.

RESULTS

A PEEP increase results in a multi-breath build-up of end-expiratory lung volume. End-expiratory oesophageal pressure did not increase further after the first expiration, constituting half of the change in ΔEELV following a PEEP increase, even though end-expiratory volume continued to increase. This resulted in a successive left shift of the chest wall pressure-volume curve. Even at a PEEP of 12 cmH(2) O did the end-expiratory oesophageal (pleural) pressure remain negative.

CONCLUSIONS

A PEEP increase resulted in a less than expected increase in end-expiratory oesophageal pressure, indicating that the chest wall and abdomen gradually can accommodate changes in lung volume. The rib cage end-expiratory spring-out force stretches the diaphragm and prevents the lung from being compressed by abdominal pressure. The increase in transpulmonary pressure following a PEEP increase was closely related to the increase in PEEP, indicating that lung compliance can be calculated from the ratio of the change in end-expiratory lung volume and the change in PEEP, ΔEELV/ΔPEEP.

Relationship between Abdominal Pressure, Pulmonary Compliance, and Cardiac Preload in a Porcine Model

Rationale. Elevated intra-abdominal pressure (IAP) may compromise respiratory and cardiovascular function by abdomino-thoracic pressure transmission. We aimed (1) to study the effects of elevated IAP on pleural pressure, (2) to understand the implications for lung and chest wall compliances and (3) to determine whether volumetric filling parameters may be more accurate than classical pressure-based filling pressures for preload assessment in the setting of elevated IAP. Methods. In eleven pigs, IAP was increased stepwise from 6 to 30 mmHg. Hemodynamic, esophageal, and pulmonary pressures were recorded. Results. 17% (end-expiratory) to 62% (end-inspiratory) of elevated IAP was transmitted to the thoracic compartment. Respiratory system compliance decreased significantly with elevated IAP and chest wall compliance decreased. Central venous and pulmonary wedge pressure increased with increasing IAP and correlated inversely (r = -0.31) with stroke index (SI). Global end-diastolic volume index was unaffected by IAP and correlated best with SI (r = 0.52). Conclusions. Increased IAP is transferred to the thoracic compartment and results in a decreased respiratory system compliance due to decreased chest wall compliance. Volumetric filling parameters and transmural filling pressures are clearly superior to classical cardiac filling pressures in the assessment of cardiac preload during elevated IAP.

Large-animal models of acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is characterized by an acute inflammatory response that compromises alveolar-capillary membrane integrity. Clinical symptoms include refractory hypoxemia, noncardiogenic edema, and decreased lung compliance. The purpose of this review is to summarize the different ARDS large-animal models in terms of similarity to the clinical disease and underlying pathophysiology. The repeated lavage, oleic acid, endotoxin, and smoke/burn ARDS models will be discussed in this review. While each model has significant benefits, none is without weaknesses. Thus, the choice of large-animal ARDS model must be carefully considered based upon the study focus and investigative team experience.

Pulmonary mechanics during mechanical ventilation

The use of mechanical ventilation has become widespread in the management of hypoxic respiratory failure. Investigations of pulmonary mechanics in this clinical scenario have demonstrated that there are significant differences in compliance, resistance and gas flow when compared with normal subjects. This paper will review the mechanisms by which pulmonary mechanics are assessed in mechanically ventilated patients and will review how the data can be used for investigative research purposes as well as to inform rational ventilator management.

Extrapolation in the analysis of lung aeration by computed tomography: a validation study

INTRODUCTION

Computed tomography (CT) is considered the gold standard for quantification of global or regional lung aeration and lung mass. Quantitative CT, however, involves the exposure to ionizing radiation and requires manual image processing. We recently evaluated an extrapolation method which calculates quantitative CT parameters characterizing the entire lung from only 10 reference CT-slices thereby reducing radiation exposure and analysis time. We hypothesized that this extrapolation method could be further validated using CT-data from pigs and sheep, which have a different thoracic anatomy.

METHODS

We quantified volume and mass of the total lung and differently aerated lung compartments in 168 ovine and 55 porcine whole-lung CTs covering lung conditions from normal to gross deaeration. Extrapolated volume and mass parameters were compared to the respective values obtained by whole-lung analysis. We also tested the accuracy of extrapolation for all possible numbers of CT slices between 15 and 5. Bias and limits of agreement (LOA) were analyzed by the Bland-Altman method.

RESULTS

For extrapolation from 10 reference slices, bias (LOA) for the total lung volume and mass of sheep were 18.4 (-57.2 to 94.0) ml and 4.2 (-21.8 to 30.2) grams, respectively. The corresponding bias (LOA) values for pigs were 5.1 (-55.2 to 65.3) ml and 1.6 (-32.9 to 36.2) grams, respectively. All bias values for differently aerated lung compartments were below 1% of the total lung volume or mass and the LOA never exceeded ± 2.5%. Bias values diverged from zero and the LOA became considerably wider when less than 10 reference slices were used.

CONCLUSIONS

The extrapolation method appears robust against variations in thoracic anatomy, which further supports its accuracy and potential usefulness for clinical and experimental application of quantitative CT.

Use of computed tomography scanning to guide lung recruitment and adjust positive-end expiratory pressure

PURPOSE OF REVIEW

We discuss the possible role of computed tomography (CT) to guide protective mechanical ventilation in acute lung injury/acute respiratory distress syndrome (ALI/ARDS), especially tidal volume (VT) and positive-end expiratory pressure (PEEP) settings and recruitment manoeuvres.

RECENT FINDINGS

CT should be used as early as possible after the onset of ALI/ARDS and then repeated after 1 week in the absence of clinical improvement. Advantages of CT include: the regional response to recruitment can be determined; it is objective; the morphofunctional correlations obtained are useful for a comprehensive patient evaluation. CT should be performed at different pressure levels to identify potential for recruitment. Initially, one single whole-lung CT scan is performed at end-expiration at PEEP 5-10 cmH2O to evaluate aeration and compute lung weight. Afterwards, two lung CT slices are performed to assess lung recruitability (at PEEP = 5-10 cmH2O; inspiratory plateau pressure of the respiratory system = 45 cmH2O).

SUMMARY

In ALI/ARDS patients, CT reveals discrepancies between bedside chest radiograph and various clinical and physiological parameters, and it is essential to assess lung morphology and recruitability. Specific algorithms, including or not CT, should be used to better identify ALI/ARDS with potential of recruitment and setting of VT and PEEP.

Open lung approach vs acute respiratory distress syndrome network ventilation in experimental acute lung injury

Background

Setting and strategies of mechanical ventilation with positive end-expiratory pressure (PEEP) in acute lung injury (ALI) remains controversial. This study compares the effects between lung-protective mechanical ventilation according to the Acute Respiratory Distress Syndrome Network recommendations (ARDSnet) and the open lung approach (OLA) on pulmonary function and inflammatory response.

Methods

Eighteen juvenile pigs were anaesthetized, mechanically ventilated, and instrumented. ALI was induced by surfactant washout. Animals were randomly assigned to mechanical ventilation according to the ARDSnet protocol or the OLA (n=9 per group). Gas exchange, haemodynamics, pulmonary blood flow (PBF) distribution, and respiratory mechanics were measured at intervals and the lungs were removed after 6 h of mechanical ventilation for further analysis.

Results

PEEP and mean airway pressure were higher in the OLA than in the ARDSnet group [15 cmH₂O, range 14–18 cmH₂O, compared with 12 cmH₂O; 20.5 (sd 2.3) compared with 18 (1.4) cmH₂O by the end of the experiment, respectively], and OLA was associated with improved oxygenation compared with the ARDSnet group after 6 h. OLA showed more alveolar overdistension, especially in gravitationally non-dependent regions, while the ARDSnet group was associated with more intra-alveolar haemorrhage. Inflammatory mediators and markers of lung parenchymal stress did not differ significantly between groups. The PBF shifted from ventral to dorsal during OLA compared with ARDSnet protocol [−0.02 (−0.09 to −0.01) compared with −0.08 (−0.12 to −0.06), dorsal–ventral gradients after 6 h, respectively].

Conclusions

According to the OLA, mechanical ventilation improved oxygenation and redistributed pulmonary perfusion when compared with the ARDSnet protocol, without differences in lung inflammatory response.

Chest wall mechanics and abdominal pressure during general anaesthesia in normal and obese individuals and in acute lung injury

PURPOSE OF REVIEW

This article discusses the methods available to evaluate chest wall mechanics and the relationship between intraabdominal pressure (IAP) and chest wall mechanics during general anaesthesia in normal and obese individuals, as well as in acute lung injury/acute respiratory distress syndrome.

RECENT FINDINGS

The interactions between the abdominal and thoracic compartments pose a specific challenge for intensive care physicians. IAP affects respiratory system, lung and chest wall elastance in an unpredictable way. Thus, transpulmonary pressure should be measured if IAP is more than 12 mmHg or if chest wall elastance is compromised for other reasons, even though the absolute values of pleural and transpulmonary pressures are not easily obtained at bedside. We suggest defining intraabdominal hypertension (IAH) as IAP at least 20 mmHg and abdominal compartment syndrome (ACS) as IAP at least 20 mmHg associated with failure of one or more organs, although further studies are required to confirm this hypothesis. Additionally, in the presence of IAH, controlled mechanical ventilation should be applied and positive end-expiratory pressure individually titrated. Prophylactic open abdomen should be considered in the presence of ACS.

SUMMARY

Increased IAP markedly affects respiratory function and complicates patient management. Frequent assessment of IAP is recommended.

Distribution of tidal ventilation during volume-targeted ventilation is variable and influenced by age in the preterm lung

PURPOSE

Synchronised volume-targeted ventilation (SIPPV + VTV) attempts to reduce lung injury by standardising volume delivery to the preterm lung. The aim of this study is to describe the regional distribution and variability of ventilation within the preterm lung during SIPPV + VTV.

METHODS

Twenty-seven stable, supine, preterm infants with <32 weeks gestation receiving SIPPV + VTV were studied. From each infant, the anterior-to-posterior impedance change due to tidal ventilation (∆Z (VT); countless units) was determined during every breath from three, 30-s, electrical impedance tomography recordings. ∆Z (VT) within the anterior, middle and posterior thirds of the chest were compared using area under the curve analysis. The coefficient of variation (CV) of ∆Z (VT) in the anterior and posterior hemithoraces, inflation pressure and, where available, V (T) at airway opening were compared. Infants were sub-grouped by age (≤7 and >7 days), supplemental oxygen requirement and set tidal volume.

RESULTS

In all sub-groups, the middle third of the chest accounted for the greatest ∆Z (VT) [p < 0.0001, repeated-measures analysis of variance (ANOVA)]. The middle third of the chest constituted a greater relative ∆Z (VT) in infants aged >7 days compared with ≤7 days (p < 0.0001, repeated-measures ANOVA). Set tidal volume and oxygen requirement did not significantly influence the regional distribution of ∆Z (VT). The mean (standard deviation, SD) CV of ∆Z (VTANT) and ∆Z (VTPOST) were 30.6% (14.0%) and 31.9% (12.7%). ∆Z (VTANT) and ∆Z (VTPOST) expressed greater breath-to-breath variability than the variation in inflation pressure and V (T) at airway opening (p = 0.012 and p < 0.0001, respectively, paired t-tests).

CONCLUSION

During SIPPV + VTV the preterm infant exhibits marked breath-to-breath variability in regional ventilation which is influenced by age.

Quantitative imaging of alveolar recruitment with hyperpolarized gas MRI during mechanical ventilation

The aim of this study was to assess the utility of (3)He MRI to noninvasively probe the effects of positive end-expiratory pressure (PEEP) maneuvers on alveolar recruitment and atelectasis buildup in mechanically ventilated animals. Sprague-Dawley rats (n = 13) were anesthetized, intubated, and ventilated in the supine position ((4)He-to-O(2) ratio: 4:1; tidal volume: 10 ml/kg, 60 breaths/min, and inspiration-to-expiration ratio: 1:2). Recruitment maneuvers consisted of either a stepwise increase of PEEP to 9 cmH(2)O and back to zero end-expiratory pressure or alternating between these two PEEP levels. Diffusion MRI was performed to image (3)He apparent diffusion coefficient (ADC) maps in the middle coronal slices of lungs (n = 10). ADC was measured immediately before and after two recruitment maneuvers, which were separated from each other with a wait period (8-44 min). We detected a statistically significant decrease in mean ADC after each recruitment maneuver. The relative ADC change was -21.2 ± 4.1 % after the first maneuver and -9.7 ± 5.8 % after the second maneuver. A significant relative increase in mean ADC was observed over the wait period between the two recruitment maneuvers. The extent of this ADC buildup was time dependent, as it was significantly related to the duration of the wait period. The two postrecruitment ADC measurements were similar, suggesting that the lungs returned to the same state after the recruitment maneuvers were applied. No significant intrasubject differences in ADC were observed between the corresponding PEEP levels in two rats that underwent three repeat maneuvers. Airway pressure tracings were recorded in separate rats undergoing one PEEP maneuver (n = 3) and showed a significant relative difference in peak inspiratory pressure between pre- and poststates. These observations support the hypothesis of redistribution of alveolar gas due to recruitment of collapsed alveoli in presence of atelectasis, which was also supported by the decrease in peak inspiratory pressure after recruitment maneuvers.

Ventilator-induced lung injury: the anatomical and physiological framework

Since its introduction into the management of the acute respiratory distress syndrome, mechanical ventilation has been so strongly interwoven with its side effects that it came to be considered as invariably dangerous. Over the decades, attention has shifted from gross barotrauma to volutrauma and, more recently, to atelectrauma and biotrauma. In this article, we describe the anatomical and physiologic framework in which ventilator-induced lung injury may occur. We address the concept of lung stress/strain as applied to the whole lung or specific pulmonary regions. We challenge some common beliefs, such as separately studying the dangerous effects of different tidal volumes (end inspiration) and end-expiratory positive pressures. Based on available data, we suggest that stress at rupture is only rarely reached and that high tidal volume induces ventilator-induced lung injury by augmenting the pressure heterogeneity at the interface between open and constantly closed units. We believe that ventilator-induced lung injury occurs only when a given threshold is exceeded; below this limit, mechanical ventilation is likely to be safe.

Maintaining end-expiratory transpulmonary pressure prevents worsening of ventilator-induced lung injury caused by chest wall constriction in surfactant-depleted rats

OBJECTIVE

To see whether in acute lung injury 1) compression of the lungs caused by thoracoabdominal constriction degrades lung function and worsens ventilator-induced lung injury; and 2) maintaining end-expiratory transpulmonary pressure by increasing positive end-expiratory pressure reduces the deleterious effects of chest wall constriction.

DESIGN

Experimental study in rats.

SETTING

Physiology laboratory.

INTERVENTIONS

Acute lung injury was induced in three groups of nine rats by saline lavage. Nine animals immediately killed served as a control group. Group L had lavage only, group LC had the chest wall constricted with an elastic binder, and group LCP had the same chest constriction but with positive end-expiratory pressure raised to maintain end-expiratory transpulmonary pressure. After lavage, all groups were ventilated with the same pattern for 1½ hrs.

MEASUREMENTS AND MAIN RESULTS

Transpulmonary pressure, measured with an esophageal balloon catheter, lung volume changes, arterial blood gasses, and pH were assessed during mechanical ventilation. Lung wet-to-dry ratio, albumin, tumor necrosis factor-α, interleukin-1β, interleukin-6, interleukin-10, and macrophage inflammatory protein-2 in serum and bronchoalveolar lavage fluid and serum E-selectin and von Willebrand Factor were measured at the end of mechanical ventilation. Lavage caused hypoxemia and acidemia, increased lung resistance and elastance, and decreased end-expiratory lung volume. With prolonged mechanical ventilation, lung mechanics, hypoxemia, and wet-to-dry ratio were significantly worse in group LC. Proinflammatory cytokines except E-selectin were elevated in serum and bronchoalveolar lavage fluid in all groups with significantly greater levels of tumor necrosis factor-α, interleukin-1β, and interleukin-6 in group LC, which also exhibited significantly worse bronchiolar injury and greater heterogeneity of airspace expansion at a fixed transpulmonary pressure than other groups.

CONCLUSIONS

Chest wall constriction in acute lung injury reduces lung volume, worsens hypoxemia, and increases pulmonary edema, mechanical abnormalities, proinflammatory mediator release, and histologic signs of ventilator-induced lung injury. Maintaining end-expiratory transpulmonary pressure at preconstriction levels by adding positive end-expiratory pressure prevents these deleterious effects.

Open lung approach associated with high-frequency oscillatory or low tidal volume mechanical ventilation improves respiratory function and minimizes lung injury in healthy and injured rats

INTRODUCTION

To test the hypothesis that open lung (OL) ventilatory strategies using high-frequency oscillatory ventilation (HFOV) or controlled mechanical ventilation (CMV) compared to CMV with lower positive end-expiratory pressure (PEEP) improve respiratory function while minimizing lung injury as well as systemic inflammation, a prospective randomized study was performed at a university animal laboratory using three different lung conditions.

METHODS

Seventy-eight adult male Wistar rats were randomly assigned to three groups: (1) uninjured (UI), (2) saline washout (SW), and (3) intraperitoneal/intravenous Escherichia coli lipopolysaccharide (LPS)-induced lung injury. Within each group, animals were further randomized to (1) OL with HFOV, (2) OL with CMV with "best" PEEP set according to the minimal static elastance of the respiratory system (BP-CMV), and (3) CMV with low PEEP (LP-CMV). They were then ventilated for 6 hours. HFOV was set with mean airway pressure (PmeanHFOV) at 2 cm H2O above the mean airway pressure recorded at BP-CMV (PmeanBP-CMV) following a recruitment manoeuvre. Six animals served as unventilated controls (C). Gas-exchange, respiratory system mechanics, lung histology, plasma cytokines, as well as cytokines and types I and III procollagen (PCI and PCIII) mRNA expression in lung tissue were measured.

RESULTS

We found that (1) in both SW and LPS, HFOV and BP-CMV improved gas exchange and mechanics with lower lung injury compared to LP-CMV, (2) in SW; HFOV yielded better oxygenation than BP-CMV; (3) in SW, interleukin (IL)-6 mRNA expression was lower during BP-CMV and HFOV compared to LP-CMV, while in LPS inflammatory response was independent of the ventilatory mode; and (4) PCIII mRNA expression decreased in all groups and ventilatory modes, with the decrease being highest in LPS.

CONCLUSIONS

Open lung ventilatory strategies associated with HFOV or BP-CMV improved respiratory function and minimized lung injury compared to LP-CMV. Therefore, HFOV with PmeanHFOV set 2 cm H2O above the PmeanBP-CMV following a recruitment manoeuvre is as beneficial as BP-CMV.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

Modeling the complex dynamics of derecruitment in the lung

Recruitment maneuvers using deep inflations (DI) have long been used clinically with the objective of recruiting collapsed regions of the lung. Considerable uncertainty continues to exist, however, as to how best to employ recruitment maneuvers or even if they should be used routinely at all for patients receiving mechanical ventilation. Much of this uncertainty may arise from a lack of understanding about the dynamic nature of recruitment and derecruitment. To shed some light on this complex issue, we developed a time-dependent computational model of recruitment and derecruitment in the lung based on a symmetrically bifurcating airway tree in which each branch has a critical closing and opening pressure as well as pressure-dependent opening and closing speeds. Starting from the fully open state, the model underwent regular ventilation for 8 min followed by a series of identical DIs separated by 5 min of identical regular ventilation. We found that the geographical nature and extent of derecruitment before and 5 min after each DI were not always the same, demonstrating that the model exhibits multiple stable states. We conclude that the effectiveness of a recruitment maneuver is not only simply a function of the duration and magnitude of a DI, but may also have an unpredictable component arising from the distributed bi-stable nature of the derecruitment process itself.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

Assisted ventilation modes reduce the expression of lung inflammatory and fibrogenic mediators in a model of mild acute lung injury

PURPOSE

The goal of the study was to compare the effects of different assisted ventilation modes with pressure controlled ventilation (PCV) on lung histology, arterial blood gases, inflammatory and fibrogenic mediators in experimental acute lung injury (ALI).

METHODS

Paraquat-induced ALI rats were studied. At 24 h, animals were anaesthetised and further randomized as follows (n = 6/group): (1) pressure controlled ventilation mode (PCV) with tidal volume (V (T)) = 6 ml/kg and inspiratory to expiratory ratio (I:E) = 1:2; (2) three assisted ventilation modes: (a) assist-pressure controlled ventilation (APCV1:2) with I:E = 1:2, (b) APCV1:1 with I:E = 1:1; and (c) biphasic positive airway pressure and pressure support ventilation (BiVent + PSV), and (3) spontaneous breathing without PEEP in air. PCV, APCV1:1, and APCV1:2 were set with P (insp) = 10 cmH(2)O and PEEP = 5 cmH(2)O. BiVent + PSV was set with two levels of CPAP [inspiratory pressure (P (High) = 10 cmH(2)O) and positive end-expiratory pressure (P (Low) = 5 cmH(2)O)] and inspiratory/expiratory times: T (High) = 0.3 s and T (Low) = 0.3 s. PSV was set as follows: 2 cmH(2)O above P (High) and 7 cmH(2)O above P (Low). All rats were mechanically ventilated in air and PEEP = 5 cmH(2)O for 1 h.

RESULTS

Assisted ventilation modes led to better functional improvement and less lung injury compared to PCV. APCV1:1 and BiVent + PSV presented similar oxygenation levels, which were higher than in APCV1:2. Bivent + PSV led to less alveolar epithelium injury and lower expression of tumour necrosis factor-alpha, interleukin-6, and type III procollagen.

CONCLUSIONS

In this experimental ALI model, assisted ventilation modes presented greater beneficial effects on respiratory function and a reduction in lung injury compared to PCV. Among assisted ventilation modes, Bi-Vent + PSV demonstrated better functional results with less lung damage and expression of inflammatory mediators.

Regional lung aeration and ventilation during pressure support and biphasic positive airway pressure ventilation in experimental lung injury

INTRODUCTION

There is an increasing interest in biphasic positive airway pressure with spontaneous breathing (BIPAP+SBmean), which is a combination of time-cycled controlled breaths at two levels of continuous positive airway pressure (BIPAP+SBcontrolled) and non-assisted spontaneous breathing (BIPAP+SBspont), in the early phase of acute lung injury (ALI). However, pressure support ventilation (PSV) remains the most commonly used mode of assisted ventilation. To date, the effects of BIPAP+SBmean and PSV on regional lung aeration and ventilation during ALI are only poorly defined.

METHODS

In 10 anesthetized juvenile pigs, ALI was induced by surfactant depletion. BIPAP+SBmean and PSV were performed in a random sequence (1 h each) at comparable mean airway pressures and minute volumes. Gas exchange, hemodynamics, and inspiratory effort were determined and dynamic computed tomography scans obtained. Aeration and ventilation were calculated in four zones along the ventral-dorsal axis at lung apex, hilum and base.

RESULTS

Compared to PSV, BIPAP+SBmean resulted in: 1) lower mean tidal volume, comparable oxygenation and hemodynamics, and increased PaCO2 and inspiratory effort; 2) less nonaerated areas at end-expiration; 3) decreased tidal hyperaeration and re-aeration; 4) similar distributions of ventilation. During BIPAP+SBmean: i) BIPAP+SBspont had lower tidal volumes and higher rates than BIPAP+SBcontrolled; ii) BIPAP+SBspont and BIPAP+SBcontrolled had similar distributions of ventilation and aeration; iii) BIPAP+SBcontrolled resulted in increased tidal re-aeration and hyperareation, compared to PSV. BIPAP+SBspont showed an opposite pattern.

CONCLUSIONS

In this model of ALI, the reduction of tidal re-aeration and hyperaeration during BIPAP+SBmean compared to PSV is not due to decreased nonaerated areas at end-expiration or different distribution of ventilation, but to lower tidal volumes during BIPAP+SBspont. The ratio between spontaneous to controlled breaths seems to play a pivotal role in reducing tidal re-aeration and hyperaeration during BIPAP+SBmean.