An objective analysis of the pressure-volume curve in the acute respiratory distress syndrome

Effects of Individualised High Positive End-Expiratory Pressure and Crystalloid Administration on Postoperative Pulmonary Function in Patients Undergoing Robotic-Assisted Radical Prostatectomy: A Prospective Randomised Single-Blinded Pilot Study

OBJECTIVES

Robotic-assisted laparoscopic prostatectomy (RALP) is typically conducted in steep Trendelenburg position (STP). The aim of the study was to evaluate whether crystalloid administration and individual management of positive end-expiratory pressure (PEEP) improve peri- and post-operative pulmonary function in patients undergoing RALP.

DESIGN

Prospective randomised single-centre single-blinded explorative study.

SETTING

Patients were either allocated to a standard PEEP (5 cmH₂O) group or an individualised high PEEP group. Furthermore, each group was divided into a liberal and a restrictive crystalloid group (8 vs. 4 mL/kg/h predicted body weight). Individualised PEEP levels were determined by means of preoperative recruitment manoeuvre and PEEP titration in STP.

PARTICIPANTS

Informed consent was obtained from 98 patients scheduled for elective RALP.

INTERVENTIONS

The following intraoperative parameters were analysed in each of the four study groups: ventilation setting (peak inspiratory pressure [PIP], plateau pressure, driving pressure [P_driv], lung compliance [LC] and mechanical power [MP]) and postoperative pulmonary function (bed-side spirometry). The spirometric parameters Tiffeneau index (FEV₁/FVC ratio) and mean forced expiratory flow (FEF_25-75) were measured pre- and post-operatively. Data are shown as mean ± standard deviation (SD), and groups were compared with ANOVA. A p-value of <0.05 was considered significant.

RESULTS

The two individualised high PEEP groups (mean PEEP 15.5 [±1.71 cmH₂O]) showed intraoperative significantly higher PIP, plateau pressure and MP levels but significantly decreased P_driv and increased LC. On the first and second postoperative day, patients with individualised high PEEP levels had a significantly higher mean Tiffeneau index and FEF_25-75. Perioperative oxygenation and ventilation and postoperative spirometric parameters were not influenced by restrictive or liberal crystalloid infusion in either of the two respective PEEP groups.

CONCLUSIONS

Individualised high PEEP levels (≥14 cmH₂O) during RALP improved intraoperative blood oxygenation and resulted in more lung-protective ventilation. Furthermore, postoperative pulmonary function was improved for up to 48 h after surgery in the sum of the two individualised high PEEP groups. Restrictive crystalloid infusion during RALP seemed to have no effect on peri- and post-operative oxygenation and pulmonary function.

Smart Hydrogels: Preparation, Characterization, and Determination of Transition Points of Crosslinked N-Isopropyl Acrylamide/Acrylamide/Carboxylic Acids Polymers

Smart hydrogels (SH) were prepared by thermal free radical polymerization of N-isopropyl acrylamide (NIPAAm), acrylamide (AAm) with acrylic acid (A) or maleic acid (M), and N,N'-methylene bisacrylamide. Spectroscopic and thermal characterizations of SHs were performed using FTIR, TGA, and DSC. To determine the effects of SHs on swelling characteristics, swelling studies were performed in different solvents, solutions, temperatures, pHs, and ionic strengths. In addition, cycle equilibrium swelling studies were carried out at different temperatures and pHs. The temperature and pH transition points of SHs are calculated using a sigmoidal equation. The pH transition points were calculated as 5.2 and 4.2 for SH-M and SH-A, respectively. The NIPAAm/AAm hydrogel exhibits a critical solution temperature (LCST) of 28.35 °C, while the SH-A and SH-M hydrogels exhibit the LCST of 34.215 °C and 28.798 °C, respectively, and the LCST of SH-A is close to the body. temperature. Commercial (CHSA) and blood human serum albumin (BHSA) were used to find the adsorption properties of biopolymers on SHs. SH-M was the most efficient SH, adsorbing 49% of CHSA while absorbing 16% of BHSA. In conclusion, the sigmoidal equation or Gaussian approach can be a useful tool for chemists, chemical engineers, polymer and plastics scientists to find the transition points of smart hydrogels.

Effects of postural differences on intrapleural pressure during chest wall compression in healthy males

[Purpose] This study aimed to investigate the difference in intrapleural pressure between the supine and lateral decubitus positions during manual chest wall compression. [Participants and Methods] Eight healthy males participated in this study. The same physiotherapist performed chest wall compression on participants lying supine, and on their right and left sides. We noted changes in intrapleural pressure and lung volume in each participant during quiet breathing and chest wall compression. [Results] During chest wall compression, intrapleural pressure at the end-expiratory lung volume and the end-inspiratory lung volume were lower in the right and left decubitus positions than in the supine position. We observed the following low inflection points in the pressure-volume loops during chest wall compression: all participants in the supine position, no participants in the right decubitus position, and two participants in the left decubitus position. [Conclusion] Chest wall compression in the bilateral decubitus positions may not cause excessive intrapleural pressure on the airway and alveoli as compared to chest wall compression in the supine position.

Early detection of skeletal muscle bioenergetic deficit by magnetic resonance spectroscopy in cigarette smoke-exposed mice

Skeletal muscle dysfunction is a common complication and an important prognostic factor in patients with chronic obstructive pulmonary disease (COPD). It is associated with intrinsic muscular abnormalities of the lower extremities, but it is not known whether there is an easy way to predict its presence. Using a mouse model of chronic cigarette smoke exposure, we tested the hypothesis that magnetic resonance spectroscopy allows us to detect muscle bioenergetic deficit in early stages of lung disease. We employed this technique to evaluate the synthesis rate of adenosine triphosphate (ATP) and characterize concomitant mitochondrial dynamics patterns in the gastrocnemius muscle of emphysematous mice. The fibers type composition and citrate synthase (CtS) and cytochrome c oxidase subunit IV (COX4) enzymatic activities were evaluated. We found that the rate of ATP synthesis was reduced in the distal skeletal muscle of mice exposed to cigarette smoke. Emphysematous mice showed a significant reduction in body weight gain, in the cross-sectional area of the total fiber and in the COX4 to CtS activity ratio, due to a significant increase in CtS activity of the gastrocnemius muscle. Taken together, these data support the hypothesis that in the early stage of lung disease, we can detect a decrease in ATP synthesis in skeletal muscle, partly caused by high oxidative mitochondrial enzyme activity. These findings may be relevant to predict the presence of skeletal bioenergetic deficit in the early stage of lung disease besides placing the mitochondria as a potential therapeutic target for the treatment of COPD comorbidities.

Predictive Virtual Patient Modelling of Mechanical Ventilation: Impact of Recruitment Function

Mechanical ventilation is a life-support therapy for intensive care patients suffering from respiratory failure. To reduce the current rate of ventilator-induced lung injury requires ventilator settings that are patient-, time-, and disease-specific. A common lung protective strategy is to optimise the level of positive end-expiratory pressure (PEEP) through a recruitment manoeuvre to prevent alveolar collapse at the end of expiration and to improve gas exchange through recruitment of additional alveoli. However, this process can subject parts of the lung to excessively high pressures or volumes. This research significantly extends and more robustly validates a previously developed pulmonary mechanics model to predict lung mechanics throughout recruitment manoeuvres. In particular, the process of recruitment is more thoroughly investigated and the impact of the inclusion of expiratory data when estimating peak inspiratory pressure is assessed. Data from the McREM trial and CURE pilot trial were used to test model predictive capability and assumptions. For PEEP changes of up to and including 14 cmH₂O, the parabolic model was shown to improve peak inspiratory pressure prediction resulting in less than 10% absolute error in the CURE cohort and 16% in the McREM cohort. The parabolic model also better captured expiratory mechanics than the exponential model for both cohorts.

Biomedical engineer's guide to the clinical aspects of intensive care mechanical ventilation

Background

Mechanical ventilation is an essential therapy to support critically ill respiratory failure patients. Current standards of care consist of generalised approaches, such as the use of positive end expiratory pressure to inspired oxygen fraction (PEEP–FiO₂) tables, which fail to account for the inter- and intra-patient variability between and within patients. The benefits of higher or lower tidal volume, PEEP, and other settings are highly debated and no consensus has been reached. Moreover, clinicians implicitly account for patient-specific factors such as disease condition and progression as they manually titrate ventilator settings. Hence, care is highly variable and potentially often non-optimal. These conditions create a situation that could benefit greatly from an engineered approach. The overall goal is a review of ventilation that is accessible to both clinicians and engineers, to bridge the divide between the two fields and enable collaboration to improve patient care and outcomes. This review does not take the form of a typical systematic review. Instead, it defines the standard terminology and introduces key clinical and biomedical measurements before introducing the key clinical studies and their influence in clinical practice which in turn flows into the needs and requirements around how biomedical engineering research can play a role in improving care. Given the significant clinical research to date and its impact on this complex area of care, this review thus provides a tutorial introduction around the review of the state of the art relevant to a biomedical engineering perspective.

Discussion

This review presents the significant clinical aspects and variables of ventilation management, the potential risks associated with suboptimal ventilation management, and a review of the major recent attempts to improve ventilation in the context of these variables. The unique aspect of this review is a focus on these key elements relevant to engineering new approaches. In particular, the need for ventilation strategies which consider, and directly account for, the significant differences in patient condition, disease etiology, and progression within patients is demonstrated with the subsequent requirement for optimal ventilation strategies to titrate for patient- and time-specific conditions.

Conclusion

Engineered, protective lung strategies that can directly account for and manage inter- and intra-patient variability thus offer great potential to improve both individual care, as well as cohort clinical outcomes.

Patient-specific optimization of mechanical ventilation for patients with acute respiratory distress syndrome using quasi-static pulmonary P-V data

Quasi-static, pulmonary pressure-volume (P-V) curves were combined with a respiratory system model to analyze tidal pressure cycles, simulating mechanical ventilation of patients with acute respiratory distress syndrome (ARDS). Two important quantities including 1) tidal recruited volume and 2) tidal hyperinflated volume were analytically computed by integrating the distribution of alveolar elements over the affected pop-open pressure range. We analytically predicted the tidal recruited volume of four canine subjects and compared our results with similar experimental measurements on canine models for the validation. We then applied our mathematical model to the P-V data of ARDS populations in four stages of Early ARDS, Deep Knee, Advanced ARDS and Baby Lung to quantify the tidal recruited volume and tidal hyperinflated volume as an indicator of ventilator-induced lung injury (VILI). These quantitative predictions based on patient-specific P-V data suggest that the optimum parameters of mechanical ventilation including PEEP and Tidal Pressure (Volume) are largely varying among ARDS population and are primarily influenced by the degree in the severity of ARDS.

Assessing Respiratory System Mechanical Function

The main goals of assessing respiratory system mechanical function are to evaluate the lung function through a variety of methods and to detect early signs of abnormalities that could affect the patient's outcomes. In ventilated patients, it has become increasingly important to recognize whether respiratory function has improved or deteriorated, whether the ventilator settings match the patient's demand, and whether the selection of ventilator parameters follows a lung-protective strategy. Ventilator graphics, esophageal pressure, intra-abdominal pressure, and electric impedance tomography are some of the best-known monitoring tools to obtain measurements and adequately evaluate the respiratory system mechanical function.

Optimal esophageal balloon volume for accurate estimation of pleural pressure at end-expiration and end-inspiration: an in vitro bench experiment

BACKGROUND

Esophageal pressure, used as a surrogate for pleural pressure, is commonly measured by air-filled balloon, and the accuracy of measurement depends on the proper balloon volume. It has been found that larger filling volume is required at higher surrounding pressure. In the present study, we determined the balloon pressure-volume relationship in a bench model simulating the pleural cavity during controlled ventilation. The aim was to confirm whether an optimal balloon volume range existed that could provide accurate measurement at both end-expiration and end-inspiration.

METHODS

We investigated three esophageal balloons with different dimensions and materials: Cooper, SmartCath-G, and Microtek catheters. The balloon was introduced into a glass chamber simulating the pleural cavity and volume-controlled ventilation was initiated. The ventilator was set to obtain respective chamber pressures of 5 and 20 cmH₂O during end-expiratory and end-inspiratory occlusion. Balloon was progressively inflated, and balloon pressure and chamber pressure were measured. Balloon transmural pressure was defined as the difference between balloon and chamber pressure. The balloon pressure-volume curve was fitted by sigmoid regression, and the minimal and maximal balloon volume accurately reflecting the surrounding pressure was estimated using the lower and upper inflection point of the fitted sigmoid curve. Balloon volumes at end-expiratory and end-inspiratory occlusion were explored, and the balloon volume range that provided accurate measurement at both phases was defined as the optimal filling volume.

RESULTS

Sigmoid regression of the balloon pressure-volume curve was justified by the dimensionless variable fitting and residual distribution analysis. All balloon transmural pressures were within ±1.0 cmH₂O at the minimal and maximal balloon volumes. The minimal and maximal balloon volumes during end-inspiratory occlusion were significantly larger than those during end-expiratory occlusion, except for the minimal volume in Cooper catheter. Mean (±standard deviation) of optimal filling volume both suitable for end-expiratory and end-inspiratory measurement ranged 0.7 ± 0.0 to 1.7 ± 0.2 ml in Cooper, 1.9 ± 0.2 to 3.6 ± 0.3 ml in SmartCath-G, and 2.2 ± 0.2 to 4.6 ± 0.1 ml in Microtek catheter.

CONCLUSIONS

In each of the tested balloon, an optimal filling volume range was found that provided accurate measurement during both end-expiratory and end-inspiratory occlusion.

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.

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.

Acute Respiratory Failure

Acute respiratory failure accounts for 25–40 % of ICU admissions and carries a mortality rate of 30 % or more. In this chapter, we classify acute respiratory failure in two main types, based on their primary physiologic abnormality:

Disorders of the airways, where increase of airway resistance to gas flow determines pharmacologic treatment and ventilatory strategies. These disorders are mainly asthma and chronic obstructive pulmonary disease.

Disorders of the alveoli, where a decrease of lung compliance mandates the use of higher ventilatory pressures that can recruit but also damage the lung. These disorders include the acute respiratory distress syndrome, pneumonia, acute cardiogenic pulmonary edema, and influenza.

Additional types of acute respiratory failure are described elsewhere in this book: disorders that result from neuromuscular disease in Chap. 10.1007/978-3-319-19668-8_19 and pulmonary disorders of the circulation, including pulmonary thromboembolism, in Chap. 10.1007/978-3-319-19668-8_27. Finally, we provide a section on weaning from mechanical ventilation, which includes the pathophysiology of the ventilatory load imposed by the prolonged acute respiratory failure, the possible ways to support the weakened respiratory system, and the current process of screening and testing for readiness to remove the ventilator.

End-Expiratory Volume and Oxygenation: Targeting PEEP in ARDS Patients

INTRODUCTION

Changes in end-expiratory lung volume (∆EELV) in response to changes in PEEP (∆PEEP) have not been reported in mechanically ventilated patients with ARDS. The purpose of this study was to determine the utility of measurements of ∆EELV in determining optimal PEEP in ARDS patients.

METHODS

Nine patients with ARDS were prospectively recruited. ∆EELV was measured using magnetometers during serial decremental PEEP trials. Changes in PaO2 (∆PaO2) were simultaneously measured. Static respiratory system compliance (CRS), ∆PaO2/∆PEEP, and ∆EELV/∆PEEP were calculated at each level of PEEP.

RESULTS

For the group, ∆EELV decreased by 1.09 ± 0.13 L (mean ± SD) as PEEP was reduced from 20 to 0 cm H2O with the greatest changes in ∆EELV occurring over the mid range of the decremental PEEP curve. Optimal values for CRS, ∆EELV/∆PEEP, and ∆PaO2/∆PEEP could be identified for each patient and occurred at PEEP levels ranging from 10 to 17.5 cm H2O. There was a significant correlation (r = 0.712, p = 0.047) between ∆PaO2/∆PEEP and ∆EELV/∆PEEP.

CONCLUSIONS

∆EELV can be measured from a decremental PEEP curve. Since ∆EELV is highly correlated with ∆PaO2, measures of ∆PaO2/∆PEEP may provide a surrogate for measures of ∆EELV/∆PEEP. Combining measures of ∆EELV/∆PEEP with measures of CRS may provide a novel means of determining optimal PEEP in patients with ARDS.

The acute respiratory distress syndrome

The acute respiratory distress syndrome (ARDS) is a major cause of acute respiratory failure. Its development leads to high rates of mortality, as well as short- and long-term complications, such as physical and cognitive impairment. Therefore, early recognition of this syndrome and application of demonstrated therapeutic interventions are essential to change the natural course of this devastating entity. In this review article, we describe updated concepts in ARDS. Specifically, we discuss the new definition of ARDS, its risk factors and pathophysiology, and current evidence regarding ventilation management, adjunctive therapies, and intervention required in refractory hypoxemia.

Beyond low tidal volumes: ventilating the patient with acute respiratory distress syndrome

The cornerstone of lung protective ventilation in patients with acute respiratory distress syndrome (ARDS) is a pressure- and volume-limited strategy. Other interventions have also been investigated. Although no method for positive end-expiratory pressure (PEEP) titration has proven most advantageous, experimental and clinical data support the use of higher PEEP in patients with moderate/severe ARDS. There is no benefit to the early use of high-frequency oscillatory ventilation (HFOV) in patients with moderate/severe ARDS, although it may be considered as rescue therapy. Further investigations of novel methods of bedside monitoring of mechanical ventilation may help identify the optimal ventilatory strategy.

Liver growth factor treatment reverses emphysema previously established in a cigarette smoke exposure mouse model

Chronic obstructive pulmonary disease (COPD) is an inflammatory lung disease largely associated with cigarette smoke exposure (CSE) and characterized by pulmonary and extrapulmonary manifestations, including systemic inflammation. Liver growth factor (LGF) is an albumin-bilirubin complex with demonstrated antifibrotic, antioxidant, and antihypertensive actions even at extrahepatic sites. We aimed to determine whether short LGF treatment (1.7 μg/mouse ip; 2 times, 2 wk), once the lung damage was established through the chronic CSE, contributes to improvement of the regeneration of damaged lung tissue, reducing systemic inflammation. We studied AKR/J mice, divided into three groups: control (air-exposed), CSE (chronic CSE), and CSE + LGF (LGF-treated CSE mice). We assessed pulmonary function, morphometric data, and levels of various systemic inflammatory markers to test the LGF regenerative capacity in this system. Our results revealed that the lungs of the CSE animals showed pulmonary emphysema and inflammation, characterized by increased lung compliance, enlargement of alveolar airspaces, systemic inflammation (circulating leukocytes and serum TNF-α level), and in vivo lung matrix metalloproteinase activity. LGF treatment was able to reverse all these parameters, decreasing total cell count in bronchoalveolar lavage fluid and T-lymphocyte infiltration in peripheral blood observed in emphysematous mice and reversing the decrease in monocytes observed in chronic CSE mice, and tends to reduce the neutrophil population and serum TNF-α level. In conclusion, LGF treatment normalizes the physiological and morphological parameters and levels of various systemic inflammatory biomarkers in a chronic CSE AKR/J model, which may have important pathophysiological and therapeutic implications for subjects with stable COPD.

Noninvasive quantitative assessment of pulmonary blood flow with 18F-FDG PET

Pulmonary blood flow (PBF) is a critical determinant of oxygenation during acute lung injury (ALI). PET/CT with (18)F-FDG allows the assessment of both lung aeration and neutrophil inflammation as well as an estimation of the regional fraction of blood (FB) if compartmental modeling is used to quantify (18)F-FDG pulmonary uptake. The aim of this study was to validate the use of FB to assess PBF, with PET and compartmental modeling of (15)O-H2O kinetics as a reference method, in both control animals and animals with ALI. For the purpose of studying a wide range of PBF values, supine and prone positions and various positive end-expiratory pressures (PEEPs) and tidal volumes (V(T)s) were selected.

METHODS

Pigs were randomized into 3 groups in which ALI was induced by HCl inhalation: pigs studied in the supine position with a low PEEP (5 ± 3 [mean ± SD] cm of H2O; n = 9) or a high PEEP (12 ± 1 cm of H2O; n = 8) and pigs studied in the prone position with a low PEEP (6 ± 3 cm of H2O; n = 9). Also included were a control group that did not have ALI (n = 6) and 2 additional groups (n = 6 each) that had a high V(T) to maintain a transpulmonary pressure of greater than or equal to 35 cm of H2O and that either received HCl inhalation or did not receive HCl inhalation. PBF and FB were measured with PET and compartmental modeling of (15)O-H2O and (18)F-FDG kinetics in 10 lung regions along the anterior-to-posterior lung dimension, and both were expressed in each region as a fraction of their values in the whole lung.

RESULTS

PBF and FB were strongly correlated (R(2) = 0.9), with a slope of the regression line close to unity and a negligible intercept. The mean difference between PBF and FB was 0, and the 95% limits of agreement were -0.035 to 0.035. This good agreement between methods was obtained in both normal and injured lungs and under a wide range of V(T), PEEP, and regional PBF values (7-71 mL/kg, 0-15 cm of H2O, and 24-603 mL·min(-1)·100 mL of lung(-1), respectively).

CONCLUSION

FB assessed with (18)F-FDG is a good surrogate for PBF in both normal animals and animals with ALI. PET/CT has the potential to be used to study ventilation, perfusion, and lung inflammation with a single tracer.

Three-dimensional simultaneous optical coherence tomography and confocal fluorescence microscopy for investigation of lung tissue

Although several strategies exist for a minimal-invasive treatment of patients with lung failure, the mortality rate of acute respiratory distress syndrome still reaches 30% at minimum. This striking number indicates the necessity of understanding lung dynamics on an alveolar level. To investigate the dynamical behavior on a microscale, we used three-dimensional geometrical and functional imaging to observe tissue parameters including alveolar size and length of embedded elastic fibers during ventilation. We established a combined optical coherence tomography (OCT) and confocal fluorescence microscopy system that is able to monitor the distension of alveolar tissue and elastin fibers simultaneously within three dimensions. The OCT system can laterally resolve a 4.9 μm line pair feature and has an approximately 11 μm full-width-half-maximum axial resolution in air. confocal fluorescence microscopy visualizes molecular properties of the tissue with a resolution of 0.75 μm (laterally), and 5.9 μm (axially) via fluorescence detection of the dye sulforhodamine B specifically binding to elastin. For system evaluation, we used a mouse model in situ to perform lung distension by application of different constant pressure values within the physiological regime. Our method enables the investigation of alveolar dynamics by helping to reveal basic processes emerging during artificial ventilation and breathing.

Acute respiratory distress syndrome: A clinical review

The acute respiratory distress syndrome (ARDS) is a complex disorder of heterogeneous etiologies characterized by a consistent, recognizable pattern of lung injury. Extensive epidemiologic studies and clinical intervention trials have been conducted to address the high mortality of this disorder and have provided significant insight into the complexity of studying new therapies for this condition. The existing clinical investigations in ARDS will be highlighted in this review. The limitations to current definitions, patient selection, and outcome assessment will be considered. While significant attention has been focused on the parenchymal injury that characterizes this disorder and the clinical support of gas exchange function, relatively limited focus has been directed to hemodynamic and pulmonary vascular dysfunction equally prominent in the disease. The limited available clinical information in this area will also be reviewed. The current standards for cardiopulmonary management of the condition will be outlined. Current gaps in our understanding of the clinical condition will be highlighted with the expectation that continued progress will contribute to a decline in disease mortality.

A sigmoidal fit for pressure-volume curves of idiopathic pulmonary fibrosis patients on mechanical ventilation: clinical implications

OBJECTIVE

Respiratory pressure-volume curves fitted to exponential equations have been used to assess disease severity and prognosis in spontaneously breathing patients with idiopathic pulmonary fibrosis. Sigmoidal equations have been used to fit pressure-volume curves for mechanically ventilated patients but not for idiopathic pulmonary fibrosis patients. We compared a sigmoidal model and an exponential model to fit pressure-volume curves from mechanically ventilated patients with idiopathic pulmonary fibrosis.

METHODS

Six idiopathic pulmonary fibrosis patients and five controls underwent inflation pressure-volume curves using the constant-flow technique during general anesthesia prior to open lung biopsy or thymectomy. We identified the lower and upper inflection points and fit the curves with an exponential equation, V = A-B.e-k.P, and a sigmoid equation, V = a+b/(1+e-(P-c)/d).

RESULTS

The mean lower inflection point for idiopathic pulmonary fibrosis patients was significantly higher (10.5 ± 5.7 cm H2O) than that of controls (3.6 ± 2.4 cm H₂O). The sigmoidal equation fit the pressure-volume curves of the fibrotic and control patients well, but the exponential equation fit the data well only when points below 50% of the inspiratory capacity were excluded.

CONCLUSION

The elevated lower inflection point and the sigmoidal shape of the pressure-volume curves suggest that respiratory system compliance is decreased close to end-expiratory lung volume in idiopathic pulmonary fibrosis patients under general anesthesia and mechanical ventilation. The sigmoidal fit was superior to the exponential fit for inflation pressure-volume curves of anesthetized patients with idiopathic pulmonary fibrosis and could be useful for guiding mechanical ventilation during general anesthesia in this condition.

A MRI and polarized gases compatible respirator and gas administrator for the study of the small animal lung: volume measurement and control

We have developed over the past years an experimental magnetic resonance imaging (MRI) and polarized gases compatible mechanical respirator for the study of the small experimental animal. The respirator has been successfully used for experiments both in the MRI setting for polarized (3)He, (19)F, and proton imaging as well as for functional measurements of the lungs. The new main pneumatic valve with the two integrated sensors for simultaneous lung pressure and volume measurements and the proportional valve to set the tidal volume of the respiration are described. It is shown how the device measures and controls the tidal volume of the lungs.

PEEP titration guided by ventilation homogeneity: a feasibility study using electrical impedance tomography

Introduction

Lung protective ventilation requires low tidal volume and suitable positive end-expiratory pressure (PEEP). To date, few methods have been accepted for clinical use to set the appropriate PEEP. The aim of this study was to test the feasibility of PEEP titration guided by ventilation homogeneity using the global inhomogeneity (GI) index based on electrical impedance tomography (EIT) images.

Methods

In a retrospective study, 10 anesthetized patients with healthy lungs mechanically ventilated under volume-controlled mode were investigated. Ventilation distribution was monitored by EIT. A standardized incremental PEEP trial (PEEP from 0 to 28 mbar, 2 mbar per step) was conducted. During the PEEP trial, "optimal" PEEP level for each patient was determined when the air was most homogeneously distributed in the lung, indicated by the lowest GI index value. Two published methods for setting PEEP were included for comparison based on the maximum global dynamic compliance and the intra-tidal compliance-volume curve.

Results

No significant differences in the results were observed between the GI index method (12.2 ± 4.6 mbar) and the dynamic compliance method (11.4 ± 2.3 mbar, P > 0.6), or between the GI index and the compliance-volume curve method (12.2 ± 4.9 mbar, P > 0.6).

Conclusions

According to the results, it is feasible and reasonable to use the GI index to select the PEEP level with respect to ventilation homogeneity. The GI index may provide new insights into the relationship between lung mechanics and tidal volume distribution and may be used to guide ventilator settings.

[The basics on mechanical ventilation support in acute respiratory distress syndrome]

Acute Respiratory Distress Syndrome (ARDS) is understood as an inflammation-induced disruption of the alveolar endothelial-epithelial barrier that results in increased permeability and surfactant dysfunction followed by alveolar flooding and collapse. ARDS management relies on mechanical ventilation. The current challenge is to determine the optimal ventilatory strategies that minimize ventilator-induced lung injury (VILI) while providing a reasonable gas exchange. The data support that a tidal volume between 6-8 ml/kg of predicted body weight providing a plateau pressure < 30 cmH₂O should be used. High positive end expiratory pressure (PEEP) has not reduced mortality, nevertheless secondary endpoints are improved. The rationale used for high PEEP argues that it prevents cyclic opening and closing of airspaces, probably the major culprit of development of VILI. Chest computed tomography has contributed to our understanding of anatomic-functional distribution patterns in ARDS. Electric impedance tomography is a technique that is radiation-free, but still under development, that allows dynamic monitoring of ventilation distribution at bedside.

Automated measurement of the lower inflection point in a pediatric lung model

OBJECTIVE

To determine which flow setting most accurately detects the lower inflection point (Pflex) using an automated constant flow method and varying endotracheal tube (ETT) sizes with and without an airleak in a pediatric lung model.

DESIGN

Interventional laboratory study.

SETTING

Children's hospital research center.

INTERVENTIONS

A pediatric lung model was created with Pflexs of the inspiratory pressure-volume (P-V) curve set at 5 and 10 cm H2O using the ASL 5000 Test Lung (IngMar Medical, Pittsburgh, PA). Three ETT sizes (3.0, 4.0, 5.0 mm) were tested with and without a 25% airleak. P-V curves were obtained using an automated constant flow method at ten different flow rates.

MEASUREMENTS AND MAIN RESULTS

Without an ETT airleak, the lowest flow of 0.5 L/min led to the most accurate determination of Pflex regardless of ETT size or set Pflex (p < 0.001). When a 25% leak was introduced, accuracy of measured Pflex depended on both ETT size (p < 0.001) and flow rate (p < 0.001). Optimum flow rates for Pflex determination were 0.5, 1.0, and 1.5 L/min at Pflex of 5 cm H2O, and 2.0, 3.5, and 4.5 L/min at 10 cm H2O for 3.0, 4.0, and 5.0 mm ETTs, respectively (p < 0.001).

CONCLUSIONS

Estimation of Pflex can be achieved using automated P-V curves with ETTs appropriate for pediatric use, with and without an airleak. ETT size and flow rate affect the accuracy of these measurements when an airleak is present, and use of increased flow rates to create the automated P-V curves can reduce error. These data support the idea that a low-flow technique provides the most accurate determination of Pflex in pediatric patients without a leak around their ETT, whereas increased flows are needed to compensate when an ETT airleak is present.

Reduced surface tension normalizes static lung mechanics in a rodent chronic heart failure model

RATIONALE

Chronic elevation of pulmonary microvascular pressure in chronic heart failure results in compensatory changes in the lung that reduce alveolar fluid filtration and protect against pulmonary microvascular rupture.

OBJECTIVES

To determine whether these compensatory responses may have maladaptive effects on lung function.

METHODS

Six weeks after myocardial infarction (chronic heart failure model) rat lung composition, both gross and histologic; air and saline mechanics; surfactant production; and immunological mediators were examined.

MEASUREMENTS AND MAIN RESULTS

An increase in dry lung weight, due to increased insoluble protein, lipid and cellular infiltrate, without pulmonary edema was found. Despite this, both forced impedance and air pressure-volume mechanics were normal. However, there was increased tissue stiffness in the absence of surface tension (saline pressure-volume curve) with a concurrent increase in both surfactant content and alveolar type II cell numbers, suggesting a novel homeostatic phenomenon.

CONCLUSIONS

These studies suggest a compensatory reduction in pulmonary surface tension that attenuates the effect of lung parenchymal remodeling on lung mechanics, hence work of breathing.

A comparison of methods to identify open-lung PEEP

PURPOSE

Many methods exist in the literature for identifying PEEP to set in ARDS patients following a lung recruitment maneuver (RM). We compared ten published parameters for setting PEEP following a RM.

METHODS

Lung injury was induced by bilateral lung lavage in 14 female Dorset sheep, yielding a PaO(2) 100-150 mmHg at F(I)O(2) 1.0 and PEEP 5 cmH(2)O. A quasi-static P-V curve was then performed using the supersyringe method; PEEP was set to 20 cmH(2)O and a RM performed with pressure control ventilation (inspiratory pressure set to 40-50 cmH(2)O), until PaO(2) + PaCO(2) > 400 mmHg. Following the RM, a decremental PEEP trial was performed. The PEEP was decreased in 1 cmH(2)O steps every 5 min until 15 cmH(2)O was reached. Parameters measured during the decremental PEEP trial were compared with parameters obtained from the P-V curve.

RESULTS

For setting PEEP, maximum dynamic tidal respiratory compliance, maximum PaO(2), maximum PaO(2) + PaCO(2), and minimum shunt calculated during the decremental PEEP trial, and the lower Pflex and point of maximal compliance increase on the inflation limb of the P-V curve (Pmci,i) were statistically indistinguishable. The PEEP value obtained using the deflation upper Pflex and the point of maximal compliance decrease on the deflation limb were significantly higher, and the true inflection point on the inflation limb and minimum PaCO(2) were significantly lower than the other variables.

CONCLUSION

In this animal model of ARDS, dynamic tidal respiratory compliance, maximum PaO(2), maximum PaO(2) + PaCO(2), minimum shunt, inflation lower Pflex and Pmci,i yield similar values for PEEP following a recruitment maneuver.

Effect of the chest wall on pressure-volume curve analysis of acute respiratory distress syndrome lungs

OBJECTIVE

Previously published methods to assess the chest wall effect on total respiratory system pressure-volume (P-V) curves in acute respiratory distress syndrome have been performed on the lung and chest wall in isolation. We sought to quantify the effect of the chest wall by considering the chest wall and lung in series.

DESIGN

Prospective study.

SETTING

Academic health center medical and surgical intensive care units.

PATIENTS

Twenty-two patients with acute respiratory distress syndrome/acute lung injury.

INTERVENTIONS

Using a sigmoidal equation, we fit the pressure-volume data of the lung alone, and defined for each curve the pressure at the point of maximum compliance increase (Pmci), decrease (Pmcd), and the point of inflection (Pinf). We calculated the pressure to which the total respiratory system must be inflated to achieve a volume that would place the lung at each point of interest. We compared these "corrected" pressures (Pmci,c, Pmcd,c, and Pinf,c) to the measured values of the total respiratory system.

MEASUREMENTS AND MAIN RESULTS

The average difference between Pmci and Pmci,c was 0.12 cm H2O on inflation (2sd = 5.6 cm H2O) and -1.4 cm H2O on deflation (2sd = 5.0 cm H2O); between Pmcd and Pmcd,c was 1.73 cm H2O on inflation (2sd = 4.5 cm H2O) and -0.15 cm H2O on deflation (2sd = 4.9 cm H2O); and between Pinf and Pinf,c was 0.14 cm H2O on inflation (2sd = 6.7 cm H2O) and -0.35 cm H2O on deflation (2sd = 5.0 cm H2O).

CONCLUSIONS

This method of "correcting" the total respiratory system P-V curve for the chest wall allows for calculation of an airway pressure that would place the lung at a desired volume on its P-V curve. For most patients, the chest wall had little influence on the total respiratory system P-V curve. However, there were patients in whom the chest wall did potentially have clinical significance.

Mechanical ventilation guided by esophageal pressure in acute lung injury

BACKGROUND

Survival of patients with acute lung injury or the acute respiratory distress syndrome (ARDS) has been improved by ventilation with small tidal volumes and the use of positive end-expiratory pressure (PEEP); however, the optimal level of PEEP has been difficult to determine. In this pilot study, we estimated transpulmonary pressure with the use of esophageal balloon catheters. We reasoned that the use of pleural-pressure measurements, despite the technical limitations to the accuracy of such measurements, would enable us to find a PEEP value that could maintain oxygenation while preventing lung injury due to repeated alveolar collapse or overdistention.

METHODS

We randomly assigned patients with acute lung injury or ARDS to undergo mechanical ventilation with PEEP adjusted according to measurements of esophageal pressure (the esophageal-pressure-guided group) or according to the Acute Respiratory Distress Syndrome Network standard-of-care recommendations (the control group). The primary end point was improvement in oxygenation. The secondary end points included respiratory-system compliance and patient outcomes.

RESULTS

The study reached its stopping criterion and was terminated after 61 patients had been enrolled. The ratio of the partial pressure of arterial oxygen to the fraction of inspired oxygen at 72 hours was 88 mm Hg higher in the esophageal-pressure-guided group than in the control group (95% confidence interval, 78.1 to 98.3; P=0.002). This effect was persistent over the entire follow-up time (at 24, 48, and 72 hours; P=0.001 by repeated-measures analysis of variance). Respiratory-system compliance was also significantly better at 24, 48, and 72 hours in the esophageal-pressure-guided group (P=0.01 by repeated-measures analysis of variance).

CONCLUSIONS

As compared with the current standard of care, a ventilator strategy using esophageal pressures to estimate the transpulmonary pressure significantly improves oxygenation and compliance. Multicenter clinical trials are needed to determine whether this approach should be widely adopted. (ClinicalTrials.gov number, NCT00127491.)

Respiratory System Model for Quasistatic Pulmonary Pressure-Volume (P-V) Curve: Generalized P-V Curve Analyses

A normalized P-V curve is proposed for quantitative comparisons of quasistatic P-V curves from different sources, including data from different investigators, airway pressure-volume curves versus transpulmonary pressure-volume curves, normal versus injured respiratory system, and animal tests versus clinical data. Similarities and differences among five different data groups we analyzed are shown to be quantified through the nondimensional pressure range of an individual data set, combined with the magnitudes of two nondimensional parameters of the inflation limb, derived from a respiratory system model previously reported.

Do newer monitors of exhaled gases, mechanics, and esophageal pressure add value?

The current understanding of lung mechanics and ventilator-induced lung injury suggests that patients who have acute respiratory distress syndrome should be ventilated in such a way as to minimize alveolar over-distension and repeated alveolar collapse. Clinical trials have used such lung protective strategies and shown a reduction in mortality; however, there is data that these "one-size fits all" strategies do not work equally well in all patients. This article reviews other methods that may prove useful in monitoring for potential lung injury: exhaled breath condensate, pressure-volume curves, and esophageal manometry. The authors explore the concepts, benefits, difficulties, and relevant clinical trials of each.

Respiratory System Model for Quasistatic Pulmonary Pressure-Volume (P-V) Curve: Inflation-Deflation Loop Analyses

A respiratory system model (RSM) is developed for the deflation process of a quasistatic pressure-volume (P-V) curve, following the model for the inflation process reported earlier. In the RSM of both the inflation and the deflation limb, a respiratory system consists of a large population of basic alveolar elements, each consisting of a piston-spring-cylinder subsystem. A normal distribution of the basic elements is derived from Boltzmann statistical model with the alveolar closing (opening) pressure as the distribution parameter for the deflation (inflation) process. An error minimization by the method of least squares applied to existing P-V loop data from two different data sources confirms that a simultaneous inflation-deflation analysis is required for an accurate determination of RSM parameters. Commonly used terms such as lower inflection point, upper inflection point, and compliance are examined based on the P-V equations, on the distribution function, as well as on the geometric and physical properties of the basic alveolar element.

A decremental PEEP trial for determining open-lung PEEP in a rabbit model of acute lung injury

A positive end-expiratory pressure (PEEP) above the lower inflection point (LIP) of the pressure-volume curve has been thought necessary to maintain recruited lung volume in acute lung injury (ALI). We used a strategy to identify the level of open-lung PEEP (OLP) by detecting the maximum tidal compliance during a decremental PEEP trial (DPT). We performed a randomized controlled study to compare the effect of the OLP to PEEP above LIP and zero PEEP on pulmonary mechanics, gas exchange, hemodynamic change, and lung injury in 26 rabbits with ALI. After recruitment maneuver, the lavage-injured rabbits received DPTs to identify the OLP. Animals were randomized to receive volume controlled ventilation with either: (a) PEEP = 0 cm H2O (ZEEP); (b) PEEP = 2 cm H2O above OLP (OLP + 2); or (c) PEEP = 2 cm H2O above LIP (LIP + 2). Peak inspiratory pressure and mean airway pressure were recorded and arterial blood gases were analyzed every 30 min. Mean blood pressure and heart rate were monitored continuously. Lung injury severity was assessed by lung wet/dry weight ratio. Animals in OLP + 2 group had less lung injury as well as relatively better compliance, more stable pH, and less hypercapnia compared to the LIP + 2 and ZEEP groups. We concluded that setting PEEP according to the OLP identified by DPTs is an effective method to attenuate lung injury. This strategy could be used as an indicator for optimal PEEP. The approach is simple and noninvasive and may be of clinical interest.

Using pressure-volume curves to set proper PEEP in acute lung injury

The evolution of respiratory care on patients with acute respiratory distress syndrome (ARDS) has been focused on preventing the deleterious effects of mechanical ventilation, termed ventilator-induced lung injury (VILI). Currently, reduced tidal volume is the standard of ventilatory care for patients with ARDS. The current focus, however, has shifted to the proper setting of positive end-expiratory pressure (PEEP). The whole lung pressure-volume (P/V) curve has been used to individualize setting proper PEEP in patients with ARDS, although the physiologic interpretation of the curve remains under debate. The purpose of this review is to present the pros and cons of using P/V curves to set PEEP in patients with ARDS. A systematic analysis of recent and relevant literature was conducted. It has been hypothesized that proper PEEP can be determined by identifying P/V curve inflection points. Acquiring a dynamic curve presents the key to the curve's bedside application. The lower inflection point of the inflation limb has been shown to be the point of massive alveolar recruitment and therefore an option for setting PEEP. However, it is becoming widely accepted that the upper inflection point (UIP) of the deflation limb of the P/V curve represents the point of optimal PEEP. New methods used to identify optimal PEEP, including tomography and active compliance measurements, are currently being investigated. In conclusion, we believe that the most promising method for determining proper PEEP settings is use of the UIP of the deflation limb. However, tomography and dynamic compliance may offer superior bedside availability.

Parameters derived from the pulmonary pressure volume curve, but not the pressure time curve, indicate recruitment in experimental lung injury

BACKGROUND

In acute lung injury, ventilation avoiding tidal hyperinflation and tidal recruitment has been proposed to prevent ventilator-associated lung injury. Information about dynamic recruitment may be obtained from the characteristics of pressure-volume (PV) curves or the profile of pressure-time (Paw-t) curves.

METHODS

Six anesthetized pigs with lung lavage-induced acute lung injury were ventilated with lung-protective settings. We measured the effects of a standard recruitment maneuver on hysteresis area and ratio obtained from the PV curve and on the stress index obtained from the Paw-t curve and correlated this with aerated and nonaerated lung volumes as measured by multislice computed tomography.

RESULTS

Hysteresis area and ratio correlated with aerated lung volume (r = 0.886). The recruitment maneuver resulted in an increase in aerated (+12%) and a decrease (-18%) in nonaerated lung. Hysteresis area correlated with alveolar recruitment, represented by an increase in aerated lung (r = 0.886) and a decrease in nonaerated lung (r = -0.829) during tidal ventilation. The stress index was always >1 and indicated tidal hyperinflation only. Values did not change after the recruitment maneuver and did not correlate with any other lung volume.

CONCLUSIONS

Parameters derived from the PV curve may help in characterizing the lung aeration of the lung and in indicating recruitment. In the presence of lung-protective ventilator settings, the stress index derived from the Paw-t curve was not able to indicate recruitment.

Reference values for dynamic and static pulmonary compliance in men

The aim of the present study was to determine new reference values and predictive variables for dynamic and static pulmonary compliance in men. The investigation was conducted as a prospective study in healthy, non-smoking men with normal pulmonary function parameters including spirometry, bodyplethysmography and CO diffusing capacity. The esophageal pressure method was used to measure dynamic compliance (Cdyn), specific dynamic compliance (Cdyn/ITGV), static compliance (Cstat) and specific static compliance (Cstat/ITGV). Lung recoil pressures were recorded at different levels of total lung capacity (TLC). A total of 208 men aged 20-69 years were included in the study. The mean values for the compliance parameters were: Cdyn: 2.91+/-1.08 L/kPa; Cdyn/ITGV: 0.71 +/- 0.30 kPa (-1); Cstat: 3.34 +/- 1.04 L/kPa; Cstat/ITGV: 0.82 +/- 0.31 kPa (-1). Cdyn, Cdyn/ITGV and Cstat/ITGV were significantly correlated with age and Cstat was related to height, but in multiple regression analyses the predictability for compliance parameters was very low. Lung recoil pressures at all TLC levels significantly decreased with ageing. In conclusion, we demonstrated that the contribution of anthropometric variables to the regression equations of pulmonary compliance was low. With ageing the static pressure-volume curve of the lung shifted to the left without substantial alteration of the slope.

Airway pressure release and biphasic intermittent positive airway pressure ventilation: are they ready for prime time?

Airway pressure release ventilation and biphasic positive airway pressure ventilation are being used increasingly as alternative strategies to conventional assist control ventilation for patients with acute respiratory distress syndrome (ARDS) and acute lung injury. By permitting spontaneous breathing throughout the ventilatory cycle, these modes offer several advantages over conventional strategies to improve the pathophysiology in these patients, including gas exchange, cardiovascular function, and reducing or eliminating the need for heavy sedation and paralysis. Whether these surrogate outcomes will translate into better patient outcomes remains to be determined. The purpose of this review is to summarize the rationale behind the use of these ventilatory strategies in ARDS, the clinical experience with the use of these modes, and their future applications in trauma patients.

Two methods of setting positive end-expiratory pressure in acute lung injury: an experimental computed tomography volumetric study

This study was conducted to observe effects of two methods of setting positive end-expiratory pressure (PEEP) based on the pressure-volume (PV) curve. After lung injury was induced by oleic acid in six mongrel adult dogs, the inflation PV curve was traced and the lower inflection point (LIP) was measured. The 'PEEP(INF)' was defined as LIP+2 cmH(2)O. After recruitment maneuver to move the lung physiology to the deflation limb of PV curve, decremental PEEP was applied. The lowest level of PEEP that did not result in a significant drop in PaO(2) was defined as the 'PEEP(DEF)'. Arterial blood gases, lung mechanics, hemodynamics, and lung volumes (measured on computed tomography during end-expiratory pause) were measured at PEEP of 0 cmH(2)O, PEEP(INF) and PEEP(DEF) sequentially. The median PEEP(INF) was 13.4 cm H(2)O (interquartile range, 12.5-14.3) and median PEEP(DEF) was 12.0 cm H(2)O (10.0-16.5) (p=0.813). PEEP(DEF) was associated with significantly higher PaO(2) and lung volumes, and significantly lower shunt fraction and cardiac index when compared to PEEP(INF) (p<0.05). Setting the PEEP based on the deflation limb of the PV curve was useful in improving oxygenation and lung volumes in a canine lung injury model.

Correlation between alveolar recruitment /derecruitment and inflection points on the pressure-volume curve

OBJECTIVE

To determine whether individual alveolar recruitment/derecruitment (R/D) is correlated with the lower and upper inflections points on the inflation and deflation limb of the whole-lung pressure-volume (P-V) curve.

DESIGN AND SETTING

Prospective experimental study in an animal research laboratory.

SUBJECTS

Five anesthetized rats subjected to saline-lavage lung injury.

INTERVENTIONS

Subpleural alveoli were filmed continuously using an in vivo microscope during the generation of a whole-lung P-V curve using the super syringe technique. Alveolar R/D was correlated to the calculated inflection points on both limbs of the P-V curve.

MEASUREMENTS AND RESULTS

There was continual alveolar recruitment along the entire inflation limb in all animals. There was some correlation (R2=0.898) between the pressure below which microscopic derecruitment was observed and the upper inflection point on the deflation limb. No correlation was observed between this pressure and the lower inflection point on the inflation limb.

CONCLUSIONS

In this physiological experiment in lungs with pure surfactant deactivation we found that individual alveolar recruitment measured by direct visualization was not correlated with the lower inflection point on inflation whereas alveolar derecruitment was correlated with alveolar derecruitment on deflation. These data suggest that inflection points on the P-V curve do not always represent a change in alveolar number.

Comparative study of four sigmoid models of pressure-volume curve in acute lung injury

Background

The pressure-volume curve of the respiratory system is a tool to monitor and set mechanical ventilation in acute lung injury. Mathematical models of the static pressure-volume curve of the respiratory system have been proposed to overcome the inter- and intra-observer variability derived from eye-fitting. However, different models have not been compared.

Methods

The goodness-of-fit and the values of derived parameters (upper asymptote, maximum compliance and points of maximum curvature) in four sigmoid models were compared, using pressure-volume data from 30 mechanically ventilated patients during the early phase of acute lung injury.

Results

All models showed an excellent goodness-of-fit (R² always above 0.92). There were significant differences between the models in the parameters derived from the inspiratory limb, but not in those derived from the expiratory limb of the curve. The within-case standard deviations of the pressures at the points of maximum curvature ranged from 2.33 to 6.08 cmH₂O.

Conclusion

There are substantial variabilities in relevant parameters obtained from the four different models of the static pressure-volume curve of the respiratory system.