Lung Reaeration Research Articles

The role of lung ultrasound in procalcitonin-guided antibiotic discontinuation in ventilator-associated pneumonia.

ABSTRACTBackground and Aims:Starting antibiotic therapy at the proper time is the cornerstone of the management of ventilator-associated pneumonia (VAP). However, using antibiotics for a long duration of therapy in intensive care leads to increased bacterial resistance, financial burden and adverse drug reactions. We hypothesised that lung ultrasound may have a role in guiding antibiotic discontinuation in patients with VAP that will help to reduce the antibiotic duration and decrease the resistance.Methods:This was a prospective blinded cohort study from October 2020 to September 2021 in which 62 VAP patients were recruited. Antibiotics were started, procalcitonin (PCT) level and clinical pulmonary infection score were estimated and lung ultrasound (US) was performed on day 1 and repeated on day 7. On day 7, discontinuation of antibiotics was recommended if the PCT level was <0.25 μg/L. A lung reaeration score was recorded.Results:Based on the PCT levels, antibiotics were discontinued in 40 patients. The computed tomography findings of VAP had improved in all. The ultrasound reaeration score showed a highly significant negative correlation with the PCT on day 7 (-0.718, P < 0.001). A cut-off of 5 for the US score showed a sensitivity of 92.5%, specificity of 95.5%, positive predictive value of 97.4% and negative predictive value of 87.5% in detecting a low PCT score on day 7.Conclusion:Lung US is a non-invasive and safe method that can be used to guide antibiotic therapy in VAP.

Randomized field trial comparing the efficacy of florfenicol and oxytetracycline in a natural outbreak of calf pneumonia using lung reaeration as a cure criterion.

BackgroundRespiratory infections are the main indication for antimicrobial use in calves. Optimal treatment duration currently is unknown, but shorter duration would likely decrease selection for antimicrobial resistance.Hypothesis/ObjectivesDetermine differences in cure rate and healing time between animals treated with florfenicol and oxytetracycline in a natural outbreak of respiratory disease using reaeration observed on thoracic ultrasound examination as healing criterion.AnimalsCommercial farm housing 130, 3 to 9 month old Belgian blue beef calves.MethodsRandomized clinical trial during an outbreak of respiratory disease. Metaphylactic treatment was initiated, randomly treating animals with either florfenicol or oxytetracycline. Ultrasonographic follow‐up was done the first day and every other day for a 14‐day period. At the individual animal level, treatment was discontinued when reaeration of the lungs occurred. Differences in cure rate and healing time were determined.ResultsOf the 130 animals studied, 67.7% developed a lung consolidation ≥0.5 cm. The mean ultrasonographic healing time was 2.5 days in the florfenicol group compared to 3.1 days in the oxytetracycline group (P = .04). After single treatment, 80.6% and 60.3% had no consolidations in the florfenicol and oxytetracycline groups, respectively (P = .01). A Mycoplasma bovis strain was genetically and phenotypically determined to be susceptible to both antimicrobials.Conclusions and Clinical ImportanceUltrasonographic lung reaeration shows potential as a cure criterion to rationalize antimicrobial use for outbreaks of pneumonia. In our study, florfenicol resulted in a faster cure and higher reduction in antimicrobial usage than did oxytetracycline.

Lung ultrasound and the role of lung aeration score in patients with acute respiratory distress syndrome on extracorporeal membrane oxygenation.

This was a pilot study to determine the utility of daily lung ultrasound (LUS) in patients requiring veno-venous extracorporeal membrane oxygenation (VV-ECMO) for acute respiratory distress syndrome (ARDS). This was a prospective, observational study. The study took place in the intensive care unit at Royal Papworth Hospital in Cambridge, UK. We recruited adult patients receiving VV-ECMO for ARDS. All patients received a lung computed tomography (CT) scan and LUS on admission. Bedside chest radiography (CXR) and LUS were done on a daily basis until patients were decannulated. Daily LUS aeration scores were calculated according to the appearance of four defined patterns. An independent radiologist calculated corresponding scores for CT and CXR, retrospectively. These were checked for correlation with LUS aeration scores. There were statistically significant correlations between LUS versus CT (r = 0.868, p = 0.002) and LUS versus CXR (r = 0.498, p = 0.018) with good agreement and no evidence of proportional bias. LUS was able to detect 13.5% of pleural effusions and 54.2% of pneumothorax that were not picked up on CXR.In most of the patients who were weaned off VV-ECMO, a progressive reduction of LUS aeration scores corresponding to lung re-aeration was observed. LUS correlated with findings on CT and CXR for quantifying lung aeration and the clinical presentation of patients. LUS also picked up more pleural effusions and pneumothorax than CXR. Together with traditional imaging techniques, the routine use of LUS should be considered for this patient group.

Assessment of Lung Parenchyma Recovery after Antibiotic Administration using Lung Ultrasound in Critically Ill Patients with Pneumonia

Background: Pneumonia is a common cause of Intensive care unit (ICU) admission, requiring frequent imaging for following up parenchymal lung involvement and antibiotic response. Being bedside and non-invasive technique; lung ultrasound (US) is increasingly used in ICU. Objectives: Assessing accuracy of lung ultrasound in detecting parenchymal lung recovery following antibiotic administration in critically ill patients with pneumonia. Methods: Fifty patients with pneumonia were included in the study with time-dependent analysis for APACHEII, CURB-65 and modified CPIS. Lung US at day 0 described basal lung condition then according to changes in lung parenchyma, US score could be first calculated at day 3. At day 5 US score was calculated again and changes in score (delta score) was calculated to asses ability of US to predict early good antibiotic response and finally lung US was repeated at day 7, score calculated to detect lung parenchyma recovery and compared with follow up CT for accuracy and agreement. Air bronchogram was reported whenever seen, described as static or dynamic and assessed in follow up examinations to be compared with CT follow up. Results: Lung US score ranged from -2 to 17 with mean value of 8.75 ± 3.88 for improving patients, while worsening patients showed lung US score of -11 to -20 with mean value of -10.08 ± 6.95 with high statistical significance (p&lt;0.001).The best cutoff value of lung US score changes for detecting good response to antibiotic was 2.5, detected using area under the curve (AUC) (p&lt;0.001). Ultrasound score on day seven showed excellent sensitivity and specificity of 91.89% and 92.31% respectively when compared to CT with PPV of 97.14% and NPV 80% and accuracy 92% with strong statistical significance (p&lt;0.001). Air bronchogram showed sensitivity of 61.5% and specificity of 89.1% and with PPV of 66.67% and NPV of 86.84% and accuracy of 82% and moderate agreement (0.52) with CT while B-lines were significant for assessing lung reaeration with sensitivity of 69.2% and specificity of 67.5% and accuracy of 68% but with fair (0.31) agreement with CT (p&lt;0.027) in detecting parenchymal lung recovery. Conclusion: Lung US is a reasonable bedside method for quantifying parenchymal lung recovery in patients with pneumonia who are successfully treated with antibiotics.

Assessment of Lung Reaeration at 2 Levels of Positive End-expiratory Pressure in Patients With Early and Late COVID-19-related Acute Respiratory Distress Syndrome.

Patients with novel coronavirus disease (COVID-19) frequently develop acute respiratory distress syndrome (ARDS) and need invasive ventilation. The potential to reaerate consolidated lung tissue in COVID-19-related ARDS is heavily debated. This study assessed the potential to reaerate lung consolidations in patients with COVID-19-related ARDS under invasive ventilation. This was a retrospective analysis of patients with COVID-19-related ARDS who underwent chest computed tomography (CT) at low positive end-expiratory pressure (PEEP) and after a recruitment maneuver at high PEEP of 20 cm H2O. Lung reaeration, volume, and weight were calculated using both CT scans. CT scans were performed after intubation and start of ventilation (early CT), or after several days of intensive care unit admission (late CT). Twenty-eight patients were analyzed. The median percentages of reaerated and nonaerated lung tissue were 19% [interquartile range, IQR: 10 to 33] and 11% [IQR: 4 to 15] for patients with early and late CT scans, respectively (P=0.049). End-expiratory lung volume showed a median increase of 663 mL [IQR: 483 to 865] and 574 mL [IQR: 292 to 670] after recruitment for patients with early and late CT scans, respectively (P=0.43). The median decrease in lung weight attributed to nonaerated lung tissue was 229 g [IQR: 165 to 376] and 171 g [IQR: 81 to 229] after recruitment for patients with early and late CT scans, respectively (P=0.16). The majority of patients with COVID-19-related ARDS undergoing invasive ventilation had substantial reaeration of lung consolidations after recruitment and ventilation at high PEEP. Higher PEEP can be considered in patients with reaerated lung consolidations when accompanied by improvement in compliance and gas exchange.

The role of ultrasound in the diagnosis of atelectasis and reaeration after recruitment maneuvers compared with chest computed tomography in critically ill, mechanically ventilated patients

Introduction In critically ill patients, atelectasis is an extremely common condition, so accurate diagnosis of atelectasis with available diagnostic methods and trials of atelectasis improvement would be very helpful. First, computed tomography (CT) is believed to be the gold standard method for atelectasis or consolidation detection and consequently a gold standard for the evaluation of reaeration after different recruitment maneuvers. Ultrasound (US) has been underestimated for a long period and thought to be of limited value in the field of chest and lung diseases, but US has proved with accurate usage and qualified application techniques to be a very important bedside tool for the diagnosis of chest diseases and may has values over CT techniques in some clinical issues. US is playing an important role in the diagnosis of many diseases, which gives many advantages of US in ICU wards as ultrasonography is of low cost, available, lightweight, gives real-time image, and diagnosis with low radiation exposure,. Portable could be used in the ward without patient transportation and immediate results. Objective The aim of this study was to assess the role of ultrasonography in the diagnosis of lung reaeration after recruitment maneuvers compared with CT chest in critically ill mechanically ventilated patients using chest ultrasonography. Materials and methods The study was carried out on 30 intubated and mechanically ventilated patients, who were admitted to ICUs (postsurgical and general ICUs). Ultrasonography was repeated every 12 h for 2 days together with arterial blood gases sampling to monitor reaeration and resolving of hypoxia. After 48 h, CT chest was done to monitor the efficacy of ultrasonography in detecting signs of reaeration. Results There was no statistically significant difference in the diagnostic ability of US before and after recruitment; diagnostic ability of US before recruitment was 93.3% and after recruitment it decreased to 86.7%, which was not significant. The role of US in the diagnosis of recruitment was very high as its sensitivity 95% confidence interval=86.7 (77.9–99.18) as well as its positive predictive value (95% confidence interval) was 100 (87.66–100). Conclusion Lung US has proved its diagnostic value besides it having advantages as a bedside, real-time image, low-radiation, and of lower cost than CT in recruitment maneuver assessment.

Modified Lung Ultrasound Examinations in Assessment and Monitoring of Positive End-Expiratory Pressure-Induced Lung Reaeration in Young Children With Congenital Heart Disease Under General Anesthesia.

Lung ultrasound can reliably diagnose pulmonary atelectasis. The object of this study is to determine the most efficient region to assess changes in atelectasis in children with congenital heart disease under general anesthesia. Randomized controlled trial. Operating room at university-affiliated children's hospital. Children between 3 months and 3 years old, scheduled for elective congenital heart disease surgery under general anesthesia. Forty children with congenital heart disease were randomly allocated to either a 5 cm H2O positive end-expiratory pressure group or a standard therapy control group. Preoperative lung ultrasound was performed twice in each patient-after 1 and 15 minutes of mechanical ventilation. Atelectatic areas and B-lines were compared between two examinations. Different ultrasound regions were evaluated using Bland-Altman plots. The occurrence rate of atelectasis was much higher in inferoposterior lung regions (Scans 4-6) than in anterior and lateral regions (Scans 1-3). The median (interquartile range) lung ultrasound scores were lower in the positive end-expiratory pressure group than in the control group after treatment: 8 (3.3-9.8) versus 13 (8.3-17.5; p < 0.001). The atelectatic area was significantly decreased after treatment in the positive end-expiratory pressure group: 128 mm (34.5.5-213.3 mm) versus 49.5 mm (5.3-75.5 mm; p < 0.001). Bland-Altman plots revealed concordance between measurements in Scans 1-6 and those in Scans 4-6. In the posterior axillary line regions, changes in atelectatic area were significantly larger in the positive end-expiratory pressure group than in the control group (p = 0.03, 0.007, and 0.018). Lung ultrasound in inferoposterior lung regions may be more likely to reflect changes in atelectasis and save examination time; 5 cm H2O positive end-expiratory pressure may be useful in lung reaeration and can reduce, but not eliminate, atelectasis in children with congenital heart disease.

Ultrasound assessment of antibiotic-induced pulmonary reaeration in ventilator-associated pneumonia

Objective In this study we assessed lung reaeration by means of bedside chest radiography, lung computed tomography (CT), and lung ultrasound (LUS) in patients with ventilator-associated pneumonia (VAP) treated by antibiotics. Introduction VAP is a subtype of hospital-acquired pneumonia that occurs in patients under mechanical ventilation for at least 48 h and is characterized by the presence of a new or progressive infiltrate, signs of systemic infection (temperature, blood cell count), changes in sputum characteristics, and detection of the causative agent. Ultrasound examination is increasingly being used as a valuable bedside method for the diagnosis of various thoracic conditions in ICUs, including pleural or pericardial effusion, empyema, pneumothorax, pulmonary embolism, and pneumonia. To date, only a few studies have investigated the use of LUS in the diagnosis of pneumonia in the emergency department or in the ICU. In critically ill patients with acute lung injury, pulmonary aeration may be reliably assessed using bedside LUS. Design This is a randomized prospective comparative trial. Setting The study was conducted in the Critical Care Department of Alexandria Main University Hospital. Patients and methods This study was carried out on 60 patients with a preliminary diagnosis of VAP who matched the inclusion and exclusion criteria in the Critical Care Medicine Department in Alexandria University. In all patients chest ultrasound (US),chest radiography, and chest CT (the gold standard) were performed in sequence at day 0 and day 7 after antibiotic administration. Thereafter, all patients were categorized on the basis of the radiological findings and finally the effectiveness of each method was calculated and statistical comparisons were made. Results In our study, we compared lung reaeration by bedside chest radiography, lung CT, and LUS in patients with VAP treated with antibiotics. We included 60 patients with a preliminary diagnosis of VAP (33.3% were male and 16.7% were female); their ages ranged between 31 and 70 years (mean±SD: 48.3±11.063 years). In our study, we found a highly significant correlation between LUS score and CT reaeration findings (P Conclusion This study can affirm the superiority of chest US over chest radiographs for quantifying lung reaeration in patients with VAP who are successfully treated with antibiotics. LUS is a reliable, dynamic, rapid, noninvasive, bedside technique. Thus, considering the benefits of chest US versus the multiple drawbacks of chest CT, such as the requirement for a special request, the dose of radiation, and the major problem of patient transfer, chest US can be reported to be a reasonable substitute, allowing the early detection of antibiotic-induced lung reaeration or the extension of lung infection in cases of antimicrobial therapy failure. Ultrasound can be considered a reasonable bedside ‘gold standard’ in the critically ill.

Lung Reaeration and Reventilation after Aspiration of Pleural Effusions. A Study Using Electrical Impedance Tomography

Lung reexpansion after pleural effusion aspiration is composed of reaeration and reventilation. Previous studies evaluated only the immediate reaeration, and the reventilation was not evaluated using a direct lung ventilation measurement method. Also, indirect evidence indicates that the effusion could cause ventilator asynchrony between the lungs. The electrical impedance tomography can directly and reliably measure lung reaeration, reventilation, and synchrony. To evaluate lung reaeration, reventilation, and ventilator synchrony before and over 1 hour after a pleural aspiration. A prospective and observational study using electrical impedance tomography to measure the lung reaeration, reventilation, and ventilatory synchrony between the lungs (through phase angle) before and over 1 hour after the pleural aspiration of 22 patients with unilateral malignant effusions. The ipsilateral (affected by the effusion) (P < 0.001) and contralateral (P = 0.008) lung reaerated immediately without further reaeration over the next hour. However, the reventilation response was heterogeneous, with patients increasing, maintaining, or decreasing ipsilateral lung ventilation after the aspiration. The pleural effusion had caused ventilatory asynchrony (93 ± 71 degrees) that was immediately reversed by the aspiration. In some patients, the asynchrony was so extreme that one lung was inflating while the other was deflating, causing paradoxical ventilation. After a pleural effusion aspiration, the ipsilateral and contralateral lungs reaerate immediately without further reaeration over the next hour. The reventilation shows a heterogeneous response, with patients increasing, maintaining, or decreasing the ipsilateral lung ventilation. The pleural effusion causes a ventilatory asynchrony between the lungs that is immediately decreased by the aspiration. In some patients, that asynchrony is so intense that it causes paradoxical ventilation.

Bedside Ultrasound Assessment of Positive End-Expiratory Pressure–induced Lung Recruitment

In the critically ill patients, lung ultrasound (LUS) is increasingly being used at the bedside for assessing alveolar-interstitial syndrome, lung consolidation, pneumonia, pneumothorax, and pleural effusion. It could be an easily repeatable noninvasive tool for assessing lung recruitment. Our goal was to compare the pressure-volume (PV) curve method with LUS for assessing positive end-expiratory pressure (PEEP)-induced lung recruitment in patients with acute respiratory distress syndrome/acute lung injury (ARDS/ALI). Thirty patients with ARDS and 10 patients with ALI were prospectively studied. PV curves and LUS were performed in PEEP 0 and PEEP 15 cm H₂O₂. PEEP-induced lung recruitment was measured using the PV curve method. Four LUS entities were defined: consolidation; multiple, irregularly spaced B lines; multiple coalescent B lines; and normal aeration. For each of the 12 lung regions examined, PEEP-induced ultrasound changes were measured, and an ultrasound reaeration score was calculated. A highly significant correlation was found between PEEP-induced lung recruitment measured by PV curves and ultrasound reaeration score (Rho = 0.88; P < 0.0001). An ultrasound reaeration score of +8 or higher was associated with a PEEP-induced lung recruitment greater than 600 ml. An ultrasound lung reaeration score of +4 or less was associated with a PEEP-induced lung recruitment ranging from 75 to 450 ml. A statistically significant correlation was found between LUS reaeration score and PEEP-induced increase in Pa(O₂) (Rho = 0.63; P < 0.05). PEEP-induced lung recruitment can be adequately estimated with bedside LUS. Because LUS cannot assess PEEP-induced lung hyperinflation, it should not be the sole method for PEEP titration.

Ultrasound assessment of antibiotic-induced pulmonary reaeration in ventilator-associated pneumonia*

To compare lung reaeration measured by bedside chest radiography, lung computed tomography, and lung ultrasound in patients with ventilator-associated pneumonia treated by antibiotics. Computed tomography, chest radiography, and lung ultrasound were performed before (day 0) and 7 days following initiation of antibiotics. A 26-bed multidisciplinary intensive care unit in La Pitié-Salpêtrière hospital (University Paris-6). : Thirty critically ill patients studied over the first 10 days of developing ventilator-associated pneumonia. : Antibiotic administration. Computed tomography reaeration was measured as the additional volume of gas present within both lungs following 7 days of antimicrobial therapy. Lung ultrasound of the entire chest wall was performed and four entities were defined: consolidation; multiple irregularly spaced B-lines; multiple abutting ultrasound lung "comets" issued from the pleural line or a small subpleural consolidation; normal aeration. For each of the 12 regions examined, ultrasound changes were measured between day 0 and 7 and a reaeration score was calculated. An ultrasound score >5 was associated with a computed tomography reaeration >400 mL and a successful antimicrobial therapy. An ultrasound score <-10 was associated with a loss of computed tomography aeration >400 mL and a failure of antibiotics. A highly significant correlation was found between computed tomography and ultrasound lung reaeration (Rho = 0.85, p < .0001). Chest radiography was inaccurate in predicting lung reaeration. Lung reaeration can be accurately estimated with bedside lung ultrasound in patients with ventilator-associated pneumonia treated by antibiotics. Lung ultrasound can also detect the failure of antibiotics to reaerate the lung.

Lung reaeration after surfactant instillation - caused by surfactant or caused by instillation?

In this issue of Critical Care, Lu and colleagues [1] report signifi cant and prolonged lung reaeration after intratracheal bolus instillation of porcine-derived surfactant. In this substudy of a larger randomized controlled trial of patients with acute lung injury [2], the investigators elegantly demonstrate increased volumes of gas in poorly and non-aerated lung of patients who were treated with surfactant as compared with patients who received routine care. Do these encouraging fi ndings truly support the rationale for exogenous surfactant replacement as an indication for lung reaeration in patients with acute lung injury? Th is can be questioned. First, intratracheal bolus instillation of surfactant required recruitment maneuvers. Indeed, larger tidal volumes (TVs) and higher levels of positive end-expiratory pressure (PEEP) were used for as long as 30 minutes after instillation of surfactant to each lung. Th e instillation procedure per se could be (solely) responsible for increased volumes of gas in poorly and non-aerated lung. In addition, bolus instillation of surfactant resulted in temporarily severe hypoxemia in more than half of the patients treated with surfactant [2]. Th ough not reported in the original study or the present study, rescue therapies such as prone ventilation, repeated recruitment maneuvers, and higher levels of PEEP could have been used more intensively in these patients. Th ese rescue maneuvers, in response to surfactant instillation-induced hypoxemia, could also be responsible for the fi ndings by Lu and colleagues [1].

Computed tomography assessment of exogenous surfactant-induced lung reaeration in patients with acute lung injury

IntroductionPrevious randomized trials failed to demonstrate a decrease in mortality of patients with acute lung injury treated by exogenous surfactant. The aim of this prospective randomized study was to evaluate the effects of exogenous porcine-derived surfactant on pulmonary reaeration and lung tissue in patients with acute lung injury and acute respiratory distress syndrome (ALI/ARDS).MethodsTwenty patients with ALI/ARDS were studied (10 treated by surfactant and 10 controls) in whom a spiral thoracic computed tomography scan was acquired before (baseline), 39 hours and 7 days after the first surfactant administration. In the surfactant group, 3 doses of porcine-derived lung surfactant (200 mg/kg/dose) were instilled in both lungs at 0, 12 and 36 hours. Each instillation was followed by recruitment maneuvers. Gas and tissue volumes were measured separately in poorly/nonaerated and normally aerated lung areas before and seven days after the first surfactant administration. Surfactant-induced lung reaeration was defined as an increase in gas volume in poorly/non-aerated lung areas between day seven and baseline compared to the control group.ResultsAt day seven, surfactant induced a significant increase in volume of gas in poorly/non-aerated lung areas (320 ± 125 ml versus 135 ± 161 ml in controls, P = 0.01) and a significant increase in volume of tissue in normally aerated lung areas (189 ± 179 ml versus -15 ± 105 ml in controls, P < 0.01). PaO2/FiO2 ratio was not different between the surfactant treated group and control group after surfactant replacement.ConclusionsIntratracheal surfactant replacement induces a significant and prolonged lung reaeration. It also induces a significant increase in lung tissue in normally aerated lung areas, whose mechanisms remain to be elucidated.Trial registrationNCT00742482.

Use of chest sonography to estimate alveolar recruitment in general anesthesia

Chest sonography evaluates extravascular lung water as the number of ultrasound lung comets (ULC). The goal of this study was to assess the potential role of chest sonography to detect lung reaeration after alveolar recruitment maneuvers (RM) using the PaO2/FiO2 ratio as reference parameter.

Spontaneous breathing improves lung aeration in oleic acid-induced lung injury.

Experimental and clinical studies have shown reduction in intrapulmonary shunt with improved oxygenation by spontaneous breathing with airway pressure release ventilation (APRV) in acute lung injury. The mechanisms of these findings are not clear. The authors hypothesized that spontaneous breathing results in better aeration of lung tissue and that improvement in oxygenation can be explained by these changes. This hypothesis was studied in a porcine model of oleic acid-induced lung injury. Two hours after induction of lung injury, 24 pigs were randomly assigned to APRV with or without spontaneous breathing at a positive end-expiratory pressure of 5 cm H(2)O. Hemodynamics, spirometry, and end-expiratory lung volume by nitrogen washout were measured at baseline, after 2 h of lung injury, and after 2 and 4 h of mechanical ventilation in the specific mode. Finally, spiral computed tomography of the chest was performed at end-expiratory lung volume in 22 pigs. Arterial carbon dioxide tension and mean and end-inspiratory airway pressures were comparable between settings. Four hours of APRV with spontaneous breathing resulted in improved oxygenation compared with APRV without spontaneous breathing (arterial oxygen tension, 144 +/- 65 vs. 91 +/- 50 mmHg, P < 0.01 for interaction time x mode), higher end-expiratory lung volume (786 +/- 320 vs. 384 +/- 148 ml, P < 0.001), and better aeration. End-expiratory lung volume and venous admixture were both correlated with the amount of lung reaeration (r(2) = 0.62 and r(2) = 0.61, respectively). The results support the hypothesis that spontaneous breathing during APRV improves oxygenation mainly by recruitment of nonaerated lung and improved aeration of the lungs.