Assessment of PEEP-induced reopening of collapsed lung regions in acute lung injury: are one or three CT sections representative of the entire lung?

Computed Tomography Assessment of Tidal Lung Overinflation in Domestic Cats Undergoing Pressure-Controlled Mechanical Ventilation During General Anesthesia

Objective

This study aimed to evaluate lung overinflation at different airway inspiratory pressure levels using computed tomography in cats undergoing general anesthesia.

Study Design

Prospective laboratory study.

Animals

A group of 17 healthy male cats, aged 1.9–4.5 years and weighing 3.5 ± 0.5 kg.

Methods

Seventeen adult male cats were ventilated in pressure-controlled mode with airway pressure stepwise increased from 5 to 15 cmH₂O in 2 cmH₂O steps every 5 min and then stepwise decreased. The respiratory rate was set at 15 movements per min and end-expiratory pressure at zero (ZEEP). After 5 min in each inspiratory pressure step, a 4 s inspiratory pause was performed to obtain a thoracic juxta-diaphragmatic single slice helical CT image and to collect respiratory mechanics data and an arterial blood sample. Lung parenchyma aeration was defined as overinflated, normally-aerated, poorly-aerated, and non-aerated according to the CT attenuation number (−1,000 to −900 HU, −900 to −500 HU, −500 to −100 HU, and −100 to +100 HU, respectively).

Result

At 5 cmH₂O airway pressure, tidal volume was 6.7± 2.2 ml kg⁻¹, 2.1% (0.3–6.3%) of the pulmonary parenchyma was overinflated and 84.9% (77.6%−87.6%) was normally inflated. Increases in airway pressure were associated with progressive distention of the lung parenchyma. At 15 cmH₂O airway pressure, tidal volume increased to 31.5± 9.9 ml kg⁻¹ (p < 0.001), overinflated pulmonary parenchyma increased to 28.4% (21.2–30.6%) (p < 0.001), while normally inflated parenchyma decreased 57.9% (53.4–62.8%) (p < 0.001). Tidal volume and overinflated lung fraction returned to baseline when airway pressure was decreased. A progressive decrease was observed in arterial carbon dioxide partial pressure (PaCO₂) and end-tidal carbon dioxide (ETCO₂) when the airway pressures were increased above 9 cmH₂O (p < 0.001). The increase in airway pressure promoted an elevation in pH (p < 0.001).

Conclusions and Clinical Relevance

Ventilation with 5 and 7 cmH₂O of airway pressure prevents overinflation in healthy cats with highly compliant chest walls, despite presenting acidemia by respiratory acidosis. This fact can be controlled by increasing or decreasing respiratory rate and inspiratory time.

Advances in medical imaging to evaluate acute respiratory distress syndrome

Acute respiratory distress syndrome is a refractory respiratory syndrome with a high prevalence in the Intensive Care Unit. Though much improvement has been achieved over the last 50 decades, the disease continues to be under-recognized and under-treated, and its mortality remains high. Since the first report, the radiologic examination has been an essential part in evaluating this disease. Chest X-ray radiography and computed tomography are conventional imaging techniques in routine clinical practice. Other image modalities, including lung ultrasound, electrical impedance tomography, positron emission tomography, have demonstrated their respective advantages over recent years but have not yet been broadly applied in clinical practice. Among these modalities, computed tomography and its quantitative analysis have shown an irreplaceable power in diagnosis, intervention evaluation and prognostic prediction. In this review, we briefly introduced the basics of acute respiratory distress syndrome and summarized imaging advances. In addition, we focused on the computed tomography modality and highlighted the value of its quantitative assessment.

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<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<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<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<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.

Dynamic single-slice CT estimates whole-lung dual-energy CT variables in pigs with and without experimental lung injury

Background

Dynamic single-slice CT (dCT) is increasingly used to examine the intra-tidal, physiological variation in aeration and lung density in experimental lung injury. The ability of dCT to predict whole-lung values is unclear, especially for dual-energy CT (DECT) variables. Additionally, the effect of inspiration-related lung movement on CT variables has not yet been quantified.

Methods

Eight domestic pigs were studied under general anaesthesia, including four following saline-lavage surfactant depletion (lung injury model). DECT, dCT and whole-lung images were collected at 12 ventilatory settings. Whole-lung single energy scans images were collected during expiratory and inspiratory apnoeas at positive end-expiratory pressures from 0 to 20 cmH₂O. Means and distributions of CT variables were calculated for both dCT and whole-lung images. The cranio-caudal displacement of the anatomical slice was measured from whole-lung images.

Results

Mean CT density and volume fractions of soft tissue, gas, iodinated blood, atelectasis, poor aeration, normal aeration and overdistension correlated between dCT and the whole lung (r² 0.75–0.94) with agreement between CT density distributions (r 0.89–0.97). Inspiration increased the matching between dCT and whole-lung values and was associated with a movement of 32% (SD 15%) of the imaged slice out of the scanner field-of-view. This effect introduced an artefactual increase in dCT mean CT density during inspiration, opposite to that caused by the underlying physiology.

Conclusions

Overall, dCT closely approximates whole-lung aeration and density. This approximation is improved by inspiration where a decrease in CT density and atelectasis can be interpreted as physiological rather than artefactual.

ERS statement on chest imaging in acute respiratory failure

Chest imaging in patients with acute respiratory failure plays an important role in diagnosing, monitoring and assessing the underlying disease. The available modalities range from plain chest X-ray to computed tomography, lung ultrasound, electrical impedance tomography and positron emission tomography. Surprisingly, there are presently no clear-cut recommendations for critical care physicians regarding indications for and limitations of these different techniques.The purpose of the present European Respiratory Society (ERS) statement is to provide physicians with a comprehensive clinical review of chest imaging techniques for the assessment of patients with acute respiratory failure, based on the scientific evidence as identified by systematic searches. For each of these imaging techniques, the panel evaluated the following items: possible indications, technical aspects, qualitative and quantitative analysis of lung morphology and the potential interplay with mechanical ventilation. A systematic search of the literature was performed from inception to September 2018. A first search provided 1833 references. After evaluating the full text and discussion among the committee, 135 references were used to prepare the current statement.These chest imaging techniques allow a better assessment and understanding of the pathogenesis and pathophysiology of patients with acute respiratory failure, but have different indications and can provide additional information to each other.

Atelectasis is inversely proportional to transpulmonary pressure during weaning from ventilator support in a large animal model

BACKGROUND

In mechanically ventilated, lung injured, patients without spontaneous breathing effort, atelectasis with shunt and desaturation may appear suddenly when ventilator pressures are decreased. It is not known how such a formation of atelectasis is related to transpulmonary pressure (P_L ) during weaning from mechanical ventilation when the spontaneous breathing effort is increased. If the relation between P_L and atelectasis were known, monitoring of P_L might help to avoid formation of atelectasis and cyclic collapse during weaning. The main purpose of this study was to determine the relation between P_L and atelectasis in an experimental model representing weaning from mechanical ventilation.

METHODS

Dynamic transverse computed tomography scans were acquired in ten anaesthetized, surfactant-depleted pigs with preserved spontaneous breathing, as ventilator support was lowered by sequentially reducing inspiratory pressure and positive end expiratory pressure in steps. The volumes of gas and atelectasis in the lungs were correlated with P_L obtained using oesophageal pressure recordings. Work of breathing (WOB) was assessed from Campbell diagrams.

RESULTS

Gradual decrease in P_L in both end-expiration and end-inspiration caused a proportional increase in atelectasis and decrease in the gas content (linear mixed model with an autoregressive correlation matrix; P < 0.001) as the WOB increased. However, cyclic alveolar collapse during tidal ventilation did not increase significantly.

CONCLUSION

We found a proportional correlation between atelectasis and P_L during the 'weaning process' in experimental mild lung injury. If confirmed in the clinical setting, a gradual tapering of ventilator support can be recommended for weaning without risk of sudden formation of atelectasis.

Impact of Recruitment on Static and Dynamic Lung Strain in Acute Respiratory Distress Syndrome

BACKGROUND

Lung strain, defined as the ratio between end-inspiratory volume and functional residual capacity, is a marker of the mechanical load during ventilation. However, changes in lung volumes in response to pressures may occur in injured lungs and modify strain values. The objective of this study was to clarify the role of recruitment in strain measurements.

METHODS

Six oleic acid-injured pigs were ventilated at positive end-expiratory pressure (PEEP) 0 and 10 cm H2O before and after a recruitment maneuver (PEEP = 20 cm H2O). Lung volumes were measured by helium dilution and inductance plethysmography. In addition, six patients with moderate-to-severe acute respiratory distress syndrome were ventilated with three strategies (peak inspiratory pressure/PEEP: 20/8, 32/8, and 32/20 cm H2O). Lung volumes were measured in computed tomography slices acquired at end-expiration and end-inspiration. From both series, recruited volume and lung strain (total, dynamic, and static) were computed.

RESULTS

In the animal model, recruitment caused a significant decrease in dynamic strain (from [mean ± SD] 0.4 ± 0.12 to 0.25 ± 0.07, P < 0.01), while increasing the static component. In patients, total strain remained constant for the three ventilatory settings (0.35 ± 0.1, 0.37 ± 0.11, and 0.32 ± 0.1, respectively). Increases in tidal volume had no significant effects. Increasing PEEP constantly decreased dynamic strain (0.35 ± 0.1, 0.32 ± 0.1, and 0.04+0.03, P < 0.05) and increased static strain (0, 0.06 ± 0.06, and 0.28 ± 0.11, P < 0.05). The changes in dynamic and total strain among patients were correlated to the amount of recruited volume. An analysis restricted to the changes in normally aerated lung yielded similar results.

CONCLUSION

Recruitment causes a shift from dynamic to static strain in early acute respiratory distress syndrome.

Semiautomatic segmentation of longitudinal computed tomography images in a rat model of lung injury by surfactant depletion

Quantitative analysis of computed tomography (CT) is essential to the study of acute lung injury. However, quantitative CT is made difficult by poor lung aeration, which complicates the critical step of image segmentation. To overcome this obstacle, this study sought to develop and validate a semiautomated, multilandmark, registration-based scheme for lung segmentation that is effective in conditions of poor aeration. Expiratory and inspiratory CT images were obtained in rats (n = 8) with surfactant depletion of incremental severity to mimic worsening aeration. Trained operators manually delineated the images to provide a comparative landmark. Semiautomatic segmentation originated from a single, previously segmented reference image obtained at healthy baseline. Deformable registration of the target images (after surfactant depletion) was performed using the symmetric diffeomorphic transformation model with B-spline regularization. Registration used multiple landmarks (i.e., rib cage, spine, and lung parenchyma) to minimize the effect of poor aeration. Then target images were automatically segmented by applying the calculated transformation function to the reference image contour. Semiautomatically and manually segmented contours proved to be highly similar in all aeration conditions, including those characterized by more severe surfactant depletion and expiration. The Dice similarity coefficient was over 0.9 in most conditions, confirming high agreement, irrespective of poor aeration. Furthermore, CT density-based measurements of gas volume, tissue mass, and lung aeration distribution were minimally affected by the method of segmentation. Moving forward, multilandmark registration has the potential to streamline quantitative CT analysis by enabling semiautomatic image segmentation of lungs with a broad range of injury severity.

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.

Visual anatomical lung CT scan assessment of lung recruitability

PURPOSE

The computation of lung recruitability in acute respiratory distress syndrome (ARDS) is advocated to set positive end-expiratory pressure (PEEP) for preventing lung collapse. The quantitative lung CT scan, obtained by manual image processing, is the reference method but it is time consuming. The aim of this study was to evaluate the accuracy of a visual anatomical analysis compared with a quantitative lung CT scan analysis in assessing lung recruitability.

METHODS

Fifty sets of two complete lung CT scans of ALI/ARDS patients computing lung recruitment were analyzed. Lung recruitability computed at an airway pressure of 5 and 45 cm H(2)O was defined as the percentage decrease in the collapsed/consolidated lung parenchyma assessed by two expert radiologists using a visual anatomical analysis and as the decrease in not aerated lung regions using a quantitative analysis computed by dedicated software.

RESULTS

Lung recruitability was 11.3 % (interquartile range 7.39-16.41) and 15.5 % (interquartile range 8.18-21.43) with the visual anatomical and quantitative analysis, respectively. In the Bland-Altman analysis, the bias and agreement bands between the visual anatomical and quantitative analysis were -2.9 % (-11.8 to +5.9 %). The ROC curve showed that the optimal cutoff values for the visual anatomical analysis in predicting high versus low lung recruitability was 8.9 % (area under the ROC curve 0.9248, 95 % CI 0.8550-0.9946). Considering this cutoff, the sensitivity, specificity, and diagnostic accuracy were 0.96, 0.76, and 0.86, respectively.

CONCLUSIONS

Visual anatomical analysis can classify patients into those with high and low lung recruitability allowing more intensivists to get access to lung recruitability assessment.

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.

Extracorporeal membrane oxygenation as a bridge to liver transplantation for acute respiratory distress syndrome-induced life-threatening hypoxaemia aggravated by hepatopulmonary syndrome

Introduction

Combined with massive lung aeration loss resulting from acute respiratory distress syndrome, hepatopulmonary syndrome, a liver-induced vascular lung disorder characterized by diffuse or localized dilated pulmonary capillaries, may induce hypoxaemia and death in patients with end-stage liver disease.

Methods

The case of such a patient presenting with both disorders and in whom an extracorporeal membrane oxygenation was used is described.

Results

A 51-year-old man with a five-year history of alcoholic cirrhosis was admitted for acute respiratory failure, platypnoea and severe hypoxaemia requiring emergency tracheal intubation. Following mechanical ventilation, hypoxaemia remained refractory to positive end-expiratory pressure, 100% of inspired oxygen and inhaled nitric oxide. Two-dimensional contrast-enhanced (agitated saline) transthoracic echocardiography disclosed a massive right-to-left extracardiac shunt, without patent foramen ovale. Contrast computed tomography (CT) of the thorax using quantitative analysis and colour encoding system established the diagnosis of acute respiratory distress syndrome aggravated by hepatopulmonary syndrome. According to the severity of the respiratory condition, a veno-venous extracorporeal membrane oxygenation was implemented and the patient was listed for emergency liver transplantation. Orthotopic liver transplantation was performed at Day 13. At the end of the surgical procedure, the improvement in oxygenation allowed removal of extracorporeal membrane oxygenation (Day 5). The patient was discharged from hospital at Day 48. Three months after hospital discharge, the patient recovered a correct physical autonomy status without supplemental O₂.

Conclusions

In a cirrhotic patient, acute respiratory distress syndrome was aggravated by hepatopulmonary syndrome causing life-threatening hypoxaemia not controlled by standard supportive measures. The use of extracorporeal membrane oxygenation, by controlling gas exchange, allowed the performing of a successful liver transplantation and final recovery.

Effects of positive end-expiratory pressure on anesthesia-induced atelectasis and gas exchange in anesthetized and mechanically ventilated sheep

OBJECTIVE

To evaluate the effects of 10 cm H(2)O of positive end-expiratory pressure (PEEP) on lung aeration and gas exchange in mechanically ventilated sheep during general anesthesia induced and maintained with propofol.

ANIMALS

10 healthy adult Bergamasca sheep.

PROCEDURES

Sheep were sedated with diazepam (0.4 mg/kg, IV). Anesthesia was induced with propofol (5 mg/kg, IV) and maintained with propofol via constant rate infusion (0.4 mg/kg/min). Muscular paralysis was induced by administration of vecuronium (25 microg/kg, bolus IV) to facilitate mechanical ventilation. After intubation, sheep were positioned in right lateral recumbency and mechanically ventilated with pure oxygen and zero end-expiratory pressure (ZEEP). After 60 minutes, 10 cm H(2)O of PEEP was applied for 20 minutes. Spiral computed tomography of the thorax was performed, and data were recorded for hemodynamic and gas exchange variables and indicators of respiratory mechanics after 15 (T(15)), 30 (T(30)), and 60 (T(60)) minutes of ZEEP and after 20 minutes of PEEP (T(PEEP)). Computed tomography images were analyzed to determine the extent of atelectasis before and after PEEP application.

RESULTS

At T(PEEP), the volume of poorly aerated and atelectatic compartments was significantly smaller than at T(15), T(30), and T(60), which indicated that there was PEEP-induced alveolar recruitment and clearance of anesthesia-induced atelectasis. Arterial oxygenation and static respiratory system compliance were significantly improved by use of PEEP.

CONCLUSIONS AND CLINICAL RELEVANCE

Pulmonary atelectasis can develop in anesthetized and mechanically ventilated sheep breathing pure oxygen; application of 10 cm H(2)O of PEEP significantly improved lung aeration and gas exchange.

Extrapolation from ten sections can make CT-based quantification of lung aeration more practicable

PURPOSE

Clinical applications of quantitative computed tomography (qCT) in patients with pulmonary opacifications are hindered by the radiation exposure and by the arduous manual image processing. We hypothesized that extrapolation from only ten thoracic CT sections will provide reliable information on the aeration of the entire lung.

METHODS

CTs of 72 patients with normal and 85 patients with opacified lungs were studied retrospectively. Volumes and masses of the lung and its differently aerated compartments were obtained from all CT sections. Then only the most cranial and caudal sections and a further eight evenly spaced sections between them were selected. The results from these ten sections were extrapolated to the entire lung. The agreement between both methods was assessed with Bland-Altman plots.

RESULTS

Median (range) total lung volume and mass were 3,738 (1,311-6,768) ml and 957 (545-3,019) g, the corresponding bias (limits of agreement) were 26 (-42 to 95) ml and 8 (-21 to 38) g, respectively. The median volumes (range) of differently aerated compartments (percentage of total lung volume) were 1 (0-54)% for the nonaerated, 5 (1-44)% for the poorly aerated, 85 (28-98)% for the normally aerated, and 4 (0-48)% for the hyperaerated subvolume. The agreement between the extrapolated results and those from all CT sections was excellent. All bias values were below 1% of the total lung volume or mass, the limits of agreement never exceeded ± 2%.

CONCLUSION

The extrapolation method can reduce radiation exposure and shorten the time required for qCT analysis of lung aeration.

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.

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.

Use of dynamic compliance for open lung positive end-expiratory pressure titration in an experimental study

OBJECTIVE

We tested whether the continuous monitoring of dynamic compliance could become a useful bedside tool for detecting the beginning of collapse of a fully recruited lung.

DESIGN

Prospective laboratory animal investigation.

SETTING

Clinical physiology research laboratory, University of Uppsala, Sweden.

SUBJECTS

Eight pigs submitted to repeated lung lavages.

INTERVENTIONS

Lung recruitment maneuver, the effect of which was confirmed by predefined oxygenation, lung mechanics, and computed tomography scan criteria, was followed by a positive end-expiratory pressure (PEEP) reduction trial in a volume control mode with a tidal volume of 6 mL/kg. Every 10 mins, PEEP was reduced in steps of 2 cm H2O starting from 24 cm H2O. During PEEP reduction, lung collapse was defined by the maximum dynamic compliance value after which a first measurable decrease occurred. Open lung PEEP according to dynamic compliance was then defined as the level of PEEP before the point of collapse. This value was compared with oxygenation (Pao2) and CT scans.

MEASUREMENTS AND MAIN RESULTS

Pao2 and dynamic compliance were monitored continuously, whereas computed tomography scans were obtained at the end of each pressure step. Collapse defined by dynamic compliance occurred at a PEEP of 14 cm H2O. This level coincided with the oxygenation-based collapse point when also shunt started to increase and occurred one step before the percentage of nonaerated tissue on the computed tomography exceeded 5%. Open lung PEEP was thus at 16 cm H2O, the level at which oxygenation and computed tomography scan confirmed a fully open, not yet collapsed lung condition.

CONCLUSIONS

In this experimental model, the continuous monitoring of dynamic compliance identified the beginning of collapse after lung recruitment. These findings were confirmed by oxygenation and computed tomography scans. This method might become a valuable bedside tool for identifying the level of PEEP that prevents end-expiratory collapse.

Alveolar recruitment assessed by positron emission tomography during experimental acute lung injury

OBJECTIVES

To compare changes in aerated lung volumes measured by positron emission tomography (PET) and inflation volume-pressure curve (V-P) of the respiratory system, and to evaluate the reliability of PET to assess alveolar recruitment.

DESIGN AND SETTING

Experimental study in six anesthetized and mechanically ventilated pigs in a PET facility in an experimental university laboratory.

INTERVENTIONS

Lung injury was induced by oleic acid. Animals were randomly studied in four conditions: PEEP 0cmH(2)O (ZEEP) in supine position (SP), PEEP 10cmH(2)O in SP, ZEEP in prone position (PP) and PEEP in PP, each applied for 30min.

MEASUREMENTS AND RESULTS

With PET aerated lung volume was obtained from pulmonary density analysis using transmission scan (VA(trans)) and from nitrogen-13 kinetics on emission scan (VA(em)). Changes in VA(trans) and VA(em) were computed as the difference in aerated volume between conditions. VA(trans) and VA(em) did not differ between SP and PP, on either ZEEP or PEEP, suggesting no modification in relaxation volume of the respiratory system induced by posture. Changes in VA(trans) or VA(em) were significantly correlated with changes in aerated volume assessed from superimposed V-P curves (R (2)=0.74 and 0.75, respectively). Alveolar recruitment assessed by PET was significantly correlated with both PaO(2) (R (2)=0.61) and PaCO(2) (R (2)=0.40) variations induced by PEEP.

CONCLUSIONS

PET is a new reliable tool of scientific interest to image lung volume and alveolar recruitment during acute lung injury.

Computed tomography studies of lung mechanics

The study of lung mechanics has progressed from global descriptions of lung pressure and volume relationships to the high-resolution, three-dimensional, quantitative measurement of dynamic regional mechanical properties and displacements. X-ray computed tomography (CT) imaging is ideally suited to the study of regional lung mechanics in intact subjects because of its high spatial and temporal resolution, correlation of functional data with anatomic detail, increasing volumetric data acquisition, and the unique relationship between CT density and lung air content. This review presents an overview of CT measurement principles and limitations for the study of regional mechanics, reviews some of the early work that set the stage for modern imaging approaches and impacted the understanding and management of patients with acute lung injury, and presents evolving novel approaches for the analysis and application of dynamic volumetric lung image data.

Analysis of Atelectasis, Ventilated, and Hyperinflated Lung During Mechanical Ventilation by Dynamic CT

STUDY OBJECTIVE

To study the dynamics of lung compartments by dynamic CT (dCT) imaging during uninterrupted pressure-controlled ventilation (PCV) and different positive end-expiratory pressure (PEEP) settings in healthy and damaged lungs.

DESIGN

Experimental animal investigation.

SETTING

Experimental animal facility of a university department.

INTERVENTIONS

In seven anesthetized pigs, static inspiratory pressure volume curves were obtained to identify the individual lower inflection point (LIP) before and after saline solution lung lavage. During PCV, PEEP was adjusted 5 millibars (mbar) below the individually determined LIP (LIP - 5), at the LIP, and 5 mbar above the LIP (LIP + 5).

MEASUREMENTS AND RESULTS

Measurements were repeated before and after induction of lung damage. Hemodynamics, arterial and mixed venous blood gases, and dCT imaging in one juxtadiaphragmatic slice (effective temporal resolution of 100 ms) were assessed during uninterrupted PCV in series of three successive respiratory cycles. The mean fractional area (FA) of the hyperinflated lung (FA-H), mean FA of ventilated lung, mean FA of poorly ventilated lung, and mean FA of nonventilated lung (FA-NV), and the change in FA of the whole lung area (DeltaFA) were compared at different PEEP settings. Calculated pulmonary shunt (Qs/Qt) was compared to FA-NV. LIP + 5 decreased the amount of atelectasis (FA-NV) and increased hyperinflation (FA-H) in healthy and injured lungs. Cyclic changes of atelectasis (DeltaFA-NV) and hyperinflation (DeltaFA-H) were observed in both healthy and injured lungs. In the injured but not in the healthy lungs, the amount of cyclic changes of atelectasis and hyperinflation were independent from the adjusted PEEP level. FA-NV correlated with the calculated Qs/Qt, with a slight overestimation (mean +/- SEM, 2.1 +/- 4.1%).

CONCLUSIONS

dCT imaging allows the following: (1) the quantification of the extent of atelectasis, ventilated, poorly ventilated, and hyperinflated lung parenchyma during ongoing mechanical ventilation; (2) the detection and quantification of repeated recruitment and derecruitment, as well as hyperinflation; and (3) an estimation of Qs/Qt. dCT adds promising functional information for the respiratory treatment of early ARDS.

Regional ventilation and lung mechanics using X-Ray CT

Advances in computed tomographic (CT) imaging of the lung in the past decade, particularly with increased speed, resolution, gating capability, and rapidly expanding volumetric image acquisition, along with advances in image processing, have expanded the repertoire of imaging methods beyond anatomic visualization into the noninvasive study of regional lung physiological function. Recognizing that significant local disease or dysfunction can exist before global measures begin to deteriorate, the motivation for the development and application of these regional techniques is to further our understanding of the basic pathophysiological characteristics of evolving lung disease and, ultimately, develop sensitive measures for its early detection. This review emphasizes the key elements of ventilation and lung mechanics relevant for regional approaches and CT measurement principles available for their study. Examples of established and evolving methods for imaging regional ventilation and mechanics, including the xenon CT ventilation method; the relationship between changing regional CT density and air volume change; and registration-based methods for examining regional lung expansion and strain, are presented.

Methodologic Aspects of Attenuation Distributions From Static and Dynamic Thoracic CT Techniques in Experimental Acute Lung Injury

BACKGROUND

In acute lung injury, thoracic CT is used to gain information about lung aeration and consolidation. This can be done either during breath-holding by spiral CT scanning of the entire lung or dynamically by scanning lung slices without interrupting ventilation. We hypothesized that attenuation distribution is dependent on static or dynamic scanning techniques. We also studied whether a variation in the CT cut level, corresponding to the diaphragm movement over a breath, had any effect on the attenuation distribution.

METHODS

Twenty-two pigs with oleic acid-induced lung injury were randomly assigned to receive pressure-controlled mechanical ventilation with or without spontaneous breathing. Transversal dynamic CT scans of the chest were performed in apical and juxtadiaphragmatic regions, and end-expiratory and end-inspiratory slices were selected. In addition, after clamping the tube at end-expiration and end-inspiration, respectively, spiral CTs were performed. Guided by morphologic structures, spiral CT slices matching the dynamic scan slice and three additional neighbored slices above the diaphragm were selected. Distributions of CT attenuation were calculated and summarized in ranges for comparison.

RESULTS

No significant difference in attenuation distributions between the two scanning methods or an interaction with the factors ventilation mode, ventilation phase, and attenuation range were found. In addition, attenuation distributions of four neighbored juxtadiaphragmatic slices, 8 mm thick, from the spiral CT did not differ statistically.

CONCLUSION

In an animal model of oleic acid-induced lung injury, analyses of transverse thoracic slices based on dynamic or static CT scanning showed comparable distributions of attenuation. Variations on the CT cut level of 24 mm had no significant effect on the distribution of Hounsfield unit numbers. CT attenuation distributions of transversal juxtadiaphragmatic slices were not dependent on exact position.

Inspiratory vs. expiratory pressure-volume curves to set end-expiratory pressure in acute lung injury

OBJECTIVE

To study the effects of two levels of positive end-expiratory pressure (PEEP), 2 cm H(2)O above the lower inflection point of the inspiratory limb and equal to the point of maximum curvature on the expiratory limb of the pressure-volume curve, in gas exchange, respiratory mechanics, and lung aeration.

DESIGN AND SETTING

Prospective clinical study in the intensive care unit and computed tomography ward of a university hospital.

PATIENTS

Eight patients with early acute lung injury.

INTERVENTIONS

Both limbs of the static pressure-volume curve were traced and inflection points calculated using a sigmoid model. During ventilation with a tidal volume of 6 ml/kg we sequentially applied a PEEP 2 cm H(2)O above the inspiratory lower inflection point (15.5+/-3.1 cm H(2)O) and a PEEP equal to the expiratory point of maximum curvature (23.5+/-4.1 cmH(2)O).

MEASUREMENTS AND RESULTS

Arterial blood gases, respiratory system compliance and resistance and changes in lung aeration (measured on three computed tomography slices during end-expiratory and end-inspiratory pauses) were measured at each PEEP level. PEEP according to the expiratory point of maximum curvature was related to an improvement in oxygenation, increase in normally aerated, decrease in nonaerated lung volumes, and greater alveolar stability. There was also an increase in PaCO(2), airway pressures, and hyperaerated lung volume.

CONCLUSIONS

High PEEP levels according to the point of maximum curvature of the deflation limb of the pressure-volume curve have both benefits and drawbacks.

A quantitative computed tomography assessment of brain weight, volume, and specific gravity in severe head trauma

BACKGROUND

Computed tomography DICOM images analysis allows a quantitative measurement of organ weight, volume and specific gravity in humans.

METHODS

The brain weight, volume and specific gravity of 15 traumatic brain-injury patients (3+/-2 days after trauma) were computed using a specially designed software (BrainView). Data were compared with those obtained from 15 healthy subjects paired for age and overall intracranial volume.

RESULTS

Hemisphere weight were 91 g higher in patients than in controls (1167+/-101 vs 1076+/-112 g; p<0.05). Specific gravity of hemispheres (1.0367+/-0.0017 vs 1.0335+/-0.0012 g/ml; p<0.001), brainstem (1.0302+/-0.0016 vs 1.0277+/-0.0015 g/ml; p<0.001) and cerebellum (1.0396+/-0.0020 vs 1.0375+/-0.0015 g/ml; p<0.05) was significantly higher in traumatic brain injury (TBI) patients than in controls (all p<0.0001 without interaction). This increase in specific gravity was evenly distributed between the hemispheres, the brainstem and the cerebellum, and the grey and white matter. It was more pronounced in the rostral than in the caudal areas of the hemispheres. It was independent of the volume of brain contusion, of the mechanism of head injury, of natremia and of initial Glasgow coma score.

CONCLUSION

Human TBI patients present a diffuse increase in specific gravity. This observation is in sharp opposition with the data derived from the experimental literature.

Low spatial resolution computed tomography underestimates lung overinflation resulting from positive pressure ventilation*

OBJECTIVE

In acute lung injury, lung overinflation resulting from mechanical ventilation with positive end-expiratory pressure (PEEP) can be assessed using lung computed tomography. The goal of this study was to compare lung overinflation measured on low and high spatial resolution computed tomography sections.

DESIGN

Lung overinflation was measured on thick (10-mm) and thin (1.5-mm) computed tomography sections obtained at zero end-expiratory pressure (ZEEP) and PEEP 10 cm H2O using a software including a color-coding system.

SETTING

A 20-bed surgical intensive care unit of a university hospital.

PATIENTS

Thirty mechanically ventilated patients with acute lung injury.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Overinflated lung volume was measured as the end-expiratory volume of lung regions with computed tomography attenuations <-900 Hounsfield units. Lung overinflation, expressed in percentage of the total lung volume, was significantly underestimated by thick computed tomography sections compared with thin computed tomography sections (0.4 +/- 1.6% vs. 3.0 +/- 4.0% in ZEEP and 1.9 +/- 4% vs. 6.8 +/- 7.3% in PEEP, p < .01). In patients with a diffuse loss of aeration, the overinflated lung volumes of thick and thin computed tomography sections were, respectively, 0.6 +/- 0.8 mL vs. 16 +/- 10 mL in ZEEP (p < .01) and 8 +/- 9 mL vs. 73 +/- 62 mL in PEEP (p < .05). In patients with a focal loss of aeration, this underestimation was more pronounced: 18 +/- 56 mL vs. 127 +/- 140 mL in ZEEP (p < .01) and 85 +/- 161 mL vs. 322 +/- 292 mL in PEEP (p < .01).

CONCLUSIONS

In patients with acute lung injury, an accurate computed tomography estimation of lung overinflation resulting from positive pressure mechanical ventilation requires high spatial resolution computed tomography sections, particularly when the lung morphology shows a focal loss of aeration.

Tomographic Study of the Inflection Points of the Pressure–Volume Curve in Acute Lung Injury

The inflection points of the pressure-volume curve have been used for setting mechanical ventilation in patients with acute lung injury. However, the lung status at these points has never been specifically addressed. In 12 patients with early lung injury we traced both limbs of the pressure-volume curve by means of a stepwise change in airway pressure, and a computed tomography (CT) scan slice was obtained for every pressure level. Although aeration (increase in normally aerated lung) and recruitment (decrease in nonaerated lung) were parallel and continuous along the pressure axis during inflation, loss of aeration and derecruitment were only significant at pressures below the point of maximum curvature on the deflation limb of the pressure-volume curve. This point was related to a higher amount of normally aerated tissue and a lower amount of nonaerated tissue when compared with the lower inflection point on both limbs of the curve. Aeration at the inflection points was similar in lung injury from pulmonary or extrapulmonary origin. There were no significant changes in hyperinflated lung tissue. These results support the use of the deflation limb of the pressure-volume curve for positive end-expiratory pressure setting in patients with acute lung injury.

Airway pressure-time curve profile (stress index) detects tidal recruitment/hyperinflation in experimental acute lung injury

OBJECTIVE

To evaluate whether the shape of the airway pressure-time (Paw-t) curve during constant flow inflation corresponds to radiologic evidence of tidal recruitment or tidal hyperinflation in an experimental model of acute lung injury.

DESIGN

Prospective randomized laboratory animal investigation.

SETTING

Department of Clinical Physiology, University of Uppsala, Sweden.

SUBJECTS

Anesthetized, paralyzed, and mechanically ventilated pigs.

INTERVENTIONS

Acute lung injury was induced by lung lavage. During constant inspiratory flow, the Paw-t curve was fitted to a power equation: airway pressure =a x time + c, where coefficient b (stress index) describes the shape of the curve:b = 1, straight curve; b < 1, progressive increase in slope; and b > 1, progressive decrease in slope. Tidal volume (Vt) was 6 mL/kg, and positive end-expiratory pressure was set to obtain a b value between 0.9 and 1.1 before (b = 1) and after (b = 1 after recruiting maneuver) application of a recruiting maneuver. Positive end-expiratory pressure was decreased and Vt increased to obtain 0.9 >b > 0.8 and 0.8 >b > 0.6, whereas positive end-expiratory pressure and Vt were both increased to obtain 1.3 >b > 1.1 and 1.5 >b > 1.3. Experimental conditions sequence was random.

MEASUREMENTS AND MAIN RESULTS

Pulmonary computed tomography was obtained during end-expiratory and end-inspiratory occlusions. Tidal recruitment was quantified as nonaerated (between -100 and +100 Hounsfield units) lung area at end-expiration minus end-inspiration. Tidal hyperinflation was quantified as hyperinflated (between -900 and -1000 Hounsfield units) lung area at end-inspiration minus end-expiration. Computed tomography images showed that tidal recruitment and tidal hyperinflation corresponded to b < 1 and b > 1, respectively. Stress index values and tidal recruitment and tidal hyperinflation values were significantly correlated (R =.917 and R =.911, p <.0001, respectively).

CONCLUSIONS

Shape of the Paw-t curve detects tidal recruitment and tidal hyperinflation.

Lung density distribution in dynamic CT correlates with oxygenation in ventilated pigs with lavage ARDS † †This study contains parts of the doctoral thesis of Elena Ribel

BACKGROUND

Fast dynamic computed tomography (dCT) has been used to assess regional dynamics of lung inflation and deflation processes. The aim of this study was to relate ventilation-induced changes in lung density distribution, as measured over several respiratory cycles by dCT, to oxygenation and shunt fraction in a lavage acute respiratory distress syndrome model.

METHODS

Six anaesthetized pigs underwent pressure-constant ventilation (FIO2=1.0, inspiratory:expiratory ratio=1:1) before and after induction of lung damage by saline lavage. Mean airway pressure (Paw) was varied (8, 13, 18, 23, 28, 33, and 38 cm H2O) in random order. At each Paw level, dCT acquisitions were performed over several respiratory cycles (Somatom Plus4, Siemens; supradiaphragmatic transverse slice; thickness=1 mm; temporal resolution=100 ms). During scanning at each Paw, arterial and mixed venous blood were obtained for blood gas analysis and shunt calculation. In each CT image, fractional areas (FA) of defined density ranges representing ventilated lung and atelectasis were determined by planimetry using dedicated software. The FA data of individual 100 ms scans were averaged over several respiratory cycles, and expressed as mean FA in percentage of total lung area at each Paw. For atelectatic lung parenchyma a quantitative relationship of the respective mean FA to shunt fraction was studied using regression analysis.

RESULTS

Under steady-state conditions, mean FA of atelectasis correlated linearly with the calculated shunt fraction (healthy lungs, r=+0.76; lavaged lungs, r=+0.89). There is a non-linear relationship between mean FA of ventilated lung parenchyma and mean FA of atelectasis with PaO2.

CONCLUSIONS

We conclude that dCT allows assessment of the effects of ventilator adjustments and resultant Paw; changes upon lung aeration and oxygenation rapidly, and with good spatial and temporal resolution. This may benefit patients with acute lung injury, whose ventilatory pattern may be optimized as early as during their first diagnostic workup.

Acute respiratory distress syndrome: lessons from computed tomography of the whole lung

OBJECTIVE

This review aims to show how computed tomography of the whole lung has modified our view of acute respiratory distress syndrome, and why it impacts on the optimization of the ventilatory strategy.

DATA SOURCES

Computed tomography allows an accurate assessment of the volumes of gas and lung tissue, respectively, and lung aeration. If computed tomographic sections are contiguous from the apex to the lung base, quantitative analysis can be performed either on the whole lung or, regionally, at the lobar level. Analysis requires a manual delineation of lung parenchyma and is facilitated by software, including a color-coding system that allows direct visualization of overinflated, normally aerated, poorly aerated, and nonaerated lung regions. In addition, lung recruitment can be measured as the amount of gas that penetrates poorly aerated and nonaerated lung regions after the application of positive intrathoracic pressure.

DATA SUMMARY

The lung in acute respiratory distress syndrome is characterized by a marked increase in lung tissue and a massive loss of aeration. The former is homogeneously distributed, although with a slight predominance in the upper lobes, whereas the latter is heterogeneously distributed. The lower lobes are essentially nonaerated, whereas the upper lobes may remain normally aerated, despite a substantial increase in regional lung tissue. The overall lung volume and the cephalocaudal lung dimensions are reduced primarily at the expense of the lower lobes, which are externally compressed by the heart and abdominal content when the patient is in the supine position. Two opposite radiologic presentations, corresponding to different lung morphologies, can be observed. In patients with focal computed tomographic attenuations, frontal chest radiography generally shows bilateral opacities in the lower quadrants and may remain normal, particularly when the lower lobes are entirely atelectatic. In patients with diffuse computed tomographic attenuations, the typical radiologic presentation of "white lungs" is observed. If these patients lie supine, lung volume is preserved in the upper lobes and reduced in the lower lobes, although the loss of aeration is equally distributed between the upper and lower lobes. This observation does not support the "opening and collapse concept" described as the "sponge model." In fact, interstitial edema, alveolar flooding, or both, not collapse, are histologically present in all regions of the lung in acute respiratory distress syndrome. Compression atelectasis is observed only in caudal parts of the lung, where external forces (such as cardiac weight, abdominal pressure, and pleural effusion) tend to squeeze the lower lobes. When a positive intrathoracic pressure is applied to patients with focal acute respiratory distress syndrome, poorly aerated and nonaerated lung regions are recruited, whereas lung regions that are normally aerated at zero end-expiratory pressure tend to be rapidly overinflated, increasing the risk of ventilator-induced lung injury.

CONCLUSION

Selection of the optimal positive end-expiratory pressure level should not only consider optimizing alveolar recruitment, it should also focus on limiting lung overinflation and counterbalancing compression of the lower lobes by maneuvers such as appropriate body positioning. Prone and semirecumbent positions facilitate the reaeration of dependent and caudal lung regions by partially relieving cardiac and abdominal compression and may improve gas exchange.

Lung computed tomography during a lung recruitment maneuver in patients with acute lung injury

OBJECTIVE

To assess the acute effect of a lung recruitment maneuver (LRM) on lung morphology in patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS).

PATIENTS

Ten patients with ALI/ARDS on mechanical ventilation.

DESIGN

Prospective clinical study.

SETTING

Computed tomography (CT) scan facility in a teaching hospital.

INTERVENTIONS

An LRM performed by stepwise increases in positive end-expiratory pressure (PEEP) of up to 30-40 cm H(2)O. Lung basal CT sections were taken at end-expiration (patients 1 to 5), and at end-expiration and end-inspiration (patients 6 to 10). Arterial blood gases and static compliance (C(st)) were measured before, during and after the LRM.

MEASUREMENTS AND MAIN RESULTS

Poorly aerated and non-aerated tissue at PEEP 10 cm H(2)O accounted for 60.0+/-29.1% of lung parenchyma, while only 1.1+/-1.8% was hyperinflated. Increasing PEEP to 20 and 30 cm H(2)O, compared to PEEP 10 cm H(2)O, decreased poorly aerated and non-aerated tissue by 16.2+/-28.0% and 33.4+/-13.8%, respectively ( p<0.05). This was associated with an increase in PaO(2) and a decrease in total static compliance. Inspiration increased alveolar recruitment at all PEEP levels. Hyperinflated tissue increased up to 2.9+/-4.0% with PEEP 30 cm H(2)O, and to a lesser degree with inspiration. No barotrauma or severe hypotension occurred.

CONCLUSIONS

Lung recruitment maneuvers improve oxygenation by expanding collapsed alveoli without inducing too much hyperinflation in ALI/ARDS patients. An LRM during the CT scan gives morphologic and functional information that could be useful in setting ventilatory parameters.

Influence of low tidal volumes on gas exchange in acute respiratory distress syndrome and the role of recruitment maneuvers

BACKGROUND Use of a low tidal volume (V(T)) strategy in the treatment of acute respiratory distress syndrome can lead to a decrease in oxygenation. This study evaluated the safety and efficacy of a recruitment maneuver (RM) in this setting.

METHODS

Twelve patients with acute respiratory distress syndrome were studied within 48 hours of diagnosis. Baseline gas exchange, hemodynamics, and respiratory mechanics were determined and patients were placed on a V(T) of 6 mL/kg. Measurements were repeated and an RM of 30 cm H20 for 40 seconds was performed. Measurements were repeated at 30 minutes and 2 hours post-RM.

RESULTS

Decreasing V(T) resulted in a decrease in arterial oxygenation (from 91 +/- 9 mm Hg to 75 +/- 9 mm Hg, p < 0.01), an increase in shunt (from 19 +/- 3.7% to 23 +/- 5%, p < 0.01), and a decrease in lung compliance (from 37 mL/cm H2O to 33 mL/cm H2O, < 0.05). At 30 minutes post-RM, oxygenation improved to 99 +/- 16 mm Hg, shunt decreased to 17 +/- 3%, and lung compliance improved to 39 mL/cm H2O (p < 0.05). Two hours later, oxygenation fell (86 +/- 12 mm Hg), shunt increased (20 +/- 3%), and compliance diminished (36 mL/cm H2O). There were no hemodynamic or barotraumatic complications.

CONCLUSION

An RM transiently improves gas exchange during low V(T)ventilation. RMs are well tolerated and no hemodynamic consequences were seen.