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Ann Thorac Surg 2006;82:261-266
© 2006 The Society of Thoracic Surgeons
a Division of Pulmonary and Critical Care, Beth Israel Deaconess Medical Center
b Division of Cardiology, Beth Israel Deaconess Medical Center
c Department of Medicine, Brigham and Women's Hospital
d Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts
e Department of Surgery, Massachusetts General Hospital, Boston, Massachusetts
f Department of Pulmonary and Critical Care Division, University Hospitals of Cleveland, Cleveland, Ohio
Accepted for publication February 6, 2006.
* Address correspondence to Dr Cho, Brookline Avenue KS-B23, Boston, MA 02215 (Email: mcho{at}bidmc.harvard.edu).
| Abstract |
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METHODS: We performed a retrospective analysis of 53 patients who underwent open lung biopsy for clinical ARDS (based on American European Consensus Conference criteria) between 1989 and 2000.
RESULTS: Sixteen patients (30.2%) developed an air leak lasting more than 7 days or died with an air leak. Univariate analyses showed no significant correlation with age, gender, sex, corticosteroid use, diabetes, immunocompromised status, or pathologic diagnosis. A lower risk of air leak was associated with lower peak airway pressure and tidal volume, use of pressure-cycled ventilation, and use of an endoscopic stapling device. In multivariate analyses, only peak airway pressure remained a significant predictor. The risk of prolonged air leak was reduced by 42% (95% confidence interval [CI: 17% to 60%]) for every 5 cm H2O reduction in peak airway pressure.
CONCLUSIONS: The use of a lung-protective ventilatory strategy that limits peak airway pressures is strongly associated with a reduced risk of postoperative air leak after open lung biopsy in ARDS. Using such a strategy may allow physicians to obtain information from open lung biopsy to make therapeutic decisions without undue harm to ARDS patients.
| Introduction |
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The most common postoperative complication of open lung biopsy noted in several studies of patients with acute respiratory failure is prolonged air leak, with an incidence rate of 3% to 22% [37]. Identification of baseline characteristics predictive of persistent air leak may allow for better risk stratification of patients. In addition, identification of risk factors that can be intervened upon may make biopsy safer.
Prolonged air leak is also the most common complication prolonging hospital stay in routine pulmonary resection [8], and studies have identified risk factors for prolonged postoperative air leaks in this setting. However, the majority of these cases are for lung cancer secondary to tobacco abuse, and thus emphysema in the underlying lung tissue is fairly common. In fact, many of the described risk factors such as forced expiratory volume in one second (FEV1), smoking, steroid use, and male gender may actually represent surrogate measures of emphysema severity. The relevance of these data to the ARDS setting, where underlying emphysema is rare, is therefore unclear. No studies, to our knowledge, have identified risk factors for this complication in ARDS. Thus, the goal of this study was to identify predictors of persistent air leak after open lung biopsy in patients with ARDS.
| Material and Methods |
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For thoracotomy, a standard anterolateral or lateral muscle sparing incision was made. For thoracoscopy, patients were reintubated with a double lumen endotracheal tube. Two intercostal access sites were chosen in standard orientation. Inspection with a thoracoscope was performed, and manipulation was performed using small Duval forceps. For both procedures, after inspection of the lung, biopsy specimens were taken from at least two different areas representing the spectrum of the disease process, usually in dependent areas of the middle lobe (or lingula) and the lower lobe. Biopsies were performed with serial application of a stapler. Staple lines were inspected for bleeding and air leak. In addition, if the patient was able to tolerate lateral decubitus positioning, the site was also tested for air leak by submersion in warm saline. Glues and sealants were not used. One or two chest tubes were inserted before closing the chest at the discretion of the surgeon. Chest tubes were placed to suction at 20 cm H2O for 48 hours, and were placed to water seal after that point if there was no evidence of air leak. Postoperative evaluation for air leak occurred at least once daily. No major changes in surgical technique or postoperative care at our institution occurred over the course of the study except for the introduction in mid-1995 of a surgical stapling device (Endo-GIA stapler, U.S. Surgical, Norwalk, CT) designed for endoscopic use for the resection of lung parenchyma.
Potential predictors of air leak considered in this study included baseline characteristics (age, gender, chronic steroid use, immune status, etc), ventilator strategy (tidal volumes, peak pressures, positive end expiratory pressure [PEEP], mode of ventilation), surgical technique (thoracotomy versus thoracoscopy, use of endoscopic stapler), and underlying diagnosis (infection, diffuse alveolar damage).
Summary data are presented as mean ± SD for normally distributed variables and as median and range otherwise. Differences between groups were compared with the Fisher exact test for dichotomous variables, the Student t test for continuous variables with a normal distribution, and the Wilcoxon rank sum test for non-normally distributed variables. Pearson correlations were used to assess associations between continuous variables. Logistic regression was performed to identify risk factors for persistent air leak and death in univariate analyses. Variables with p less than 0.1 in univariate analysis were then used as independent variables in a stepwise logistic regression analysis, with a p less than 0.05 criterion for retention of variables in the final model. The multivariate procedure was validated by bootstrap bagging with 1,000 samples as has been previously described [10]. In the bootstrap procedure, repeated samples were generated with replacement from the original set of observations. For each sample, stepwise logistic regression was performed entering the predictors with p less than 0.1 at univariate analysis. The stability of the final stepwise model can be assessed by identifying the variables that enter most frequently in the repeated bootstrap models and comparing with the variables in the final stepwise model. If the final stepwise model variables occur in a majority (>50%) of the bootstrap models, the original final stepwise regression model can be judged stable. All analyses were performed using SAS version 8.0 (SAS Institute, Cary, NC).
| Results |
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Sensitivity analyses were performed considering the five patients who died with an air leak before day 7 as not having an air leak and then excluding them completely. No substantial difference in our findings was observed. A higher peak ventilatory pressure was significantly associated with an increased risk of persistent air leak [data not shown].
| Comment |
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Several risk factors have been identified for air leak in more routine lung resection. These include age; male gender; low forced expiratory volume in one second (FEV1) and ratio of FEV1 to forced vital capacity (FEV1/FVC); steroid use; concurrent infection; diabetes; malnutrition; presence of pleural adhesions; upper versus lower lobectomy; and individual surgeon [8, 1012]. While the presence of diffuse lung disease has not been prospectively studied, in patients with chronic obstructive pulmonary disease undergoing lung volume reduction surgery the presence of additional pathology appears to increase the risk for persistent air leak [13]. We assessed several of these previously determined risk factors (age, gender, smoking, concurrent infection, diabetes) and found no significant correlation with air leak risk. Several operative techniques, such as buttressing the staple line, glues and sealants, and pleural tents, as well as postoperative chest tube management have been explored as solutions to prevent or control air leaks, with variable success [11, 1419]. During the time period of our study, the surgical stapler at our institution was changed to a disposable device that had better staple formation on thicker tissue and placed an additional row of staples to secure the lung. Use of this stapler was associated with reduced risk of persistent air leak. We were unable to evaluate other aspects of operative and postoperative care, including chest tube management, as they did not vary across the cohort.
Controlling airway pressures has been recommended both for the management and prevention of persistent air leaks [20, 21]. In surgically created bronchopleural fistulae in animals, the rate of gas flow was primarily influenced by mean airway pressure [22]; thus, presumably reducing the mean airway pressures would reduce flow through the fistula and lead to faster resolution. However, more specific data are lacking. While high frequency ventilation has been used in management of bronchopleural fistulas, we are not aware of any data concerning the effect of ventilator mode (volume-cycled versus pressure-cycled) on risk of persistent air leak. We found a reduced risk associated with the use of pressure-cycled ventilation. This may be due to a better ability to ensure against brief rises in airway pressure with this modality.
Although the etiology differs, barotrauma resulting in persistent air leaks can be seen in the setting of ARDS. This complication may increase the duration of mechanical ventilation, hospital length of stay, and hospital mortality [23]. The striking difference in the incidence of barotrauma in recent trials compared with those reported 10 to 15 years ago lends support to the hypothesis that higher airway pressure is a major contributor [24]. For example, Amato and colleagues [25] found a marked reduction in pneumothorax risk using a lung protective strategy with lower tidal volumes and ventilatory pressures. More recently, a review of ARDS clinical trials found lower plateau pressures (below 35 cm H2O) were associated with a reduction in risk of barotrauma [26].
While our data support a correlation between reduction in ventilator pressures and risk of persistent air leak, there are several limitations that should be noted. First, the pressure across the lung parenchyma (the transpulmonary or transalveolar pressure) more accurately reflects the potentially injurious distending pressure on the lung than the airway opening pressures measured and applied by the ventilator. While true transpulmonary pressures are difficult to measure without additional equipment, plateau pressure is likely a more reliable surrogate than peak pressure; however, none of these data were available for our study. Although we acknowledge this limitation, we would argue that the airway opening pressure is a more practical variable for the treating clinician to monitor and (or) modify.
Second, we cannot discount the possibility that a lower peak airway pressure in our population was simply a reflection of less diseased lung. However, high peak airway pressures were more strongly associated with prolonged air leak than other measures of lung injury severity such as PaO 2/FIO 2 ratio. The lack of correlation between peak pressure and PaO 2/FIO 2 ratio further suggests that peak pressure was an independent variable determined by the treating clinician rather than a dependent variable of lung injury severity.
A notable finding of our study was the large drop in the risk of air leak over the 12-year period of this study. Our data suggest the fall in peak pressures used to ventilate patients during this time explains this temporal drop in risk. However, other changes may also explain this finding. For example, the endoscopic stapler was introduced into general use at our institution in mid-1995. Other unmeasured aspects of care may also have changed over time; aspects such as better nutritional support, development of more effective antibiotics, and improvements in overall ICU care. Because of the observational nature of this study, we cannot definitively establish whether minimizing airway pressures in this setting leads to improvements in clinical outcome.
While the results of the ARDSNet study advocating lower tidal volumes and ventilatory pressures have generally been accepted, aspects of the study have been questioned [27] and ventilation with low tidal volumes is still not routinely applied [28]. Despite the limitations in our study, the strong correlation between inspiratory pressures, tidal volumes, and risk of prolonged air leak lends support to the notion that reducing airway pressures and tidal volume may reduce the risk of postoperative air leak. Further research to test this hypothesis is merited. Whatever the etiology, the risk of morbidity from open lung biopsy in ARDS at our institution has dropped substantially since the 1980s. In light of these improvements, more frequent use of this diagnostic modality should be considered. Finally, the lack of correlation between corticosteroid administration and persistence of air leak should allay fears about employing this treatment in those cases where biopsy results support the use of these agents.
| References |
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