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Ann Thorac Surg 2000;70:1211
© 2000 The Society of Thoracic Surgeons
a Divisions of Pulmonary and Critical Care Medicine and General Medical Sciences, Washington University School of Medicine, St. Louis, MO 63110-1093, USA,
e-mail: yusenr{at}msnotes.wustl.edu
Baldwin and colleagues found a significant correlation between preoperative emphysema heterogeneity and change in forced expiratory volume in 1 second (FEV1) after bilateral excisional lung volume reduction surgery. Heterogeneity was determined by grading chest radiographs according to defined criteria. The severity of emphysema was assessed, and the difference in severity scores between the total scores of the two worst quadrants and the total scores of the two best quadrants determined the heterogeneity index. Although the authors determined heterogeneity from a radiologic standpoint, the severity of emphysema in potential surgical resection sites and nonresection sites was the issue at hand. For example, large areas of mostly destroyed emphysematous lung provided "target areas" for surgical resection, whereas large areas of lung relatively spared from emphysema were not targeted for surgical resection. The readers assume that the target areas visualized on the chest radiographs were the areas of lung that were actually resected.
Did resection of radiologic target areas of emphysema actually cause the change in FEV1? The evidence for causation can be assessed by addressing issues of (1) study validity, (2) biologic plausibility, (3) temporal sequence, (4) magnitude of effect, (5) dose–response effect, and (6) consistency of results. Although the study authors did not fully elucidate the validity and reliability of the heterogeneity measurement technique, they may have used an adequate method. Further validation of techniques that assess emphysema heterogeneity is required.
Assuming that the techniques used to measure heterogeneity were valid, causation was supported by the biologic plausibility of the relationship between heterogeneity and change in FEV1. If a severely damaged lung was surgically resected and a relatively good lung was not, then improvements in elastic recoil of the nonresected lung may have led to improvements in airflow obstruction. Situations with less dramatic heterogeneity may have led to less dramatic changes in airflow.
The temporal sequence criterion for causation was clearly met. Preoperative heterogeneity and FEV1 were measured. Three months after surgery, the FEV1 was remeasured and the change in FEV1 was calculated.
The magnitude of the association between preoperative heterogeneity and postoperative change in FEV1 was significant and clinically meaningful. In addition, a dose–response effect was present: greater heterogeneity was associated with greater improvements in FEV1.
Importantly, this study’s findings were consistent with the results of previous studies. In other studies, heterogeneity has been measured using computed tomography and lung scintigraphy as well as chest radiography, and similar findings have been described; more heterogeneous emphysema and resection of the most damaged areas of lung have been associated with greater improvements in FEV1 after surgery.
The evidence for causation may have been limited by confounding, a situation in which other factors related to preoperative heterogeneity may really be driving the postoperative change in FEV1. However, our institutional experience supports the findings of Baldwin and colleagues. Heterogeneity may be the major factor driving the improvement in FEV1 after surgery.
The study of Baldwin and researchers raises important questions. What is the best method for judging heterogeneity? What other factors predict improvements in airflow obstruction? How do we best judge the success of lung volume reduction surgery anyway? Should we not evaluate the success of lung volume reduction surgery by using quality of life and mortality as the main outcomes? Hopefully, the refinement of methods used to evaluate patients for lung volume reduction surgery will lead to improved patient outcomes.
Related Article
Ann. Thorac. Surg. 2000 70: 1208-1211.
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