Can Computed Tomography Predict Functional Benefit from Lung Volume Reduction Surgery for Emphysema?

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Lung volume reduction surgery (LVRS) has been widely practiced since its reintroduction by Cooper and coworkers in 1995 (1). Patient selection criteria have evolved, but identification of patients who are more likely to have a functional benefit remains problematic. Yet this issue is critical because between 20% and as much as 50% of patients undergoing LVRS show little or no improvement in FEV₁ (2). Several studies have shown that the functional benefit from LVRS depends in part on the distribution of areas of lung destruction by emphysema (3). Although these studies used different imaging techniques (chest X-ray, computed tomography [CT], perfusion scintigraphy) and indexes to assess the distribution of the disease, they all found a trend toward greater functional gains in patients with emphysematous lesions involving principally the upper lung fields; conversely, LVRS for lower lobe predominant emphysema yielded poor functional results.

In the study by Nakano and coworkers (pp. 2195-2199) published in this issue of the American Journal of Respiratory and Critical Care Medicine (8), CT measurements of lung attenuation obtained before LVRS were used to quantify the severity of emphysema in the core and the rind of the lung, and the percentage of severe emphysema in these regions was correlated with functional outcome after surgery. The investigators found that a greater proportion of severe emphysema (corresponding to lesions > 5 mm) in the rind of the upper lung predicted greater increases in FEV₁ and maximal exercise capacity, presumably because emphysematous lesions in the upper rind are more accessible to surgery. The authors did not report if using a cutoff value corresponding to more severe emphysema (e.g., lesions > 10 mm) improved the prediction.

The technique used by Nakano and coworkers (8) provides estimates of the severity of emphysema derived from the distribution curve of attenuation values. Recently, Mishima and coworkers (9) proposed to quantify emphysema by measuring the actual size of clusters of adjacent voxels of low attenuation. In a preliminary study, they showed that the cumulative size distribution of such clusters follows a power law characterized by an exponent D and that D reflects the extent of terminal airspace enlargement. This technique thus measures the actual size of emphysematous lesions. It would be of interest to determine whether functional outcome after LVRS is better predicted by the severity of emphysema as quantified by the method of Mishima and coworkers (9) or Nakano and coworkers (8).

The results obtained by Nakano and coworkers (8) are consistent with theoretical studies suggesting that the functional benefit from LVRS should increase as areas that are more hyperinflated are removed (10). Similarly, Thurnheer and coworkers (6) have recently reported that a greater preoperative degree of hyperinflation (as measured by an increased residual volume/total lung capacity [RV/TLC] ratio) may predict larger functional improvements after LVRS. The RV/TLC ratio, however, reflects the overall hyperinflation of the lung rather than the hyperinflation of target areas amenable to resection. It might be useful to explore whether CT obtained at full inspiration and full expiration might be used to compute regional estimates of RV/TLC ratios in these areas. This assessment may prove complementary to the quantification of emphysema in the rind of the lung as performed by Nakano and coworkers (8).

The aforementioned studies (6, 8, 10) thus suggest a significant impact of the morphology and function of the resected lung parenchyma on functional gains from LVRS, but these gains should also be influenced by the characteristics of the lung remaining within the chest after surgery, i.e., the less emphysematous this lung, the greater the functional improvement. Interestingly, data reported by Nakano and coworkers (8) do not demonstrate such a relationship. It is not known, however, whether the cutoff value of lung expansion proposed in this study to identify normal lung (< 6.0 ml/g) may be used in patients with emphysema. Because this cutoff is derived from CT data obtained at TLC in normal subjects (11), it may not be valid when applied to areas of normal, yet relatively collapsed, lung as may be found in these patients. Further studies are needed to explore this point and develop reliable CT methods to investigate the quality of lung parenchyma in areas that are not amenable to resection.

In addition to radiologic variables, the use of preoperative physiologic variables has been proposed to predict the functional response to LVRS. For example, it has been shown that the increase in FEV₁ is greater in patients with high values of inspiratory lung conductance before surgery, presumably because this identifies patients with predominant emphysema as opposed to intrinsic airways disease (7). However, measurement of lung conductance is not widely available and requires the use of an esophageal balloon. In a prior study in smokers, Nakano and coworkers (12) have provided CT measurements of airway dimensions and have shown that airway wall thickness ranges from normal to markedly increased in patients with emphysema. It would be of interest to investigate whether combined CT measurements of airway dimensions and emphysema can be used as a substitute for inspiratory lung conductance to identify patients with predominant emphysema.

It is unlikely that a single preoperative variable will ever allow one to predict accurately functional outcome after LVRS. The correlations that have been reported so far between radiologic or physiologic indexes and functional gains have been at best moderate (3). Future studies will demonstrate whether the prediction of functional outcome might be improved by developing tools that assess more specifically the morphologic and physiologic alterations of target areas for resection, and of areas that will be left behind after surgery. The study by Nakano and coworkers (8) is a first step in this direction.

PIERRE ALAIN GEVENOIS, M.D.

MARC ESTENNE, M.D.

Departments of Radiology and Chest Medicine

Erasme University Hospital

Brussels, Belgium

DOI:

	References

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