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Prediction of pulmonary function in COPD on the basis of CT measurements of bronchial wall thickness.

Editor: With the latest technical advances in computed tomography (CT), in vivo measurements of human airway dimensions, such as lumen area and wall thickness, have now become feasible even for small bronchi. Authors of recent studies have explored the correlations between these bronchial dimensions and pulmonary function. In this respect, we read with interest the article by Dr Orlandi and colleagues (1), in the February 2005 issue of Radiology, on the associations between wall thickening and lung function in chronic obstructive pulmonary disease (COPD). They assessed wall thickening by measuring percentage wall area (PWA) and wall thickness-to-diameter ratio (TDR) and subsequently normalized PWA for body weight. In a multiple regression analysis they found that a combination of PWA, TDR, and PWA per kilogram of body weight could significantly predict gas exchange (diffusing capacity of the lung for carbon monoxide [DLCO]) and airway obstruction (forced expiratory volume in 1 second–vital capacity ratio [FEV1/VC]) (both expressed as percentage of predicted value). However, it is unclear why these three parameters were included in the analysis, as they basically measure the same feature (ie, wall thickness relative to lumen size). Moreover, the inclusion of highly correlated predictors such as PWA and TDR is undesirable, since it causes unstable regression coefficient estimates (2). Stable estimates of regression coefficients and their confidence intervals are essential for the interpretation of a model, but these were not provided by the authors. However, our main concern is that the authors probably have introduced a confounder. The normalization of PWA to body weight suggests that weight was associated with airway dimensions (the authors did not elucidate this matter). By entering both original PWA and weight-normalized PWA as predictor variables, it is likely that a confounding factor has been introduced, as an association between body weight and DLCO (or FEV1/VC) could explain the association found by means of the multiple regression. A relationship between body weight and DLCO and FEV1/VC is conceivable, since weight loss is related to the progression of COPD (3). We suggest that the authors test whether the combination of PWA, PWA normalized to body weight, and weight significantly improves the prediction compared with a model that includes only weight.