Response From

We thank Dr Milne for his thoughtful comments about our recent article (1). The diagnostic utility of small vessel measurements obtained with CT has been demonstrated previously. For example, the ratio between the diameters of the segmental artery and lobar bronchiole in patients with pulmonary hypertension (PH) and chronic thromboembolic disease was found to be elevated in 50% of patients compared with 7% of healthy control subjects (2). In another study on patients with various parenchymal lung diseases, analysis of a combination of main pulmonary artery diameter and the segmental artery-tobronchus ratio afforded a sensitivity of 86% in terms of detecting PH (3). This sensitivity is remarkably similar to that in our study, in which only pulmonary artery diameter was considered. It is important to note that, in patients with lung fibrosis, traction bronchiectasis (which is common) will likely confound small vessel measurements (4). Further, the clinical significance of tortuous peripheral arteries, which reflect plexogenic arteriopathy and are occasionally detected with CT, is difficult to interpret when pulmonary fibrosis is present. Thus, we believe that neither measuring small vessels nor searching for tortuous peripheral arteries substantially improves sensitivity or specificity of PH detection in the context of fibrotic lung disease. In his correspondence, Dr Milne correctly describes the roles played by hypoxia-driven vasoconstriction, and struc tural changes in the small muscular arteries and arterioles, in the genesis of secondary PH. However,as pointed out in our study, exercise-induced hypoxemia caused blood viscosity to rise, and the consequent increase in shear stress affords a potential explanation for progressive dilatation of the pulmonary artery. Such dilatation was thus independent of pulmonary hemodynamic changes noted in the study by Boerrigter et al (5). PH is prognostically important in patients with ILD. However, the fact that only weak-to-moderate correlation was found between Spo2 and mean PAP in both our study and the work of others (6) indicates that pulse oximetry cannot be reliably used to screen for PH. Thus, right heart catheterization remains the gold standard for PH diagnosis. Improved noninvasive methods for accurately detecting PH are required.