Reply to commentary on: Confusion in reporting pulmonary diffusion capacity for nitric oxide and the alveolar-capillary membrane conductance for nitric oxide

Abstract

We read with interest the commentary by Gerald Zavorsky and Colin Borland on our paper on lung diffusion for nitric oxide (DLNO) vs lung diffusion for carbon monoxide (DLCO) in normal subjects and in patients with heart failure.1 Both authors are worldwide recognised experts. We are therefore indebted with them for the time and thought they gave to our work. The criticisms that they raise are both methodological and physiological. Indeed, DLNO measurement for clinical purposes has been introduced only a few years ago since the commercial apparatus has been available. Studies about simultaneous DLNO/DLCO evaluation have been conducted with different analysers and different NO concentrations so the method is still far from being standardised. However, we recognise that our DLNO values in normal subjects, obtained in a sizeable number of subjects, are lower than those previously reported by Zavorsky et al. 2 In any case the apparatus we used (Jaeger/Vyasis PFT Masterscreen) was calibrated for gas analysis using automated procedures and the linearity of analysers was factory checked. The formula used is: Concerning breath hold time, we used 4 s, which is the same as Aguilaniu et al. 8 used whose data were the basis of the normal standards for European equations for carbon monoxide (CO) and nitric oxide (NO). Anyway, with a longer breath hold time we would expect an underestimation of DLNO, as reported by Dressler et al., 9 and not an overestimation as suggested by Zavorsky and Borland. In fact as breath hold time is increased, NO expiratory concentration reduces, reaching the detection limit of the NO analyser. This is an issue when using electrochemical sensors, as we 1 and others did, 2,5,8 and not with a highsensitivity chemiluminescence NO analyser as other authors did. 10,11 As regards the membrane diffusion for carbon monoxide (DmCO), membrane diffusion for nitric oxide (DmNO) and capillary volume (Vcap), we prefer to call the capillary volume Vcap and not Vc as frequently done to avoid confusion with the vital capacity, and we do not agree with the Zavorsky and Borland critique. We recognise, however, that we erroneously quoted, and always have done so previously, the original paper by Roughton and Forster 12 but actually used the more recent description of DLCO partitioning.