Lung diffusing capacity for nitric oxide as a marker of fibrotic changes in idiopathic interstitial pneumonias

Lung diffusing capacity for carbon monoxide (DL_CO) is decreased in both usual interstitial pneumonia-idiopathic pulmonary fibrosis (UIP-IPF) and nonspecific interstitial pneumonia (NSIP), but is moderately related to computed tomography (CT)-determined fibrotic changes. This may be due to the relative insensitivity of DL_CO to changes in alveolar membrane diffusive conductance (DM_CO). The purpose of this study was to determine whether measurement of lung diffusing capacity for nitric oxide (DL_NO) better reflects fibrotic changes than DL_CO. DL_NO-DL_CO were measured simultaneously in 30 patients with UIP-IPF and 30 with NSIP. Eighty-one matched healthy subjects served as a control group. The amount of pulmonary fibrosis was estimated by CT volumetric analysis of visually bounded areas showing reticular opacities and honeycombing. DM_CO and pulmonary capillary volume (VC) were calculated. DL_NO was below the lower limit of normal in all patients irrespective of extent and nature of disease, whereas DL_CO was within the normal range in a non-negligible number of patients. Both DL_NO and DL_CO were significantly correlated with visual assessment of fibrosis but DL_NO more closely than DL_CO. DM_CO was also below the lower limit of normal in all UIP-IPF and NSIP patients and significantly correlated with fibrosis extent in both diseases, whereas VC was weakly correlated with fibrosis in UIP-IPF and uncorrelated in NSIP, with normal values in half of patients. In conclusion, measurement of DL_NO may provide a more sensitive evaluation of fibrotic changes than DL_CO in either UIP-IPF or NSIP, because it better reflects DM_CO.

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