Value of lung diffusing capacity for nitric oxide in systemic sclerosis

A decreased lung diffusing capacity for carbon monoxide (DL(CO)) in systemic sclerosis (SSc) is considered to reflect losses of alveolar membrane diffusive conductance for CO (DM(CO)), due to interstitial lung disease, and/or pulmonary capillary blood volume (V(C)), due to vasculopathy. However, standard DL(CO) does not allow separate DM(CO) from V(C). Lung diffusing capacity for nitric oxide (DL(NO)) is considered to be more sensitive to decrement of alveolar membrane diffusive conductance than DL(CO). Standard DL(CO) and DL(NO) were compared in 96 SSc subjects with or without lung restriction. Data showed that DL(NO) was reduced in 22% of subjects with normal lung volumes and DL(CO), whereas DL(CO) was normal in 30% of those with decreased DL(NO). In 30 subjects with available computed tomography of the chest, both DL(CO) and DL(NO) were negatively correlated with the extent of pulmonary fibrosis. However, DL(NO) but not DL(CO) was always reduced in subjects with ≥ 5% fibrosis, and also decreased in some subjects with < 5% fibrosis. DM(CO) and V(C) partitioning and Doppler ultrasound‐determined systolic pulmonary artery pressure could not explain individual differences in DL(CO) and DL(NO). DL(NO) may be of clinical value in SSc because it is more sensitive to DM(CO) loss than standard DL(CO), even in nonrestricted subjects without fibrosis, whereas DL(CO) partitioning into its subcomponents does not provide information on whether diffusion limitation is primarily due to vascular or interstitial lung disease in individual subjects. Moreover, decreased DL(CO) in the absence of lung restriction does not allow to suspect pulmonary arterial hypertension without fibrosis.