Value of lung diffusing capacity for nitric oxide in systemic sclerosis

A decreased lung diffusing capacity for carbon monoxide (DLCO) in systemic sclerosis (SSc) is considered to reflect losses of alveolar membrane diffusive conductance for CO (DMCO), due to interstitial lung disease, and/or pulmonary capillary blood volume (V-C), due to vasculopathy. However, standard DLCO does not allow separate DMCO from V-C. Lung diffusing capacity for nitric oxide (DLNO) is considered to be more sensitive to decrement of alveolar membrane diffusive conductance than DLCO. Standard DLCO and DLNO were compared in 96 SSc subjects with or without lung restriction. Data showed that DLNO was reduced in 22% of subjects with normal lung volumes and DLCO, whereas DLCO was normal in 30% of those with decreased DLNO. In 30 subjects with available computed tomography of the chest, both DLCO and DLNO were negatively correlated with the extent of pulmonary fibrosis. However, DLNO but not DLCO was always reduced in subjects with >= 5% fibrosis, and also decreased in some subjects with DMCO and V-C partitioning and Doppler ultrasound-determined systolic pulmonary artery pressure could not explain individual differences in DLCO and DLNO. DLNO may be of clinical value in SSc because it is more sensitive to DMCO loss than standard DLCO, even in nonrestricted subjects without fibrosis, whereas DLCO 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 DLCO in the absence of lung restriction does not allow to suspect pulmonary arterial hypertension without fibrosis.