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.