Pulmonary function tests in systemic sclerosis-associated interstitial lung disease: new directions and future prospects

Abstract

Axial CT scan acquired 2 cm below the dome of the right hemidiaphragm in supine position in a representative 61-year-old woman, former smoker (9 pack-year), with limited cutaneous SSc. Noteworthy, the discrepancy between normal lung volumes and standard DL CO and decreased DL NO values despite extensive (41%) fibrosis. Both sPap (systolic pulmonary artery pressure) and TAPSE (tricuspid annular plane systolic excursion), measured by Doppler echocardiography, are within normal limits. Other abbreviations are as in the text. • Spirometry and lung volumes show good specificity but poor sensitivity to pulmonary fibrosis in systemic sclerosis. • Lung diffusing capacity for nitric oxide is more sensitive than lung diffusing capacity for carbon monoxide. • Lung diffusing capacity for nitric oxide may be reduced even in the absence of lung restriction or fibrosis. • Evaluation of ventilatory efficiency during exercise may help interpreting breathlessness. Forced vital capacity and lung diffusing capacity for carbon monoxide (DL CO ) are traditionally used to assess lung involvement in systemic sclerosis (SSc) whereas total lung capacity is less frequently measured as marker for progression of systemic sclerosis-associated interstitial lung disease (SSc-ILD). Oscillometry has been recently proposed to disentangle the complexity of lung mechanical abnormalities in SSc-ILD, but the findings were inconsistent possibly due to device-related discrepancies. In the last three decades, the lung diffusing capacity for nitric oxide (DL NO ) has been introduced because is more sensitive to membrane diffusion limitation than DL CO . Thus, DL NO may be abnormal even in the absence of lung restriction or fibrosis at computed tomography suggesting that it may reveal gas exchange alterations in SSc-ILD not detectable by other measurements. Finally, exercise studies have shown that higher slopes of expiratory minute ventilation versus carbon dioxide output relationship may predict mortality from SSc-ILD or pulmonary vasculopathy.