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

Forced vital capacity and lung diffusing capacity for carbon monoxide (DLCO) 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 (DLNO) has been introduced because is more sensitive to membrane diffusion limitation than DLCO. Thus, DLNO 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.