Performance capacity evaluated using the 6-minute walk test: 5-year results in patients with diffuse systemic sclerosis and initial interstitial lung disease

Objective. To identify factors indicating exercise-induced oxygen desaturation during the 6-minute walk test (6MWT) in patients with diffuse systemic sclerosis (SSc) and initial interstitial lung disease (ILD). Methods. The study involved 121 consecutive adult anti-Scl 70 autoantibody-positive SSc patients with initial ILD, 93 of whom were followed up for five years. Before enrolment and then annually, the patients underwent high-resolution computed tomography (HRCT), functional lung tests, with carbon monoxide diffusion capacity of the lung (DLCO) and its components (alveolar-capillary membrane [Dm] and pulmonary blood volume [Vc]), the evaluation of dyspnea before and after the 6MWT using the Borg scale, and transthoracic echocardiography. A decrease in peripheral capillary oxygen saturation (SpO(2)) of >= 4% during the 6MWT was used to define desaturation, the appearance of which led to the patient being withdrawn from follow-up. Results. There were no significant differences in HRCT score during the follow-up, but 32 patients (35%) desaturated during the 6MWT, including 12 (37%) who experienced a severe decrease SpO(2), to <= 88%, indicating a high risk of mortality. At baseline, there was no statistically significant difference in any considered clinical characteristics between the desaturating and non-desaturating patients but, at the time of desaturation, the desaturators had lower minimum SpO(2)% levels during the 6MWT (p<0.0001), and lower DLCO (p<0.0001) and Dm (p<0.0001). Comparison of the desaturators defined on the basis of a reduction in SpO(2), to <= 88% and those defined on the basis of a decrease in SpO(2), of >= 4% showed that, at baseline, the former had lower minimum SpO(2)% levels during 6MWT (p<0.001), lower DLCO (p=0.01), a lower DLCO/VA ratio (p=0.05), lower Dm (p<0.005) and Vc values (p<0.5), and. higher RVsystP (p=0.01). At the time of desaturation, the desaturators minimum SpO(2), levels during the 6MWT correlated with their DLCO (r=0.78; p<0.001), Dm (r=0.65; p<0.01), Vc (r=0.52;p<0.05) and RV-systP values (r = -0.53; p<0.05). Conclusion. Our data seem to confirm the close interdependence between pulmonary diffusion and oxygen desaturation during exercise. In SSc combined 6MWT, DLCO and its components may indicate patients at increased risk of developing pulmonary hypertension.