Reference value for the 6-min walking distance in women with systemic sclerosis considering the impact of muscle strength

Background: Early triage, the search for new therapies, and closer monitoring of patients with systemic sclerosis before their lung function irreversibly deteriorates are urgent concerns. Because it is an independent predictor of systemic sclerosis-related mortality, the 6-min walk test is a potentially useful tool to evaluate outcomes, along with pulmonary function and computed tomography. This study aimed to establish a reference value for the 6min walking distance in women with diffuse cutaneous systemic sclerosis-associated interstitial lung disease that takes into account the effects of muscle and lung function. Methods: This was a cross-sectional study in which 69 women with systemic sclerosis underwent the 6-min walk test, Health Assessment Questionnaire-Disability Index, pulmonary function, handgrip strength test, and quadriceps strength test. Findings: The mean 6-min walking distance was 447 +/- 78 m, and 43.5% of the participants did not reach 80% of their predicted value. 6-min walking distance correlated positively with quadriceps strength (r = 0.418, P = 0.0004), forced vital capacity (r = 0.306, P = 0.011), pulmonary diffusion (r = 0.360, P = 0.002), maximum inspiratory pressure (r = 0.268, P = 0.029), and maximum expiratory pressure (r = 0.288, P = 0.019) and negatively with age (r = -0.378, P = 0.001), body mass index (r = -0.248, P = 0.039), and Health Assessment Questionnaire-Disability Index (r = -0.438, P = 0.0001). In the multiple linear regression analysis, quadriceps strength, body mass index, pulmonary diffusion, age, and maximum expiratory pressure explained 72% of the 6min walking distance variability. Interpretation: Muscle function and, to a lesser extent, lung function are key contributors in determining the reference value for the 6-min walking distance in women with diffuse cutaneous systemic sclerosis-associated interstitial lung disease.