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Submaximal exercise testing in the assessment of interstitial lung disease secondary to systemic sclerosis: reproducibility and correlations of the 6-min walk test

¹ University of Michigan Scleroderma Program, University of Michigan Health System, Ann Arbor, Michigan, USA
² Centre for Rheumatology, Royal Free Campus, Royal Free and University College Medical School, London, UK
³ Division of Rheumatology, UCLA School of Medicine, University of California, Los Angeles, California, USA
⁴ Department of Internal Medicine, Hopital Cochin, Assistance Publique-Hopitaux de Paris, Universite Rene Descartes, Paris, France
⁵ Division of Pulmonary and Critical Care Medicine, Department of Medicine, University of California, San Diego, La Jolla, California, USA
⁶ Interstitial Lung Disease Unit, Royal Brompton Hospital, Fulham, London, UK
⁷ Division of Rheumatology, Department of Medicine, University of Florence, Florence, Italy
⁸ Department of Rheumatology and Clinical Immunology, Charite University Hospital, Berlin, Germany
⁹ Academic Unit of Musculoskeletal Disease, University of Leeds, Leeds, UK
¹⁰ Actelion, Allschwil, Switzerland

Correspondence to:
Correspondence to:
Professor J R Seibold
University of Michigan Scleroderma Program, 3918 Taubman Centre, Box 0358, 1500 E Medical Center Drive, Ann Arbor, MI 48109-0358, USA; jseibold{at}umich.edu

Background: The 6-min walk test (6MWT) is increasingly used as an outcome measure in interstitial lung disease (ILD).

Aim: To evaluate the usefulness of the 6MWT in a cohort of patients with ILD secondary to systemic sclerosis (SSc) and to correlate with established physiological parameters.

Methods: 163 patients with SSc-ILD were recruited for a multicentre, randomised, double-blind clinical trial. Available data at protocol screening included repeated 6MWTs, pulmonary function testing with diffusing capacity, Doppler echocardiography and high-resolution computed tomography of the thorax. Borg Dyspnoea Index was evaluated before and after 6MWT.

Results: Mean (standard deviation (SD)) distance walked during walk test 1 was 396.6 (84.55) m compared with 399.5 (86.28) m at walk test 2. The within-subject, intertest correlation as determined by Pearson’s correlation coefficient testing was 0.95 (p<0.001). However, only weak correlations of 6MWT with percentage forced vital capacity and the Borg Dyspnoea Index were observed, and no correlation was observed with percentage diffusing capacity.

Conclusion: These data confirm the high reproducibility of the 6MWT in patients with SSc-ILD and therefore the validity of the test in this cohort. The lack of correlation of 6MWT with standard physiological parameters of ILD suggests a multifactorial basis for limited exercise capacity in patients with SSc and calls into question the utility of the 6MWT as a measure of outcome in future studies on SSc-ILD.

Abbreviations: 6MWT, six-min walk test; DL_CO, single-breath diffusing capacity; FVC, forced vital capacity; HRCT, high-resolution computed tomography; IIP, idiopathic interstitial pneumonia; ILD, interstitial lung disease; OMERACT, outcome measures in rheumatological clinical trials; PAH, pulmonary arterial hypertension; SSc, systemic sclerosis

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