Cardiopulmonary factors affecting 6-min walk distance in patients with idiopathic inflammatory myopathies

Naoki Mugii, Fujiko Someya

Idiopathic inflammatory myopathies involve skeletal muscles and can be associated with interstitial lung disease and/or heart dysfunction, which may reduce exercise capacity. We aimed to clarify cardiopulmonary factors affecting the 6-min walk distance in patients who were able to walk without leg pain or fatigue. Twenty-three patients with inactive adult idiopathic inflammatory myopathies, and 18 age- and gender-matched healthy controls were evaluated for hemodynamic responses using noninvasive impedance cardiography during the 6-min walk test. The patients were also examined by the pulmonary function test for forced vital capacity and diffusing capacity for carbon monoxide (DLCO), and by echocardiography for left ventricular ejection fraction and right ventricular systolic pressure. Interstitial lung disease was diagnosed in 19 patients using high-resolution computed tomography. There was no difference in 6-min walk distance or cardiac output after walking between the patients and healthy controls. However, stroke volume during the 6-min walk test was significantly lower in the patients than in healthy controls, suggesting malfunction in the heart. Moreover, the increased heart rate matched the cardiac output. Spearman's correlation analysis demonstrated a correlation between 6-min walk distance and stroke volume, cardiac output after walking and DLCO, but not left ventricular ejection fraction or right ventricular systolic pressure, as this study lacked the patients with pulmonary hypertension. In conclusion, impaired DLCO due to interstitial lung disease was suggested to be a fundamental parameter affecting exercise capacity, in addition to heart involvement, in patients with idiopathic inflammatory myopathies.