Foot-to-foot lower-limb phase angle as a predictor of reduced exercise capacity in patients with interstitial lung disease: A comparative study with muscle mass and pulmonary function.

BACKGROUND: Reduced exercise capacity, measured by the 6-min walk distance (6MWD), is a critical prognostic factor in interstitial lung disease (ILD). However, conventional physiological markers often fail to identify early functional decline. We aimed to evaluate whether the foot-to-foot lower-limb phase angle (F-F PhA), a non-invasive surrogate marker of muscle quality, predicts reduced exercise capacity better than traditional muscle mass and pulmonary function indices.

METHODS: In this retrospective study of 68 patients with ILD, F-F PhA was measured using bioelectrical impedance analysis, and erector spinae muscle area (ESM_CSA) via computed tomography. Reduced exercise capacity was defined as 6MWD < 450 m. Multivariable logistic regression models adjusted for age and pulmonary function (%DLCO or %FVC), with internal validation performed via 1000-resample bootstrapping.

RESULTS: In multivariable analysis, F-F PhA was independently associated with a 6MWD < 450 m when adjusted for age and %DLCO (OR: 0.228, 95% CI: 0.088-0.591, p = 0.002; bootstrap p = 0.005) or %FVC (OR: 0.213, 95% CI: 0.091-0.496, p = 0.001; bootstrap p = 0.001). ROC analysis showed F-F PhA had the highest discriminative accuracy (AUC: 0.802), outperforming %DLCO (AUC: 0.729) and ESM_CSA (AUC: 0.701). Combining age, %DLCO, and F-F PhA achieved the highest performance (AUC: 0.833).

CONCLUSIONS: F-F PhA serves as a robust, independent indicator for predicting reduced exercise capacity in ILD, outperforming conventional muscle mass. As a non-invasive tool, F-F PhA complements established functional tests in screening for early physical decline.