Muscle Ultrasonographic Parameters as Independent Predictors of Functional Impairment in COPD: Validation of Cutoff Points and Their Correlation with the ABE Classification

Abstract

Introduction: Chronic Obstructive Pulmonary Disease (COPD) is a clinical condition characterized by usually progressive airflow limitation, airway inflammation, and impaired pulmonary function. In addition to respiratory system damage, COPD presents several systemic effects, including skeletal muscle dysfunction (ventilatory and peripheral), which appears to result in a significant reduction in functional capacity, increased dyspnea, and worsening quality of life. Objectives: To establish and validate optimal cutoff points for kinesiological ultrasound parameters in predicting functional limitation in stable patients with COPD and to examine their performance across GOLD stages and ABE classifications. Methods: A cross-sectional study was conducted with 124 volunteers living with COPD (mean age 61.5 ± 9.5 years). All participants underwent a comprehensive clinical assessment, including spirometry, the six-minute walk test (6MWT), handgrip strength, and standardized muscle ultrasound evaluation of the quadriceps (thickness, cross-sectional area, contractility index) and diaphragm (thickening fraction). Functional limitation was defined as 6MWT < 65% of the predicted value. Receiver operating characteristic (ROC) curve analysis determined optimal cutoff points. Performance was evaluated according to GOLD stages and ABE classifications. Results: Eighty-one patients (65.3%) presented functional limitation. The contractility index demonstrated excellent discriminatory ability (AUC 0.910, 95% CI: 0.854–0.966), with an optimal cutoff < 34.5%, resulting in 71.6% sensitivity and 100% specificity. Quadriceps cross-sectional area < 6.85 cm² showed an AUC of 0.864 (72.8% sensitivity, 97.7% specificity). All muscle parameters showed progressive decline across GOLD stages (p < 0.001) and strong correlations with functional capacity (r = 0.68–0.82). Notably, peripheral and respiratory muscle parameters demonstrated high correlation (r = 0.78–0.82), suggesting systemic involvement. In multivariate analysis, muscle parameters remained significant independent predictors after adjustment for FEV₁ (p < 0.05). ABE classification analysis revealed progressive muscle deterioration from class A to E, with the prevalence of functional limitation increasing from 32.1% to 81.2%. Conclusions: Muscle ultrasound parameters, particularly the contractility index and quadriceps cross-sectional area, are excellent independent predictors of functional limitation in COPD. The validated cutoff points provide clinically actionable thresholds for risk stratification and optimal timing of intervention. Strong correlations between peripheral and respiratory musculature support systemic rather than localized therapeutic approaches. The ABE classification adds complementary prognostic information beyond the traditional GOLD classification.