Ultrasound assessment of the rectus femoris in patients with chronic obstructive pulmonary disease predicts poor exercise tolerance: an exploratory study

BACKGROUND: Reduced exercise tolerance is an important clinical feature of chronic obstructive pulmonary disease (COPD) and is associated with poor prognosis. The 6-min walk test (6MWT) is widely used to assess exercise capacity; however, it is not commonly administered in primary medical institutions because it requires a suitable site and professional training. Ultrasound has great potential for evaluating skeletal muscle dimensions in COPD. However, whether skeletal muscle ultrasound can predict impaired exercise tolerance is unclear. METHODS: The study included 154 stable patients with COPD, who were randomly divided into a development set and a validation set. The thickness (RF(thick)) and cross-sectional area (RF(csa)) of the rectus femoris were measured using ultrasound. Standardized RF(thick) (STD- RF(thick)) and Standardized RF(csa) (STD-RFcsa) were obtained via standardization of RF(thick) and RF(csa) by patients' height. RESULTS: Strong correlations were observed between the 6MWD and RF(thick) (r = 0.84, p < 0.001) and between the 6MWD and RF(csa) (r = 0.81, p < 0.001). In the development set, the optimal cut-off values for men and women for predicting poor exercise tolerance were < 3.098 cm/m and < 3.319 cm/m for STD-RF(thick) and < 4.052 cm(2)/m and < 4.366 cm(2)/m for STD-RF(csa), respectively. In the validation set, the area under the curve (AUC) values for the prediction of a 6MWD < 350 by STD-RF(thick) and STD-RF(csa) were 0.881 and 0.903, respectively. Finally, the predictive efficacy of STD-RF(thick) (AUC: 0.922), STD-RF(csa) (AUC: 0.904), and the derived nomogram model (AUC: 0.98) for exercise tolerance was superior to that of the sit-to-stand test and traditional clinical features. CONCLUSIONS: Rectus femoris ultrasound has potential clinical application to predict impaired exercise tolerance in patients with COPD. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12890-021-01663-8.