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.
(© 2021. The Author(s).)