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

Abstract

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 cm²/m and < 4.366 cm²/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.

Keywords: COPD; Exercise tolerance; Rectus femoris; Ultrasound.

Fig. 1

Ultrasound of the rectus femoris. The arrow indicates the direction of the scan

Fig. 2

Relationships of rectus femoris thickness (RF_thick) and rectus femoris cross-sectional area (RF_csa) with the clinical features of COPD patients. A: Relationships of RF_thick (left) and RF_csa (right) with the clinical features; red: statistically significant (p < 0.05). The 6MWD is significantly positively correlated with RF_thick (B) and RF_csa (C). The histograms on the right and top of the figure represent the distribution of the data. The more data in this section, the higher the column

Fig. 3

Receiver operating characteristic curve analysis of STD-RF_thick and STD-RF_csa for the prediction of poor exercise tolerance (6MWD < 350 m) in the development set (A) and the validation set (B)

Fig. 4

Receiver operating characteristic curve analysis of STD-RF_thick, STD-RF_csa, the 5STS, and the 30STS for the prediction of exercise tolerance (6MWD < 350 m)

Fig. 5

Construction of a nomogram model. A A nomogram was constructed to predict poor exercise tolerance (6MWD < 350 m) in COPD patients. B Calibration curves for the nomogram for the prediction of poor exercise tolerance (6MWD < 350 m) in COPD patients. C ROC curve analysis showing that the highest AUC value corresponded to the nomogram model. D Decision curve analysis showing the net benefit of the nomogram model for the prediction of poor exercise tolerance (6MWD < 350 m)