Ultrasound measurement of rectus femoris cross-sectional area and the relationship with quadriceps strength in COPD

Background: Quadriceps weakness and loss of muscle mass predict mortality in chronic obstructive pulmonary disease (COPD). It was hypothesised that a reduced quadriceps cross-sectional area could be detected by ultrasound in patients with COPD compared with healthy subjects, and that measurements relate to strength and fat-free mass (FFM).

Methods: Rectus femoris muscle cross-sectional area (RFCSA) was measured by ultrasound and whole-body FFM estimated using electrical bioimpedance. Quadriceps strength was measured by maximum voluntary contraction and twitch tension (TwQ) following magnetic femoral nerve stimulation.

Results: 26 healthy volunteers of mean (SD) age 63 (9) years and 30 patients with COPD of mean (SD) age 67 (9) years and percentage predicted forced expiratory volume in 1 s (FEV1) 48.0 (20.8)% with a similar FFM (46.9 (9.3) kg vs 46.1 (7.3) kg, p = 0.193) participated in the study. Mean RFCSA was reduced in patients with COPD by 25% of the mean value in healthy subjects(−115 mm2; 95% CI −177 to −54, p = 0.001) and was related to MRC dyspnoea scale score, independent of FFM or sex. Maximum voluntary contraction strength was linearly related to RFCSA in patients with COPD (r = 0.78, p<0.001). TwQ force per unit of RFCSA was similar in both healthy individuals and those with COPD (mean (SD) 17 (4) g/mm2 vs 18 (3) g/mm2, p = 0.657). Voluntary contraction strength per unit of RFCSA was dependent on central quadriceps activation and peripheral oxygen saturation in COPD.

Conclusion: Ultrasound measurement of RFCSA is an effort-independent and radiation-free method of measuring quadriceps muscle cross-sectional area in patients with COPD that relates to strength.