Longitudinal change in Ultrasound derived Rectus Femoris cross sectional area (RFCSA) in patients with COPD

Introduction: Ultrasound derived RFCSA is an effort-independent and non-invasive surrogate marker of quadriceps muscle mass. Previous cross-sectional data have shown that RFCSA correlates with quadriceps maximum voluntary contraction (QMVC) and is reduced by 25% in stable patients with COPD compared to healthy controls (Seymour et al. Thorax 2009; 64:418-428). However limited longitudinal data exist. Aim: To quantify longitudinal change in RFCSA, and assess relationship with changes in quadriceps strength (QMVC) and exercise capacity over 12 months in patients with COPD. Methods: In 169 patients with COPD, RFCSA, QMVC, five repetition sit to stand (5STS) and Incremental shuttle walk test (ISWT) were measured at baseline and at 12 months. Results: Baseline characteristics: 53% male; mean(SD) age 70(9.4) years; BMI 27.9(6.0) kg/m2; FEV1 52.6(18.8) % predicted; RFCSA 521.3(197.5) mm2; QMVC 30.6(13.1) kg; 5STS 12.8(8.5)secs; MRC dyspnoea 3(1). RFCSA at baseline correlated with QMVC (r=0.51; p<0.001), QMVC/height (r=0.49 p<0.001), QMVC % predicted (r=0.41 p<0.001), ISWT (r=0.329 p<0.001) and 5STS (r=-0.261 p<0.001). Mean (95% CI) change in RFSCA over 12 months was -33.7(-55.6 to -11.9) mm2. Change in RFCSA did not correlate with change in QMVC (r=0.065 p=0.469) change in 5STS (r=-0.044 p=0.595) or change in ISWT (r=0.091 p=0.311). RFCSA %change did not correlate with QMVC %change (r=-0.049 p=0.58). Conclusion: RFCSA reduces over 12 months in patients with COPD, but does not relate to changes in quadriceps strength, lower limb function nor exercise capacity.