Estimation of minimal clinically important difference for quadriceps and inspiratory muscle strength in older outpatients with chronic obstructive pulmonary disease: a prospective cohort study

Objective: To estimate the minimal clinically important difference (MCID) of quadriceps and inspiratory muscle strength after a home-based pulmonary rehabilitation program (PRP) in chronic obstructive pulmonary disease (COPD). Method: Eighty-five COPD patients were included. Quadriceps maximal voluntary contraction (QMVC) was measured. We measured maximal inspiratory mouth pressure (PImax), the 6-minute walk distance (6MWD), the chronic respiratory questionnaire (CRQ) and the modified Medical Research Council dyspnoea score (mMRC). All measurements were conducted at baseline and at the end of the PRP. The MCID was calculated using anchor-based (using 6MWD, CRQ, and mMRC as possible anchor variables) and distribution-based (half standard deviation and 1.96 standard error of measurement) approaches. Changes in the five variables were compared in patients with and without changes in QMVC or PImax >MCID for each variable. Results: Sixty-nine COPD patients (age 75±6 years) were analysed. QMVC improved by 2.4 (95%CI 1.1-3.7) kgf, PImax by 5.8 (2.7-8.8) cmH₂O, 6MWD by 21 (11-32) meters and CRQ by 3.9 (1.6-6.3) points. The MCID of QMVC and PImax was 3.3-7.5 kgf and 17.2-17.6 cmH₂O, respectively. The MCID of QMVC (3.3 kgf) could differentiate individuals with significant improvement in 6MWD and PImax from those without. Conclusion: The MCID of QMVC (3.3 kgf) can identify a meaningful change in quadriceps muscle strength after a PRP. The MCID of PImax (17.2 cmH₂O) should be used with careful consideration, because the value is estimated using distributionbased method.