Biological variation, reference change value (RCV) and minimal important difference (MID) of inspiratory muscle strength (PImax) in patients with stable chronic heart failure.

Abstract

Despite the widespread application of measurements of respiratory muscle force (PImax) in clinical trials there is no data on biological variation, reference change value (RCV), or the minimal important difference (MID) for PImax irrespective of the target cohort. We addressed this issue for patients with chronic stable heart failure. From the outpatients' clinic of the University of Heidelberg we retrospectively selected three groups of patients with stable systolic chronic heart failure (CHF). Each group had two measurements of PImax: 90 days apart in Group A (n=25), 180days apart in Group B (n=93), and 365days apart in Group C (n=184). Stability was defined as (a) no change in NYHA class between visits and (b) absence of cardiac decompensation 3months prior, during, and 3months after measurements. For each group, we determined within-subject (CVI), between-subject (CVG), and total (CVT) coefficient of variation (CV), the index of individuality (II), RCV, reliability coefficient, and MID of PImax. CVT was 8.7, 7.5, and 6.9% for groups A, B, and C, respectively. The II and RCV were 0.21, 0.20, 0.16 and 13.6, 11.6, 10.8%, respectively. The reliability coefficient and MID were 0.83, 0.87, 0.88 and 1.44, 1.06, 1.12kPa, respectively. Results were similar between age, gender, and aetiology subgroups. In patients with stable CHF, measurements of PImax are highly stable for intervals up to 1year. The low values for II suggest that evaluation of change in PImax should be performed on an individual (per patient) basis. Individually significant change can be assumed beyond 14% (RCV) or 1.12kPa (MID).