Abstract
Introduction: Quadriceps muscle dysfunction is an important complication of COPD occurring in mild and more advanced disease. It is largely driven by physical inactivity and is characterised by atrophy and a shift towards a less aerobic phenotype, with reduced Type I fibre proportions and oxidative enzymes. We hypothesised that physical inactivity and fibre type shift would be associated with intramuscular fat and that this could be a non-invasive and non-volitional marker of muscle quality.
Methods: Mid-thigh cross-sectional area (MTCSA) and percentage intramuscular fat were assessed using computed tomography (CT) image analysis. Tissues were differentiated using standard attenuation ranges; fat -190 to -30 and skeletal muscle -29 to 150 Hounsfield Units. Daily step count and physical activity level (PAL) were recorded using an armband accelerometer (SenseWear, Bodymedia). Type I and II fibre proportions were determined from vastus lateralis samples using immunohistochemistry. Associations were determined using multivariate regression models incorporating MTCSA and age.
Results: CT data were available for 102 patients (61 male, mean (SD) age 65(8) years, FEV1 41(20)% predicted) and 10 age-matched healthy controls. Accelerometer and biopsy data were obtained in 69 and 58 participants respectively. Percentage intramuscular fat was higher in the patient group (6.7(3.5) vs 4.3(1.2)%, p<0.01) and was independently associated with PAL (r=-0.33, p=0.02) and Type I fibre proportion (r=-0.33, p<0.01).
Conclusion: Intramuscular fat assessed using CT reflects multiple aspects of muscle dysfunction in patients with COPD and may provide a valuable biomarker in this group.
- © 2013 ERS
