Alterations in the muscle-to-capillary interface in patients with different degrees of chronic obstructive pulmonary disease

Background: It is hypothesized that decreased capillarization of limb skeletal muscle is implicated in the decreased exercise tolerance in COPD patients. We have recently demonstrated decreased number of capillaries per muscle fibre (CAF) but no changes in CAF in relation to fibre area (CAFA), which is based on the diffusion distance between the capillary and muscle fibre. The aim of the current study is to investigate the muscle-to-capillary interface which is an important factor involved in oxygen supply to the muscle that has previously been suggested to be a more sensitive marker for changes in the capillary bed compared to CAF and CAFA. Methods: 23 COPD patients and 12 age-matched healthy subjects participated in the study. Muscle-to-capillary interface was assessed in muscle biopsies from the tibialis anterior muscle using the following parameters: 1) The capillary-to-fibre ratio (C:F-i) which is defined as the sum of the fractional contributions of all capillary contacts around the fibre 2) The ratio between C:F-i and the fibre perimeter (CFPE-index) 3) The ratio between length of capillary and fibre perimeter (LC/PF) which is also referred to as the index of tortuosity. Exercise capacity was determined using the 6-min walking test. Results: A positive correlation was found between CFPE-index and ascending disease severity with CFPE-index for type I fibres being significantly lower in patients with moderate and severe COPD. Furthermore, a positive correlation was observed between exercise capacity and CFPE-index for both type I and type IIa fibres. Conclusion: It can be concluded that the muscle-to-capillary interface is disturbed in the tibialis anterior muscle in patients with COPD and that interface is strongly correlated to increased disease severity and to decreased exercise capacity in this patient group.