• COPD mechanisms
• pulmonary rehabilitation

Skeletal muscle wasting is a debilitating consequence of respiratory disease, particularly well characterised in individuals with chronic obstructive pulmonary disease (COPD). However, it remains controversial whether peripheral muscle dysfunction is an intrinsic part of the respiratory disease process, or a generalised response to multiple atrophy stimuli. Such debate has not only informed the subtleties of pathophysiology, but has also instigated detailed analyses of the molecular mechanisms involved in the induction of muscle atrophy, identifying the ubiquitin–proteasome system (UPS) as the predominant pathway required for the rapid proteolysis seen in muscle wasting.

In this issue of Thorax (see page 113), Mainguy et al contribute to our knowledge of muscle weakness in chronic respiratory disease, characterising peripheral muscle function in patients with idiopathic pulmonary arterial hypertension (IPAH).1 Ten WHO functional class II–III patients with IPAH and 10 matched healthy controls underwent exercise capacity assessment, quadriceps strength testing and peripheral muscle morphology analysis. Patients with IPAH demonstrated a lower proportion of type I fibres in vastus lateralis muscle biopsies than healthy controls (38% vs 50%), and this was associated with reduced quadriceps strength and a relatively higher potential for anaerobic metabolism (enzymatic ratio of phosphofructokinase (PFK)/3-hydroxyacyl CoA dehydrogenase (HDAH)). While there was no correlation between the proportion of type I fibres and muscle strength with pulmonary haemodynamic parameters, a positive correlation was demonstrated for peripheral muscle strength and exercise capacity (R²=0.42, p=0.04). The authors conclude that peripheral muscle dysfunction may contribute to exercise intolerance in patients with IPAH. With relatively small numbers in the study groups, …