Exercise-limiting factors in respiratory distress.

Exercise performance data, circulatory function and respiratory and leg muscle quality, expressed as muscle fiber composition, are reviewed and together with our own data discussed as possible limiting factors for physical performance in chronic obstructive pulmonary disease (COPD). COPD is regarded as synonymous with reduced physical performance, exaggerated breathlessness or dyspnea, muscle hypotrophy and/or wasting and, frequently, malnutrition. Impaired right ventricular circulatory function seems to be essential. The observed preponderance of fast twitch (FT), 'glycogenolytic' and capillary-poor muscle fiber type in the investigated muscles might reflect endowment, a 'hypoxic vasoconstriction'-related downregulation of the other main fiber type: the slow twitch (ST), capillary-rich, fatigue-resistant fiber, and/or selective muscle trauma to ST fibers. Ischemic heart disease (IHD) patients demonstrate a similar fiber type pattern in leg muscles. Both COPD and IHD patients have low leg muscle and plasma deposits of antioxidants such as coenzyme Q10 (CoQ10) and alpha-tocopherol. This could reflect a depressed resistance to radical induced cell trauma and/or malnutrition. The magnitude of the antioxidant reduction is less pronounced in patients rich in FT fibers indicating a ST fiber-related susceptibility to trauma. Treatment of other muscle disorders including heart muscle with, e.g., CoQ10 improves performance due to a causative enhanced antioxidant potential, reduced catabolism and/or an upregulated muscle anabolism, increased mitochondrial volume/function, etc. Such data are lacking in COPD.