Exercise capacity in heart failure patients: Relative importance of heart and skeletal muscle

The knee extensor and the whole-body exercise capacities were measured in 11 chronic heart failure (CHF) patients and 11 healthy age- and sex-matched controls, and were related to ejection fraction and to biochemical and histochemical markers of the musculature. The CHF patients had a 39% lower maximal oxygen uptake measured on an ergometer cycle than the healthy controls (1.54+/-0.57 vs, 2.51+/-0.70 l min(-1), P<0.001). The low exercise capacity was markedly related to the ejection fraction (r=0.77, P<0.001). The maximal strength of m. quadriceps femoris was 15% lower in the CHF patients than in the controls (P<0.05). The cross-sectional area (CSA) of m. quadriceps femoris explained 55% (r=0.74, P<0.001) of the difference in strength between both groups. The endurance capacity of m. quadriceps femoris was 30% lower in CHF patients than in controls, partly as a result of the 25% lower capillary density (P<0.05) and the 27% lower aerobic enzyme capacity (P<0.05), as estimated by the citrate synthase activity, in the CHF patients. The citrate synthase activity correlated with the maximal oxygen uptake (r=0.61, P<0.05). Moreover, the ejection fraction, together with the CSA of m. quadriceps femoris, explained 75% (r=0.86, P<0.01) of the difference in maximal oxygen uptake between CHF patients and controls. These results demonstrate that CHF patients have both a lower local and a lower whole-body work capacity than healthy controls. This is a function of a smaller leg muscle mass and a lower capillary density and mitochondrial enzyme capacity in the CHF patients; however, a lowered pump capacity of the heart is the factor which limits the exercise capacity the most.