Previous studies suggested that alteration of systemic skeletal muscle metabolism is a major determinant of exercise tolerance in patients with chronic heart failure (CHF). The authors examined calf muscle metabolism during continuous exercise of the foot in patients with CHF compared with normal subjects using (31)P magnetic resonance spectroscopy. The subjects were patients with New York Heart Association class II CHF who had previously suffered New York Heart Association class IV heart failure. Plantarflexion of the foot was repeated for 8 min 40 s at a rate of one contraction per second against a 2 kg load inside the magnet. At rest, during exercise (divided into the first one-half [EX1] and the latter one-half [EX2]) and at recovery, (31)P magnetic resonance spectroscopy data sets were acquired every 4 min 20 s. At rest, the phosphocreatine to hexamethylphosphoric triamide (PCr:HMPT) and the inorganic phosphate (Pi) to PCr ratios in the CHF group were not different from those in the normal group. During EX1 in the normal group, PCr levels decreased and Pi levels increased. Although exercise continued, these changes improved during EX2, suggesting there was an adaptation to exercise. The degree of change in the PCr:HMPT ratio during EX1 in the CHF group was not significantly different from that during EX1 in the normal group; however, the improvement during EX2 in the CHF group was impaired. The Pi:PCr ratio of EX1 to EX2 in the CHF group was significantly greater than that in the normal group (0.74+/-0.22 versus 0.19+/-0.05, respectively, P<0.005). Thus, in CHF, adaptation to continuous exercise may be impaired by alteration of skeletal muscle metabolism and this alteration may worsen exercise capacity.
Keywords: 31P magnetic resonance spectroscopy; Calf muscle; Exercise tolerance; Heart failure.