Metabolism and aerobic capacity of skeletal muscle in chronic respiratory failure related to chronic obstructive pulmonary disease

The calf muscle energy metabolism of 8 stable chronic obstructive pulmonary disease (COPD) patients with chronic respiratory failure (arterial oxygen tension (PaO₂) 7.7 ± 0.4 kPa or 58 ± 3 mmHg) was studied, using 31-phosphorus magnetic resonance spectroscopy (³¹P MRS). MRS spectra were acquired at rest and during the course of 360 pedal movements at 20, 35 and 50% of the maximal voluntary contraction (MVC) and during recovery. Eight healthy age-matched subjects served as the control group. No significant differences between groups were observed in resting muscle, as regards intracellular pH, Pi/PCr ratio (Pi: inorganic phosphate; PCr: phosphocreatine) and the relative ATP expressed as the ratio βATP/PCr+Pi+PME (PME: phosphomonoester). Although effective power outputs were similar for both groups at each work level, COPD patients exhibited a higher Pi/PCr ratio than healthy controls (3.34 ± 0.89 vs 0.49 ± 0.05 at 50% MVC; p < 0.01) and a lower pH(i) (6.65 ± 0.11 vs 7.06 ± 0.02 at 50% MVC; p < 0.01). PCr resynthesis during recovery was slower in patients than in control subjects (t( 1/2 ) PCr 1.27 ± 0.26 min vs 0.47 ± 0.05 min; p < 0.05). These results suggest impairment of aerobic capacity in a non-ventilatory working muscle, which may be due to hypoxaemia in patients with chronic respiratory failure.