To better understand the metabolic implications of a higher ATP cost of contraction in chronic obstructive pulmonary disease (COPD), we used 31P-magnetic resonance spectroscopy (31P-MRS) to examine muscle energetics and pH in response to graded exercise. Specifically, in six patients and six well-matched healthy controls, we determined the intracellular threshold for pH (TpH) and inorganic phosphate-to-phosphocreatine ratio (TPi/PCr) during progressive dynamic plantar flexion exercise with work rate expressed as both absolute and relative intensity. Patients with COPD displayed a lower peak power output (WRmax) compared with controls (controls 25 ± 4 W, COPD 15 ± 5 W, P = 0.01) while end-exercise pH (controls 6.79 ± 0.15, COPD 6.76 ± 0.21, P = 0.87) and PCr consumption (controls 82 ± 10%, COPD 70 ± 18%, P = 0.26) were similar between groups. Both TpH and TPi/PCr occurred at a significantly lower absolute work rate in patients with COPD compared with controls (controls: 14.7 ± 2.4 W for TpH and 15.3 ± 2.4 W for TPi/PCr; COPD: 9.7 ± 4.5 W for TpH and 10.0 ± 4.6 W for TPi/PCr, P < 0.05), but these thresholds occurred at the same percentage of WRmax (controls: 63 ± 11% WRmax for TpH and 67 ± 18% WRmax for TPi/PCr; COPD: 59 ± 9% WRmax for TpH and 61 ± 12% WRmax for TPi/PCr, P > 0.05). Indexes of mitochondrial function, the PCr recovery time constant (controls 42 ± 7 s, COPD 45 ± 11 s, P = 0.66) and the PCr resynthesis rate (controls 105 ± 21%/min, COPD 91 ± 31%/min, P = 0.43) were similar between groups. In combination, these results reveal that when energy demand is normalized to WRmax, as a consequence of higher ATP cost of contraction, patients with COPD display the same metabolic pattern as healthy subjects, suggesting that skeletal muscle energy production is well preserved in these patients.
Dynamics of chest wall volume regulation during constant work rate exercise in patients with chronic obstructive pulmonary disease