Background: Reduced muscle aerobic capacity in COPD patients has been demonstrated in several laboratories by phosphorus magnetic resonance spectroscopy and by analysis of oxygen uptake (VO2) kinetics. COPD patients are usually elderly, hypoxemic, poorly active with muscle atrophy, and often malnourished. Under these conditions there is usually reduction of O2 delivery to the tissues (bulk O2 flow), redistribution of fiber type within the muscle, capillary rarefaction, and decreased mitochondrial function, alterations all capable of reducing muscle aerobic capacity. In COPD, the effect of reduced body mass on muscle aerobic capacity has not been investigated (to our knowledge).
Methods: We studied 24 patients with stable COPD with moderate-to-severe airway obstruction (68±5 [SD] years; FEV1, 39±12% predicted; PaO2, 66±8 mm Hg; PaCO2, 41±3 mm Hg) with poor to normal nutritional status, as indicated by a low-normal percent of ideal body weight (IBW). Each subject first underwent 1-min maximal incremental cycle ergometer exercise for determination of VO2 peak and lactate threshold (LT). Subsequently, they performed a 10-min moderate (80% of LT-VO2) constant load exercise for determination of oxygen deficit (O2DEF) and mean response time VO2 (MRT). VO2, CO2 output (VCO2), and minute ventilation were measured breath by breath.
Results: Patients displayed low VO2 peak (1,094±47 [SE] mL/min), LT-VO2 (35±3% predicted VO2 max), and higher MRT-VO2 (67±4 s). Univariate regression analysis showed that percent of IBW correlated with indexes of maximal and submaximal aerobic capacity: vs VO2 peak, R=0.53 (p<0.01); vs MRT R=−0.77 (p<0.001). Using stepwise regression analysis, MRT correlated (R2=−0.70) with percent of IBW (p<0.01) and with PaO2 (p<0.05).
Conclusions: Reduced body mass has an independent negative effect on muscle aerobic capacity in COPD patients: this effect may explain the variability in exercise tolerance among patients with comparable ventilatory limitation.