Limitation of lower limbV˙o ₂ during cycling exercise in COPD patients

Patients with chronic obstructive pulmonary disease (COPD) usually stop exercise before reaching physiological limits in terms of O2 delivery and extraction. A plateau in lower limb O2 uptake (V˙o 2) and blood flow occurs despite progression of the imposed workload during cycling in some patients with COPD, suggesting that maximal capacity to transport O2had been reached and that it had been extracted in the peripheral exercising muscles. This study addresses this observation. Symptom-limited incremental cycle exercise was performed by 14 men [62 ± 11 (SD) yr] with severe COPD (forced expiratory volume in 1 s = 35 ± 7% of predicted value). Leg blood flow was measured at each exercise step with a thermodilution catheter inserted in the femoral vein. This value was multiplied by two to account for both working legs (Q˙LEGS). Arterial and femoral venous blood was sampled at each exercise step to measure blood gases. Leg O2 consumption (V˙o 2LEGS) was calculated according to the Fick equation. Total bodyV˙o 2(V˙o 2 TOT) was measured from expired gas analysis, and tidal volume (Vt) and minute ventilation (V˙e) were derived from the flow signal. In eight patients, V˙o 2 LEGSkept increasing in parallel withV˙o 2 TOT as external work rate was increasing. In six subjects, a plateau inV˙o 2 LEGS andQ˙LEGS occurred during exercise (increment of <3% between 2 consecutive increasing workloads) despite the increase in workload and V˙o 2 TOT[corresponding mean was 110 ± 38 ml (11 ± 4%)]. These six patients also exhibited a plateau in O2 extraction during exercise. Peak exercise work rate was higher in the eight patients without a plateau than in the six with a plateau (51 ± 10 vs. 40 ± 13 W, P = 0.043). Vt,V˙e, and dyspnea were significantly greater at submaximal exercise in patients of the plateau group compared with those of the nonplateau group. These results show that, in some patients with COPD, blood flow directed to peripheral muscles and O2 extraction during exercise may be limited. We speculate that redistribution of cardiac output and O2 from the lower limb exercising muscles to the ventilatory muscles is a possible mechanism.