Limitation of lower limb Vo(2) during cycling exercise in COPD patients

Patients with chronic obstructive pulmonary disease (COPD) usually stop exe rcise before reaching physiological limits in terms of O-2 delivery and ext raction. A plateau in lower limb O-2 uptake (<V)over dot>o(2),) and blood f low occurs despite progression of the imposed workload during cycling in so me patients with COPD, suggesting that maximal capacity to transport O-2 ha d 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 COP D (forced expiratory volume in 1 s = 35 +/- 7% of predicted value). Leg blo od flow was measured at each exercise step with a thermodilution catheter i nserted in the femoral vein. This value was multiplied by two to account fo r both working legs (<(Q)over dot)(LEGS)). Arterial and femoral venous bloo d was sampled at each exercise step to measure blood gases. Leg O-2 consump tion (<(V)over dot>(2LEGS)) was calculated according to the Fick equation. Total body <(V)over dot>o(2) (<(V)over dot>o(2TOT)) was measured from expir ed gas analysis, and tidal volume (<(V)over dot>T) and minute ventilation ( <(V)over dot>E) were derived from the flow signal. In eight patients, work rate was increasing. In six subjects, a plateau in <(V)over dot>o(2LEGS) an d <(Q)over dot>(LEGS) occurred during exercise (increment of <3% between 2 consecutive increasing workloads) despite the increase in workload and <(V) over dot>o(2TOT) [corresponding mean was 110 +/- 38 ml (11 +/- 4%)]. These six patients also exhibited a plateau in O-2 extraction during exercise. Pe ak exercise work rate was higher in the eight patients without a plateau th an in the six with a plateau (51 +/- 10 vs. 40 +/- 13 W, P = 0.043). <(V)ov er dot>T, <(V)over dot>E, and dyspnea were significantly greater at submaxi mal exercise in patients of the plateau group compared with those of the no nplateau group. These results show that, in some patients with COPD, blood flow directed to peripheral muscles and O-2 extraction during exercise may be limited. We speculate that redistribution of cardiac output and O-2 from the lower limb exercising muscles to the ventilatory muscles is a possible mechanism.