Mechanisms by which COPD affects exercise tolerance

In view of the recent advances in our understanding of the pathophysiology of COPD, we felt that it would be appropriate to examine the contribution of several abnormalities, not hitherto examined, to exercise limitation in this disease. These included: (I) The ability to exceed maximum expiratory flow (determined during forced maneuvers from TLC) during partial expiratory maneuvers. This is referred to as Delta FEV1. (2) Shape of the flow-volume curve (Shape). (3) Susceptibility to develop dynamic hyperinflation (dynamic hyperinflation index, DHI). (4) Ventilatory response to exercise ((V) over dot E-max/(V) over dot E-pred). Twenty-four COPD patients (FEV1 = 42 +/- 13% pred) underwent symptom-limited progressive exercise. Delta FEV1, shape, DHI and (V) over dot E-max/(V) over dot E-pred were determined. All values were normalized to eliminate the effects of age, sex, and body size. Shape had no impact on peak (V) over dot O-2 (r = 0.8). Delta FEV1 (r = 0.50), DHI (r = 0.50) and (V) over dot E-max/(V) over dot E-pred (r = 0.46) correlated significantly with peak (V) over dot O-2 with all three exceeding FEV1 (r = 0.43). DHI and Delta FEV1 correlated significantly with each other (r = 0.43) suggesting that the latter exerts its beneficial effects by reducing the tendency to develop DH. We conclude that variability among patients in ventilatory response to exercise and in Delta FEV1 (likely an expression of extent of regional mechanical heterogeneity) contribute importantly to variability of exercise tolerance in COPD.