Breathing pattern and gas exchange at peak exercise in COPD patients with and without tidal flow limitation at rest

Expiratory flow limitation (FL) at rest is frequently present in chronic obstructive pulmonary disease (COPD) patients. It promotes dynamic hyperinflation with a consequent decrease in inspiratory capacity (IC). Since in COPD resting IC is strongly correlated with exercise tolerance, this study hypothesized that this is due to limitation of the maximal tidal volume (VT,max) during exercise by the reduced IC. The present study investigated the role of tidal FL at rest on: 1) the relationship of resting IC to VT,max; and 2) on gas exchange during peak exercise in COPD patients. Fifty-two stable COPD patients were studied at rest, using the negative expiratory pressure technique to assess the presence of FL, and during incremental symptom-limited cycling exercise to evaluate exercise performance. At rest, FL was present in 29 patients. In the 52 patients, a close relationship of VT,max to IC was found using non-normalized values (r=0.77; p < 0.0001), and stepwise regression analysis selected IC as the only significant predictor of VT,max. Subgroup analysis showed that this was also the case for patients both with and without FL (r=0.70 and 0.76, respectively). In addition, in FL patients there was an increase (p < 0.002) in arterial carbon dioxide partial pressure at peak exercise, mainly due to a relatively low VT,max and consequent increase in the physiological dead space (VD)/VT ratio. The arterial oxygen partial pressure also decreased at peak exercise in the FL patients (p < 0.05). In conclusion, in chronic obstructive pulmonary disease patients the maximal tidal volume, and hence maximal oxygen consumption, are closely related to the reduced inspiratory capacity. The flow limited patients also exhibit a significant increase in arterial carbon dioxide partial pressure and a decrease in arterial oxygen partial pressure during peak exercise.