THE TIDAL VOLUME RESPONSE TO INCREMENTAL EXERCISE IN COPD

Patients with severe COPD often exhibit a ventilatory limit to exercis e. This is associated with a shallow breathing pattern when compared w ith normal control subjects. It is unclear, however, what factors affe ct differences in breathing patterns within this patient population. T o further investigate the tidal volume (VT) response to exercise in se vere COPD, nine patients were recruited to undergo a maximal increment al exercise test. Pulmonary function tests, collection of expired gase s, and continuous pulse oximetry were performed. As a group, the resul ts were as follows (mean +/- SD): The FEV1 was 31 +/- 7 percent of pre dicted, the FRC was 143 +/- 18 percent of predicted, and the Dco was 4 7 +/- 15 percent of predicted. Exercise testing showed an oxygen consu mption at peak exercise of only 44 +/- 9 percent predicted, a dyspnea index of 101 +/- 19 percent predicted, a heart rate at peak exercise o f 75 +/- 12 percent predicted, a tidal volume at peak exercise (PKVT) of 1.23 +/- .35, a respiratory rate at peak exercise (PKfb) of 26 +/- 6, and an oxygen saturation at peak exercise (PKo2sat) of 96 +/- 4 per cent. An anaerobic threshold (AT) occurred in seven of the nine patien ts at a mean of 31 +/- 8 percent predicted maximal oxygen consumption. Regression analysis showed the PKVT to be inversely correlated with t he FEV1 (r = -0.76; p = 0.01) and the PKfb (r= -0.85; p = 0.003), whil e positively correlating with the FRC (r = +0.80; p = 0.01) and the PK o2sat (r = +0.69; p = 0.04). Additionally, there was a trend for the P KVT to be inversely related with the AT (r = - 0. 72; p = 0.06). In CO PD, the more severe the obstruction and hyperinflation, the larger the VT response to exercise. This may serve to avoid a deleterious increa se in autoPEEP by promoting a lengthening of the expiratory time. Furt hermore, given the association of an earlier AT and a higher PKo2sat w ith a larger PKVT, this would suggest that such a response may minimiz e the effects of dead space and/or autoPEEP on O2 delivery (early AT).