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Exercise tolerance is impaired in chronic obstructive pulmonary disease (COPD), in part because of a reduction in ventilatory capacity and excessive dyspnea experienced. The authors reasoned that proportional assist ventilation (PAV), a ventilator mode in which the level of support varies proportionately with patient effort, could be used during exercise to assist ventilation. The purpose of this study was to evaluate the efficacy of PAV to improve exercise endurance and related physiologic parameters in COPD.In 8 patients (age = 62.8 years mean, ± 6.9 standard deviation) with severe COPD (forced expiratory volume in 1 second = 0.70 ± 0.21 L) flow, volume, dyspnea, leg fatigue, arterial blood gases, and gas exchange were measured during constant workrate exercise (37 ± 18 watts; i.e., 80% previously determined maximum oxygen consumption). Crossover exercise trials were performed in random order: while spontaneously breathing through the experimental circuit without assistance (control trial) and with PAV (using 9.8 ± 2.1 cm H2O/L and 3.3 ± 1.0 cm H2O/L/sec of volume assist and flow assist, respectively).The application of PAV during exercise was well tolerated by each subject. Compared with the control measurement at equivalent time during exercise, PAV improved breathing pattern and arterial blood gases while dyspnea was reduced. Consequently, there was a significant increase in exercise duration with PAV (323 ± 245 seconds during the control trial compared with 507 ± 334 seconds with PAV, P = 0.02).Proportional assist ventilation can improve performance during constant workrate exercise in severe COPD.