METHODS: 15 severe COPD patients with hypercapnia were placed on the three levels (PS, PS-, PS+) of mask -PSV in random sequence, after the assessment of unassisted spontaneous breathing (SB). The PS level was titrated by patient’s comfort before the experiment. To further demonstrate the physiological responds of the varying pressure, 25% decrease and increase by baseline PS were applied. (PS-=75%PS PS+=125%PS). Each level lasted at least 20 minutes. Respiratory rate (RR), tidal volume (Vt), inspiratory effort (PTPin/min), neuro-ventilatory coupling (VE/RMS%) were measured.