We have been treating a 50 year old patient with undefined neuromuscular disease. He retains limb mobility, but was recently tracheostomized due to cardiac arrhythmia. He was weaned from oxygen and has a SPO2 of 96-98 on pressure support via tracheostomy with the following settings: PS 22 PEEP 6 RR 14, cycle-off 50% of peak flow. We deflated the cuff and the patient could speak well, although with a “wet” voice. With the tube capped we switched the patient to volume targeted mouthpiece ventilation, but he always develops desaturation even at high tidal volumes (2.5 L), which corrects quickly to normal when he is returned to pressure support trach ventilation. Even when we tried pressure targeted mouthpiece ventilation to achieve volumes as high as 3 L, we failed. Of note his back up rate on mouthpiece ventilation is 14/min and PEEP=0. Could you suggest any possible solutions?