Non-invasive positive pressure ventilation (NIPPV or NPPV or NIV), overnight or during the day, has not been shown to help most people with stable COPD and daytime hypercarbia.
Citing their own previous findings, Dreher, Windisch et al argue that's because you can't just use wimpy ordinary settings -- you've got to crank that sucker to 11. They report another series in CHEST. Although 4 of 17 faint-hearted patients couldn't stand the mean inspiratory pressure of 29 millibars (mbar and cm H2O are roughly equivalent), those 13 who toughed it out had a reduction in their nocturnal paCO2, without worsened sleep quality.
BUT, their sleep quality was also no better, there's still no convincing evidence of benefit to nocturnal NIPPV in stable COPD, and and authors acknowledge that randomized trials will be necessary to identify any.
Dreher M et al. Noninvasive Ventilation in COPD: Impact of Inspiratory Pressure Levels on Sleep Quality. CHEST 2011;140:939-945.