If air trapping causes symptoms and limitation in people with emphysema, why not un-trap the air? Say, by poking holes in bronchi leading to areas of severe emphysema, and putting paclitaxel-eluting stents in place to maintain patency? The air should naturally become freed, flow out, and improve respiratory mechanics.
Only, it didn't. Shah et al randomized 315 severely hyperinflated COPD patients to get the above procedure (Exhale stents) or placebo/sham, in a 2:1 ratio. At 6 months, there was no improvement in the treatment arm.
Why? Probably collateral ventilation, or interalveolar air drift through the pores of Kohn -- basically tiny intercommunications that let those emphysematous areas refill as soon as they are decompressed.
Shah PL et al. Bronchoscopic lung-volume reduction with Exhale airway stents for emphysema (EASE trial): randomised, sham-controlled, multicentre trial. Lancet 2011; 378:997 - 1005.