air trapping

Imagine trying to fully empty a balloon whose neck keeps pinching shut as it deflates: a little air gets stuck inside every time. Air trapping is that problem in the lungs — with each breath out, some air cannot escape, so it accumulates and the lungs stay fuller than they should.

Mechanistically, air trapping is abnormal retention of air due to incomplete exhalation. In obstructive disease, narrowed or floppy small airways collapse during expiration before they empty, sealing air behind them. The faster or harder a person breathes (as in exercise or an exacerbation), the less time there is to exhale, so trapping worsens — a phenomenon called dynamic hyperinflation.

On testing it shows up as a raised residual volume and functional residual capacity, and on CT as patchy areas that stay dark (low density) on expiratory images. Air trapping is a key reason people with asthma or COPD feel they cannot get a full breath in: their lungs are already partly inflated, leaving less room to inhale and forcing the breathing muscles to work at a disadvantage.