Why does airway obstruction in chronic lung disease lead to hypercapnia?

Hypercapnea in COPD: This is the hallmark of copd.Patient can inhale , but cannot exhale completely, so when the next breath comes in the patient still has some air remaining in the lung with high content of co2. The new breath of air with o2 will not get all in the lung due to that. That why we train patients to prolong exhalation time to be able to evacuate the previous batch of air with high co2.

Not alwasy: The more inefficient the damaged lung becomes at eliminating co2 the more you have to work to breath and the more work the more co2 you make. Once co2 is up for a period of time your body stops responding to theco2 &switches to o2 for the drive. Thus co2 rises even more as the body stops response to co2. This makes o2 dangerous to use. Most COPD get no rise in co2. You need a blood test to know.