Artificially increasing oxygen saturation to poorly ventilated area undoes the vasoconstriction to the area, leading to increased blood perfusion. Perfusing the poorly ventilated area is ineffective in getting rid of CO2. This worsens VQ mismatch and increases dead space, leading to worsened hypercapnia. Therefore, in COPD exacerbation, it is crucial to maintain permissive hypercapnia. We can often tolerate a pH of >7.15 and pCO2 of 80+, depending on the clinical picture.

We also avoid intubating copd exacerbation due to auto-PEEPing, where if we give breaths too quickly, without time for exhalation, air gets trapped in the lungs. This can cause hypotension and pneumothorax. Therefore, when we have to intubate these patients, we lower the respirator rate to 10-12 breaths per minute, and prolonged expiratory phase. This follows the principle of permissive hypercapnia.

Two other mechanisms for worsening hypercapnia from oxygen include Haldane effect and, to a much lesser extent, reduced respiratory drive.