Hello and thank you for the Excellent question John.
Answers given are for informational purposes only and are not meant to replace an exam by an in-person.
I am Dr. Sohaib, a practicing Internal Medicine Specialist and i will do my best to help you today.
Brain of the patients with COPD are chronically exposed to low oxygen environment; and hence the respiratory drive (stimulus) is gradually shifted from "low oxygen tension" to "levels of Carbondioxide".
For a normal person, then respiratory drive is triggered by low oxygen levels in blood.
For a COPD patient, the respiratory drive is dependent upon carbondioxide levels, so it this is removed from the blood by supplying more/rapid oxygen, the respiratory stimulus is gone and patient's respiratory rate may fall abnormally which will lead to serious outcomes (like death) because patient wont be breathing in the oxygen you are giving.
I hope this clarifies :)
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