MDC3 Practice Test

In COPD patients who chronically retain CO2, giving supplemental oxygen can blunt their hypoxic drive and worsen CO2 retention, leading to somnolence. The brain’s breathing drive in these individuals becomes more sensitive to low oxygen levels than to high CO2, so increasing O2 reduces the stimulant to breathe, causing hypoventilation and a rise in PaCO2 (hypercapnia). Oxygen can also worsen V/Q mismatch in damaged lungs, further elevating CO2. If this happens, check arterial blood gas to confirm hypercapnia and adjust oxygen to a target saturation around 88–92% (or 90–92% depending on guidelines). Consider ventilatory support if CO2 retention is significant. Hypoglycemia, pneumonia, and electrolyte imbalance can cause altered mental status, but they aren’t specifically precipitated by oxygen therapy in this way, which is why the CO2 retention mechanism best explains the somnolence after oxygen.