You respond to a nursing home for a call about a patient with a history of COPD. You are told that the patient complained of difficulty breathing, so oxygen was applied at 2 liters per minute by a simple face mask. After several minutes, the patient became drowsy and then unconscious. As you assess the patient, you find him barely breathing and begin assisted ventilations with a bag-valve mask. After a few minutes, the patient improves and begins waking up. Why did the patient rapidly deteriorate after oxygen was applied?

The following subject areas are covered in the EMT Basic Exam:    
- Airway techniques    
- Patient assessment    
- Medical emergencies    
- Trauma    
- Treatment of infants and children    
- Operations    
- Advanced airway maneuvers    
- Anatomy and physiology