USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 31
Practice exam for USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment.
Question 1
MEDIUMA 58-year-old man presents to the emergency department with progressive shortness of breath and wheezing for 2 days. He has a history of chronic obstructive pulmonary disease and hypertension. Medications include albuterol and hydrochlorothiazide. He is in moderate respiratory distress. Temperature is 37.0°C (98.6°F), blood pressure is 146/88 mm Hg, pulse is 112/min, respiratory rate is 28/min, and oxygen saturation is 88% on room air. He is started on supplemental oxygen. Thirty minutes later, an arterial blood gas on 2 L/min nasal cannula shows:
pH: 7.33 PaCO2: 60 mm Hg PaO2: 70 mm Hg HCO3−: 31 mEq/L
Which of the following best explains why supplemental oxygen can worsen hypercapnia in this patient?
In some patients with severe COPD and chronic CO2 retention, hypoxemia contributes to ventilatory drive. Supplemental oxygen can reduce hypoxic drive, decreasing alveolar ventilation and leading to increased PaCO2. A key distractor is reduced diffusion capacity, which can worsen hypoxemia but does not directly explain oxygen-induced hypercapnia; ventilation-perfusion changes and reduced ventilatory drive are the classic mechanisms.
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