USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 02
Practice exam for USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment.
Question 1
MEDIUMA 67-year-old man presents to the emergency department with progressive shortness of breath and wheezing for 2 days. He has chronic obstructive pulmonary disease and hypertension. Medications include tiotropium and hydrochlorothiazide. He is afebrile. Blood pressure is 148/86 mm Hg, pulse is 104/min, respiratory rate is 26/min, and oxygen saturation is 88% on room air. He is started on supplemental oxygen. Thirty minutes later, his oxygen saturation improves to 96%, but he becomes increasingly somnolent. Arterial blood gas shows pH 7.29, PaCO2 68 mm Hg, PaO2 82 mm Hg. Which of the following best explains the increase in PaCO2 after oxygen therapy in this patient?
In COPD, supplemental oxygen can raise PaCO2 primarily by reversing hypoxic pulmonary vasoconstriction, which increases perfusion to poorly ventilated alveoli and worsens V/Q mismatch, raising CO2 retention. Decreased central chemoreceptor drive is a commonly cited contributor but is not the main mechanism and does not fully explain rapid CO2 rise. Intracardiac shunt is not induced by oxygen therapy and would typically worsen hypoxemia rather than cause hypercapnia.
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