USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 29
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 for 2 days. He has a history of chronic obstructive pulmonary disease and heart failure with reduced ejection fraction. Medications include furosemide and albuterol. Temperature is 36.8°C (98.2°F), blood pressure is 148/86 mm Hg, pulse is 104/min, respiratory rate is 26/min, and oxygen saturation is 86% on room air. He is placed on supplemental oxygen, and his oxygen saturation increases to 96%. Thirty minutes later, he becomes more somnolent. Arterial blood gas shows pH 7.29, PaCO2 68 mm Hg, and PaO2 78 mm Hg. Which of the following best explains the rise in PaCO2 after oxygen administration?
In COPD, supplemental oxygen can raise PaCO2 in part via the Haldane effect: oxygenation of hemoglobin reduces its capacity to carry CO2 (as carbaminohemoglobin), increasing PaCO2. While reduced hypoxic drive may contribute, the Haldane effect and worsening V/Q mismatch are classic mechanisms; here the best single explanation among options is the Haldane effect. Hypoxic pulmonary vasoconstriction would decrease with oxygen, not increase. Dead space changes do not explain hypercapnia here.
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