USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 34
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 shortness of breath and confusion for 1 day. He has severe COPD and uses home oxygen intermittently but increased the flow rate overnight because he “felt air hungry.” Temperature is 37.0°C (98.6°F), blood pressure is 136/78 mm Hg, pulse is 96/min, respiratory rate is 10/min, and oxygen saturation is 98% on 4 L/min nasal cannula. Arterial blood gas shows pH 7.28, PaCO2 68 mm Hg, and PaO2 92 mm Hg. Which of the following best explains the mechanism of his decreased ventilatory drive after increasing supplemental oxygen?
In some patients with chronic hypercapnia, central chemoreceptors become less responsive to CO2, and ventilation relies more on hypoxic drive mediated by peripheral chemoreceptors (carotid bodies). Increasing PaO2 with supplemental oxygen decreases peripheral chemoreceptor firing, reducing ventilatory drive and worsening CO2 retention. Central chemoreceptors would increase ventilation in response to CO2, not decrease it.
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