Mar 25, 2012
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  1. Medical Student
A 50-year-old man comes to the emergency department because of a 2-week history of progressive shortness of breath. His pulse is 90/min, respirations are 26/min, and blood pressure is 120/80 mm Hg. Physical examination shows no other abnormalities. Laboratory studies show:

Arterial Pco2 30 mm Hg Arterial Po2 96 mm Hg Arterial O2 content 12 vol% (N=17%–21%) Mixed venous Po2 36 mm Hg Mixed venous O2 content 8 vol% (N=10%–16%)

Which of the following is the most likely explanation for these findings?

A) Anemia B) Drug-induced alveolar hypoventilation C) Residence at a high altitude D) Severe regional mismatching of alveolar ventilation and pulmonary capillary perfusion E) Voluntary hyperventilation

I understand why it's A but I can't rule out D either. Why can't it be D as well? Or E?


2+ Year Member
Jun 22, 2016
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  1. Medical Student
A V/Q mismatch will decrease PaO2. Hyperventilation will not decrease arterial O2 content.

In general, if the arterial O2 content is decreased in the setting of normal PaO2, that's anemia.
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