nbme 12 respi

[firstaid2012]

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A 24yo woman is brought to ER by roommate cause of vomiting for 4 hrs. Also has 2day history of fatigue adn dizziness on standing. severe heartburn past 3 months. Antacids help provide relief. Pulse 110, RR 25, BP 95/70, Physical exam shows marked pallor, Hb 6 and Hct is 18. What is decreased in this patient?

a. alveolar Po2
b. arterial O2 carrying capacity
c. Arterial oxyhemoglobin saturation
d. arterial p50
e. arterial Po2

please explain

 

[ashik33]

B. arterial O2 carrying capacity

A can't be the answer. The patient is breathing room air and does not have any respiratory compromise. So alveolar pO2 level should be fine. It would have been correct in high altitude scenarios (decreased O2 in atmosphere) or some form of obstructive diseases and so on

E can't be the answer as there's no problem with the O2 transport across the alveolar membrane (there's no V/Q mismatch and diffusion is good). So arterial pO2 is fine. It would have been correct in cases of V/Q mismatch or pulmonary disorders with compromised diffusion (all these factors increase the alveolar-arterial O2 gradient). Also, when alveolar pO2 is already low, it may eventually lead to low arterial pO2 despite good pulmonary function

C and D can't be the answer as there's no problem with hemoblogin quality here and there're no factors that could have shifted the O2 dissociation curve either way. CO poisoning would have led to answer D.

In this question the patient is having decrease in Hb level. So according to the formula O2 content = (Hb * 1.34) * SaO2 + PaO2 * 0.003, total O2 content in the blood has decreased because of decreased Hb level and not any other factors.