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NBME 15 Answers

nbme15/Block 2/Question#26 (15.1 difficulty score)
A 47-year-old man is admitted to the hospital ...
Increased pulmonary capillary pressure🔍

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submitted by ergogenic22(303),
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medstudent  Doesn’t this also result in decreased alveolar ventilation since the fluid blocks air from getting to the alveoli? +3  
cassdawg  ^I would agree, but I think the primary cause of hypoxemia in pulmonary edema is actually the diffusion defect rather than strictly the decrease in alveolar ventilation so the better (more NBME) answer would simply be the increased pulmonary capillary pressure as this is the root cause of all of the issues in this guy's oxygenation. +  
cassdawg  Another way of saying this is that if the defect was purely due to a decrease in alveolar ventilation, the A-a gradient would be unchanged and CO2 would be increased. However, since it is edema, the A-a gradient is increased because there is a diffusion defect, and CO2 is not significantly increased. +1  

submitted by trazobone(14),

Acute MI and mitral regurg from the murmur leads to LV failure and backflow of blood into the lungs.

This leads to increased pulmonary hydrostatic capillary pressure. This will lead to excess volume leaking from the pulmonary capillaries into the interstitial and this will manifest as pulmonary edema (crackles).

Pulmonary edema will interfere with gas exchange leading to hypoxemia.