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NBME 24 Answers
An otherwise healthy 35-year-old man sustains a ...
Decreased blood volume
Doesn’t supine position compress IVC leading to decreased venous return? (This is the pathophys of supine hypotension syndrome.) There was a UWorld questions about this ...
@medstruggle *Supine position* decreases blood pooling in the legs and decreases the effect of gravity. *Supine hypotension syndrome*, on the other hand, seems specific to a pregnant female, since the gravid uterus will compress the IVC; in an average pt, there wouldn’t be the same postural compression.
this was the exact same reasoning I used, but I thought the RAAS would inactivate which would lead to less aldosterone and less sodium retention
You gotta be preggers to compress your IVC
Could you also think of it in a purely "rest/digest" vs "fight/fright/flight" response, i.e. you're PNS is active, so your HR and subsequently your CO is less? But the explanation given above does make sense. Also because I think just saying someone is one bed rest leaves a lot up for interpretation, maybe not with this patient because his pelvis is broken, but lots of people on bed rest aren't lying flat.... ?
I think it might just be what NBME feels “decreased plasma sodium concentration” means, since through the mechanism that BV is lowered in bedrest, you would definitely have a decreased plasma sodium concentration compared to not-bedrest. However you won’t be presenting with any pathologic signs of hyponatremia because the Na+ would still be maintained in normal limits. Low blood volume is the cause of one of the main pathologic states associated w/bedrest -- cardiac deconditioning and postural hypotension once out of bedrest. Seems like poor wording though.
Remember that changes in sodium concentration over a long period time need to be due to a water dysregulation problem (like SIADH, polydipsia, HF, etc.). Here we just have physiologically increased effective circulating volume, and the body will compensate by diuresing, and since Na+ (and K+) are regulated ions, their plasma levels will not fluctuate.