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NBME 23 Answers

nbme23/Block 4/Question#45

A 32-year-old man has a diastolic blood pressure ...

Precapillary resistance

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 +15  upvote downvote
submitted by ferrero(15),

A very similar question I have seen in Qbanks will ask why a patient with right heart failure does not develop edema and the answer is increased lymphatic drainage. I got this question wrong originally because I answered along this line of reasoning but I think in this case it all has to do with WHERE the extra pressure is coming from. In this question the pt has diastolic hypertension so you can think about the pressure as coming "forward" so constricting precapillary sphincters can prevent an increase in pressure in the capillary bed. However for right heart failure this extra fluid is coming from the OPPOSITE direction (backwards from the right heart) and constricting precapillary sphincters can do nothing (on opposite side of capillary bed) - the only way to prevent edema is to increase lymphatic drainage.

seagull  The question clearly lead us to think about Osmotic pressure by talking about protein and urine. I wonder how many people used that line of reasoning (like myself)? +10  
mousie  Great explanation, I chose lymphatic drainage for the same reasoning (similar Q on different bank) +1  
sympathetikey  My reasoning was much more simplistic (maybe too simple) but in my mind, systolic BP is determined by Cardiac Output and diastolic BP is determined by arterioles. Therefore, what comes before the capillary and regulates resistance? Arterioles. That's why I said that pre-capillary resistance. +11  
cr  the main difference between the 2 cases is that in this case the patient has high BP +  
link981  So in kindergarten language the question is essentially asking how high pressure in the arterial system is NOT transmitted to the venous system (which is where EDEMA develops). But you know they have to add all this info to try confuse a basic principle and make you second guess yourself. (Got it wrong by the way) because of what @ferrero said of Qbank questions. +  




 +0  upvote downvote
submitted by usmleuser007(87),

Aortic Diastolic Pressure

  • High TPR = high DP
  • High HR = high DP
  • High SV = high DP

Aortic Systolic Pressure

  • High Contractility = high SP
  • High SV = high SP
  • Low Compliance = high SP





 +0  upvote downvote
submitted by ferrero(15),

A very similar question I have seen in Qbanks will ask why a patient with right heart failure does not develop edema and the answer is increased lymphatic drainage. I got this question wrong originally because I answered along this line of reasoning but I think in this case it all has to do with WHERE the extra pressure is coming from. In this question the pt has diastolic hypertension so you can think about the pressure as coming "forward" so constricting precapillary sphincters can prevent an increase in pressure in the capillary bed. However for right heart failure this extra fluid is coming from the OPPOSITE direction (backwards from the right heart) and constricting precapillary sphincters can do nothing (on opposite side of capillary bed) - the only way to prevent edema is to increase lymphatic drainage.





 +0  upvote downvote
submitted by notadoctor(47),

I've been searching for my source for this but can't seem to find it. However, the way I thought about it was that edema happens via the capillaries. If there is increased resistance via the precapillary sphincters as much blood wouldn't be able to get into the capillaries. The blood would instead get shunted via anastomoses to the veins. This article from cvphysiology.com explains it a little better: CV Physiology: Tissue Edema and General Principles of Transcapillary Fluid Exchange





 -1  upvote downvote
submitted by usmleuser007(87),
1. Aortic Diastolic Pressure 1. High TPR = high DP 2. High HR = high DP 3. High SV = high DP 2. Aortic Systolic Pressure 1. High Contractility = high SP 2. High SV = high SP 3. Low Compliance = high SP
yex  https://cvphysiology.com/Microcirculation/M012 This helps somehow, the first part about capillary pressure. +