NBME 22 Increased blood HCO3− (NBME Answers)

Retired NBME 22 Answers

nbme22/Block 2/Question#27 (reveal difficulty score)
A 50-year-old man who has smoked 2 packs of ...
Increased blood HCO3− 🔍 / 📺 / 🌳 / 📖
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submitted by ∗armymed88(49)

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emphysema leads to CO2 trapping leading to increase paCO2 in the blood, which gives you a respiratory acidosis Proper renal compensation will increase bicard reabs and decrease excretion- giving you increased bicarb in the blood

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meningitis Increased blood HCO3 could have easily been interpreted as increased blood pH aswell. FOllowing your explanation, since the pt had acidosis, the increased HCO3 will just make it a normal pH. Another way to think of the question is: if there is decreased exhalation due to COPD --> increased CO2 --> increased CO2 transported in blood by entering the RBC's with Carbonic Anhydrase and HCO3 is released into blood stream. So increased CO2 -> increased HCO3 seeing as this type of CO2 transport is 70% of total CO2 content in blood. +27

drmohandes I thought you could never fully compensate, so your pH will never normalize. Primary problem = respiratory acidosis → pH low. Compensatory metabolic alkalosis will increase blood HCO3-, but not enough to normalize pH, it will just be 'less' low, but still an acidosis. +7

mtkilimanjaro I also think decreased blood PCO2 and increased blood pH are very similar (less CO2 in the blood means less acidic, pH could go up) therefore I ruled both of them out just from that +1

brise Aka this is the Bohr effect! +2

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submitted by ∗waterloo(126)

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if you also think, when someone has increased CO2 (as in this patient) they have respiratory ACIDOSIS right...so increased blood pH cannot be the right answer.

Also, compensatory mechanism for respiratory acidosis happens in three ways.

ECF: where due to mass action increase of CO2 leads to increase in H+ and HCO3- (but this is minimal - like 3%)
ICF: most of the buffering happens here with CO2 going into the cell.
Kidney: then over sometime, there will be some renal compensation (not correction) where the kidney will start to let go of H+ and reabsorb new HCO3 to help adjust pH towards normal.

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submitted by usmlehulk(7)

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can anyone please explain why option E which is increased in Urinary pH is not the correct answer. becasue hydrogen wasting is also a form of correcting respiratory acidosis.

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wfma888 right- you will increase excretion of protons which will LOWER the pH! +2

an1 ↑ urine excretion of HCO3 (represents the alkaline pH/ higher pH) and ↑ urine pH are the same thing. Yes, you may wonder now why the ↑ blood HCO3 didn't cause an alkalosis but that's because the PRIMARY effect was the retention of CO2 leading to decreased blood pH +

an1 HCO3 is alkaline compared to H is what I meant to say ^ +

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submitted by medjay7(5)

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The easiest way I think of to see this is: 1) You are retaining a lot of CO2 2) The principal way in which we find CO2 in blood is as HCO3 3) You have a lot of HCO3

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