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NBME 23 Answers

nbme23/Block 3/Question#45 (31.8 difficulty score)
A 44-year-old woman comes to the physician ...
Decreased arterial Pco2πŸ”,πŸ“Ί
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 +10 
submitted by seagull(1677),
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Tihs is a pinca takca.t vptnHlireyinteao orsdp p2CO aeiglnd to a ryesaotripr ikla.solsa o2p si etivaelryl tcnufadefe no(td' aks em ?w)oh

sympathetikey  Yeah haha I had the same conundrum. +  
sajaqua1  If she's breathing deep as she breathes fast, then oxygen is still reaching the alveoli , so arterial pO2 would not be effected. +25  
imnotarobotbut  lmao i'm so freaking dumb i thought she was having alcohol withdrawals because it was relieved by alcohol +2  
soph  Maybe Po2 is unaffected bc its perfusion (blood) limited not difusion limited (under normal circumstances). +2  
charcot_bouchard  PErioral tingling- due to transient hypocalcemia induced by resp alkalosis. +1  
rainlad  I believe CO2 diffuses ~20x faster than O2, so increases in her respiratory rate have more effect on her PCO2 than her PO2 +1  
usmile1  adding onto Charcot_bouchards comment, I found this: Respiratory alkalosis secondary to hyperventilation is probably the most common cause of acute ionised hypocalcaemia. Binding between calcium and protein is enhanced when serum pH increases, resulting in decreased ionised calcium. Respiratory alkalosis can induce secondary hypocalcaemia that may cause cardiac arrhythmias, conduction abnormalities and various somatic symptoms such as paraesthesia, PErioral numbness, hyperreflexia, convulsive disorders, muscle spasm and tetany. https://www.sciencedirect.com/science/article/pii/S1110184913000615 +3  
fatboyslim  Wow very cool. I didn't know alkalosis causes hypocalcemia or why hyperventilation causes perioral tingling. I thought the tingling sensation was due to hypocapnia-induced cerebral vasoconstriction and hence decreased cerebral perfusion. Sort of like when u get choked out you feel tingling (not recommended lol) +