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nbme24/Block 3/Question#16 (50.5 difficulty score)
An 18-year-old woman is brought to the ...
Respiratory acidosis🔍
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 +11 
submitted by keycompany(309),
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Thsi ipenatt ash a emxho.ounrpat ivtyiHlrtpolaeenn is nto gnohue to cametosepn fro eht veollra recedsea ni ngul casuefr .aear

_yeetmasterflex  Could the pneumothorax also cause less ventilation due to decreased lung surface, retaining more CO2 causing respiratory acidosis? That's how I got to the answer at least. +6  
duat98  I think pneumothorax would increase RR because you're probably hypoxic. Also I'm sure when you have a lung collapse on you you'd be scared and that would trigger your sympathetic so your RR will go up either way. +3  
kateinwonderland  Arterial blood gas studies may show respiratory alkalosis caused by a decrease in CO2 as a result of tachypnea but later hypoxemia, hypercapnia, and acidosis. The patient's SaO2 levels may decrease at first, but typically return to normal within 24 hours. (https://journals.lww.com/nursing/Fulltext/2002/11000/Understanding_pneumothorax.52.aspx) +1  
linwanrun1357  How about choice C, --ARDS? +2  
bullshitusmle  there is no bilateral lung opacities as you would see in ARDS +4  
jesusisking  Was thinking some sort of infection b/c of the atelectasis so picked empyema but this makes sense! +  
djeffs1  does it need to be ARDS to cause "diffuse alveolar damage"? +  
makingstrides  Not only that, does having a collapsed lung affect the alveoli? +  



 +5 
submitted by jucapami(11),
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xr-ay ecsnsoodrrp ot a etiosnn oxtmounaerph = neniimmt ytrrripaoes iurealf fi .eettandur Rhitg lnug is yufll dcaeloslp, irsngineca otcni-rachtiar sse,eprur maiigrpn 2O agecxhne u(ed to mssa ftfcee rtwado etlf g,unl dan cldlseopa irhtg o),ne ecehn nuglumiccaat O2C ni( ood,b)l iiudcgnn ptyarreorsi assi.icod

j44n  how is it a tension if there is not tracheal deviation? +2  
djeffs1  I can see that its resp. Acidosis, but wouldn't the most risky potential complication be diffuse alveolar damage (If you arent able to reinflate sometime? +  
sexymexican888  @j44n honestly you cant even see the damn trachea on this! lol +  





 +2 
submitted by pingra(3),

I thought of this as a giant physiologic shunt (ie, due to the pneumothorax there is no ventilation to an entire lung, as a consequence you retain CO2) - not sure if this is the actual mechanism but it helped me get this question right

hopefully this helps!

drdoom  this definitely makes sense to me, especially if it happens “acutely”/suddenly. if someone gets a lung or lobe removed, e.g., cancer, my guess is that the reminaing lung would “remodel” over time and recoup at least some of that lost surface area — in the same way new anastomoses form in the weeks or months after near-complete artery blockage (as guided by VEGF elaboration) +  
drdoom  but in the case where it happens “all of sudden”, i totally agree you’re going to get CO2 retention +  



 +1 
submitted by faus305(12),

Keycompany gave the quick, simple explanation.

but from a less-clinical perspective: If anyone has ever been "Lit-up" on the football field, or just generally ever had the "wind-knocked out" of them, you know that your breathing for the next 5 minutes is very shallow because it just hurts to breath too deep.

This girl broke a rib so likely can't breath very deep, so even though she is breathing rapidly the CO2 is likely remaining in her lungs and causing a respiratory acidosis.

Also, here is why the other answers don't make sense:

Amyloidosis & carcinoid - long term problems not related to a broken rib and acute presentation Empyema- "collection of pus" wouldn't form this soon and if it did the percussion would not reveal increased tympany. Pulmonary edema- percussion would not reveal increased tympany. PE- Could cause similar symptoms but percussion would not reveal increased tympany, and the x-ray and history of trauma tells you that this is definitely a pneumothorax.