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NBME 21 Answers

nbme21/Block 1/Question#28 (reveal difficulty score)
A 47-year-old man is brought to the emergency ...
Mixed venous oxygen tension πŸ” / πŸ“Ί / 🌳

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submitted by βˆ—usmleuser007(435),
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Troeubus lriescoss = sh-l"aaef o"psts = pa-vadlohse eraas leap rasae no fn en.oitks d/t rohtmaasam

submitted by βˆ—nicnac20(18),
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xMeid vonuse xgneoy ntsione si teh ntauom of xgyneo nduob to mlgbonihoe as ti teursrn ot teh rghit ised of eht thear refta rlatgnevi to het tsess.iu

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pparalpha  A helpful equation is CO=rate of O2 consumption/(arterial O2 content-venous O2 content): Fick principle If CO is decreased, then the difference between arterial O2 content and venous O2 content is increased +17  
misterdoctor69  To clarify a little bit, the tissues wouldn't necessarily extract MORE oxygen from the passing RBCs than they normally would. They would extract the same amount as they normally would to carry out their own functions. However, because the starting amount of O2 delivered to tissues is lower, you'd end up with less O2 bound to hemoglobin returning to the right heart. +5  
doso2222  Heart failure --> decreased cardiac output --> slower blood circulation time (blood is more stagnant) --> translate to increased oxygen extraction by tissues and therefore lowers oxygen tension by the time it returns to the right heart. +  

submitted by βˆ—kentuckyfan(44),
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I tge ywh the idxme evousn xeygno nonetsi .oadeceH,ew, eerdsvr tin's the tyeismcs acsavrul serncietas aslo es?careded

yb_26  no, decreased CO => peripheral vasoconstriction => SVR will be increased +7  
yssya1992  No SVR will increase due to RAAS and SAN thats why we decrease afterload in HF treatment ( ACEI, ARBs ) +5  
snafull  Wouldn't pulmonary vascular resistance also be decreased here due to pulmonary vasodilation in the setting of an MI? +  
cienfuegos  @snafull: my initial thought is that we would see pulmonary vasoconstriction because of the relatively low oxygen tension (that results from the low cardiac output). +3  

submitted by paloma(9),
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hatW aobtu olrnyuamp aalsrucv ernscsie?at tI ololfws hte sscimtye vralusac aietsn?cesr

cooldudeboy1  crackles are heard bilaterally so there is pulm patho which leads to increased pulm vascular resistance, since systemic blood flows into the lungs. any block in the flow ahead (lungs) will increase resistance in flow behind ( systemic ) +3  
mrglass  This patient is hypoxic increased diffusion distance. This causes pulmonary vasoconstriction. Ordinarily this response is designed to shunt blood to parts of the lungs that are well ventilated, but the response is maladaptive in global hypoxia +4  
happyhib_  I dont believe decreased venous oxygen tension would lead to pulmonary vasoconstriction (this is typically in the setting of low PAO2 you see this; shunting blood away from poorly oxygenated alveoli). You can get to increased pulmonary vascular resistance due to pulm edema from Left heart failing leading to fluid overload in pulm vasculature). +  

submitted by βˆ—hayayah(1145),
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exdMi ousvne egyonx satianutor v2S(O) si daeremsu in hte munpolyar e.arytr O2Sv maepssl eht teru diemx enusov olobd egivnal teh grith ntsrMameeeu fo dxeim euvosn geonyx orstaaunti )O2(vS mfor eth urympanlo etryra ahs bene dceodatva as na nicretdi indxe of tisseu oniyegan.txo

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