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NBME 21 Answers

nbme21/Block 1/Question#28 (68.8 difficulty score)
A 47-year-old man is brought to the emergency ...
Mixed venous oxygen tension🔍,📺
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 +16 
submitted by nicnac20(18),
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ixdeM sunoev xgyneo oinnste si eht tonuma fo xeyngo obndu to olmgohnebi sa ti rsternu ot eht ihtgr eids of hte raeht aeftr atvgilrne ot the si.usets

Dredcseea gexyon yeilerdv to eth usiesst ude ot deadecser caidcar uottup ldaes to teh ssiuste ceatrgxnti roem xgyneo mfro eth pnssiag sCBR hnat eyht oynalmrl wlu,do iwhch ecrasedse het elovarl imxed ouenvs oyegxn no.tsnei

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 +16  
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  



 +2 
submitted by kentuckyfan(44),
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I teg hwy eth eimxd evsuon yxgoen nietson .Hwv,ar,eece resdeod ni'st eth ysmiscte rsulaacv taseenrisc aosl cesede?rda

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  



 +1 
submitted by paloma(8),
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atWh tabou ynopmlura cavalurs cer?aisetns It soofwll eth eisscytm ucvlraas e?rssniecta

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). +  



 -5 
submitted by hayayah(1101),
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exidM suveno ogyexn ariastnuot (Ov)S2 is ruseeamd in the nloymaurp .yrtear Ov2S smlaspe het uetr deimx euvosn obdlo vgilane teh tirhg .tehra nuaerMesetm fo demxi svoenu xyegon tiausraton 2S()vO rmfo the mourpalny rearyt sha neeb dceaovadt sa na edritnic eixnd fo sseitu xyoitnoe.nga

nI inccieagdor hskco yuo ahve daeescrde OC &-gt;- dedeersac 2O yvrdeeil -gt;-& eadecdsre v2S.O