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NBME 24 Answers

nbme24/Block 4/Question#11
A 53-year-old woman comes to the physician ...
Fixed cardiac output in spite of increased demand🔍

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 +9  upvote downvote
submitted by neonem(452),

this patient has symptomatic aortic stenosis. This can be identified by the ventricular hypertrophy (to compensate for increased functional afterload from non-compliant aortic valve), midsystolic murmur and the location at the normal aortic area.

Per UpToDate on Clinical manifestations of Aortic Stenosis:

"Dizziness and syncope — Syncope occurs as a presenting symptom in approximately 10 percent of patients with symptomatic severe AS (or approximately 3 percent of all patients with severe AS) [3]. There are several proposed explanations for exertional dizziness (presyncope) or syncope in patients with AS, both of which reflect decreased cerebral perfusion. Exercise-induced vasodilation in the presence of an obstruction with fixed cardiac output can result in hypotension."

guillo12  What does "fixed cardiac output" signify? +  
usmleuser007  "fixed cardiac output" might mean that with the stenosis (ie. narrowed aortic valve) there is a limited or rather reduced cardiac output. Exercise would not increase cardiac output because the stenosis is caused by a mechanical (physical) rather than a biochemical process. Therefore, At any given moment the heart can not increase its output no matter how forcefully it contracts. +3  
fallot4logy  why not option A?arterial compression ? +1  
sunshinesweetheart  @fallot4logy LVH does not lead to coronary artery compression. only reallyyyy rarely will pulmonary artery dilation cause coronary artery compression. plus that would cause angina but probably wouldnt decrease cerebral bloodflow to syncope. her murmur + LVH point us toward aortic stenosis which does cause those --> fixed CO +1  
drpatinoire  @fallot4logy LVH can cause coronary artery compression, but typically leading to coronary ischemia after exercise (i.e. stable angina in this patient). The question is asking what leads to her syncope. Syncope actually means her brain is lacking blood supply abruptly. +2  
rainlad  how do we rule out mitral valve prolapse in this case? +  
spow  @rainlad murmurs at the right upper sternal border are aortic in nature. Mitral murmurs are heard at left 5th intercostal at the midaxillary line. +  

 +4  upvote downvote
submitted by usmile1(55),

from Boards&Beyond- Aortic stenosis leads to Syncope, Angina, and Left heart failure. Syncope is due to failure to increase cardiac output due to increased afterload. Angina is due to increased LVEDP which leads to decreased coronary blood flow. And left heart failure is due to increased LVEDP.

described a classical murmur of AS (late peaking, mid systolic, radiates to carotids b/l), no lesions on angio graph, ECG - LVH d/t AS most likely. its relieved on rest, rule our b) coronary spasm, an LVH wouldn't compress coronary artery (think a bit of anatomy). Mitral valve prolapse would a mid systolic click. im sure no one picked vasopressor induced hypotension; vasopressors increase blood pressure, this pt bp is 120/80 and a transient increase wouldn't lead to syncope.

the explanation of the right answer is given in other comments, just some thoughts of mine for wrong answers

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