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NBME 24 Answers

nbme24/Block 4/Question#11

A 53-year-old woman comes to the physician because ...

Fixed cardiac output in spite of increased demand

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 +8  upvote downvote
submitted by neonem(257),

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. +1  
fallot4logy  why not option A?arterial compression ? +  
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 +  




 +0  upvote downvote
submitted by usmile1(12),

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.