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NBME 21 Answers

nbme21/Block 2/Question#5 (64.1 difficulty score)
A 2-week-old male newborn has a patent ductus ...
Higher than normal left ventricular cardiac output🔍

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seagull  doesnt pre-load also decrease which would drop the C.O.? +  
hungrybox  @seagull I think it would increase preload b/c more blood is going into the pulmonary arteries -> lungs -> pulmonary veins -> eventually more blood in left atrium/ventricle -> inc preload +30  

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felxordigitorum  The "steal" from the aorta during diastole requires increased cardiac output to compensate. Extremely premature infants have limited ability to increase stroke volume and thus use increased heart rate to increase cardiac output. +2  

 +3  upvote downvote
submitted by d_holles(137),
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lovebug  very helpful! thanks! +  

 +1  upvote downvote
submitted by didelphus(44),
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A PDA ayesnlestil seaecrt a hfogilh-w haret erifual ttioasnui ni teh yab.b Sienc a infrtaoc fo teh LV utotup si rereundt otthwui grhnceia the by,do ni erdro ot naaitmni a nlomra CO ot hte boyd hte ftle ilrcnveet umts mppu a gheihr .mveuol hTsi oulwd alos cuesa hehgri ahtn nmlrao plornyamu lcaapilyr wflo.

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didelphus  *another user noted that this wouldn't impact RV oxygen because the blood is added to the pulmonary artery, which has exited the RV. +3  

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usmleuser007  1) higher than normal CO b/c blood is shunted from aorta to pulmonary arteries. This blood is added to the volume that was pumped into the pulmonary arteries by the RV. Now when the oxygenated blood returns to the LA & LV, the O2 content would be greater d/t higher blood volume. Also for that same reason more blood is returning to the LV (d/t LV volume plus fraction of RV volume). This increased the CO. Right--> Left shunts have late cyanosis b/c the RV is pushing against the excess pressure generated by the LV. This leads to Eisenmenger Syndrome as RV enlarges and pushes against the pressure from the LV in the PDA. Thus shifting Left to right to right to Left and thus the late cyanosis +2  
temmy  The anatomy is aorta-pulmonary artery-pulmonary veins-left atrium-left ventricle Notice that the blood did not come across the right heart at all and because of the LEFT TO RIGHT shunt of the PDA, we add more volume to the LEFT side. Hence the increased left ventricular output +2  

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necrotizingfasciitis  Going off of the comments people have posted above & kinda bringing things together: PDA flows from aorta to pulmonary arteries, which reverses after birth. This means de-oxygenated blood flow from the pulmonary arteries to the aorta & less volume being sent to the LF side of the heart. This results in a decreased afterload because there is less blood flowing from the lungs to re-fill the LF ventricle, & the heart is still pumping with the same force as before, so the same volume of blood is leaving, but less in entering the LF side of the heart. From here, you use CO = SV x HR SV = preload - afterload (which is decreased due to the PDA) This results in SV being larger than normal, so when you plug that into CO = SV x HR you get a higher number for cardiac output. +  
didelphus  The ductus arteriosus flows from PA --> aorta in utero to bypass the lungs, which have extremely high resistance to flow. This reverses after birth due to a drop in PGE2 (which was supplied by the placenta) and increase in left-sided systemic resistance. So a PDA typically flows aorta --> PA (assuming there are no other defects). +  

 +0  upvote downvote
submitted by jandj19(0),

This is a very cool concept explained in the Rapid Review pathology book, page 257.

Left-sided to right-sided heart shunts results in volume overload in the right side of the heart, which increases LV volume (preload) due to more blood returning from the right heart to the left heart. An increase in LV preload is equal more cardiac output and more work for the heart. Later in life that will lead to an eccentric type of LVH with decrease cardiac output, pulmonary hypertension and a concentric RVH due to high afterload that the right heart has to pump against.

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submitted by djinn(3),

Low afterload > high CO

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submitted by hello(246),
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fkstpashls  It's L to R +1