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NBME 23 Answers

nbme23/Block 3/Question#50
A 32-year-old woman, gravida 2, para 1, at 39 ...
Increased fetal systemic vascular resistance🔍

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From lnsetick on reddit: "This was my reasoning: you're compressing either the blood supply to the baby, or the blood flow away. If you compress the supply to the baby, pressure would tank and regulatory systems would get its heart to work harder. If you compress the blood flow away, then SVR would rise and regulatory systems would reflexively slow the heart down."

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submitted by krewfoo99(58),

Afterload and heart rate share an inverse relationship.

As the umbilical cord is compressed, there is an increase in systemic vascular resistance (Think of how the pressure would increase if you were to press down on a water hose). Thus, the afterload is increased and there is a compensatory decrease in heart rate.

divya  yeah, i too thought similarly. btw increase afterload --> increase in bp --> baroreceptor firing --> decrease in heart rate. is that it? +2  

Blood flow in series increases the resistance; blood flow in parallel decreases the resistance (TPR).

By blocking the umbilical veins you have in respect limited the excess flood flow to the placenta.

This reduces the flow in parallel circulation; thus increases the TPR. (MAP = cardiac out put times TPR)

This is because the total cross sectional area is reduced.

From this, there is an increased pressure in the fetal circulation.

The baroreceptors located immediately distal to the bifurcation of the common carotid artery would sense a high pressure and increase their afferent signal via CN9.

This Reduces the sympathetics and increases the parasympathetics via CN10 (vagus).

Thus, reducing the heart rate!

usmleuser007  correction: meant to say umbilical arteries (2 of them from the fetal heart to the placenta) +  

I see what they're saying (this was my second choice) but at the same time I feel like a backup of blood would activate the baroreceptors and cause decreased sympathetic activity to the SA & AV node.

sympathetikey  (choice E) +  
meningitis  Could you elaborate? Is this related to: less "preload" from mother circulation causes lowered HR? +  
meningitis  Or backflow of blood and causes a Reflex Bradycardia? still confused on this question. +  
kentuckyfan  So I think the subtle difference in choice E is that there would be a negative CHRONOTROPIC effect, no inotropic effect (contractility). +7  
maxillarythirdmolar  if anything, inotrophy could go UP not down as diastole prolongs and LVEDV increases --> Starling equation bullshit +  

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