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.
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!
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.
Compression of the umbilical cord is what doctors do when the baby is delivered, this causes an increase in SVR! The increase will be felt by the baby and cause a compensatory decrease in HR. This is also the reason why carotid massage tricks the body into decreasing HR (it thinks the pressure is high... like the pressure is high on you... to do well on this test...or fail.. and drop out... and raise sheep on a farm...where you can sell sheep wool... and catch anthrax... and have to get medical attention.. so you end up in the hospital as the Illuminati intended).
Increased fetal vascular resistance=> reflex bradycardia (decreased HR)
submitted by โimnotarobotbut(184)
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."