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Hi, I'm Becky Bagley, Director of the Nurse 
Midwifery Education program at East Carolina University, and I'm going to talk with you today
about fetal heart rate decelerations and the physiology behind them. 
First of all, we have variable decelerations, and variable decelerations are 
caused by cord compression. The cord can be compressed 
by the baby grabbing onto it or the umbilical cord could be in between 
the baby's body and the uterus. So the maternal position 
can make a difference with that. So variable decelerations are called 
variables because they vary in shape and they can vary in where they occur
in relationship to the contraction. Variable decelerations can 
be "U" shaped, "W" shaped, or "V" shaped. They typically 
drop quickly and return to the baseline quickly. 
Variable decelerations will often times have shoulders around the decelerations 
which means the heart rate will accelerate before it drops, and then 
it accelerates again before it comes back to the baseline. The reason 
for this is because of the physiology of the umbilical cord. 
The umbilical cord has two arteries and one vein. 
Opposite of how the body works after the baby's born, the arteries carry 
the wastes from the baby back to the mother, and the vein 
carries the oxygenated blood from the mother to the baby. 
So the vein is the larger vessel. It's more
thin-walled. So it's more easily compressed. So typically, 
with cord compression, the vein is compressed first which causes 
a sympathetic response in the baby's heart rate causing that 
fight or flight reaction which accelerates the heart rate. 
That's what causes the initial shoulder prior to the deceleration. 
Then, barorecptors kick in and that's what drops the heart rate 
down. Then once the pressure is released off the umbilical cord, 
the heart rate accelerates again before it returns to the baseline. 
Early decelerations are caused by head compression. 
They are also called mirror images of the contraction because as the 
contraction starts to rise, the heart rate will drop. As the contraction 
starts to fall, the heart rate increases. So if you were to 
cover-up the contraction and the deceleration at the same time, 
you wouldn't even see the deceleration. It occurs at the same time
as the contraction. So early decelerations are a 
normal type of deceleration. It is a sign that the mother is 
probably progressing well in labor and needs to have her cervix examined 
as it may be time for her to start pushing. 
The last type of deceleration is called late decelerations. These 
are the decelerations that we worry about. As the contraction starts to 
increase and gets to the peak of the contraction, 
that's when the heart rate will start to drop. As the contraction decreases, 
the heart rate slowly begins to increase, but is still 
on it's way back up to the baseline as the contraction is over. 
Late decelerations are caused by utero placental insufficiency. 
What this means is that there's a poor 
transfusion between the uterus and the placenta. So the baby is not getting the oxygen that 
it needs. Here you can see a cross section of the placenta
and within the intervilla space, which is that blue part, you can see
the spiral arteries which carries the deoxygenated blood from the 
baby back to the mother. Then the mother's blood vessels pick up 
the deoxygenated blood but also take oxygenated blood 
from greater concentration of the mother to the lesser 
concentration of the baby. To help you understand this, I'm going to use my hands. 
If you would imagine this hand being the blood vessels within the placenta 
that are coming from the mother, and this hand is the blood vessels going to the 
baby. The oxygen goes from greater concentration of mom to the 
lesser concentration of the baby. When mom has a contraction, 
all of these blood vessels are going to be compressed. So the baby's not 
getting any oxygen at all during the contraction. As soon as the contraction 
is over, the blood vessels are released and the baby's able to breathe again. 
When you have utero placental insufficiency, for some reason, 
some of those blood vessels are clotted off. 
This could be because of hypertension. It could be because of 
diabetes. It could be because mom's a smoker,
or possibly she's post-dates. So now when you throw a contraction 
on top of this baby, instead of the baby getting a good, deep breath
to help him go through the contraction, it's as if he gets a 
small breath. By the time he gets to the peak of the contraction, the baby realizes 
he doesn't have any more oxygen on board, and that's when his heart rate begins to 
fall. Then the contraction lets up, and the baby gets his oxygen
again. At that point is when the heart rate slowly starts picking back up. 
Thus, the deceleration occurs late 
in the contraction. I hope this was helpful in your understanding 
of the physiology behind decelerations. 
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