Know fetal circulation (which vessels have the least/most O ); rem2mber that the baby is NOT
exchanging blood with O in 2he lungs. Pulmonary vessels in the fetus look like they have
pulmonary HTN – they are so thick that it is extremely hard to get blood through the pulmonary
artery into the LV b/c very little blood can go there – this is why baby needs a patent ductus to
get blood out. Where is O com2ng from? Coming from chorionic villus dipping into lake of
blood, which derives from mom’s spiral arterioles. Have chorionic villi dipping into blood and
extracting O 2rom it. Obviously, this is not as good an O sour2e as the lungs; therefore, you
want a high affinity Hb to be able to get what little O is2down in the area – this is why babies
have HbF, b/c of its high affinity to grab O from the blood. Bad news is that it gets the O , but
doesn’t want to give it up (says mine) – it left shifts the curve. What is compensatory response?
This left shift causes tissue hypoxia, which will cause EPO to be released and the kid will have an
18 gram Hb – b/c of this, all newborns (in a sense) have polycythemia. This is the way around
HbF’s high affinity for O 2 more RBC’s made, more Hb, and baby gets more O . 2
Order of O p2ssing: O goes2through syncytiotrophoblast of chorionic villus, into the
cytotrophoblast, then through the myxomatous stroma of the chorionic villus, then into the
blood vessel. The blood vessels of the chorionic villis all coalesce to form the umbilical vein.
This has the highest O c2ntent. It goes to the liver and it can go two ways: 1) into the hepatic
sinusoids and recollects into the hepatic vein and gets dumped into the IVC; and 2) ductus
venosis and straight into the IVC. Then it goes up the right side of the heart; the foramen ovale
is open in all fetuses (its not closed) – so all this blood is coming up the IVC – will it go straight
across, through the foramen ovale and into the left atrium, or will it go into the IVC into the
right atrium, down to through the tricuspid valve, and into the right ventricle? It will go through
the foramen ovale. So, all this oxygenated blood will go directly from the right atrium of the
foramen ovale into the left atrium, then the left ventricle and out the aorta. What about SVC
blood valve? It is coming from the superior part of the right atrium (its not gonna make a left
turn and go through the foramen ovale). It will go straight down, through the tricuspid valve
into the right ventricle. Now, it will go out the pulmonary artery. This is a PROBLEM b/c the
pulmonary vessels are too thick and it’s encountering this tremendous amount of pressure. To
counter this problem, kept the patent ductus open (which is kept open by the PGE2, a
vasodilator, made by the placenta) – so, there is a right to left shunt and blood can get out of
the pulmonary artery and dumped back into the aorta. Then, when the baby is born and takes
its first breath, the pulmonary vessels (that were all shut), all open within a millisecond, and
blood is going through those pulmonary arteries and gas exchange is occurring through the
lungs in literally seconds. Also, the patent ductus closes and forms the ligamentum arteriosum.
This is normal fetal circulation. Vessels with the least O ar2 the 2 umbilical arteries, and the
one with the most amount of O is th2 umbilical vein.
Look at O 2aturations (this is how they dx them – they catheterize, measure O saturati2ns in
different chambers, and know which direction the shunts are going. Need to get used to two terms – step up and step down.
If you have a left to right shunt, and have oxygenated blood going into unO ’d 2lood, what is
happening to O s2turation on the right side? Step up b/c mixing O ’d wi2h unO ’d blo2d.
If you have a right to left shunt with unO 2d blood going into the O ’d2blood? Step down.
The O 2aturation on the right side of the heart in blood returning from the body is 75%. The O 2
saturation on the left side is 95%.
Who’s stronger - left or right ventricle? Left, therefore the direction of the shunt is left to right.
So, oxygenated blood will be dumped into the right ventricle, leading to step up. Also, it will
pump it out of the pulmonary artery, leading to step up. So, you have a step up of O in r2ght
vent and pul artery. What if this is not corrected? With this mech, you are volume overloading
the right side of the heart b/c of all that blood coming over. The outcome of this will be
pulmonary HTN (the pulmonary artery has to deal with more blood and must contract more –
leading to pul HTN) – Once pul HTN occurs, right ventricle will have a problem contracting and it
will get hypertrophied. Suddenly, you run the risk of reversing a shunt b/c then right ventricle
could eventually be stronger than the left. So, it will be a right to left shunt – this is called
Eisenmenger’s syndrome. So, an uncorrected left to right shunt has the potential for producing
Eisenmenger’s syndrome. After reversal of the shunt occurs, pt will have cyanosis (aka cyanosis
tardive). Most VSD’s close spontaneously and some need to be patched.
Normal for a fetus to have a patent foramen ovale; it is not normal once they are born. Which
direction will blood go through the foramen ovale? Left to right (b/c the left side is always
stronger than the right). Therefore, what will happen to the right atrium? Step up – so it will go
from 75 to 80%. What will happen to the right ventricle and pulmonary artery? Step up. So,
what is the main diff in O2 saturations in VSD vs ASD? ASD is step up of O2 also in the right
atrium. Are you volume overloading the right heart? Yes. So do you run a risk for
Eisenmenger’s? Yes. What else are at increased risk for? Paradoxical embolization. What if you
weren’t lucky enough to have a DVT in the leg, and it embolize up and the pressures of the right
side of the heart are increasing, and you have a patent foramen ovale – will there be an
embolus that can go from the right atrium to the left atrium and will have a venous clot in
arterial circulation? Yes – this occurs in pts with ASD. MC teratogen that has ASD associated
with it? Fetal alcohol syndrome (1/5000)
It’s normal in a fetus but not when they are born. Connection between the aorta and
pulmonary artery – which is stronger? Aorta. So, oxygenated blood goes from left and get
dumped in the pulmonary artery before going into the lungs. So, what happens in the
pulmonary artery? Step up. So, now its 80% O2 saturation – the pulmonary artery is the only
thing that has a step up of O2. Then will go under the lungs and the pulmonary vein will have
the normal 95% O2 sat. B/c there is an opening between these, there is blood going back and
forth during systole and diastole – machinery murmur – where is it heard best? Between shoulder blades. Can you vol. overload the right heart? Yes. Pulmonary HTN? Yes. Now which
way will the shunt go? Will go the same way when it was a fetus; you will have unO2’d blood
dumping into the aorta. Where does the ductus empty? Distal to the subclavian artery – so, the
baby will have pink on top and blue on bottom b/c dumping unO2’d blood below the subclavi