“Tetralogy of Fallot: Basic Anatomy and Pathophysiology,” by Peter Lang, MD, for OPENPediatrics

Tetralogy of Fallot: Basic Anatomy and Pathophysiology,
by Dr. Peter Lang. I’m Peter Lang. I’m a cardiologist at Children’s
Hospital Boston, and today we’re going to discuss Tetralogy of Fallot. What we’re going
to talk about is some understanding of what Tetralogy of Fallot is, and some of the ways
of understanding the anatomy, the pathology, and the treatment. These are ideas that have
developed over decades here in Boston. I hope to show you the approach that has worked
for us. Different strategies develop in different institutions, but this will give you an idea
of one set of ideas that has been effective over the years. Anatomy. Tetralogy of Fallot– what most of you will
hear in text books is the four parts that make up Tetralogy of Fallot, or four pieces
of pathology. And the four are that there is a ventricular septal defect, that there
is pulmonary stenosis, that there is an overriding aorta, and the fourth is that there is right
ventricular hypertrophy. And the first thing that I want to do is show you that these four
points are actually one. But let’s talk a little bit about the four
things that Fallot described. He described a ventricular septal defect, which, in truth
is one type of ventricular septal defect. It’s an anterior malalignment type. It’s a
VSD that’s always in the same position. That doesn’t close spontaneously. There’s pulmonary stenosis, which is obstruction
to blood flow from the right ventricle to the pulmonary artery. It’s usually under the
pulmonary valve, but it’s very, very variable. Overriding aorta, and this is based on the
malalignment of the ventricular septal defect. The aorta overrides the ventricular septum,
and then because of the obstruction for blood leaving the right ventricle and the big VSD,
the right ventricle works at high pressure. So there’s right ventricular hypertrophy. Point of clarification. Pulmonary stenosis
in Tetralogy of Fallot may involve stenosis of the pulmonary valve, but it can also potentially
involve stenosis of the distal pulmonary artery. The degree of stenosis varies in patients
based on their individual anatomy. Now the first thing I want to do is say that
conceptually there really aren’t four parts of Tetralogy of Fallot, but there is one part.
I learned from Richard van Praagh, who’s a cardiologist and a terrific cardiac pathologist. Let me draw you a very simple heart. And this is oversimplified. We’ve got a right
ventricle. Coming into the right ventricle is a tricuspid valve. Leaving it is a pulmonary
artery which branches into a right and left side. The muscular part of the ventricular
septum, the outflow portion of the ventricular septum. A left ventricle. Mitral valve coming
into the left ventricle, and the aorta. So let me tell you that this portion up here
of the ventricular septum we’re going to call the conal septum. And the first concept I
want to make is that there’s really one thing that makes Tetralogy of Fallot. And that is
when there is a malalignment between the conal septum, and the rest of the ventricular septum,
everything else follows. So the conal septum, instead of coming in here we’re going to get
rid of it, and I’m just going to remove the pulmonary artery for a moment. And remove
the lower part of the aorta for a moment, and I’m going to take the conal septum, and
I’m going to bring it over here. And so it is malaligned with the rest of the
ventricular septum. And this is going to be rightward, and it’s going to be anterior,
and it’s going to be superior. And when this happens there is a ventricular septal defect.
It’s big, and it’s not going to close spontaneously. What happens then is the pulmonary artery,
and the pulmonary outflow tract, is squeezed between this conal septum and what’s going
to be the right ventricular free wall. And so we’re going to have the VSD, which
is our first part of Tetralogy of Fallot. And we’re going to have the subpulmonary narrowing,
which is the second part of Tetralogy of Fallot. And the aorta is going to override the septum
because it’s going to be coming over this way, and instead of being closed by a ventricular
septum in the normal position, it’s going to appear to override the ventricular septum. And sometimes it actually will move quite
rightward. And then finally– so that’s the third part of Tetralogy of Fallot– and the
last part has to do with the right ventricular muscle, which is going to hypertrophy because
there is systemic pressure in the right ventricle, because there’s a huge VSD and the pressure
equalizes on both sides. And so Tetralogy of Fallot has the four components
which Fallot talked about. But there really is one primary, ideologic anatomic problem,
and that is, once again, the malalignment of the conal septum with the rest of the ventricular
septum, because it’s anterior, superior, and to the right. You get your VSD. You get your
subpulmonary stenosis. You get your overriding aorta, and the right ventricular hypertrophy
follows suit. So that’s Tetralogy of Fallot. Pathology. Now, Tetralogy is the most common
form of cyanotic congenital heart disease that we see. There’s a little distinction.
It’s not the most common form of cyanotic congenital heart disease that we see in newborns,
because not all kids with Tetralogy present with cyanosis. And so transposition of the
great arteries is the most common form of cyanotic congenital heart disease that present
in the newborn period, but Tetralogy is the most common form of cyanotic congenital heart
disease period. Tetralogy varies a lot in its presentation,
its time of presentation, and that all follows from the anatomy. So let’s take this example
that I drew for you I’ll get rid of the numbers. And what we’ve got here is a right ventricle,
a left ventricle. We have our VSD. We have our narrowing under the pulmonary valve, under
the pulmonary artery, and we have our aorta. Now, if this pathway out of the right ventricle
from here to the pulmonary artery is not all that narrow, then at birth, what’s this kid
going to be like? Well, this kid is going to have– right ventricular blood is going
to come in from the right atrium, and it’s going to go to the pulmonary artery. Left
ventricular blood is going to come in from the left atrium and it’s going to go to the
aorta. And if blood is staying on its own side, then
this kid may look normal as a newborn, because there is pink blood going to the aorta. There’s
blue blood going off to the pulmonary arteries, and there may not be much shunting in either
direction. The kid may have a little bit of murmur, because there’s turbulence here. But
this would be a kid with Tetralogy of Fallot who is not necessarily cyanotic, has enough
pulmonary blood flow, and things are pretty good. And this would be a kid who has a balanced
circulation, one of the things that we’d say. So let me just jump ahead to show you the
two extremes, and then we’ll come back and look in a little bit more detail in both.
Let’s say that Tetralogy of Fallot– that there was very little malalignment of the
conal septum. I’m going to get the pulmonary artery a little bit bigger, get the aorta
closer to where it wants to be. And what I’m going to say then is this is our circulation
where we do have a malalignment ventricular septal defect, but not very much malalignment. When this kid is born, just like any child
with a big ventricular septal defect, pulmonary resistance and systemic arterial resistance
is about equal, so we have a balanced circulation. But then over the course of the first weeks
to months of life, pulmonary resistance drops, and it’s easier for blood to go to the pulmonary
arteries than to go to the body. And so right atrial blood goes through the right ventricle
and comes up across this essentially unobstructed right ventricular outflow tract, goes to the
pulmonary artery. Left atrial blood returning from the lungs
goes across the mitral valve and left ventricle goes to the aorta. But if the blood is here
and the pulmonary resistance is lower than the systemic arterial resistance, you might,
in fact, have more blood shunting from left to right. And so a very mild form of Tetralogy
of Fallot might act like a kid with a ventricular septal defect. The kid is pink, because the only blood that’s
going to the aorta is fully saturated blood. The child may actually– in rare instances–
can go into congestive heart failure, because there’s so much pulmonary blood flow. So this
would be a child with so-called pink Tetralogy of Fallot, because they don’t have arterial
desaturation, can have congestive heart failure. And that’s their initial presentation. They
will mimic somebody with a ventricular septal defect. In cases of Tetralogy of Fallot with mild
pulmonary stenosis, the patient may initially maintain normal oxygen saturations. However,
over time, the degree of subpulmonary obstruction will progress and eventually cause oxygen
saturations to fall. Thus, it is important to note that patients with mild Tetralogy
of Fallot will become cyanotic with time if the defect is not repaired. Let’s just leave that alone for awhile, and
let’s take the opposite extreme. Instead of having very little malalignment, let’s have
an awful lot of malalignment. Let’s get rid of this and move our conal septum way over
here. And we have a tiny little right ventricular outflow tract. So what happens in that case? Child is born.
Blood comes from the right atrium into the right ventricle and has a very hard time going
to the pulmonary artery, because there’s a lot of obstruction. And so a little bit goes
that way. But the majority of the desaturated blood coming from the systemic veinous atrium,
the right atrium, goes across the tricuspid valve. Most of it goes off and goes into the
aorta. What blood does go to the pulmonary artery
comes back to the left atrium, gets saturated, full of oxygen, and comes through mitral valve,
left ventricle, aorta. But it’s mixed– this saturated blood mixes with desaturated blood,
and we end up with desaturation. So fully saturated blood would have 95% saturation
coming from the left atrium. So we have some of that. But then we’ve got a lot of blood
with, let’s say, 65% saturation. And so when you put these two together, you
might have blood with 80% saturation. The child is cyanotic, looks blue. There may be
a lot of– a loud murmur from this obstruction. And so this would be an example of a kid with
Tetralogy of Fallot, moderate to severe obstruction. Does not present like the other kid in congestive
heart failure and is pink, but presents desaturated, looks blue, has a loud murmur. If we take that example one little step further
and malalign this ventricular septal defect all the way over so, in fact, we end up with
the most severe form of Tetralogy of Fallot– Tetralogy of Fallot with pulmonary atresia–
when a child like that is born, what happens? Well, the kid– when this child is born and
there is no antegrade pulmonary blood flow– well, since there is likely to be a ductus
arteriosus– though not everybody has one– pulmonary blood flow is not going to come
from the right ventricular outflow tract. It’s all occluded. All of the blood that comes into the right
atrium, right ventricle goes across the malalignment VSD. All of the pulmonary venous return comes
from the left atrium mitral valve, left ventricle aorta. And how does the 95% saturation blood
get to the left atrium? Well, it comes from the pulmonary veins. It comes from the pulmonary
artery. But it’s only the blood that enters the pulmonary artery from either a PDA or
perhaps a collateral. So if the child is dependent on the PDA for
pulmonary blood flow, then early on, some 95% blood, some 65% blood starts out, 80%
blood. The ductus will close over the course of the first days of life. There is less 95%
blood. So a smaller proportion of 95% blood, still the 65% blood, and the saturation goes
down and down. And you start getting into trouble. Severe Tetralogy of Fallot with pulmonary
atresia can often have complex arrangement of the pulmonary arteries. Management of patients
with this condition is beyond the scope of this talk, as it requires more complex planning
for surgical repair depending upon the patient-specific pulmonary artery anatomy. So we have– I think I’ve just gone through
three ways that kids can present with Tetralogy of Fallot. One, not very interesting– they’re
pink. They may have a bit of a murmur. They’re not going to be sick as newborns. Two, modest
degree of pulmonary stenosis, well-balanced circulation, relatively pink, stable. Get
a little bit worse, more pulmonary stenosis, but still adequate pulmonary blood flow. You’re
doing OK. And then finally, the most severe form of
Tetralogy of Fallot– Tetralogy of Fallot with pulmonary atresia where you’re dependent
on alternative means of getting blood flow to your lungs– a ductus or aortic to pulmonary
artery collaterals where you can present quite blue as a newborn and get bluer in a hurry
if you’re dependent on a ductus arteriosus and that ductus closes. So that’s the broad
sweep of Tetralogy of Fallot– what’s the matter– that is what’s the matter anatomically–
and how it is very variable. Please help us improve the content by providing
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