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.
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