Tetralogy of Fallot, by Dr. Mjaye Mazwi.
My name is Mjaye Mazwi. I'm a Fellow in cardiology and critical care at Children's Hospital Boston.
And I'm going to talk to you very briefly about Tetralogy of Fallot. This is not an
exhaustive discussion but rather a chance to give you a brief overview of some of the
very important things to understand about Tetralogy of Fallot.
Epidemiology.
So let's just start out with the epidemiology. This is an important form of cardiac cyanosis
to understand, because it's very common. It's approximately 10% of all congenital heart
disease. And then just as important, particularly in the developing world, is that it's the
most survivable form of cyanotic heart disease. So as a form of cyanotic heart disease, its
prevalence increases with age.
Physiology. In understanding how it comes about, there's a single inciting event as
the heart develops that explains all four features of Tetralogy of Fallot, and that
is anterior malalignment of the conal or infundibular septum. What that creates is the four classic
features of Tetralogy, specifically aortic override, a large unrestricted ventricular
septal defect, right ventricular outflow tract obstruction, and then concentric hypertrophy
of the right ventricle.
The physiology that results from those four features is easy to understand if you grasp
one central principle, and that is the VSD is unrestrictive, and what that means is that
the pressure in the right ventricle is the same as the pressure in the left ventricle.
So from a functional standpoint, both of those ventricles function as a single unit. What
that means is that if a red blood cell is in one of those ventricles, the direction
that it takes leaving the heart, i.e. either going to the lungs or going to the body, is
entirely dependent on the resistance to blood flow. Blood will flow in the direction of
lowest resistance.
If the pulmonary stenosis is severe, and it's easier for blood to go to the body than it
is to go to the lungs, then that causes cyanosis because that causes a right to left shunt
at the VSD. If the pulmonary stenosis is fairly mild and it's easier for the blood to go to
the lungs than it is to go to the body, then the VSD is left to right, and the patient
can be quite pink and overcirculated and develop heart failure. And so Tetralogy of Fallot
can exist along a spectrum from patients who are overcirculated and behave like patients
with large VSDs to patients that have critical pulmonary stenosis, where there's not a large
enough opening in the right ventricular outflow tract to allow enough pulmonary blood flow
to sustain life, and those are patients that are dependent on a patent ductus arteriosus
or neonatal shunt for survival.
Point of clarification. In cases of Tetralogy of Fallot with severe obstruction to the pulmonary
outflow tract, it may be necessary to place a shunt in order to provide a direct pulmonary
blood flow from the systemic circulation. This may be achieved using methods such as
placement of a synthetic conduit. The location of the surgical shunt placement is often determined
at the discretion of the cardiac surgeon, but can involve placement of a shunt between
the right ventricle and main pulmonary artery or between the subclavian artery and the ipsilateral
pulmonary artery. It is important to note that specific management of patients with
severe pulmonary outflow tract obstruction is beyond the scope of this talk, as it requires
complex planning for surgical repair based upon the patient's specific anatomy.
There's also an additional element of Tetralogy physiology that's important to understand,
and that is the dynamic right ventricular outflow tract obstruction that can be superimposed
upon this baseline physiology that we've just discussed. The dynamic obstruction is when
muscle bundles that exist in the right ventricular outflow tract basically, more or less, go
into spasm. That dramatically increases resistance to blood flow going to the lungs, and so blood
flow preferentially goes to the body, causing a very large right to left shunt at the VSD,
and making the patient extremely blue because blood can't go out to the lungs and pick up
oxygen. These are also known as tet spells or hypercyanotic spells, and I want you to
understand this physiology because we will talk a little bit more about this when we
talk about how Tetralogy is managed.
Clinical Features.
So features that might make you think that a patient that you are seeing with cyanosis
that is cardiac in origin has Tetralogy are the following. Patients with Tetralogy are
typically cyanotic and relatively comfortable. The majority of patients with Tetralogy are
blue and not pink.
There are chest X-ray features that suggest Tetralogy, that is a boot-shaped heart with
the apex lifted off the diaphragm. And the exam features, although helpful, are not specific.
Tetralogy can sometimes be associated with a tapping apex as a result of the hypertrophic
right ventricle. And there is a systolic murmur that you can hear in Tetralogy that is the
murmur of pulmonary stenosis. It's not a murmur associated with the VSD because there is no
restriction to blood flow across the ventricular septum. [MURMUR SOUNDS]
Being a pulmonary stenosis murmur, it's a murmur that's heard loudest in the pulmonic
area, and it radiates superiorly into the back. One of the interesting features of Tetralogy
is because the VSD is unrestrictive, when patients have more severe pulmonary stenosis,
quite often the murmur actually gets quieter, and that's because blood can go across the
VSD and out to the body rather than passing through the area of obstruction. So the loudness
of the murmur does not predict, in Tetralogy, the degree of pulmonary stenosis.
Diagnosis. Let's talk a little bit about how you would diagnose Tetralogy if you thought
that a patient that you were seeing had this diagnosis. The diagnosis is confirmed by echocardiography.
So you'd have clinical features that suggested the diagnosis, and you'd perform an echo to
confirm that diagnosis.
In addition to confirming the four features of Tetralogy that we'll run through and trying
to understand how those four features explain the physiology that you're seeing, there's
one additional very important feature of Tetralogy to talk about in the diagnosis. And that is
an understanding of the coronary anatomy. The anatomy of the coronary arteries in Tetralogy
can vary in ways that affect your ability to perform a surgical repair.
Classically, for instance, if the left anterior descending coronary artery comes from the
right coronary artery, or if the left anterior descending artery is supplied by both the
right and the left coronary systems, there are coronary branches that cross areas of
the infundibulum, or right ventricular outflow tract, that complicate the process of repair,
because they could be injured in the process of repair.
Management. The management of Tetralogy is variable and is dependent upon the degree
of right ventricular outflow tract obstruction. Milder degrees of Tetralogy, which are far
more common, present in the newborn period with cyanosis and the murmur that we discussed,
as well as the chest X-ray findings that we've already talked about. Milder degrees of Tetralogy
can be associated with heart failure due to pulmonary overcirculation and behave much
more like patients with just a very large VSD.
Those patients are typically monitored for the first year of life and are typically repaired
at some point within that first year. Worldwide, the timing of repair varies because these
phenotype varies, as well, but typically sometime between three and six months of age the defect
is repaired to limit the exposure of the patient to the effects of chronic cyanosis.
One indication to repair patients much earlier than this is tet spells. A patient who's having
a tet or a hypercyanotic spell does need to have a repair. That is an absolute indication
to take that patient for either a repair or a shunt.
If that degree is severe, it can be called critical. Critical pulmonary stenosis is a
ductal-dependent lesion, and those patients require prostaglandins until they can have
a more definitive intervention like a repair or shunt.
One of the interesting things about Tetralogy physiology is because the right ventricular
outflow tract obstruction is fixed, patients that start out with milder degrees of obstruction
will tend to develop more and more severe obstruction as they get bigger because as
the rest of the heart grows, that portion of the heart does not. So whatever the cyanosis
is when the patient is first born will typically be progressive unless there is an intervention.
Hypercyanotic "Tet Spells."
Finally, I want to talk about tet spells, or hypercyanotic spells, because these are
true medical emergencies. We discuss the physiology as being a dynamic obstruction to right ventricular
outflow, creating effectively critical pulmonary stenosis, causing a very large right to left
shunt at the VSD and severe cyanosis.
The management for hypercyanotic spells involves placing the patient in a knee-chest position--
so folding the knees up against the chest-- because that increases systemic vascular resistance
and makes that red blood cell sitting in the ventricle more likely to try to go through
the lungs than to go to the body.
With that measure, some of these spells can be terminated. But assuming that the spell
is progressive, other options are morphine. The exact mechanism by which morphine works
is not fully understood. But a dose of morphine, a bolus of fluid-- a bolus of fluid increases
intracardiac mixing and can improve saturations.
Beta blockers. Beta blockers are thought to decrease the dynamic obstruction to right
ventricular outflow by causing muscle relaxation. And that can improve saturations by decreasing
the magnitude of the right to left shunt.
And then finally, if you really, really are stuck, phenylephrine, or norepinephrine, or
epinephrine, an agent that suddenly increases systemic vascular resistance in a way that
makes it easier for that red cell to go to the lungs and improve saturations than to
go to the body.
Summary.
Tetralogy of Fallot is a congenital cardiac defect consisting of Fallot's four components--
a ventricular septal defect, pulmonary stenosis, overriding aorta, and right ventricular hypertrophy.
Patient presentation can vary depending upon unique anatomy and the degree of obstruction
to the right ventricular outflow tract.
While patients with Tetralogy of Fallot may appear cyanotic, present with certain characteristic
features on chest x-ray such as a boot-shaped heart, or have an audible systolic murmur
upon auscultation. A diagnosis of Tetralogy of Fallot should be confirmed by an echocardiogram.
Patient specific factors including, but not limited to, anomalous coronary artery anatomy
and degree of pulmonary stenosis can complicate management of patients with Tetralogy of Fallot,
therefore more complex planning that accounts for the patient's individual anatomy is required
prior to surgical repair.
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