Tetralogy of Fallot (TET)
Tetra, as in tetralogy, is latin for “four”, or four defects. Hence the title of this defect. Tetralogy of Fallot encompasses four defects; overriding aorta, pulmonary stenosis, right ventricular hypertrophy and a VSD.
This is one of the more common defects in congenital heart disease and affects about 3.5 % of all infants born with congenital heart defects, or about one in every 3500 births.
It is a complex cyanotic abnormality resulting from a failure of the interventricular outlet septum (LVOT or left ventricular outflow tract) and the pulmonary outflow infundibulum (RVOT or right ventricular outflow tract) to form normally and attach properly to their respective great artery truncal vessels. Further, as a result of the infundibular malformation, there is (typically) sub-pulmonic stenosis.
As a result, there is large VSD, an overriding aorta, right ventricular hypertrophy and pulmonary stenosis. This malformation at the crux of the heart may cause the aorta to override minimally (most left ventricular outflow proceeds out the aorta), or the aorta may be so malaligned to the right that most right ventricular outflow proceeds out the aorta and the pulmonary artery.
There are varying degrees to which the aorta may override the interventricular septum, in other words how “committed” the aorta is to one ventricle or the other. The aorta may override but be primarily committed to the left ventricle, or it may be doubly committed to both ventricles, or it may be primarily committed to the right ventricle. The level of commitment to the right ventricle varies between 5 to 100%.
If the aorta is primarily committed to the right ventricle then this would sound like a “double outlet right ventricle” (see my post on this subject), and in reality it is! If the aorta overrides to the point that it is totally committed to the right ventricle, then it can technically be a double outlet right ventricle, There is no reason that a double outlet right ventricle can not coexist with a tetralogy of fallot,
The difference is the malformation at the level of the infundibulum and the LVOT.
Typically, there is sub-pulmonic stenosis or hypertrophy at the level of infundibulum, but there may also be pulmonic valve stenosis as a result of a bicuspid pulmonic valve or a congenitally stenotic pulmonic valve, or a supravalvular stenosis of the main pulmonary artery or one of its branches.
Right ventricular hypertrophy occurs when the right ventricle can not pump blood out easily as a result of the pulmonary stenosis; this causes the right ventricle to “thicken” or hypertrophy.
The degree of cyanosis in the neonate depends much more upon the amount of pulmonary stenosis present rather than the percentage amount that the aorta overrides the ventricular septum If there is significant pulmonary stenosis, this reduces blood flow to the lungs hence enhancing cyanosis.
As the aorta is more committed to the right ventricle, this reduces systemic blood flow and mixing of the systemic and venous circuits, but the size of the VSD encourages equalization of the pressures and SaO2 levels of both ventricles.
It is important for the sonographer to evaluate the degree to that the aorta overrides, the size of the VSD and the level of pulmonary stenosis, and whether this stenosis is sub-pulmonary, pulmonic at the level of the valve, or supravalvular (superior to the valve).
Measure the size of the VSD. Is it right-to-left, left-to-right or bidirectional? Determine to what degree the aorta overrides the ventricular septum. Is the aorta more committed to the left ventricle or to the right ventricle?
Finally determine what type of pulmonary stenosis is present. Is it sub-pulmonary, at the level of the pulmonic valve, or is it supra-valvular? How much stenosis is present? Measure the pressure gradient across the stenosis. Also, measure the size of the pulmonary artery and its branches. Are they hypoplastic?
Keep in mind that there may complete atresia of the pulmonary artery (this will be discussed in another post).
Look for anomalous coronary arteries, as this will be an important surgical consideration.
A right aortic arch may be common in this defect. Document this, and determine which branches are the brachiocephalic, carotid and subclavian arteries.
Major Aortocopulmonary Arteries (MAPAC’s)
In the presence of severe pulmonic obstruction, the body tries to perfuse the lungs somehow. As a result, collateral arteries will begin to branch out from the aorta and integrate themselves into the lungs, thus providing blood flow in the pulmonary circuit.
This will be further discussed in a future post. If there is a severe lesion in the outflow portion of the pulmonary tract, then MAPCA”s should be suspected. This defect can not really be detected by echocardiography but rather by MRI.
Surgical repair is required in this defect. In the past, 2 to 4 years before permanent corrective surgery would be performed, a palliative (temporary) shunt procedure was done in order to establish a systemic arterial to pulmonary artery connection that increases blood flow to the pulmonary circulation.
Several types of shunts were typically used.:
Blalock-Taussig shunt (classic): involves the anastamosis of the right subclavian artery to the right pulmonary artery.
Blalock-Taussig shunt (modified): presently the preferred procedure that uses a homograft or synthetic tube to attach the right subclavian artery to the right pulmonary artery.
Potts shunt: the descending aorta is attached to the left pulmonary artery via a “hole” or an anastamosis.
Waterston shunt: the ascending aorta is attached to the right pulmonary artery via a “hole” or an anastamosis.
Presently, complete single-stage surgical repair at approximately 6 months of age is the preferred approach. However, severely cyanotic neonates may still require palliative shunting. In these patients, the modified Blalock-Taussig shunt is the preferred approach, although some institutions may use balloon dilation and stenting of the infundibulum. This may be done in prior to, or in addition to a complete repair.
Complete repair generally involves the following:
1) If a palliative shunt was used, it will be detached and removed.
2) The VSD outflow malformation is closed with an autograft (pericardium), or a knitted Dacron or Teflon patch.
3) If an ASD (atrial septal defect) is present, it will be closed.
4) If there is a PDA (patent ductus arteriosis) present, it will be closed.
5) Pulmonary stenosis is relieved by resection of the obstructive tissue in the infundibulum. If the pulmonary valve is involved or if there is supravalvular involvement then a valvotomy and/or a transannular patch will be used.
The object of surgery is to normalize right ventricular pressures, open the stenotic pulmonary artery, close the shunts and save the pulmonic valve if possible. Unfortunately, a patch repair tends to sacrifice the pulmonic valve.
If the pulmonic valve can not be saved, or if a pulmonary conduit is not used, then there will be severe pulmonic regurgitation. This is well tolerated by the heart for up to 20 years, but eventually it will result in chronic right ventricular dilitation and eventual failure. This condition may require at some point the implantation of a prosthetic pulmonic valve or a valved conduit.
Survival for this surgery is excellent, with over 90% of patients still alive after 30 years. Reintervention may be necessary in about 15-20% of patients later in adulthood. This typically involves resection of additional infundibular stenosis, or the above stated repair of the pulmonic valve and or its outflow tract.