Maester’s Collection: Tetralogy of Fallot

Tetralogy of Fallot (ToF) is one of the first congenital cardiac diseases described with reports dating back to the 17th and 18th centuries. It was clinically defined by Fallot during the late 19th century and with the advent of the Blalock-Taussig procedure (GoreTex graft from the subclavian artery to the pulmonary artery), clinicians finally had a procedure to treat ToF.

It has become obvious with further studies that ToF is a broad spectrum of phenotypes due to varied combination congenital cardiac abnormalities. The phenotype of ToF depends on the dominant anatomic feature of the tetrad. Right ventricular hypertrophy is essentially a secondary phenomenon and likely doesn’t contribute to the heterogeneity of presentations. Further, the physiologic significance of the override aorta is questionable and thus not a dominant factor in the presenting physiology in ToF. The infundibular stenosis and the ventricular septal defect thus remain as the essential elements determining the clinical and physiologic patterns in patients with the tetralogy of Fallot.

spectrum-physiology

The first 3 phenotypes are forms of tetralogy where the 2 components are of equal but varying severity. The final 2 phenotypes, not discussed here, are ones where one lesion is severe and the other is very mild taking on the phenotype of the dominant lesion.

Extreme Tetralogy

Severe pulmonary stenosis or atresia with large VSD

Also called pseudotruncus arteriosus particularly when there is pulmonary atresia. This pattern appears similar to transposition of the great arteries with a single ventricle and pulmonic stenosis. This phenotype produces a large right-to-left shunt with severely reduced or absent pulmonary blood flow from the right ventricle. As a result there is equalization of left and right ventricular pressures. In this pattern there is early evidence of cyanosis and collateralization to the pulmonary circulation.

These patients tend of have lower peripheral arterial saturation and can be astonishingly low on exertion. Previously, these patients died from episodes of extreme hypoxia, but if significant collaterals are formed, they may live into adulthood. Softer systolic murmur at the left sternal border with only the aortic component of S2 is heard. Sometimes a continuous murmur is heard as a result of the flow through collateral arteries to the lungs.

The right heart is enlarged on EKG. Collateral vessels can cause a nodular hilar pattern on x-ray and the heart size is usually enlarged. On barium swallow these vessels may indent the esophagus.

The hypoplasia of the main and at times of the right and left pulmonary arteries renders the establishment of a systemic anastomosis very difficult. Surgery to relieve pulmonic stenosis are not as helpful because the PAs are small or atretic.

Classic Tetralogy

Severe pulmonary stenosis with large VSD

The classic tetralogy phenotype produces a large right-to-left shunting and a small amount of left-to-right shunting. The pulmonic stenosis reduces pulmonary blood flow and thus lowers the pulmonary artery pressures.

Classic triad of early cyanosis, squatting to alleviate symptoms, and episodes of paroxysmal dypsnea. The physical exam reveals cyanosis along with clubbing. Ascultation usually reveals a systolic murmur at the left sternal border, but its location and intensity may vary. Second heart sound generally has a pulmonic component, but A2 may dominate.

EKG reveals RVH. X-rays demonstrate decreased pulmonary vascularity in the lung fields and small pulmonary arteries. Surprisingly the overall heart size is normal, but is often boot shaped by RVH. Right sided aortic is present in 1/4 of patients with classic tetralogy.

Mild Tetralogy

Mild-moderate pulmonary stenosis with small-moderate VSD

In this phenotype the congenital anatomic abnormalities are at balance with each other. The left-to-right or right-to-left shunting is generally limited by the VSD size. As a result, RV pressures may equal LV pressures, but are often less. PA pressures are generally normal or slightly elevated.

These patients tend to have milder symptoms and are acyanotic. The small size of the VSD produces a loud systolic murmur at the left sternal border in nearly all patients and often with a thrill. S2 is variable.

EKG findings tend to be variable as well depending on the degree and direction of shunting. Sometimes RVH maybe present.

Radiographs are also dependent on degree and direction of shunting. Lung field vascularity maybe normal to slightly slightly increased as the pulmonary blood is less restricted and the VSD may allow left-to-right shunting.

Treatment

PottsWester

Palliative Shunts

Palliative shunts can be used to delay surgical repair, but they are rarely or no longer used today. The Blalock-Taussig shunt is described above. Potts shunt connects the descending  aorta to the left pulmonary artery. Potts shunts have been abandoned because of difficulty in removing during corrective surgery. The Waterston shunt connects the ascending aorta to the right pulmonary artery. The Potts and Waterston shunts are sometimes still used to relieve pulmonary artery stenosis or atresia. These are not used in cases of mild tetralogy as they increase blood flow.

Surgical Correction

With advances in surgical technique, corrective surgery currently is the standard treatment. It is often performed with the goals of relieving of the RVOT obstruction, completely separating the pulmonic and systemic circulations by closing the VSD, and minimizing valve incompetence. When technically feasible, the RVOT is opened by resecting the subinfundibular muscle bundles and patching the area open. Other times, a conduit with an artificial pulmonic valve is inserted between the RV and MPA.

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