Tetralogy of Fallot includes four conditions: pulmonary artery stenosis, ventricular septal defect, aortic riding span, and right ventricular hypertrophy, the main ones being ventricular septal defect and pulmonary artery stenosis. This disease is the most common form of cyanotic congenital heart disease in older children and adult patients.
Etiology and pathology
The ventricular septal defect in this disease is located in the membranous portion of the right ventricular septum. Pulmonary orifice stenosis may be valvular, right ventricular funnel, or pulmonary artery type, with the right ventricular funnel type predominating. The aortic root is shifted to the right and rides over the defective ventricular septum, so it is more or less directly connected to both the left and right ventricles. In 20-25% of patients, the aortic arch and descending aorta are located on the right side. The right ventricular wall is significantly hypertrophied. In severe stenosis of the pulmonary artery orifice resulting in occlusion, a pseudo-arterial trunk is formed permanently.
Due to the obstruction of blood flow to the lungs caused by pulmonary stenosis, most of the blood discharged from the right ventricle passes through the ventricular septal defect into the riding aorta, and pulmonary blood flow is reduced, while arterial blood is mixed at the aorta and sent to all parts of the body, resulting in a significant decrease in arterial oxygen saturation, cyanosis, and secondary erythrocytosis. In patients with mild stenosis of the pulmonary artery orifice, there can be a bidirectional shunt at the ventricular level. The right ventricular pressure is increased and its systolic pressure is equal to that of the left ventricle and aorta, while the right atrial pressure is also increased and the pulmonary artery pressure is decreased.
Symptoms
The main manifestation is cyanosis (cyanosis), the degree and early appearance of which is related to the degree of pulmonary artery stenosis. It is mostly seen in superficial areas rich in capillaries, such as lips, finger (toe) nail beds, and bulbous conjunctiva. The shortness of breath and bruising can be aggravated by the slightest activity, such as crying, emotional excitement, physical labor, cold, etc., because of the decreased blood oxygen level and poor activity tolerance. Children often have squatting symptoms and often squat for a moment when walking or playing. At the same time, the lower limb artery is compressed and the resistance of body circulation increases, which reduces the right-to-left shunt flow, thus temporarily relieving the symptoms of hypoxia. As a result of long-term hypoxia, the capillaries at the ends of the fingers and toes expand and proliferate, and the local soft tissue and bone tissue also proliferate and hypertrophy, and then the ends of the fingers (toes) expand like a drumstick. Older children often complain of headache and dizziness, which are related to cerebral hypoxia. Infants sometimes have paroxysmal dyspnea after feeding or crying, and in severe cases, this can cause sudden fainting and convulsions, which are due to sudden muscle spasms in the funnel part of the pulmonary artery based on the stenosis there, causing temporary pulmonary artery obstruction and aggravating cerebral hypoxia. In addition, cerebral thrombosis can be caused by increased red blood cells, high blood viscosity and slow blood flow, and in case of bacterial thrombosis, brain abscess can easily be formed.
The anterior precordial region may be slightly elevated, and a grade II-III jet systolic murmur is often heard between the 2nd and 4th ribs at the left border of the sternum, usually the loudest between the 3rd ribs, with the loudness depending on the degree of pulmonary artery stenosis. In heavy stenosis, the murmur is light and short when there is little blood flowing through the pulmonary artery; in funicular spasm, the murmur disappears temporarily. All the second sounds of the pulmonary artery are diminished or absent. In combination with the aorta riding forward and positioned closer to the chest wall, sometimes only a loud, single second tone from the aorta can be heard in the pulmonary artery office area.
Common complications of tetralogy of Fallot are cerebral thrombosis, brain abscess and subacute bacterial endocarditis.
Examination
X-ray examination: the heart is normal or slightly enlarged in size, with a rounded and upward-pointing apical heart, a depressed pulmonary artery segment, a “boot-shaped” heart shadow, a reduced vascular shadow in the pulmonary hilum, reduced lung texture on both sides, and increased translucency. In cases with abundant collateral circulation, the two lung fields appear reticulo-pulmonary.
Electrocardiogram: Right-sided electrical axis, right ventricular hypertrophy, myocardial strain in severe stenosis, right atrial hypertrophy may also be seen.
Echocardiogram: The aorta rides on top of the ventricular septum, and the inner diameter is widened. The right ventricle has an enlarged internal diameter and narrowed outflow tract. The left ventricular internal diameter is narrowed. Doppler color flow imaging shows the right ventricle injecting blood directly into the riding aorta.
Cardiac catheterization: Catheters enter the aorta more easily from the right ventricle, indicating aortic riding. A catheter that enters the left ventricle from the right ventricle indicates a ventricular septal defect. A catheter that does not easily enter the pulmonary artery suggests a heavy pulmonary artery stenosis. If access to the pulmonary artery is possible, a pressure step difference between the pulmonary artery and the right ventricle can be recorded as the catheter is gradually pulled out. The patient has increased right ventricular pressure and decreased pulmonary artery pressure, and the continuous pressure curve can help identify the type of stenosis. Decreased oxygen saturation in the femoral artery demonstrates the presence of a right-to-left shunt.
Cardiac angiography: contrast is injected into the right ventricle, and the aorta and pulmonary artery are seen to be visualized almost simultaneously. The aortic shadow is thickened and positioned anteriorly and slightly to the right. In addition, the site and extent of pulmonary artery stenosis and the morphology of pulmonary artery branches can be visualized. The angiogram is more helpful in planning the procedure. Left ventriculography or coronary angiography is also required if necessary.
Magnetic resonance computed tomography: shows an enlarged ascending aorta riding on top of the ventricular septum, a ventricular septal defect, a small common pulmonary artery trunk, stenosis of the right ventricular funnel, and stenosis of the pulmonary valve annulus.
Selective cardiovascular angiography: Injecting contrast into the right ventricle through the right heart catheter will show both the aorta and the pulmonary artery, and will show whether the pulmonary stenosis is valvular, funnel, or pulmonary, in addition to the possibility of seeing contrast entering the left ventricle through the ventricular septal defect.
Treatment
Surgical treatment of this disease is palliative and corrective.
(i) Bypass surgery causes a shunt between the body and pulmonary circulation to increase blood flow to the pulmonary circulation and allow for an increase in oxygenated blood. There are methods such as anastomosis of the subclavian artery to the pulmonary artery, anastomosis of the aorta to the pulmonary artery, and anastomosis of the vena cava to the right pulmonary artery. This procedure does not change the deformity of the heart itself and is a palliative procedure, but it can create conditions for future corrective surgery.
(b) Direct vision surgery is performed under extracorporeal circulation to repair the ventricular septal defect, open the stenotic pulmonary valve or pulmonary artery, and remove the stenosis of the right ventricular funnel, which is a method to completely correct the malformation.
The prognosis of the disease is poor and most patients die before the age of 20. Causes of death include heart failure, cerebrovascular accident, infective endocarditis, brain abscess, and pulmonary infection.