Tetralogy of Fallot
Tetralogy of Fallot is the most common defect that produces cyanosis. It results from a combination of four components: pulmonic stenosis, large ventricular septal defect, right ventricular hypertrophy, and dextropositioning (overriding) of the aorta. A single conotruncal malformation (cranially displaced formation of the upper portion of the interventricular septum) is believed to result in narrowing of the right ventricular outflow tract (pulmonic stenosis), overriding of the aorta, and the ventricular septal defect. The right ventricular concentric hypertrophy is simply a consequence of the pulmonic stenosis. The pulmonic stenosis may be valvular, infundibular, or both. Breeds predisposed to tetralogy of Fallot include Keeshonds, English Bulldogs, and Wirehaired Fox Terriers. The trait is inherited in Keeshonds and presumably in other breeds. This defect has been recognized in other breeds of dogs and in cats.
The hemodynamic consequences of tetralogy of Fallot depend primarily on the severity of the pulmonic stenosis, the size of the ventricular septal defect (which is typically large and nonrestrictive), and the ratio of pulmonary to systemic vascular resistance. The direction and magnitude of the shunt through the septal defect depends in large part on the relative resistances to flow between the pulmonic circulation (obstructed by the pulmonic stenosis) and the systemic circulation. Consequences include reduced pulmonary blood flow (resulting in fatigue, shortness of breath) and generalized cyanosis (resulting in polycythemia and weakness) caused by the mixing of deoxygenated blood from the right ventricle with oxygenated blood from the left ventricle. Because of venous admixture, the kidneys release erythropoietin, resulting in polycythemia (see Erythrocytosis and Polycythemia). The increased blood viscosity associated with polycythemia can lead to sludging of blood and poor capillary perfusion. Consequences of polycythemia include ocular changes, bleeding diathesis, and neurologic abnormalities (ataxia, seizures).
Typical historical features include stunted growth, exercise intolerance, cyanosis, collapse, and seizures. A precordial thrill may be felt in the area of the pulmonic valve, and in most cases, a murmur of pulmonic stenosis is present. The intensity of the murmur is attenuated when severe polycythemia is present, and in some affected animals, a cardiac murmur is not present. Electrocardiographically, a pattern of right ventricular enlargement is usually seen (deep S waves in lead II, right axis shift), and arrhythmias are infrequent. Radiographs demonstrate variable right heart enlargement and undersized pulmonary vessels. Echocardiography confirms the diagnosis. Overriding (rightward displacement) of the aortic root, right ventricular hypertrophy, and a ventricular septal defect are evident. The left-side chambers may be small as a result of decreased pulmonary venous return. Routine contrast echocardiography demonstrates right-to-left shunting at the level of the ventricular septal defect. Flow through the defect can also be detected by Doppler echocardiography.
β-adrenergic blockade has been used to reduce the dynamic component of right ventricular outflow obstruction and to attenuate β-adrenergic–mediated decreases in systemic vascular resistance. Increases in systemic vascular resistance lower the magnitude of shunting. Polycythemia should be controlled by periodic phlebotomy to resolve clinical signs of polycythemia. The prognosis is guarded, but animals with mild to moderate shunting may reach adulthood.
Treatment options include surgical and medical management. Corrective surgery has been reported in dogs but is rarely performed. Palliative surgical techniques to relieve clinical signs associated with tetralogy of Fallot are also rarely performed and include techniques to produce systemic to pulmonary anastomoses such as a modified Blalock-Taussig shunt. These procedures increase blood flow to the lungs to reduce signs of pulmonary hypoperfusion and systemic hypoxia. In some cases, reducing pulmonic stenosis is palliative. Surgical valvuloplasty or balloon valvuloplasty of the pulmonic stenosis are also options.