The defining anatomic features of tetralogy of Fallot include right ventricular outflow obstruction (pulmonic stenosis), secondary right ventricular hypertrophy, a subaortic ventricular septal defect, and a rightward-positioned aorta. Pulmonic stenosis that occurs in combination with an isolated ventricular septal defect produces similar findings, but the infundibular septum is not malaligned, the aorta is normal in size, and the infundibulum of the right ventricle is not narrowed. These distinctions are commonly ignored in veterinary patients because corrective surgery is rarely performed.
Tetralogy of Fallot has been extensively studied in keeshond breeding colonies, and a spectrum of lesions ranging from the subclinical to the clinically complicated has been identified. Patterson et al. graded the conotruncal defects as follows:
• Grade 1: Subclinical malformations involving persistence of the conus septum fusion line, aneurysm of the ventricular septum, and absence of the papillary muscle of the conus.
• Grade 2: Pulmonic stenosis or ventricular septal defect in addition to the grade 1 lesions.
• Grade 3: Tetralogy of Fallot — pulmonic stenosis, ventricular septal defect, and dextropositioned aorta (with secondary right ventricular hypertrophy.
Additional abnormalities found in some dogs included a dilated and tortuous ascending aorta, pulmonary atresia, hypoplasia of the supraventricular crest, and anomalies of the aortic arch system. Based on extensive breeding studies and sophisticated genetic analysis, conotruncal defects have been shown to be an inherited autosomal recessive trait with variable expression.
The essential components of tetralogy of Fallot are severe right ventricular outflow tract obstruction and a ventricular septal defect. As a result of the outflow obstruction and elevated right ventricular pressure, desaturated blood shunts from the right heart through the septal defect to mix with oxygenated blood coming from the left ventricle. PulmonSry arterial blood flow and pulmonary venous return are scant, and the left atrium and left ventricle are small (underdeveloped). The addition ot unoxygenated blood from the right ventricle to the systemic side of the circulation causes arterial hypoxemia, decreased hemoglobin oxygen saturation, cyanosis, and secondary polycythemia. Systemic collateral circulation to the lung increases via the bronchial arterial system. These vessels supply blood to the capillaries of the pulmonary parenchyma either directly or via anastomosing connections with a larger pulmonary artery. A substantial portion of this blood can participate in pulmonary gas exchange. Other aspects of clinical pathophysiology have been previously described (see Clinical Evaluation of the Patient with Cyanotic Heart Disease, above).
Tetralogy of Fallot is common in the keeshond and English bulldog and in some families of other breeds. It has also been recognized in the cat. Presenting complaints and clinical signs are as previously described for cyanotic heart disease. In most cases, the murmur of tetralogy of Fallot is produced by blood flowing through the stenotic pulmonic valve. A right-sided murmur, resulting from blood (low through the VSD, may predominate when pulmonic stenosis is mild and left-to-right shunting occurs across a restrictive ventricular septal defect (i.e., an acyanotic defect). The absence of an obvious murmur suggests pulmonary atresia and/or polycythemia with hyperviscosity (which reduces blood flow turbulence) and ejection across a large, nonrestrictive ventral septal defect (VSD). Exercise or excitement may induce or enhance detection of peripheral cyanosis by accentuating right-to-left shunting by mechanisms previously described.
Radiography usually reveals a small or normal-sized heart with rounding of the right ventricular border. The main pulmonary artery is not always visibly enlarged, in contrast to the usual case of pulmonic stenosis with intact ventricular septum. The pulmonary vasculature is diminished, and the left auricle may be inconspicuous as a consequence of decreased venous return. The ECG typically exhibits criteria for right heart enlargement, including right axis deviation, although left or cranially directed vectors may be found in some cats. Echocardiographic findings include right ventricular hypertrophy, increased right ventricular chamber dimensions, reduced left atrial (LA) and LV dimensions, a large subaortic VSD, and right ventricular outflow obstruction. Doppler or contrast studies can be used to document right-to-left shunting at the ventricular outflow level.
Cardiac catheterization demonstrates equilibration of left and right ventricular systolic pressures, compatible with a large, nonrestrictive ventral septal defect (VSD). Oximetry samples reveal a step-down at the left ventricular outflow level, and the aortic blood is relatively desaturated. Angiocardiography reveals right ventricular hypertrophy, narrowing ol the right ventricular infundibulum, pulmonic stenosis with minimal poststenotic dilatation, varying degrees of pulmonary artery hypoplasia, a large subaortic VSD, a small, dorsally displaced left ventricle, an enlarged and right-ward-positioned aorta, and prominent bronchial circulation.. Bidirectional shunting across the ventricular septal defect is common in anesthetized animals. Anticoagulation therapy (e.g., heparin) should be considered to prevent cerebral emboliza-tion during and immediately after cardiac catheterization.
The natural history and survival times of dogs and cats with tetralogy of Fallot are not well characterized. Like other cyanotic heart diseases, tetralogy of Fallot can be tolerated for years if pulmonary blood flow is maintained and hyperviscos-ity is controlled. Most affected animals have severely limited exercise capacity. In cases of pulmonary atresia, pulmonary blood flow must be derived from a patent ductus arteriosus, the bronchial artery, or an elaborate network of systemic collaterals. Sudden death is common due to the combined consequences of hypoxia, hyperviscosity, or cardiac arrhythmia. Unlike with pulmonic stenosis with intact ventricular septum, congestive heart failure is an unusual outcome.
Options for treating animals with tetralogy of Fallot include medical and surgical approaches. Definitive correction of the defect (i.e., closure of the ventricular septal defect and removal or bypass of the stenosis) can be done under cardiopulmonary bypass, but such surgery is rarely performed in animals. As a general rule, the stenosis should not be relieved if the ventricular septal defect cannot be closed because the loss of right ventricular pressure results in marked left-to-right shunting with subsequent left-sided congestive heart failure. As an alternative to definitive correction, surgical palliation through the creation of a systemic to pulmonary shunt can be quite rewarding. Subclavian to pulmonary artery (Blalock-Taussig), ascending aorta to pulmonary artery (Potts), and aorta to right pulmonary artery (Waterston-Cooley) connections have been made in dogs and cats. Creation of a left-to-right shunt distal to the cyanotic defect increases pulmonary perfusion and allows a greater contribution of oxygenated blood to the systemic circulation. The size of the accessory shunt must be controlled to prevent overloading of the diminutive left ventricle and subsequent pulmonary edema. The extent to which these shunts remain patent over long periods in veterinary patients has not been reported.
Periodic phlebotomy, performed to maintain the PCV between 62% and 68%, produces a satisfactory result in many cases. Excessive bleeding should be avoided, and the blood that is withdrawn is replaced with crystalloid fluids to maintain cardiac output and tissue oxygen delivery. Some children with tetralogy of Fallot benefit from beta blockade with propranolol; however, controlled studies of the clinical efficacy of this treatment in animals are lacking. Severe hypoxic spells should be treated with cage rest, oxygen, and sodium bicarbonate (if metabolic acidosis is evident). Treatment with vasoconstrictive agents such as phenylephrine can also help reduce the amount of right-to-left shunting. Drugs with marked systemic vasodilating properties should be avoided.