The four components of tetralogy of Fallot are (1) pulmonic stenosis (valvular, infundibular or both), (2) high ventricular septai defect, (3) compensatory right ventricular hypertrophy (secondary to pulmonic stenosis) and (4) an overriding or dextraposed aorta which means the aorta may arise from both ventricles or from the right ventricle alone.
The haemodynamic abnormalities associated with tetralogy of Fallot depend largely on the size of the ventricular septal defect and degree of pulmonic stenosis. Right ventricular systolic pressure increases resulting in a variable degree of right ventricular hypertrophy. A large ventricular septal defect accompanied by a minimal degree of pulmonic stenosis results in a left to right or bidirectional shunt, pulmonary overcireulation and volume overload of the left side of the heart. A minimal amount of venous, non-oxygenated blood enters the systemic circulation via the overriding aorta and therefore cyanosis is not apparent. In comparison, severe pulmonic stenosis and a large ventricular septal defect leads to an increase in pulmonary vascular resistance and the development of a right to left vascular shunt. Hypertrophy of the right ventricular outflow tract and / or hypoplasia of the pulmonary artery may contribute to the pulmonic stenosis. Dogs with severe pulmonary hypertension may develop signs of right-sided heart failure.
Tetralogy of Fallot is more common in smaller breeds of dog (for example English bulldogs, poodles, and terrier breeds); in the keeshond breed tetralogy has a poly genie mode of inheritance.
Most dogs with tetralogy show clinical signs during the first 6-12 months of life. Clinical signs associated with severe right to left shunts include syncope, cyanosis, and dyspnoea which may be apparent even at rest. Affected dogs often appear severely stunted and show marked exercise intolerance. Chronic hypoxia, due to shunting of unsaturated blood across the ventricular septal defect, may lead to secondary polycythaemia. A systolic murmur, and occasionally a precordial thrill, typical of pulmonic stenosis can often be heard over the left 3rd intercostal space although this may become attenuated as the pressure within the right ventricle equilibrates with that in the left ventricle and blood is shunted preferentially through the aorta.
A harsher holosystolic ‘diagonal-type’ murmur more suggestive of a ventricular septal defect may be detected in cases where the pulmonic stenosis is less severe.
Most cases of tetralogy showing clinical signs have ECG changes consistent with right ventricular enlargement and a right axis shift. Signs of left-sided enlargement may be present in cases with a left to right shunting ventricular septal defect.
The classical features of tetralogy of Fallot are right ventricular enlargement and an enlarged pulmonary artcry segment. Displacement of the aorta may result in a loss of the cranial waist. A right to left shunting ventricular septal defect may result in hyperlucency of the lung fields due to pulmonary hypoperfusion.
Echocardiography can be used to image the high ventricular septal defect, pulmonic stenosis and dextraposirion of the aorta, and to confirm right ventricular hypertrophy. Other findings include reduced left atrial and left ventricular internal dimensions; hypertrophy with flattening or paradoxical motion of the inter -ventricular septum may also be apparent. A non-selective contrast (bubble) study may help to confirm the presence of a right to left shunting ventricular septal defect.
Angiocardiography and intracardiac catheterization
The non-selective injection of contrast via the jugular vein or a selective injection into the right ventricle results in simultaneous opacification of the pulmonary artery and aorta with no apparent left ventricular filling. Post-stenotic dilatation of the main pulmonary art cry is usually evident. Right ventricular pressure increases (normal less than 35 mm Hg) and a systolic pressure gradient develops across the pulmonic valve.
Tetralogy of Fallot: Treatment
Dogs with low pressure gradients across the pulmonic valve (less than 30 mm Hg) and a left to right shunt can be managed conservatively. The prognosis for more severe right to left shunting cases which are cyanotic is generally poor and few animals survive beyond 12-18 months of age. Although definitive surgical correction is only possible with cardiopulmonary bypass, creation of a systemic-pulmonary artery shunt, for example between the aorta and pulmonary artery (Potts anastomosis) or subclavian artery and pulmonary artery (Blalock-Taussig procedure), may increase pulmonary blood flow and systemic oxygenation. Animals which are significantly poly-cythaemic (PCV greater than 0,60 ll-1) may be given aspirin to minimize the risk of thromboembolic complications. The administration of beta-adrenergic blocking drugs has been advocated although their value has not been determined.