Surgical management of tetralogy of Fallot.

Pathoanatomy The primary pathoanatomy seen in dogs with Tetralogy of Fallot (ventricular septal defect, pulmonic stenosis and overriding aorta) can all be explained by embryological developmental defects of the endocardial cushions comprising the conotruncal septum. (Patterson and others 1974) This is because the sinistroventral conus cushion and sinistroinferior truncus cushion give rise to the left part of the ventricular septum, and the left pulmonary valve leaflet respectively. The dextrodorsal conus cushion and dextrosuperior truncus cushions give rise to the right portion of the ventricular septum and the right pulmonary valve leaflet respectively. Finally, the anterior valve leaflet forms from the intercalated valve cushion. Abnormalities of conotruncal septal development, therefore, give rise to a spectrum of intracardiac defects the most severe viable form of which is Tetralogy of Fallot. (Patterson 1989) The degree of ventricular hypertrophy that accompanies the primary abnormalities will depend on the severity of the pulmonic stenosis and the size of the septal defect. The right ventricular outflow tract obstruction forms the progressive component of this disease because of the contribution of secondary infundibular muscular hypertrophy.