Tetralogy of Fallot is a rare congenital defect of the heart with 3 separate developmental abnormalities, and one compensatory change :

1. Transposition of the aorta from it's normal position entering the left side of the heart, to the right so that it sits over the ventricular septum
2. Pulmonic stenosis
3. A subaortic ventricular septal defect - often large
4. Right ventricular hypertrophy (secondary change)

Cause
Tetralogy of Fallot is caused by a genetically inherited defect

Signs

Very occasionally there may be no signs, but usually affected animals show exercise intolerance  , cyanosis  of visible mucous membranes, fainting (syncope) , and retarded growth rate are seen. A compensatory increase in red blood cells (called polycythemia) is sometimes seen which causes the membranes and tongue to appear very dark - even black.

Diagnosis

Auscultation - pulmonic stenosis murmur. Ventricular septal defect murmur.

ECG - changes consistent with right ventricular enlargement * NOTE A Provet video on "ECG Interpretation" is available form the Multimedia Shop GO HERE.

X-rays - Heart size may be normal, or have right-sided enlargement. Small, less radiodense than normal pulmonary blood vessels, and a bulge in the aorta may be seen.

Echocardiography - shows most of the abnormalities

Catheterisation and angiography - shows de-oxygenated blood shunting from the right ventricle into the aorta.