This is a serious congenital cardiac lesion thought to result from conotruncal malformation in the fetus.  It is most common in the Keeshond and English bulldog, but any breed and cats can be affected.  Diagnosis is usually made with color flow doppler echocardiography, bubble echocardiography, radiographs and electrocardiograms.

There are four components of the tetralogy 1) right ventricular outflow obstruction (usually pulmonic stenosis which means narrowing of the pulmonary artery) 2) seconday right ventricular hypertrophy or thickening  3) subaortic ventricular septal defect and 4) dextropositioned or overriding aorta.  The result of all these defects is that the venous (low oxygen) blood from the right heart has difficulty entering the lungs to become oxygenated and the arterial (high oxygen) blood from the left heart become mixed with the right heart blood and low oxygen blood enters circulation to be delivered to the body.  This results in a reduced ability to exercise and syncope (fainting)  with excitement or exertion.  Many patients become polycythemic which means they have more red blood cells than normal in an attempt to increase oxygen delivery to the body.  Unfortunately, this can lead to thickened or hyperviscous blood and some patients feel better with periodic phlebotomy or removal of some blood.  The goal is to maintain the PCV less than 65%, so it is prudent to check hematocrits (pcv) every 3-4 months initially.

Some pets tolerate the defect for years provided pulmonary blood flow is maintained and hyperviscosity is controlled, however, many succumb early in life.  Sudden death is common.  Congestive heart failure is uncommon.

Definitive correction of the defects can be done with cardiopulmonary bypass, but such surgery is rarely performed in animals.  There are a few centers attempting some palliative type surgeries, but these are considered high risk procedures.  Medical therapy consists of phlebotomy as needed to keep the pcv less than 65%.  Since some children with tetralogy of Fallot benefit from beta blockade, we empirically employ these agents in some patients.  No studies have been done in veterinary patients to document efficacy of this therapy in this disease.  Severe hypoxic spells should be treated with strict rest and oxygen.  Vigorous exercise, excitement, high altitudes and drugs with vasodilating properties should be avoided.