Tetralogy of Fallot in Dogs and Cats

Tetralogy of Fallot (ToF) is a rare heart condition that both dogs and cats can be born with. It is unique because it involves four structural heart abnormalities. The unusual name originates from the Greek word “tetra,” meaning “four,” and from Étienne-Louis-Arthur Fallot, a French physician who first described the condition.

There are actually only three true defects with this condition: a pulmonic stenosis (narrowing of the pulmonary artery), a ventricular septal defect (a hole between the two ventricles), and a rightwardly misplaced aorta. The “fourth defect” is the thickening of the right ventricle in response to the high resistance to outflow of blood into the pulmonary artery and the aorta.

Because of the pulmonic stenosis, the path of least resistance for blood to flow from the right heart is into the aorta, with only some of the blood flowing into the pulmonary artery and the lungs to collect oxygen. This means that, in most cases, unoxygenated blood from the right heart mixes with oxygenated blood from the left heart and flows out into the body, resulting in a reduced oxygen concentration delivered to the body. Consequently, most animals with ToF have cyanotic (blue) gums and mucous membranes.

The lack of oxygen in the blood reaching the body, over time, results in the body producing more red blood cells (polycythemia). This causes the blood to be too thick and have a sluggish flow, and can lead to signs such as seizures.

More typically, dogs and cats become short of breath with exertion.

Diagnosis

This condition is often diagnosed at or shortly after birth. Affected animals are usually the smallest of their litter, tire easily, and may have a bluish color to their gums and tongue (called cyanosis) during activity due to low oxygen levels in the blood. During the first veterinary visit, a pet’s heart may sound abnormal. Your veterinarian may describe the sound as a systolic murmur caused by abnormal blood flow at the pulmonary valve. A diagnosis is confirmed with an ultrasound of the heart (echocardiography). Additional tests, such as blood work and an electrocardiogram (ECG), can provide more evidence of heart abnormalities.

Treatment

Most ToF cases are severe and result in clinical signs in dogs or cats within the first few years of life. The only way to fully correct the condition is through open-heart surgery, but this is rarely performed. For pets that show few or no symptoms, medication such as propranolol can help manage the condition. More commonly, affected dogs or cats are treated with regular phlebotomy (“blood letting,” or the removal of blood) to reduce the consequences of polycythemia.

Prognosis

The prognosis depends on the severity of the heart defects and how well your pet responds to treatment. However, in most cases the prognosis is often guarded to poor because the heart abnormalities are irreversible without surgery. Medication and supportive care can help manage symptoms. Still, owners should closely monitor their pets for signs of decline, such as an increased breathing rate, exercise intolerance, or changes in gum color. If any of these signs are evident, you should seek veterinary assistance immediately. Sadly, most dogs or cats with ToF die relatively early (within two to five years).

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