Tetralogy of Fallot

Patent ductus arteriosus, aortic stenosis, ventricular septal defect, pulmonic stenosis, and tetralogy of Fallot are the most frequently reported cardiac anomalies of dogs. Tetralogy of Fallot is a complex combination of four congenital heart defects that were first described by French physician Ettienne Louis Arthur Fallot. These include:

  1. Ventricular Septal Defect (VSD)
  2. Pulmonary Stenosis
  3. Right Ventricular Hypertrophy
  4. Overriding Aorta

Tetralogy of Fallot has been extensively studied in the Keeshond breed, in which the defects have been shown to be an inherited autosomal recessive trait.2 The disorder is also common in English Bulldogs. It is an uncommon congenital disease in dogs, but slightly more common in cats and horses.3

The heart has a wall called septum that separates the chambers on its left side from those on its right side. The septum prevents blood from mixing between the two sides of the heart. Ventricular septal defect is a hole in the part of the septum that separates the ventricles (lower chambers of the heart). The hole allows oxygen-rich blood to flow from the left ventricle into the right ventricle instead of flowing into the aorta (the main artery leading out to the body).

Pulmonary stenosis is a narrowing of the pulmonary valve and the passageway through which blood flows from the right ventricle to the pulmonary arteries. Normally, oxygen-poor blood from the right ventricle flows through the pulmonary valve into the pulmonary arteries and out to the lungs to pick up oxygen. In pulmonary stenosis, the heart has to work harder than normal to pump blood, and not enough blood can get to the lungs.



The right ventricular hypertrophy occurs when the right ventricle thickens because the heart has to pump harder than it should to move blood through the narrowed pulmonary valve. Overriding aorta is a defect in the location of the aorta. In a healthy heart, the aorta is attached to the left ventricle, allowing only oxygen-rich blood to go to the body. In tetralogy of Fallot, the aorta is between the left and right ventricles, directly over the ventricular septal defect. As a result, oxygen-poor blood from the right ventricle can flow directly into the aorta instead of into the pulmonary artery to the lungs.

The signs of tetralogy of Fallot vary in type and severity between individual dogs. Young animals may fail to grow normally, while dogs of all ages may have weakness or be unable to exercise normally. Affected animals are often short of breath and have a faster breathing rate that is made worse by exercise and excitement. The low level of oxygen in the blood causes the blood to appear darker than normal. This shows as a bluish-purple discoloration of the gums, particularly after exercise. In some animals the obstruction of the outlet results in a heart murmur produced by blood flowing through the narrowed pulmonic valve (pulmonic stenosis). In extreme cases, a dog may lose consciousness, suffer from seizures, or die suddenly.

To diagnose tetralogy of Fallot, a veterinarian will order several tests that will help determine the exact nature of the defects and how serious they are. Echocardiography is a harmless and painless diagnostic procedure that uses sound waves to create a moving picture of the heart. During this procedure, ultrasound waves bounce off the structures of the heart and a computer converts the sound waves into pictures on a video screen. The echocardiography test allows the doctor to clearly see any problem with the way the heart is formed or working.

Another commonly used test is cardiac catheterization during which a catheter (thin, flexible tube) is put into a vein in a limb, upper thigh, or neck, and threaded to the heart. A dye that can be seen on an X-ray is injected through the catheter into a blood vessel or a chamber of the heart. This allows the veterinarian cardiologist to see the flow of blood through the heart and blood vessels on the X-ray image. Cardiac catheterization also can be used to measure the pressure inside the heart chambers and blood vessels. It can determine whether blood is mixing between the two sides of the heart.

In many cases the treatment is surgical. Surgery involves creating a shunt between the aorta and pulmonary artery to allow sufficient blood to enter the lungs. Definitive correction of the defect (closing the VSD and removing or bypassing the stenosis) can be done under cardiopulmonary bypass, but such surgery is rarely performed. Sudden cardiac death is a late complication of surgically treated patients. Like other cyanotic heart diseases, tetralogy of Fallot can be tolerated for years, provided pulmonary blood flow is maintained and blood thickening is controlled. Most affected animals have severely limited exercise capacity. Sudden death is common because of the combined consequences of low blood oxygen level, increased thickness of the blood, and irregular heart beat.2

References

  1. Lowell Ackerman. Genetic Connection: A Guide to Health Problems in Purebred Dogs, Second Edition
  2. Stephen J. Ettinger, DVM, DACVIM and Edward C. Feldman, DVM, DACVIM. Textbook of Veterinary Internal Medicine.
  3. Dukes' Physiology of Domestic Animals. William O. Reece, Howard H. Erickson, Jesse P. Goff, Etsuro E. Uemura




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