related term: conotruncal defect

What is Tetralogy of Fallot?

Tetralogy of Fallot is a relatively uncommon but serious combination of defects that are the result of abnormal development in the embryo during the formation of the heart and great blood vessels. As the name implies, Tetralogy of Fallot consists of 4 defects. These are pulmonic stenosis, ventricular septal defect, over riding aorta and right ventricular hypertrophy secondary to the pulmonic stenosis. Evidence suggests that these defects are the result of varying degrees of abnormality in a single developmental process - the growth and fusion of the conotruncal septum. It is possible that pulmonic stenosis or a ventricular septal defect, both of which occur independently,  may be less severe manifestations of the same genetic defect. 

In pulmonic stenosis, there is partial obstruction of blood flow from the right side of the heart through the pulmonic valve. Because of the obstruction, the right side of  the heart has to work harder to pump blood to the lungs. This causes an increase in the mass of the heart muscle, or right ventricular hypertrophy, one of the hallmarks of this disorder.

A ventricular septal defect is a defect or hole in the muscular wall of the heart (the septum) that separates the right and left ventricles.

The aorta which carries blood from the left side of the heart, is mal-positioned to varying degrees with Tetralogy of Fallot.

Normally, the blood that is pumped to the body from the left side of the heart is fully saturated with oxygen. The oxygen is extracted from the blood for use in the various tissues and then the deoxygenated blood is returned to the right side of the heart. It goes to the lungs to pick up oxygen, and then is delivered back to the left side of the heart, from which it is pumped out to the tissues again. The result of the defects that make up the Tetralogy of Fallot is that poorly oxygenated blood is delivered to the body. This causes general cyanosis or a grey tone to tissues that would normally be pink. Tetralogy of Fallot is the most common cyanotic heart defect.

How is Tetralogy of Fallot inherited?

Based on studies with affected Keeshunds, the mode of inheritance is believed to be autosomal recessive with variable expression.

What breeds are affected by Tetralogy of Fallot?

There is an increased incidence of this condition in the Keeshond and English bulldog, relative to other breeds.

For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a general consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.

What does Tetralogy of Fallot mean to your dog & you?

As with other heart defects, the degree to which your dog is affected depends on the severity of the defect. If your dog has Tetralogy of Fallot with a very mild degree of pulmonic stenosis and a small ventricular septal defect, then he or she may only have a heart murmur and no associated clinical problems.

More often though, puppies with this combination of defects experience weakness, failure to thrive and grow, a reduced tolerance for exercise, and general cyanosis (blue-grey instead of pink mucous membranes). These signs are the result of the delivery of poorly oxygenated blood to the different parts of the body.

Unfortunately, these dogs rarely live beyond 1or 2 years without treatment. Treatment (described below) is helpful in about 50 per cent of cases, although these animals will have reduced activity levels.

How is Tetralogy of Fallot diagnosed?

Puppies with this disorder are weak and grow poorly. On physical examination, your veterinarian will find cyanosis and a heart murmur. X-rays and an electrocardiogram (ECG) will show severe enlargement of the right side of the heart. X-rays will also show reduced blood circulation in the lungs.

FOR THE VETERINARIAN:

  1. MURMUR: systolic ejection murmur, loudest over pulmonic valve area.
  2. ELECTROCARDIOGRAM: right ventricular hypertrophy.
  3. RADIOGRAPHS: usually see right ventricular enlargement, diminished pulmonary vasculature, mal-positioned aorta; may see post stenotic dilation in main pulmonary artery.
  4. ECHOCARDIOGRAPHY: RV hypertrophy, small left atrium, pulmonic stenosis, VSD, dextro-positioned aorta. Contrast echocardiography may show a shunt across the VSD.
  5. ANGIOGRAM: A venous angiogram taken 1 - 2 seconds after injection will show dye in the right atrium and ventricle, pulmonary artery, and ascending and descending aorta.
  6. OTHER: cyanosis at rest or after exercise, polycythemia, low arterial oxygen pressure.

How is Tetralogy of Fallot treated?

Complex open heart surgery is required to correct the condition. Surgery has a high mortality rate and is not considered a viable clinical option at this time.

There are medical and surgical treatments used to manage the condition. These include medication to reduce the muscular obstruction associated with the pulmonic stenosis (beta-adrenergic blocker), to allow more blood to flow to the lungs. There are some surgical procedures used in animals over 10 kilograms to reroute poorly oxygenated blood to the lungs, and this may help. This therapy is helpful in approximately 50 per cent of cases, although these animals will still have reduced activity levels. 

Breeding advice

Affected pups and their parents (assumed to be carriers) should not be bred. Siblings that appear normal after careful physical examination may be used for breeding with caution. Their offspring should be carefully examined and, if affected, the breeding of the parents should be discontinued.

 

FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.

 

Resources

Eyster, G. E., Gaber, C.E., Probst, M. 1993. Cardiac disorders. In D. Slatter (ed.) Textbook of Small Animal Surgery, p. 856-889. W.B. Saunders, Toronto. ( pp 908 - 911, for palliative surgical procedures).

Patterson, D.F.  1996. The genetics of canine congenital heart disease.  ACVIM-Proceedings of the 14th Annual Veterinary Medical Forum: 225-226.   This reference has good information for breeders and veterinarians regarding screening and genetic counselling for congenital heart defects.

 

Copyright 1998 Canine Inherited Disorders Database. All rights reserved.
Revised: October 30, 2001.