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matters of heart part 4: tetralogy of fallot

  • Dr. Kim
  • Posted by Dr. Kim Smyth on
    Staff Veterinarian and Pet Health Writer of Petplan

Today we’re finishing up our February series on congenital heart conditions by taking a look at a rare complicated condition called Tetralogy of Fallot (TOF). The congenital condition can affect dogs or cats (or humans, for that matter – Olympic snowboarder Shaun White underwent two corrective surgeries before age 1 to correct the defect), but is more commonly seen in dogs.


The word “tetralogy” means “group of four,” so as its name implies, this condition consists of four separate anomalies that occur during fetal development.  They are: 

1.  A large ventricular septal defect 

2.  Pulmonic stenosis 

3.  Overriding aorta

4.  Right ventricular hypertrophy 

We’ve already covered what happens in the cases of large ventricular septal defects and pulmonic stenosis, so you’re an expert on these already. But let’s take a closer look at the other two elements of the tetralogy. 

Overriding aorta

Usually, the aorta is attached to the left side of the heart, where it receives and delivers oxygenated blood to the tissues of the body. When an overriding aorta is present, it is misaligned. Instead, some amount of the aorta is positioned over either the ventricular septal defect or over the right side of the heart.


Right ventricular hypertrophy

The muscles of the right heart become enlarged (or hypertrophied), making it difficult for them to function properly. Right ventricular hypertrophy is now thought to be caused by the pulmonic stenosis that is present in cases of TOF.


The end result of all of defects that make up the TOF is that poorly oxygenated blood is circulated throughout the body. The following breeds have a predilection for the development of TOF:


  • Keeshonds
  • Poodles
  • German Shepherds
  • Fox Terriers
  • Boxers
  • Schnauzers


Symptoms are due to the circulation of oxygen poor blood. Cyanosis (or a blueish tinge to the gums and other mucous membranes), weakness, decreased exercise intolerance and failure to thrive are the most noticeable symptoms.


Tetralogy of Fallot may be suspected when your veterinarian hears a heart murmur while examining your puppy or kitten. Many things can cause heart murmurs, so don’t fret if your pet has been diagnosed with one. When a heart murmur occurs with other clinical signs like blueish mucous membranes and weakness, however, suspicion for a serious condition rises. X-rays and an echocardiogram will help your veterinarian or veterinary cardiologist make a definitive diagnosis of TOF.


Unfortunately, there are few specific treatment options for patients with TOF. In human pediatric medicine, surgeries are performed to correct the cardiac anomalies, but in veterinary medicine these surgeries are simply not practical (with the exception of balloon valvuloplasty to address pulmonic stenosis). Instead, we rely on medical therapy, such as the beta blockers mentioned in previous blogs. Thankfully, TOF is relatively uncommon, even in the breeds mentioned above, but should your pet develop a congenital heart problem, Petplan pet insurance can help you manage the costs of caring.

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