Issue Description 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.
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.
Causes 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.
Symptoms 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
Diagnosis 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.
Treatment 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.