Page 1012 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Cardiovascular system 987

VetBooks.ir TETRALOGY OF FALLOT the presence of multiple anomalies. Severe exercise
intolerance is common. Cyanosis is not responsive
Definition/overview
By definition, four congenital cardiac anomalies to oxygen therapy, in contrast to that associated with
respiratory disease.
are present in tetralogy of Fallot: overriding aorta
(aorta sitting over both ventricular outflow tracts), Differential diagnosis
pulmonic stenosis, VSD and right ventricular hyper- Other causes of cyanosis include respiratory disease
trophy. If there is also an ASD, the term pentalogy and heart failure with pulmonary oedema. Other
is used. congenital cardiac anomalies, such as right ventricu-
lar hypoplasia or persistent truncus arteriosus, may
Aetiology/pathophysiology be associated with similar clinical signs.
The conal septum, involved in the development of
the right heart outflow tract and in the septation Diagnosis
of the ventricles, develops abnormally. This results Clinical pathology may be of some value.
in narrowing of the right ventricular outflow tract Polycythaemia may occur with prolonged hypoxia.
and an inability to complete the ventricular septum. Cardiac catheterisation could be used to demonstrate
The aorta overrides both outflow tracts as a result equalisation of pressure across the VSD. Decreased
of the pulmonic stenosis. Pulmonic stenosis results pulmonary vascularity may be observed radiograph-
in right ventricular pressure overload and therefore ically. There are no characteristic electrocardio-
right ventricular hypertrophy. graphic findings. Echocardiography is required for
Systemic hypoxia occurs because of mixing of a definitive diagnosis and all four components of the
oxygenated and unoxygenated blood in the aorta. condition are often readily visualised. The VSD is
Pulmonic stenosis results in decreased pulmonary often large, making it easy to identify, and the aorta
perfusion. Equalisation of pressure may occur across is observed sitting above the VSD. The right ven-
the VSD. Cyanosis is more profound if the obstruc- tricular wall is sufficiently thickened to be evident
tion to the right heart outflow tract is great. on echocardiography. Doppler or contrast echocar-
diography may be of benefit for characterising blood
Clinical presentation flow and direction.
Resting cyanosis is uncommon, but cyanosis fol-
lowing exercise is common. A loud pansystolic mur- Management/prognosis
mur, often with a thrill, is present over the left 3rd There are no treatment options. The prognosis for
to 4th ICS. The murmur may be complex due to survival is poor and euthanasia is often indicated.

ACQUIRED CARDIAC DISEASE

Acquired cardiovascular disease is significantly more Knowledge of the age, breed, performance history
common in the horse than congenital heart dis- and desired future use are important in determining
ease. The majority of conditions involve the heart the significance of a finding and issuing a prognosis.
valves and degenerative valve disease is common.
Myocardial disease, pericardial disease and periph- VALVULAR DISEASE
eral vascular disease are less common. Acquired
arrhythmias are addressed in a separate section In the horse, most cases of valvular heart disease
(see p. 972). are acquired. Acquired valvular disease may be a
There is no breed predisposition. Certain con- result of degenerative changes, damage or rup-
ditions are more common in certain age groups. ture of chordae tendinae or bacterial endocarditis.