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966 Tetralogy of Fallot

• The overriding aorta acts as a conduit that TREATMENT • Aspirin or clopidogrel should be used if a
further contributes to the right-to-left shunt- Treatment Overview surgical shunt is performed to reduce the
VetBooks.ir • Severity of the RV obstruction often deter- There are surgical and palliative medical man- Behavior/Exercise
risk of thrombosis.
ing of blood.
agement strategies. Definitive surgical repair
mines direction and volume of shunting.
• Flow direction and volume can additionally
be altered by systemic blood pressure. may need to be delayed if patient size is not Vigorous or extended exercise restrictions should
amenable. Palliative surgical options may be
be implemented.
• Exercise and systemic hypotension are likely considered to allow patient to grow to a larger
to increase the right-to-left shunting. size for definitive repair. Alternatively, palliative Drug Interactions
• Increased blood flow through bronchial artery surgical and medical options may provide relief Hydroxyurea may be difficult to dose with the
collateral vessels increases venous admixture of symptoms for a period of time. commercially available product, especially in
and further lowers the arterial PO 2 . patients with low body weights. Compounding
• Chronic hypoxemia caused by the right-to- Acute General Treatment is frequently needed, and caution is warranted
left shunting can lead to erythrocytosis and • The asymptomatic patient may not need when chronically administering a chemothera-
signs of hyperviscosity. acute treatment. peutic drug. Periodic monitoring of the CBC
• Sedation with butorphanol 0.2-0.3 mg/kg is warranted.
DIAGNOSIS may be beneficial if the patient is anxious
and dyspneic. Avoid sedatives and anxiolytics Recommended Monitoring
Diagnostic Overview that can decrease systemic blood pressure. • Monitor PCV or hematocrit; CBC if patient
TOF should be suspected based on young • Supplemental oxygen, although of little receiving hydroxyurea
age of animal, left basilar systolic murmur, benefit with right-to-left shunts, should be • If surgical shunt is created, serial echocardio-
and cyanosis. CBC, electrocardiography, and provided when clinical signs are apparent. grams are warranted to monitor for patency.
thoracic radiographs may help raise or lower In addition to oxygen therapy, the quieter • Resting heart rate, especially when using
the likelihood of TOF, but the most useful environment of oxygen cages may provide beta-blockers; resting respiratory rate
diagnostic test is a complete echocardiogram. a buffer from hospital noise. Monitor
temperature in cage and avoid hyperthermia. PROGNOSIS & OUTCOME
Differential Diagnosis • If necessary, increase systemic vascular
• Pulmonic stenosis with VSD or ASD resistance with an alpha-adrenergic agonist Median survival time reported in one recent
• ASD, VSD, or PDA with pulmonary (phenylephrine) to reduce right-to-left publication was approximately 2 years for
hypertension (Eisenmenger’s physiology) shunting. dogs and cats being treated medically. Pallia-
• Double-chamber right ventricle with a • If packed cell volume (PCV) or hematocrit tive surgery can significantly improve clinical
VSD is > 70%, perform a phlebotomy with a outcome. A low-grade murmur appears to have
• Double-outlet right ventricle target of 10%-12% drop in PCV. Replace the poorer prognosis than a higher-grade murmur
• Truncus and pseudotruncus arteriosus removed volume with a balanced crystalloid with TOF. Sudden cardiac death is common.
• Extracardiac causes of erythrocytosis (pul- (100%-200%).
monary, renal, and bone marrow) PEARLS & CONSIDERATIONS
Chronic Treatment
Initial Database • Definitive surgical repair is available but Comments
• CBC: erythrocytosis requires cardiac bypass and open-heart Erythrocytosis is commonly seen with TOF
• Thoracic radiography: thoracic radiographs surgery. Veterinary facilities that can provide secondary to hypoxemia, but hydration status
commonly have minor abnormalities despite cardiac bypass are limited at this time. is commonly overlooked.
the significant cardiovascular changes present. • Palliative surgical options to create a systemic
RV, right atrial, and pulmonary artery (best to pulmonary shunt (e.g., modified Blalock- Prevention
seen on the ventrodorsal or dorsoventral Taussig shunt) can improve pulmonary blood Breeding of affected dogs and their immediate
views) enlargement may be seen. Thoracic flow and increase systemic arterial oxygen relatives is not recommended.
radiographs are most helpful in ruling out concentrations. This surgical option does not
primary respiratory disease. require bypass and can be performed by an Technician Tips
• Electrocardiography (ECG): RV hypertrophy experienced thoracic surgeon. If routine phlebotomy is indicated, care should
pattern is typically present. • Another palliative surgical option is balloon be taken to preserve vessels during venipuncture.
• Systemic blood pressure: clinical assess- valvuloplasty if the pulmonic stenosis is
ment of systemic vascular resistance should valvular. Caution is warranted because Client Education
be assessed before treatment with beta- overdilation with the balloon catheter can If animals acquire heartworm infection, it could
blockers. lead to severe left-to-right shunting and acute be a fatal combination with TOF. Heartworm
pulmonary edema. preventives should be given regularly, especially
Advanced or Confirmatory Testing • Control of erythrocytosis (PCV > 70%) with in endemic areas.
• Echocardiography with color Doppler: should periodic phlebotomy may be necessary.
allow confirmation of all four components • If frequent phlebotomies are needed to control SUGGESTED READING
of TOF. Contrast echocardiography (bubble PCV, consider hydroxyurea at 30-50 mg/kg Chetboul V, et al: Epidemiological, clinical, and
study) may be beneficial if there is equivo- PO q 24-48h; titrate based on PCV. echocardiographic features and survival times of
cal right-to-left shunting or bidirectional • Propranolol, a nonselective beta-adrenergic dogs and cats with tetralogy of Fallot: 31 cases
shunting. blocker, should be considered. The beta- (2003-2014). J Am Vet Med Assoc 249:909-917,
• Cardiac catheterization and angiography 2-blocking effects may reduce systemic vasodi- 2016.
are typically not needed but may be con- lation, minimizing right-to-left shunting. AUTHOR: Ryan Baumwart, DVM, DACVIM
sidered if definitive surgical correction is It may also reduce dynamic RV obstruction EDITOR: Meg M. Sleeper, VMD, DACVIM
planned. from RV hypertrophy. The dose range for
• CT/MRI are typically not needed but may propranolol is 0.5-1.0 mg/kg q 8h starting
be beneficial if definitive surgical correction at the lower dosage and titrating upward
is planned. based on symptoms and resting heart rate.

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