Tetralogy of Fallot

Introduction

The Four Defects Below Are Present in Tetrology of Fallot:

• Pulmonic Stenosis (Increases right ventricular pressure)
• Ventricular Septal Defect (Leads to shunting of blood from right to left)
• Overriding Aorta (Obtains blood from both ventricles)
• Hypertrophy of the Right Ventricle (Direct result of pulmonic stenosis; Contributes to an increase in ventricular pressure)
  

The outcome of the combined defects listed above depends on their severity. Usually the animal experiences pulmonary underperfusion and unoxygenated blood in the systemic circulation. Unoxygenated blood in the systemic circulation creates the hypoxic conditions necessary to stimulate the kidney’s release of the hormone erythropoietin. Erythropoietin travels to the bone marrow and stimulates an increase in red blood cell production. The increase in red blood cells can lead to polycythaemia. Overproduction of red blood cells increases the viscosity of blood leading to poor circulation and decreased oxygen delivery. Intravascular thrombi, haemorrhage, and stroke are just a few of the complications resulting from polycythaemia.

Signalment

Certain breeds are predisposed to the disease and these include Keeshonds, Miniature Poodles, Miniature Schnauzers, English Bulldogs and Wire-haired Fox Terriers.

Clinical Signs

May vary depending on severity but include lethargy, cyanosis, stunted growth, exercise intolerance, syncope and seizures.

Diagnosis

History and clinical examination findings, plus physical examination findings. These will often include a systolic murmur (over left heart base): due to pulmonic stenosis, a systolic murmur (over right sternal border): due to ventricular septal defect and a precordial thrill (palpable over both areas).

Radiographs of the thorax will show right ventricular hypertrophy and pulmonary underperfusion.

Echocardiographic findings will include structural abnormalities described and doppler will reveal the presence of a shunt and its abnormal blood flow.

Electrocardiographic (ECG) findings will indicate right ventricular enlargement by the presence of deep S-waves.

Treatment

Surgical treatment options include open heart surgery for complete correction (rarely done in animals).

Palliative surgery includes anastamosis of either the ascending aorta or subclavian artery to the pulmonary artery improves pulmonary blood flow.

Medical options include exercise restriction, phlebotomy (bleeding): helps to reduce blood viscosity and improve oxygen delivery to the tissues by maintaining PCV below 65% and beta-blockers: reduce shunting and protect the heart from hypertrophy.

Prognosis

Guarded, but depends on the severity of defects and the level of polycythemia.

Without treatment, animals will live only a couple of years with poor exercise tolerance. With treatment, animals can live up to seven years.

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References

Ettinger, S.J. and Feldman, E. C. (2000) Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2 (Fifth Edition) W.B. Saunders Company

Ettinger, S.J, Feldman, E.C. (2005) Textbook of Veterinary Internal Medicine (6th edition, volume 2)W.B. Saunders Company

Fossum, T. W. et. al. (2007) Small Animal Surgery (Third Edition) Mosby Elsevier