ISSN 2398-2977      

Heart: tetralogy of Fallot



  • A congenital defect of the heart, combining:
    • Right ventricular outflow tract obstruction.
    • Ventricular septal defect.
    • Overriding aorta (dextroposition).
    • Right ventricular hypertrophy.
  • Cause: abnormal formation of infundibular septum.
  • Signs: depends on degree of right ventricular outflow tract obstruction and subsequent restriction to pulmonary blood flow as well as the size of the ventricular septal defect.
  • Diagnosis: auscultation, echocardiography, hematology.
  • Treatment: none.



  • Failure of correct formation and insertion of the infundibular septum of the embryonic heart during organogenesis.
  • Malalignment and anterior deviation of the septum results in a large interventricular septal defect and apparent over-riding of the aorta. The deviation of the septum results in an obstruction to blood flow in the right ventricular outflow tract. In some cases, the pulmonary valve also fails to develop correctly resulting in variable degrees of pulmonic stenosis Heart: pulmonic valve stenosis
  • Clinical signs depend upon the degree of stenosis and restriction to pulmonary blood flow, and the volume of blood shunting from right to left.
  • Tetralogy of Fallot can also be associated with complete pulmonic valve atresia (absence of a pulmonary valve and PA orifice). Pulmonary blood flow is then dependent on a ductus arteriosus that remain patent.
  • The elevated pressure in the right ventricle results in severe right ventricular hypertrophy.


  • Right ventricular pressure overload can → the development of right sided congestive heart failure (although this is rare).
  • Right-to-left shunting of blood → variable degrees of arterial hypoxemia dependent upon the relative RV outflow through the stenotic pulmonary artery and the ventricular septal defect. 
  • If the obstruction is severe, or the pulmonic artery is atretic, there will be almost complete venous admixture and marked cyanosis.
  • In cases with a less restricted pulmonary opening, cyanosis may only occur during periods of increased oxygen demand, or when the relative resistance to blood flow from the stenosis and the systemic vasculature is changed, eg during exercise. Cyanosis may only be evident after exercise. 
  • When there is marked cyanosis, activation of the renal and extrarenal erythropoietin systems results increased red blood cell and hemoglobin synthesis. The resultant erythrocytosis and increase in blood viscosity can result in pulmonary hemorrhage, reduced capillary perfusion and increased afterload on the heart.
  • Hypoxia → reduced exercise tolerance and exercise-related dyspnea.


  • Depends upon the degree of right ventricular outflow obstruction and venous admixture associated with right-to-left shunting of blood.


  • Sporadic and rare.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Scansen B A (2019) Equine congenital heart disease. Vet Clin Equine 35 (1), 103-117 PubMed.
  • Schmitz R R, Klaus C & Grabner A (2008) Detailed echocardiographic findings in a newborn foal with tetralogy of fallot. Equine Vet Educ 20 (6), 298-303 VetMedResource.
  • Bayly W M, Reed S M, Leathers C W et al (1982) Multiple congenital heart anomalies in five Arabian foals. JAVMA 181,684-689 PubMed.
  • Vitums A & Bayly W M (1982) Pulmonary atresia with dextroposition of the aorta and ventricular septal defect in three Arabian foals. Vet Pathol 19, 160-168 PubMed.
  • Critchley K L (1976) An interventricular septal defect, pulmonary stenosis and bicuspid pulmonary valve in a Welsh pony foal. Equine Vet J 8, 176-178 PubMed.
  • Rang H & Hurtienne H (1976) Persistent truncus arteriosus in a 2-year old horse. Tierarztl Prax 4, 55-58 PubMed.
  • Prickett M E, Reeves J T & Zent W W (1973) Tetralogy of fallot in a thoroughbred foal. JAVMA 162, 552-555 PubMed.
  • Vitums A, Grant B D, Stone E C & Spencer G R (1973) Transposition of the aorta and atresia of the pulmonary trunk in a horse. Cornell Vet 63, 41-57 PubMed.
  • Rooney J R & Franks W C (1964) Congenital cardiac abnormalities in horses. Path Vet 1, 454-464 SAGE.

Other sources of information

  • Friedman W F (1992) Congenital Heart Disease in Infancy and Childhood. In: Heart Disease: A Textbook of Cardiovascular Medicine. Ed: Braunwald E. W B Saunders Co, USA.  pp 933-934. 

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