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Heart: tetralogy of Fallot

pequis
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Introduction

  • A congenital defect of the heart, combining:
    • Pulmonary stenosis.
    • Ventricular septal defect.
    • Dextroposition of the aorta.
    • Right ventricular hypertrophy.
  • Cause: abnormal anatomy of infundibular septum.
  • Signs: depends on degree of stenosis and subsequent restriction to pulmonary blood flow as well as the size of the ventricular septal defect.
  • Diagnosis: auscultation, echocardiography, hematology.
  • Treatment: none.

Pathogenesis

Etiology

  • 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 pulmonary valve also fails to develop correctly resulting in variable degrees of pulmonic stenosis   Pulmonic valve: stenosis  . 
  • Clinical signs depend upon the degree of stenosis   Pulmonic valve: stenosis   and restriction to pulmonary blood flow. 
  • Stenosis    Pulmonic valve: stenosis  ranges from mild to complete pulmonic valve atresia (absence of a pulmonary valve and PA oriface).  The tightness of the stenosis and imposition of systemic pressures on the right ventricle result in massive right ventricular hypertrophy.
  • The hallmarks of tetralogy of fallot are therefore: 

Pathophysiology

  • Right ventricular pressure overload can   →    the development of right sided congestive heart failure (although this is rare).
  • Right-to-left shunting of venous blood   →   variable degrees of arterial hypoxemia dependent upon the relative RV outflow through the stenotic pulmonary artery and the ventricular septal defect. 
  • If the pulmonic stenosis is tight, 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 increases 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.

Timecourse

  • Depends upon the degree of pulmonary stenosis   Pulmonic valve: stenosis   and venous admixture associated wtih right-to-left shunting of blood.

Epidemiology

  • Sporadic and rare.

Diagnosis

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Treatment

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • 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. pp 933-934. W B Saunders Co, USA.

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