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Tetralogy of FallotR-to-L shunt – CYANOTIC
THE most common cause of cyanotic heart disease – but only accounts for <10% of all congenital cardiac abnormalities
Has four features:
  • Large VSD – usually so large that pressures in the left and right sides of the heart are equal
  • Right ventricular outflow obstruction – often pulmonary valve stenosis – due to the overriding aorta, and sometimes, muscle overgrowth around the valve.
  • Overriding of the aorta – this is where the aorta is position directly over the ASD – and not over the left ventricle. The aorta will receive blood from both the left and right ventricles, reducing the concentration of oxygenated blood that goes into system circulation.
  • Right ventricular hypertrophy – resulting from the other three factors
Tetralogy of Fallot

Tetralogy of Fallot. This file is taken from wikimedia commons and is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.


  • The most common cyanotic heart condition
  • The most common cause of blue baby syndrome

Clinical features

  • Cyanosis – possibly at rest depending on severity
  • Dyspnoea on feeding / crying / exertion
  • Failure to thrive
  • Murmur – usually harsh, and at the left sterna edge, grade 3-6. Due to pulmonary outflow obstruction rather than VSD
    • Systolic thrill may also be present

‘Tet Spells’

  • Sudden onset dyspnoea / cyanosis
  • Typically triggered by an event that slightly reduces O2 concentration, e.g. crying, defecating, feeding, distress
  • Can be a vicious cycle, as the tet spell can make the child more distressed
  • Sometimes preceded by rapid, deep breathing
  • This increases venous return to the right ventricle, whose output is mainly via the aorta – and thus this is part of the vicious cycle
  • Tet spells typically occur in those aged 2-4M
  • Severe spells can lead to death
  • Some toddlers may adopt a squatting position – which reduces venous return to the heart, and may help increase blood O2


  • Murtagh’s General Practice. 6th Ed. (2015) John Murtagh, Jill Rosenblatt
  • Oxford Handbook of General Practice. 3rd Ed. (2010) Simon, C., Everitt, H., van Drop, F.
  • Beers, MH., Porter RS., Jones, TV., Kaplan JL., Berkwits, M. The Merck Manual of Diagnosis and Therapy

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