Left-to-right Shunts

There are two main types of ASD:

  • Secundum ASD (80% of ASDs)
  • Partial atrioventricular septal defect (primum ASD, pAVSD)

Both present with similar symptoms and signs, but their anatomy is quite different. The secundum ASD is a defect in the centre of the atrial septum involving the foramen ovale (Fig 1). Partial AVSD is a defect of the atrioventricular septum.

Fig 1: Secundum ASD


Clinical presentation

  • None (commonly)
  • Recurrent chest infections/wheeze
  • Arrhythmias (fourth decade onwards)
Physical Signs
  • An ejection systolic murmur best heard at the upper left sternal edge due to increased flow across the pulmonary valve because of the left-to-right shunt
  • A fixed and widely split second heart sound (often difficult to hear) due to the right ventricular stroke volume being equal in both inspiration and expiration
  • With a partial AVSD, an apical pansystolic murmur from atrioventricular valve regurgitation


Chest radiograph
  • Cardiomegaly
  • Enlarged pulmonary arteries
  • Increased pulmonary vascular markings
  • Secundum ASD partial right bundle branch block is common (but may occur in normal children), right axis deviation due to right ventricular enlargement
  • Partial AVSD a superior QRS axis (mainly negative in AVF). This occurs because there is a defect of the middle part of the heart where the atrioventricular node is. The displaced node then conducts to the ventricles superiorly giving the abnormal axis
  • This will delineate the anatomy and is the mainstay of diagnostic investigations


Children with significant ASD will require treatment. For secundum ASDs this is by cardiac catheterisation with insertion of an occlusion device but for partial AVSD surgical correction is required. Treatment is usually undertaken at about 3-5 years of age in order to prevent right heart failure and arrhythmias in later life.

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