Definition

  • ASD located at lower part of IAS with both AV valves at the same level.

Embryology

  • Septum primum has an inferior defect called ostium primum. Normally this closes by growth of endocardial cushions. Here it fails to close.
  • Other defects of endocardial cushion may be present like cleft mitral valve.

Incidence

  • 20% of all ASDs.
  • Females are 2 times more affected.

Hemodynamics

  • If MR is present,
    • Increase in left to right shunt leading to
      • Chance of HF in infancy
      • Earlier onset of symptoms of ASD
      • Earlier occurrence of PVOD
    • MR itself can cause heart failure
  • Does not close spontaneously.

Clinical features

  • MR-
    • PSM at apex
    • MR murmur is not modified by onset of PVOD.
  • If MR is present, the findings of ASD may be modified
    • Prominent v wave in JVP
    • LV type apex
  • IE may occur (unlike secundum ASD)

ECG

  • Incomplete RBBB pattern in v1 as in secundum ASD.
  • Features peculiar to primum ASD are-
    • First degree heart block in 50% cases.
    • Left axis deviation (-20 to -60 deg) is always present.
    • q in I & avL (counterclockwise loop).
    • qRs in v5 & v6 due to LV volume overload.

Chest X-ray

  • RA, RV and LV enlargement.
  • LA is inconspicuous despite MR due to presence of ASD. Thus enlargement of all chambers except LA.

Echocardiography

  • ASD in inferior septum.
  • AV valves at same level.
  • Cleft in AML at 12 O’clock position.
  • MR.

Cardiac catheterization

  • Catheter from RA to LA to LV has a low horizontal course.
  • PA pressure is more than in secundum ASD.
  • Goose neck deformity of LVOT may be present.

Differential diagnosis

  • Complete AVSD
  • Secundum ASD with rheumatic MR
    • PSM radiates to axilla
    • Left axis deviation excludes this diagnosis
  • Common atrium
    • Symptoms occur earlier than primum ASD
    • Mild cyanosis is present
    • Howell-Jolly bodies due to asplenia may be present

Management

  • Mitral valvuloplasty with ASD closure.
  • Done between 1 and 2 years of age even if shunt is small and even if MR is not significant.