* ASD (Atrial Septal Defect ) accounts for 10% of
all congenital heart disease.
- Ostium secundum defect is the most
common type and accounts for 60-70% of all case.
- Ostium primum type accounts for 15-20%
- Sinus venosus type ASDs are seen
*Defects may be identified in atrial septa by 2-D studies. The direction
of flow across such defects can be shown by color flow mapping and
the velocity of the jet across the defect can be measured (and hence
the pressure gradient identified) by continuous wave Doppler.
* TTE (tarnsthoracic echocardiography) and TOE (transoesophageal
echocardiography) are both good at diagnosing ostium primum or Ostium
secundum ASDs but TOE is better than TTE in diagnosing ASDs under
10mm diameter. Most clinically and haemodynamically significant
ASDs should be diagnosed by TTE. However, TOE should be considered
where left to right shunting is suspected but not proven on TTE
(even after contrast study) or where a small defect may be significant,
e.g. after trans-septal puncture at catheterization.
* Doppler echocardiography may be helpful in demonstrating flow
across the atrial septum. It typically shows biphasic pattern (systolic
and diastolic) with a small right-to-left shunt at the beginning
of systole. Largest shunt flow (left-to-right) occurs in late systole.
* Contrast studies are often useful in determining whether there
is flow across the IAS. Contrast may be seen shunting from RA to
LA in the presence of an ASD. A bubble contrast study may be positive
even when no obvious flow is detected on color flow mapping.
* Real-time echocardiography is helpful for identifying additional
abnormalities, such as mitral valve prolapse and a double orifice
mitral valve (seen in 3% of patients with ostium primum defect).
The inter-atrial septum (IAS) is often thin and in certain views
in normal individuals (especially the apical 4-chamber view) there
can appear to be a defect in a normal septum, giving the false illusion
of an ASD. This is due to an effect known as 'écho drop-out' which
happens because the reflected echo signal from the IAS is weak.
The IAS in this view is being hit along its edge by the ultrasound
beam and is at a large depth from the transducer. By examining the
IAS from other views (e.g. subcostal), it can be seen that it is