The heart is split into four parts. Two collecting chambers at the top called Atria and two pumping chambers at the bottom called Ventricles. These four chambers are divided by a horizontal barrier called the Fibrous Skeleton and a vertical wall called the Septum. These meet in the middle like the cross in the centre of a hot cross bun. An Atrioventricular septal defect (AVSD) is where this crucial centre is missing. This results in a hole between the top two chambers and the bottom two chambers. An atrial septal defect is a therefore a hole between the two top chambers - the atria. There should not normally be a hole in this position as it allows blood to mix between the atria which should normally be separated. This is a reasonably common heart defect and often not to serious so don't be suprised if cardiologist does not hurry to to close the hole (with surgery or with a catheter in some cases).
Click here to see the British Heart Foundation leaflet on Atrioventricular Septal Defect (AVSD)
or click here to go to the British Heart Foundation website.