There is a hole in the wall between the two collecting chambers of the heart (the left and right atria). All babies have a hole in this position whilst inside the womb, which should close within a few days of birth. The technical term for this is an Atrial Septal Defect.
Small holes do not allow sufficient blood across the hole to cause any problems at all. Larger holes allow enough blood to pass into the lungs, which may cause breathlessness, poor weight gain and an increased susceptibility to infections. Surprisingly, even large holes may have little effect and they are only noticed when a doctor hears a murmur.
Although the traditional method of closing the hole requires open heart surgery, the operation is very safe. The hole is closed by either stitching or patching it with a cloth material (dacron), or the lining of the patients own heart (pericardium). The patients can usually go home after 5 days and back to full activities within a month.