An ostium primum atrial septal defect (ASD) occurs when then atrial septum near the atrioventricular valves has a communication between the two atrium causing a left to right shunt. This rarely develops into Eisenmenger’s syndrome if pulmonary hypertension develops reversing the direction of the shunt to right to left.
Less common than the secundum type atrial septal defect, a primum ASD is usually small and less commonly requires intervention when it occurs in isolation. Frequently it is considered as a part of an “endocardial cushion defect” which is more common in Down Syndrome.
The image to the right shows the location of the different types of atrial septal defects while looking from the right atrium.
1 = Upper sinus venosus defect
2 = Lower sinus venosus defect
3 = Ostium secundum atrial septal defect
4 = Coronary sinus atrial septal defect
5 = Ostium Primum atrial septal defect
The murmur produced by an ASD is due to increased flow through the pulmonic valve, thus it is remarkably similar to that of pulmonic stenosis. The difference lies in the intensity and splitting pattern of the S2 heart sound. The intensity of S2 should remain unchanged and may infact be accentuated if pulmonary hypertension develops. The S2 is fixed-split in a person with an ASD. This differs from the widened split S2 seen in severe pulmonic stenosis. Also, the murmur of an ASD does not increase in intensity with inspiration like that of pulmonic stenosis.
A primum atrial septal defect is diagnosed on transthoracic echocardiography. The 12-lead ECG will show an incomplete right bundle branch block and left axis deviation (in contrast to the secundum ASD which has a right axis deviation).