The ECG characteristics of an ASD of the ostium primum type include left axis deviation and incomplete RBBB.

The differential diagnosis for an ASD, ostium primum type, includes conditions with endocardial cushion defects, such as those that include ASD, ostium primum, ventricular septal defect, and complete atrioventricular channel.

Patient With Atrial Septal Defect ECG ⁶¹

The ECG above is from a patient with an ostium primum type ASD. There is an rSR’ pattern in precordial lead V1 as well as T wave inversion, characteristics seen in volume overload and patients with RVH. There is left axis deviation with a frontal QRS axis of −45°.