An atrial septal defect should show a right bundle branch block, or RBBB ― sometimes incomplete ― on ECG. This is partially due to the right ventricular volume and pressure overload that occurs.
When an ostium primum atrial defect is present, the ECG reveals left axis deviation. When an ostium secundum atrial septal defect is present, the ECG reveals right axis deviation.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009; doi:10.1161/CIRCULATIONAHA.108.191095.