A 29-year-old primigravida woman presented to the outpatient obstetric clinic for a routine antenatal checkup. Cardiovascular system examination revealed a systolic murmur in the left upper parasternal region, which was initially thought to be a physiological murmur of pregnancy. There was no history of dyspnea, chest pain, palpitation, or lower limb swelling. Her vital parameters were within normal limits. The 12-lead electrocardiogram (ECG) is presented in the Figure.
Questions: What is the abnormality in the ECG, and what is the likely diagnosis?
The ECG in the Figure shows normal sinus rhythm with heart rate of 70 beats/min. There is complete right bundle-branch block with normal PR interval. There are no significant ST-T segment changes. However, on closer inspection, fragmented QRS segment with a notch on the R wave of inferior leads can be noticed. This sign, also eponymously known as crochetage sign, is relatively specific for ostium secundum (OS) atrial septal defect (ASD). Transthoracic echocardiography revealed a dilated right atrium and right ventricle with a large 21-mm OS-ASD, with left-to-right shunt.