American Board of Internal Medicine (ABIM) Certification Practice Exam

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What does the electrocardiogram (ECG) of an ostium secundum atrial septal defect typically show?

Left axis deviation
Right axis deviation and incomplete right bundle branch block
First-degree atrioventricular block
Normal axis with no blocks

The correct answer is: Right axis deviation and incomplete right bundle branch block

An electrocardiogram (ECG) of an ostium secundum atrial septal defect (ASD) typically demonstrates right axis deviation and may also show an incomplete right bundle branch block. This is due to the volume overload in the right atrium and right ventricle that occurs because of left-to-right shunting of blood through the defect. In pulmonary circulation, the increased blood flow leads to hypertrophy and dilation of the right heart structures, which can result in characteristic changes on the ECG. Right axis deviation occurs as the electrical forces shift due to the enlargement of the right atrium and the right ventricle. Additional evidence of right ventricular strain might show as incomplete right bundle branch block on the ECG, reflecting the changes in conduction through the heart as a consequence of the structural abnormality. Other options represent different conditions or heart issues not commonly associated with ostium secundum ASD. Thus, the changes seen in a patient with this type of atrial septal defect reflect the underlying physiology and adaptations of the heart due to altered hemodynamics.