American Board of Internal Medicine (ABIM) Certification Practice Exam

What characteristic finding is observed on the ECG in ostium primum atrial septal defect?

1. Right axis deviation

2. First-degree atrioventricular block

3. Left axis deviation, right bundle branch block

4. Normal rhythm

The correct answer is: First-degree atrioventricular block

In the context of an ostium primum atrial septal defect, the characteristic finding on the ECG is indeed associated with conduction abnormalities, particularly first-degree atrioventricular block. This is because the ostium primum defect can lead to distortion of the atrioventricular (AV) conduction system due to the proximity of the defect to the AV node and bundle of His. As a result of this anatomical anomaly, the electrical signals in the heart may take longer to be conducted through the AV node, which manifests as a prolonged PR interval on the ECG—this is indicative of first-degree AV block. Additionally, while right axis deviation and left axis deviation with right bundle branch block can occur in various cardiac conditions, they are not specifically characteristic of the ostium primum defect. Some patients with this defect may present with a normal rhythm, yet this does not capture the unique conduction issues associated with first-degree AV block resulting from the septal defect. Therefore, recognizing first-degree atrioventricular block as a finding in patients with ostium primum atrial septal defects is essential, particularly in understanding the relationship between structural heart defects and their effects on the electrophysiological activity of the heart.