Registered Cardiovascular Invasive Specialist (RCIS) Practice Test

In a 1st degree heart block, where does the delay primarily occur?

1. SA node

2. AV node

3. Bundle of His

4. Purkinje fibers

The correct answer is: AV node

In a first-degree heart block, the primary delay occurs at the AV node (atrioventricular node). This condition is characterized by a prolonged PR interval on an electrocardiogram (ECG), which reflects a delay in the electrical conduction from the atria to the ventricles through the AV node. In a healthy heart, the electrical impulse generated by the SA node travels directly to the ventricles via the AV node. In first-degree heart block, although the impulse does reach the ventricles, the delay is noted in the conduction through the AV node. This is typically not life-threatening and often does not cause symptoms but indicates that there is some impairment in the conduction system. The other options represent different components of the heart's conduction system, but they are not primarily responsible for the delay observed in first-degree heart block. The SA node usually functions normally, and while the Bundle of His and Purkinje fibers are involved in ventricular conduction, they are not the site where the delay is specifically marked in this condition. Thus, the AV node is the correct answer, highlighting its crucial role in the conduction pathway that can become impaired in first-degree heart block.