RCIS Practice Test 2026 – All-in-One Exam Prep

In first-degree heart block, the conductive delay occurs at the atrioventricular (AV) node. This condition is characterized by a prolonged PR interval on an electrocardiogram (ECG), which is the time it takes for an electrical impulse to travel from the atria to the ventricles. The AV node plays a crucial role in this conduction pathway, acting as a gateway that regulates the electrical signals moving from the atrial to the ventricular myocardium.

The prolonged PR interval indicates that while each atrial impulse is conducted to the ventricles, there is a delay in its transmission through the AV node. This means the impulse is not blocked, but it takes longer than normal for the signal to pass, which is consistent with first-degree heart block.

The other structures listed, such as the SA node, Bundle of His, and Purkinje fibers, are involved in different aspects of cardiac conduction. The SA node is responsible for initiating the heartbeat, the Bundle of His conducts impulses from the AV node to the ventricles, and Purkinje fibers distribute electrical impulses to the ventricular muscle. However, they do not specifically cause the type of delay associated with first-degree heart block.