In a first degree AV block, where does the delay usually occur?

In a first-degree AV block, the delay typically occurs at the AV node or the bundle of His. This type of block is characterized by a prolonged PR interval on the electrocardiogram (ECG), which indicates that the impulse traveling from the atria to the ventricles is delayed, rather than interrupted. The AV node is responsible for conducting electrical signals from the atria to the ventricles, and when there is a delay here, it results in the first-degree block. The impulse still reaches the ventricles, which is why it is classified as a first-degree block rather than more severe forms where the signal is completely blocked.

Understanding the role of the AV node is crucial, as other parts of the heart such as the ventricles or the atria are not the primary locations where the delay occurs in this specific type of block. The atria are responsible for initiating the impulse, and the ventricles respond to the signals; however, the key feature of first-degree AV block lies in the conduction delay at the AV junction.