In heart block, which structure is affected in Mobitz Type 1?

Mobitz Type 1, also known as Wenckebach phenomenon, is a form of second-degree heart block characterized by progressive prolongation of the PR interval until a single QRS complex is dropped. This specific pattern indicates that the site of the block is located within the atrioventricular (AV) node.

In Mobitz Type 1, the electrical signals generated by the atria successfully reach the AV node but are delayed to the point where some impulses fail to be conducted to the ventricles. The gradual increase in the PR interval suggests that the conduction through the AV node is becoming increasingly impaired.

Unlike other types of block, such as Mobitz Type 2, where the conduction impairment occurs below the AV node (in the His-Purkinje system), Mobitz Type 1 specifically reflects a dysfunction at the level of the AV node itself. Therefore, the correct identification of the affected structure in Mobitz Type 1 is the AV node.