What happens during Second Degree Heart Block Type 1 (Wenckebach)?

During Second Degree Heart Block Type 1, also known as Wenckebach, there is a characteristic pattern observed in the behavior of electrical impulses in the heart. The hallmark of this condition is that the PR interval, which represents the time it takes for the electrical impulse to travel from the atria to the ventricles, progressively lengthens with each successive beat until one impulse is eventually not conducted, resulting in a dropped beat. This pattern continues in a cycle, causing the rhythm to become irregular.

In this context, the gradual lengthening of the PR interval reflects a failure in conduction at the AV node, where the impulses are being progressively delayed. The drop in beat occurs when the length of the PR interval reaches a critical point where the AV node can no longer conduct the impulse to the ventricles. This is what differentiates Wenckebach from other types of heart block, as it does not drop every beat, and the pattern of progressive lengthening of the PR interval is a unique identifier for this condition.

This explanation highlights the dynamics of Wenckebach, making it clear why the identification of the progressive lengthening PR interval until a dropped beat is crucial in understanding this specific heart block.