Which of the following best describes the EKG characteristics of a second-degree heart block, type 1 (Wenckebach)?

The EKG characteristics of a second-degree heart block type 1, also known as Wenckebach, are best described by the pattern of progressive prolongation of the PR interval until a P wave occurs that is not followed by a QRS complex. This phenomenon happens because of a gradual increase in the time it takes for electrical impulses to travel through the AV node, culminating in a failure to conduct one of the impulses to the ventricles, hence the dropped QRS after a certain number of beats.

This characteristic behavior of fluctuating PR intervals creates the classic "grouped beating" pattern often seen in Wenckebach. After several beats of increasing PR interval duration, one P wave is not transmitted to the ventricles, resulting in a missing QRS complex. Understanding this pattern helps differentiate it from other types of heart blocks and arrhythmias.

In contrast, other options describe rhythms associated with different conduction characteristics that do not align with the unique features of a type 1 second-degree AV block.