This 67 year old man is noted to have a slightly irregular pulse. At the beginning of this ECG, he appears to be in NSR with a first-degree AV block. Twice, P waves are non-conducted. Careful measurement of the P to P interval shows that it is regular, there are no PACs noted. The PR interval changes very subtly by lengthening just before the non-conducted P waves. A hint when non-conducted P waves are noted, first check for non-conducted PACs. If the sinus rhythm is regular, check the PR interval before the non-conducted beat, and the PR interval immediately after the non-conducted beat. You will see the PRI preceding the non-conducted P is longer than the PRI after the NCP.
Wenckebach conduction is caused by RP/PR reciprocity. In other words, the shorter the RP interval, the longer the PR interval. So, as the PRI lengthens, the QRS "moves" to the right, eventually causing the next regular sinus P wave to fall into the refractory period and fail to conduct. This results in a pause, or a long RP interval, which shortens the next PRI.
If you or your students would like to review AV Blocks, go to this LINK for Dr. Grauer's excellent, FREE, self-directed tutorial.
For a slightly more advanced discussion of RP/PR reciprocity, see Jason's Blog.