This diagram is from Chou's Electrocardiography in Clinical Practice (6 ed), Chapter 19: Atrioventricular Block; Concealed Conduction; Gap Phenomenon.
(Please excuse the yellow highlighter.)
Notice how this masquerades as Mobitz II. The first clue to some other mechanism is the changing PR-interval related to QRS 2-4.
Twitter case from @ecgandrhythmRoe
Seems to be some manner of escape rhythm, punctuated by Sinus capture.
Hypothesis:
The long horizontal lines demarcate zones of Atrium (A), AV node (AV), and Ventricles (V).
The blue dots suggest a primary ventricular escape with successful retrograde conduction through the AV node, and into the Atrium.
Suddenly, there is Sinus capture (green dot) with antegrade conduction into the ventricles. This Sinus impulse attempts to control the pace (orange dots), but is impeded because of restoration of the ventricular escape.
This ladder diagram is from the following publication Atrioventricular Block with Narrow and Wide QRS: The Pause That Refreshes . The full a...
