In the words of Ken Grauer, MD
'Not all patients read the textbook'

Describe the rhythm/type of AV block in this ECG below ⬇️

If you fully understand each beat in this rhythm, you have a great grasp on rhythm interpretation - give it ago

Case details:
Male in his 80s, who has had a fall and been complaining of palpitations.

Original tracing for accurate interval mapping

Below are my thoughts on this rhythm:

Underlying Sinus rhythm with 2nd degree AV block Mobitz I (Wenckebach) and junctional escape beats.

To help with my explanation, please see the annotated pictures below

Black arrows = Sinus P waves
Purple arrows = Junctional escape beats triggered by the pauses from non-conducted sinus beats
Blue arcs = PR prolongation
Red cross = P waves failing to conduct due to Wenckebach AV block

We can identify junctional escape beats based on fixed RR coupling. The AV junction senses the pause and will depolarise when it senses a pause of 1.5 seconds (in this patient). This escape interval principle can be seen in junctional escape rhythms and isolated beats.

The rhythm or beat will only appear after a specific pause duration - in this case 1500ms/40 bpm. I like to think of it like a safety mechanism to reduce symptoms of syncope.

This rhythm may seem a little atypical at first, because the second cycle of Wenckebach conduction starts with a different PR interval to the first cycle (PR 1 is different from PR 2 - see picture below)

Normally, with cycles of Wenckebach, we see each cycle begin with the same PR - however, this is not the case in this example. Its a little tricky to explain, but I believe this is due to a shorter preceding RR interval, compared to PR 1.

PR 2 has a longer PR than PR 1 because the AV-His bundle has only just depolarised, i.e. the junctional escape beat before PR 2 has slowed conduction through the AV node for the next beat (due to AV-his disease), similar to the second beat in the first Wenckebach cycle.