The simple 5-beat rhythm strip in this case is a surprisingly effective rhythm strip for teaching a number of important principles:

• We are told that this patient has “underlying AFib” (Atrial Fibrillation) — and now has in addition Complete AV Block. We do not have the benefit of prior tracings — nor do we have more than this 7-second rhythm strip on which to base our opinion.
• The ECG diagnosis of AFib is based on the presence of an irregularly irregular rhythm without P waves in any of the 12 leads. Irregular undulations (ie, “fib waves”) may or may not be present in the baseline. Skeptic that I am — I do not have sufficient information to confirm the baseline diagnosis of “underlying AFib” because: i) We only have 1 unlabeled lead to look at (and P waves are sometimes present in some leads but not in others); and ii) We don’t have any baseline rhythm strip showing an irregularly irregular rhythm without P waves.
• Clearly — it looks as if there are “fib waves” in this tracing. That said — distinction of artifact from “fib waves” is sometimes difficult without additional information …
• The rate of the rhythm in this strip is indeed extremely slow. AFib most often presents with a rapid irregularly irregular rhythm. Presentation with a slow ventricular response may be the result of: i) overuse of rate-slowing medication; ii) recent ischemia/infarction; iii) hypothyroidism; iv) sleep apnea; v) other profound vagal stimulation; or vi) SSS ( = Sick Sinus Syndrome). Presumably the diagnosis is SSS in this example (since a permanent pacemaker was implanted) — but it is important to realize that the diagnosis of SSS can only be made after ruling out entities i)-thru-v) as potential causes of the slow ventricular response to AFib.
• Slow AFib may be the consequence of superposition of complete AV Block on a patient with underlying AFib. This rhythm used to be seen much more often in days when Digoxin was more commonly used — but it may also occur as a result of recent ischemia/infarction — use of rate—slowing medication — Sick Sinus Syndrome — and/or some combination of these entities. That said - DID YOU NOTICE that the 3rd R-R interval in this rhythm strip is approximately 0.10 second longer than the other 3 R-R intervals? Given that we only have 5 beats on the entire tracing (and 1 of the 4 R-R intervals is different from the others!) — I would want to see MORE monitoring before concluding that this rhythm strip truly represents AFib WITH complete AV Block vs simply reflecting a very slow ventricular response to a patient with AFib.