Interesting tracing. For clarity, I’ve labeled beats and have added arrows for atrial activity. Unfortunately, due to baseline artifact — atrial activity is not always easy to make out (there are some instances in the figure where there is confusion between U wave vs P wave …) — but overall, the underlying sinus rate appears fairly regular (at ~100/minute). Knowledge of the clinical setting helps immeasurably in such instances to “intuit” what the rhythm almost certainly is. By this I mean to emphasize the points Dawn made — namely that there is obvious acute inferior STEMI — and that is a situation in which when one sees definite P waves with an irregular narrow-complex ventricular rhythm showing grouped beating and changing PR intervals — that 2nd-degree AV block ( = Mobitz I or AV Wenckebach) is almost always present, often with some junctional escape beats mixed in. In such situations, as the “electrocardiographer” — my main task is greatly simplified, since all I have to do is “intuit” where are the periods of progressively increasing PR interval, and where are the likely junctional escape beats (since the “answer” = AV Wenckebach, is already almost certain to be the underlying cause of the rhythm disturbance).

NOTE: Unfortunately, we lack a long lead rhythm strip … As a result, we have to look at the “bottom line” in both 12 leads — in which QRS morphology changes because we switch from leads III to aVF to V3 to V6 … That said, we still CAN interpret the rhythm.

Of note, ECG #2 (Bottom) was obtained 10 minutes after ECG #1 (Top).  ECG #2 is easier to look at first. This begins with 2:1 AV conduction (Mobitz I) for the first 4 beats in ECG #2. Note that the PR intervals preceding beats #1,2,3,4 appear to be normal and constant. There are then two 3:2 Wenckebach cycles at the end of the tracing (beats #5,6; and 7,8 in ECG #2) — and this confirms our diagnosis!

Things were a bit more complex 10 minutes earlier in ECG #1 (Top). I believe there probably are two 2:1 cycles (beats #4 and 5) — since the PR intervals preceding beats #4 and #5 are similar to the PR intervals preceding what we KNOW are 2:1 cycles in ECG #2. But for beats #2 and 3 in ECG #1 — there is variation in the PR interval preceding these QRS complexes. So one wonders if there is higher-grade AV block at this point? Once again, knowing the clinical context is invaluable — since it is NOT uncommon to go back-and-forth between Mobitz I 2nd-degree AV block and complete AV block with narriow QRS escape during the early hours of acute inferior STEMI. This may be what we see here. Toward the end of ECG #1 — the R-R interval becomes more regular — so this probably reflects junctional escape. But because the R-R interval between beats #2-3 is LESS than the R-R interval between beats #6-7; 7-8; and 8-9 toward the end of the tracing — it is unlikely that complete AV block was present, but rather some “higher-grade” 2nd-degree. And as we see in ECG #2 — 10 minutes later, a much clearer pattern of Mobitz I with 2:1 and 3:2 AV conduction emerges.

As to the 12-lead ECGs — I’ll add to Dawn’s comments by suggesting that although ECG #2 with right-sided lead V4 confirmed the diagnosis of acute RV infarction — this additional tracing was NOT needed for diagnosis given what we already see in ECG #1. There is obvious acute inferior STEMI in ECG #1. The presence of more marked ST elevation in lead III > II suggests acute occlusion of the RCA ( = Right Coronary Artery). What is unusual about ECG #1 is the very marked ST coving and elevation in lead V1! There is ST depression in lead V2, but only minimal. Putting the clinical situation together — the relatively limited amount of ST depression we see in lead V2 reflects associated posterior involvement. In this setting, isolated ST elevation in lead V1 almost certainly indicates acute RV (right ventricular) involvement, perhaps already with some RV dilatation. That’s because lead V1 is a “right-sided” lead — and whenever you see a flat or slightly elevated ST segment in lead V1 in a patient with acute inferior ST and obvious ST depression in V2, V3 — you can suspect RV involvement. Those are the patients for whom right-sided leads DO benefit — because you can then confirm acute RV involvement with ST elevation in V2R, V3R, V4R. But when you see dramatic isolated ST elevation in lead V1 (as we do in ECG #1 here) — you already KNOW you are dealing with acute occlusion of a proximal RCA lesion. Especially in view of the associated rhythm disturbance — urgent cath/PCI is essential for optimal outcome.