No clinical history. What is the interpretation?
Image 1 - Long lead II
The rhythm is sinus at a rate of about 68 bpm. Some of the P's are hidden from view or are merged with the QRS. The QRs interval is irregular. If you see an irregular RR interval then you should right away think that some of the sinus beats were conducted. Thus, this is not complete heart block (CHB).
In CHB, because the sinus beats cannot traverse the AV node then the ventricles are under the control of another pacemaker like the AV junction or the from the ventricles. These subsidiary or back-up pacemakers generate a regular pattern.
If you rely on lead II alone then a complex-looking arrhythmia becomes impossible to decipher. Then we need simultaneous leads in full disclosure view.
Image 2 - Full disclosure view with ladder diagram
What are clues of a captured QRS?
Sudden shortening of the RR interval would mean that the R with a shorter RR interval is conducted. In this strip, it is difficult to do that. Another clue is to use the morphology of the QRS. In using QRS morphology, you can use any lead where you can spot an obvious difference. In this case, we can use V1.
R1, 4,5 and 7 shared the same QRS morphology and R2,3, 6 and 8 have the same morphology.
Using deductive reasoning, R1 is near a P wave within a conductible distance and R2 is merging with the 3rd P wave. So, R1 is a captured beat and R2 is a junctional beat. If we follow the reasoning, then R1,4,5 and 7 are captured beats and R2,3,6 and 8 are junctional beats.
To make it easier to understand, a ladder diagram is constructed. By looking at the ladder diagram in R4 and R5 then there is some form of second degree AV block type I and an episode of advanced heart block considering (4 consecutive P waves are not conducted).
For this case, the 12L captured says SR, LAFB, first degree heart block. The initial PRI I saw was ~ 300 ms. After years of watching strips, I know this P will drop and will manifest itself. Indeed it had several cycles of Wenckebach. The heart rate dropped some more and also captured this interesting portion. I know this 10 sec strip will stir imaginations and the challenge.