Image 1 Long lead II
This is an irregular wide QRS rhythm with LBBB-like morphology. The machine interpreted it as atrial fibrillation. Do you believe the machine?
Image 2 Full disclosure image
If you look at lead II, then you will be deceived but the presence of a P wave in V1 proves that this is sinus. The P to P interval is 0.72 sec or an atrial rate of about 83 bpm. If you use a caliper and march the P, you will notice that some of the P wave are hidden from view.
Before going further, why did we say that this is LBBB-like morphology or properly called intraventricular conduction defect/delay (IVCD). It is because even though there is a QS pattern in V1, there is no broad, notched, monophasic R in I.
We can right away rule-out complete heart (CHB) block because the RR interval is NOT regular. In CHB, because there is complete AV dissociation, the ventricle is always in under the control of either a junctional or ventricular pacemaker. In this case, the sinus beats are conducted.
Differentiating beats by examining QRS morphology
Image 4 Measurement and ladder diagram
I have learned from Dr. Marriott's writings that QRS morphology can give a clue to the source of the impulse. There is no rule which lead will consistently show difference in morphology. In our case, we can use lead I. There are 2 QRS morphology (red circle and blue circle). So, R1,R2,R5,R6 and R7 (red circle) with the same morphology and the same PR interval (0.54 sec) are all sinus beats with long PRI and an LBBB-like morphology.
R3, R4 and R8 (blue circles) are a challenge here. The fastest way to make sense of these beats is the progressive shortening of the RR interval from R2to R4 (0.86 sec to 0.74). The RR interval of R4R5 is 1.31 sec which is less than 2x than R3R4. This fits the logical thought of a Wenckebach. The daunting challenge is here is that the P waves are hidden in the QRS in these beats but in other strips, the P waves near the QRS. That is why I know there is a P wave in there.
So, this is sinus rhythm with a long PRI (first degree AV block), second degree AV block type I, LBBB.