Sorry, Naser. I just saw this. Will be happy to clarify further in a meeting. 

Since device is programmed to AAI, all paced beats here are captured by atrial lead. The first 7 beats are ventricular paced beats (red circle) has superior axis, hence the lead is probably deep in low RV or even apical.  

After that the PM then fails to capture the ventricle with subsequent emergence of the native sinus beat (beat # 8). 

This is followed by three more ventricular paced beats (blue circle). Note that the morphology of these last three paced beats in the rhythm very bottom rhythm strip lead II is different (positive QRS, i.e., inferior axis) than the morphology of the first 7 paced beats (negative QRS, i.e., superior axis). This suggests that RV in these last 3 beats is captured in high RV (?RVOT or high-septal area with inferior axis). 

The loss of capture in the middle of EKG and then change of morphology in the last few beats support that the atrial lead was floating around in the RV giving all the above confusing/inconsistent findings. 

Now, if we go back and look at the EKG in Fig.2a again. Since the PM was in DDD mode, the first spike should be an atrial spike and the second ventricular. 

When two spikes are there, the failure of atrial capture is easy to figure out,  but since the first spike has to be an atrial spike, then beats #1, 4, 7 and 10 should be produced by the atrial lead now passing through the TV and capturing the neighboring septal area of RV (hence the QRS is narrow like Mansour pointed out, and with inferior axis). 

It is hard to explain why the RV is captured a trigemini pattern, which can be fortuitous, or probably related the lead movement is being somehow affected by the patient’s respiration or position. 

Finally, below is the PM programmer while testing this PM when programmed to AAI and DDD respectively:

The lead was revised and repositioned successfully.

Conclusion:

In general lead malfunction is easy to diagnose and figure out when it gives consistent finding on EKG. 

Nonetheless, intermittent malfunction, or dislodged leads that migrates between different chambers, or “floating”, like in this case can create quite confusing EKGs with challenging interpretation. 

Applying basic pacing timing interval principles and programming troubleshooting with or without maneuvers and, of course, CXR evaluation help sort this out.