Let me reiterate that this ECG is 29-years old and I know virtually nothing specific about this case.  It is chock-full-o' interesting findings though so this is my official read on what's happening here.

I’m not going to count it as a true “beat” because it is only partially-inscribed, but the first waveform is presumably the T-wave of a ventricularly paced beat with a sinus P-wave perched up on top of it's apex. This gives the T-wave a taller and sharper appearance.  The next fully-inscribed beat (i.e., 1st beat) is presumably a conducted sinus beat which would probably be more correctly labeled as a ventricular capture (C) beat. This first beat is conducted with a prolonged P-R interval of 0.33s. I agree with both doctors (Raed and Ken) that this could be the result of either retrograde concealed conduction from a ventricular paced beat into the A-V node leaving it somewhat refractory OR that the ventricular paced beat could have conducted retrogradely into the fast pathway (FP) causing descending A-V conduction to “jump” over to the slow pathway (SP) for just the one sinus beat. In the laddergram, I have chosen to illustrate the mechanism as retrograde concealed conduction.  The sinus rate is slightly irregular producing the following sinus cycles: 1st = 78/min; 2nd = 79/min; 3rd and 4th = 75/min; 5th – 8th cycles = 74/min.  The 2nd and 3rd beats are conducted sinus beats with normal P-R intervals of about 0.16s.  I agree with Ruan that the inferior q-waves in III and aVF could be suggestive of a previous inferior infarction.

When the sinus rate slows to 75/min, the pacemaker starts to pace the ventricles at a faster rate of 80/min.  The 4th and 5th beats are paced fusion (F) beats with varying degrees of ventricular fusion. Beats 6, 7, and 8 are 100% ventricular paced beats that are dissociated from sinus impulses.  In lead V1, the polarity of the QRS complex is wholly negative indicting that this is probably pacing the apex of the right ventricle (RV).  Since there is no consensus between the two resident pacemaker experts (Raed and Ruan), I will just say that this is programmed to either VVI or VOO mode.  Note, in the A-V tier, that I’ve illustrated an advancing retrograde wavefront as the anterograde wavefront progressively retreats backwards to the atria.
 
There was no mention of the atrial fusion (arrow) on the 9th beat.  There are only two different types of fusion and they both occur on this ECG (i.e., ventricular and atrial).  Electrocardiographically speaking, I don’t think the word “fusion” should ever be broadly used to describe situations where unrelated waveforms overlap one another.  Case and point:  At the beginning of this tracing, there is a simultaneous P-waves and T-waves.  Some authors would refer to this as T/P fusion.  A sinus P-wave represents atrial depolarization while a T-wave represents ventricular repolarization.  These are two entirely separate events that are taking place on two different anatomic levels of the heart (i.e., upper and lower chambers).  True fusion takes place in the same chamber as I've illustrated on the laddergram.  That kind of misuse of the word “fusion” is only going to needlessly complicate and confuse those who are trying to learn about both atrial and ventricular fusion.  I'm referencing this recently released video by a website that I otherwise respect:  http://www.ecgteacher.com/index.php/arrhythmias/tachycardia-svt-1 If we were to accept this alternate definition of fusion, then common rhythms like atrial fibrillation are displaying fusion every time a fibrillatory wave coincides with a QRS complex or T-wave.

Beats 10, 11, 12, and 13 are ventricular paced beats with 1:1 retrograde ventriculoatrial (V-A) conduction to the atria.  Note in the laddergram that V-A conduction is comparable to atrioventricular (A-V) conduction. Since the R-P’ interval (0.19s) on the ventricular paced beats is comparable to the P-R interval (0.16s) on the sinus beats, one can infer that V-A conduction is taking place over the A-V node’s fast pathway (FP).  If Raed is correct in assuming that the capture beat conducts anterogradely over the SP, then retrograde conduction over the SP would certainly produce an R-P’ interval in excess of 0.33s.