Bingo! Nicely done!

The two phenomena are:

1. Electrical: Wenckebach type second degree AV block. Heart block is not complete (there is still R-R irregularities with group-beating). Baseline AV is quite long with next P hidden on the previous T makes it look peaked.

2.Mechanical: Frank-Starling Law of the heart - After the pause, the stroke volume is bigger as this is likely due to a larger diastolic filling (due to missed beat). Hence, if myocardium is otherwise intact, the beat is stronger, as shown in Oximeter tracing.

Although, there appear to be ST elevation, but on only one lead one need to be careful about calling STEMI. The motivated paramedics did keep the 12 lead EKG (below) which does not show acute STEMI, may be old inferior MI.

The patient denied acute cardiac symptoms and was in a transfer between two hospitals for non-cardiac issues.