Although the R-R cycle is irregular, the P-P interval is not.
Note the (+) P waves in the Lead II Rhythm strip and march the P waves back and forth. Observe that the PR Interval gradually increases, ending in a Wenckebach “Dropped Beat”.
The continuation of the Tachycardia, despite the non-conducted atrial stimuli indicates that the mechanism cannot be atrio-ventriculat reentry using an accessory path (orthodromic tachycardia) and is very unlikely to be due to av nodal reentry.
Most likely this represents an automatic atrial focus – Primary Atrial Tachycardia