The underlying rhythm is sinus at a rate of 70/minute. The QRS morphology of the sinus rhythm complex is LBBB (QRS ≥ 120msec, tall monophasic R in V₆). The 4^th and 8^th beats in the rhythm strip above have a similar morphology to the sinus beats but are premature. Notice that the T wave following the sinus beat and before the early beat is different – deformed by a premature ectopic P wave, suggesting that these early beats are PACs.

As a confirmation, you can demonstrate that these early beats are not AV dissociated (eg, the sinus “metronome” is reset by the early ectopic P and subsequent P waves do not appear “on time”). PACs are not fully compensated.

PVCs are fully compensated, another way of saying that they are AV dissociated and do not disturb the sinus metronome (P waves will march through the appearance of the premature beat.)

While this premature beat might look like a PAC, it is fully compensated (AV dissociated) and therefore a PVC!