Here, 3 paced atrial events were not followed by a ventricular event (AAIR mode). After that the machine switched to DDD mode. Also note the lengthening of the PRI interval in the middle. Seems to have a 3:2 AV Wenckebach cycle.
AAISafeR mode operates in the AAIR and switches to DDDR mode when criteria are met.
AVB Criteria 2 – if 3 of 12 sensed or paced atrial events are not followed by ventricular event
Ellenbogen K et al. 2011Clinical Cardiac Pacing, Defibrillation and Resynchronization Therapy 4ed. Elsevier PA