Timing by Manufacturer Manufacturer Timing System Abbott Ventricular based =>VDD & DDI,when PV>MTR in DDD. Atrial based => DOO, DDD or when PV < MTR. Biotronik Atrial based timing for DDD; DDI and VDI are ventricular based timing, however there are exceptions: i.e PAC, PVC, VSP. Boston Scientific Medtronic Atrial based timing for all devices that have atrial pacing: DDD, DDI, DVI.