These are different pacing modes your doctor can program for your device.  This tells the device how to function when it comes to pacing and sensing.  Each letter indicates something different (A = Atrium; V = Ventricle; D = Dual or both A and V; and O = none). The first letter indicates the chamber of the heart being paced. The second letter indicates the chamber of the heart being sensed or monitored. The third letter indicates how the device should respond when it senses a cardiac event (T = Triggered; I = Inhibited; D = Trigger or Inhibit; and O = none). Finally, the fourth letter, usually “R,” means that the rate modulation function in the device is turned on (R = Rate modulation on; O or blank = none or no rate modulation). In this case DDD means both the atrium and ventricle is paced, both the atrium and ventricle are sensed, and the device can trigger or inhibit sensed events. A VVI mode means the pacemaker will pace in the ventricle, sense in the ventricle, and inhibit sensed events.