DDD and VVI are pacemaker therapy modes that your doctor can program in your device. These are just two of many options. 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 is the heart chamber being sensed. 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 response in the device is turned on (R=Rate Modulation on; O or blank = none or no rate modulation).
