DDD pacemaker
http://www.patient.co.uk/doctor/pacemakers-and-pacing

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Note added at 9 mins (2014-01-17 21:39:25 GMT)
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Also called a Dual-chamber pacemaker

Pacemaker codes[1]
The North American Society of Pacing and Electrophysiology and the British Pacing and Electrophysiology Group have developed a code to describe various pacing modes. It usually consists of three letters, but some systems use four or five:

•Letter 1: chamber that is paced (A = atria, V = ventricles, D = dual-chamber).
•Letter 2: chamber that is sensed (A = atria, V = ventricles, D = dual-chamber, 0 = none).
•Letter 3: response to a sensed event (T = triggered, I = inhibited, D = dual - T and I, R = reverse).
•Letter 4: rate responsive features; an activity sensor, eg an accelerometer in the pulse generator, in single or dual-chamber pacemakers detects bodily movement and increases the pacing rate according to a programmable algorithm (R = rate responsive pacemaker).
•Letter 5: Anti-tachycardia facilities.
A pacemaker in VVI mode denotes that it paces and senses the ventricle and is inhibited by a sensed ventricular event. The DDD mode denotes that both chambers are capable of being sensed and paced.