The DDI mode provides dual-chamber sequential AV pacing with dual atrial and ventricular sensing but with no ventricular triggering of the sensed atria. AV synchrony is only provided at the current atrial pacing rate (base rate, rate response or smoothed rate). If the atrial rate is faster than the atrial pacing rate, the latter is inhibited and no AV delay is initiated; when the atria are activated spontaneously, there is no ventricular synchrony.

Thus, in the case of a complete atrioventricular block, spontaneous atrial events do not synchronize with ventricular pacing if their rate is faster than the current atrial pacing rate: functioning is therefore the same as in the VVI mode. This explains the absence of runaway ventricular pacing upon sensing an atrial arrhythmia, hence the use of DDI as a fallback mode. It is also the selected function when the pacemaker does not correctly sense atrial arrhythmias and therefore does not fallback properly, with erratic ventricular pacing. This choice is therefore not appropriate in a patient with atrioventricular block and normal sinus function (lack of P-synchronous pacing), although is conversely completely acceptable if the patient, even with a permanent atrioventricular block, also suffers from sinus dysfunction triggering permanent atrial pacing (since atrial pacing synchronizes ventricular pacing). The setting of the minimum programmed rate is therefore essential. The latter should be high, to prevent the occurrence of spontaneous atrial activation, and programmed in association with the rate response.
The ideal indication of this mode is a patient with atrioventricular block and atrial disease presenting with rapid AF episodes (no risk of runaway) and permanent sinus dysfunction after termination (AP-VP pacing).