63-year-old man with a history of paroxysmal atrial fibrillation; full right bundle branch block pattern on the electrocardiogram; 3 episodes of sudden syncope; electrophysiological study with HV interval measurement at 82 ms; implantation of an Adapta dual-chamber pacemaker; the 2 leads are well positioned with normal pacing impedance, proper sensing and satisfactory pacing thresholds; 3 days after implantation, pacemaker control; programming of various pacing modes and recording of tracings; for this first tracing, programming in ODO mode.
Tracing 7a: programming in DDI mode 60 beats/minute;
Tracing 7b: programming in DDI mode 70 beats/minute;
The DDI mode provides dual-chamber, sequential AV pacing with atrial sensing but no tracking of the sensed atria. AV synchrony is only provided at the current atrial pacing rate (base rate pacing, rate responsive pacing or rate-smoothed pacing). If the atrium is faster than the atrial pacing rate, there is inhibition of atrial pacing and absence of AV delay; when the atria are spontaneous there is no ventricular synchrony.
Thus, in the case of AV block, spontaneous atria do not synchronize ventricular pacing if they are faster than the current atrial pacing rate: functioning is equivalent to VVI mode. This explains the absence of ventricular runaway pacing in the event of atrial arrhythmia sensing, hence the use of DDI as a fallback mode. It is also the function chosen when the pacemaker does not correctly sense atrial arrhythmias and therefore does not fallback properly, with erratic ventricular pacing. This choice is therefore not satisfactory in a patient with AV block and normal sinus function (no P-wave synchrony detected), but is conversely entirely acceptable if the patient also has a sinus dysfunction causing permanent atrial pacing even with permanent AV block (since atrial pacing synchronizes ventricular pacing). The setting of the base rate is therefore essential. It should be set high in order to avoid the occurrence of spontaneous atrial activation and be associated with rate responsive pacing. In this patient, programming at 70 beats per minute ensures satisfactory atrioventricular synchrony.
The ideal indication for this type of mode is a patient with AV block and atrial disease associating passage in fast AF (no risk of runaway) and permanent sinus dysfunction after reduction (AP-VP pacing).
