This 63-year-old man presented with a history of paroxysmal atrial fibrillation (AF) and complete right bundle branch block on the electrocardiogram (ECG). He suffered 3 episodes of syncope without prodrome. An electrophysiologic study revealed an 82-ms HV interval, prompting the implantation of an Adapta® dual chamber pacemaker. Both pacing leads were properly positioned and associated with normal impedance, and satisfactory sensing and capture thresholds. The pacemaker was interrogated 3 days after the implant and recordings were obtained during the programming of various pacing modes. This first tracing was recorded in ODO mode.
Tracing 7a: DDI mode programmed at 60 bpm;
Tracing 7b: DDI mode programmed at 70 bpm;
The DDI mode provides sequential dual chamber AV pacing, with atrial as well as ventricular sensing, though without trigger by sensed atrial events. AV synchrony is guaranteed only at the rate of on-going atrial pacing (backup rate, rate responsive or smoothed rate). If the spontaneous 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, AV synchronization is absent. Thus, in case of AV block, the spontaneously activated atria do not synchronize with ventricular pacing if their rate is faster than the ongoing atrial pacing: the function is the same as a VVI mode. This explains the absence of runaway ventricular pacing in case of detection of an atrial arrhythmia, or the use of DDI as a fallback mode. It is also the function of choice when the pacemaker does not accurately detect atrial arrhythmias and does not properly fallback and paces the ventricle erratically. It is not a wise choice for a patient who presents with AV block and normal sinus function because the mode is incapable of p-synchronous pacing, although it may be acceptable for a patient presenting with AV block, even if permanent, who also suffers from sinus node dysfunction and persistent atrial pacing (since atrial pacing synchronizes ventricular pacing).
An adjustment of the backup rate is, therefore, essential, and must be a) relatively rapid in order to prevent the onset of spontaneous atrial activation, and b) associated with rate responsiveness.
In this patient, a rate programmed at 70 bpm allowed satisfactory AV synchrony, at least at rest.
The ideal indication for this type of pacing mode is a patient presenting with AV block and bradycardia-tachycardia syndrome, combining AF (no risk of runaway) and incessant sinus node dysfunction after the end of the AF episode (AP VP pacing).
