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.
Tracé 6a Programmation en mode VDD 60 battements/minute ;
Tracé 6b Programmation en mode VDD 40 battements/minute ;
The VDD mode can be programmed in a standard dual chamber pacemaker as well as in a single lead, dual chamber VDD device equipped with a) two floating (orthogonal) atrial electrodes to accomplish atrial sensing, and b) pacing/sensing bipolar ventricular electrodes. However, atrial pacing is precluded. Thus, this mode is acceptable in the absence of sinus node dysfunction, along with the programming of a slow backup rate, perhaps with a rate hysteresis to prevent VVI pacing. In this tracing, programming at 60 bpm was associated with retrograde conduction, which was sensed in the post-ventricular atrial refractory period (PVARP) and, consequently, did not trigger a pacemaker-mediated tachycardia (PMT). A PMT prevention algorithm as well as fallback must be programmed. The advantages of the single lead VDD systems are 1) the need for a single lead, which might limit the number of leads implanted and to shorten the implantation procedure, 2) the absence of injury to, and fibrotic development in the atrial wall, and 3) in patients whose sinus node function is preserved, the normal AV activation sequence is undisturbed.
The disadvantages of single lead VDD systems are 1) an occasionally suboptimal atrial sensing compared with a separate atrial lead, as the atrial dipole might move away from the wall, particularly during deep inspiration or cardiac motion, 2) likewise, the sensing of atrial arrhythmias might be poor for the same reasons, making stored information less reliable, 3) in case of paroxysmal, exercise-induced loss of atrial sensing and of AV block, the pacing rate reverts to backup with a possible abrupt fall in heart rate, possible pacemaker syndrome and risk of PMT, and 4) this mode must be avoided in presence of sinus node dysfunction.
The preferred indication for the VDD mode is, therefore, complete AV block with a preserved sinus node function.
