Limitations of the dual-chamber VDD mode

The VDD mode can be programmed on a traditional dual-chamber pacemaker but can also be obtained from a single-lead VDD dual-chamber device with two pairs of electrodes, one floating atrial pair providing atrial sensing, and one ventricular pair for ventricular pacing and sensing. On the other hand, atrial pacing is not possible. This mode is therefore acceptable in the absence of sinus dysfunction, and a low base rate should be programmed, possibly with rate hysteresis, in order to avoid VVI behavior. Indeed, on this tracing, the programming at 60 beats/minute is associated with a loss of atrioventricular synchrony.

The advantages of single-lead VDD systems are that

1. only one lead is required, allowing for a sometimes shorter implantation procedure while reducing the amount of required endovascular material;
2. the absence of trauma to the atrial wall thereby preventing the development of atrial fibrosis;
3. in patients without sinus dysfunction, the physiological atrioventricular activation sequence is preserved.

The disadvantages of single-lead VDD systems are that

1. atrial sensing is sometimes not as effective as that obtained with a separate atrial lead, since the atrial dipole can move away from the wall, especially during deep inspiration and cardiac movements;
2. similarly, the sensing of atrial arrhythmias is sometimes not as effective, for the same reasons, rendering the memory banks less reliable;
3. in the event of loss of paroxysmal atrial sensing on exertion and presence of atrioventricular block, the pacing rate returns to the base rate with a possible sudden drop in rate, along with possible pacemaker syndrome and risk of PMT;
4. this mode should be avoided in the presence of sinus dysfunction.

The preferred indication of the VDD mode is therefore complete atrioventricular block with normal sinus function.
In this patient, the recording of this periodic EGM led to a reduction of the minimum rate in order to promote atrial sensing.