Dual-chamber VDD mode

The VDD mode can be programmed on a conventional dual-chamber pacemaker but can also be obtained from a single-chamber device or a ventricular single-lead VDD pacemaker with two floating atrial electrodes for atrial sensing and the ventricular bipolar pacing/sensing electrodes. On the other hand, any atrial pacing is impossible. Sensing is therefore carried out in the atrium and in the ventricle, although pacing only takes place in the ventricle. The VDD mode provides synchronized pacing in the atrium (or VVI pacing at the minimum rate). The ventricle is paced synchronously up to the maximum synchronous rate.

This mode is therefore acceptable in the absence of sinus dysfunction; it is advisable to program a low base rate, possibly with rate hysteresis to avoid VVI behavior. Indeed, on this tracing, the programming at 60 beats/minute is associated with retrograde conduction and pacemaker syndrome. In this example, retrograde conduction is detected in the PVARP and therefore does not induce PMT. A PMT protection algorithm must be programmed. 

The advantages of single-lead VDD systems are that:

1. only one lead is required, allowing for a shorter implantation procedure and reduces the amount of endovascular material needed;
2. the absence of trauma of the atrial wall avoids the development of atrial fibrosis;
3. in patients without sinus dysfunction, the physiological atrioventricular activation sequence is respected.

The disadvantages of single-lead VDD systems are that:

1. atrial sensing is sometimes poorer than with a separate atrial lead, since the atrial dipole can stray 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 thereby making the memory less reliable;
3. in the event of loss of paroxysmal atrial sensing and AV block, the pacing rate returns to the base rate with a possible sudden drop in rate, a 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 AV block with normal sinus function.