2 relative refractory periods are designed to reduce the risk of PMT:

• Atrial Refractory Period (or Retro P watch): each PVC is followed by a 500 ms atrial refractory period; an atrial signal is sensed but no AV delay is started; cannot be generated by more than 3 consecutive cycles considered as PVC
• WARAD: Window of Atrial Rate Acceleration Detection started on each atrial sensed or paced event; an atrial signal is sensed but no AV delay is started; a short AV delay is applied on the next cycle

Anti-PMT algorithm

3 phases

Detection

A PMT is suspected when the pacemaker encounters during 8 consecutive cycles the following condition:

• atrial sensing (As)
• ventricular pacing (Vp)
• short Vp-As intervals (< 470 ms)
• stable Vp-As intervals (difference Vp-As max – Vp-As min is less than 30 ms)

Confirmation

When the device suspects a PMT, starts a confirmation phase with modification of the AV delay to differentiate PMT from sinus tachycardia.

• reduction of the AV delay except if reduction of the AV delay would cause an AV delay to be less than 30 ms or would induce pacing faster than the maximal tracking rate
• prolongation of the AV delay if reduction of the AV delay would cause an AV delay to be less than 30 ms or would induce pacing faster than the maximal tracking rate
• 50 ms AV delay modulation if the difference between the shortest and the longest Vp-As interval is <= 15 ms (strong stability criterion)
• 65 ms AV delay modulation if the difference between the shortest and the longest Vp-As interval is between 15 and 30 ms (weak stability criterion)

During the cycle following the modulation cycle:

• the stability criterion is not met: diagnosis of sinus tachycardia; the algorithm will not begin another suspicion phase for at least 100 ventricular cycles
• the stability criterion is still met: diagnosis of PMT
• the weak stability criterion is met, although before reducing the AV delay the strong criterion was met; no conclusion and a second modulation (65 ms) is applied 

  Termination

  When the algorithm confirms the presence of PMT:

  • one cycle with post-ventricular pacing atrial refractory period of 500 ms

Programmable parameter

Only one parameter can be programmed: Termination or Reprogramming.

• Termination: detection and interruption of the PMTs without modification of the AV delay
• Reprogramming: detection and interruption of the PMTs + automatic decrease of the AV delay after recurrent PMTs (10 episodes within 24h); rest and exercise AV delay are shortened by 15 ms steps to 125 ms for rest AV delay and to 80 ms for exercise AV delay