Refractory periods and PMTs in a Microport pacemaker

In this article
  1. Atrial Refractory Periods
  2. Ventricular Refractory Periods
  3. PMT response

Atrial Refractory Periods

Post-ventricular atrial blanking

  • any ventricular pacing triggers a programmable atrial blanking (nominal value 150 ms)
  • any ventricular sensing triggers an atrial blanking (nominal value 100 ms)

Window of Atrial Rate Acceleration Detection (WARAD)

The device monitors atrial events prematurity beat after beat, and is able to label each ventricular cycle as «normal» or «suspicious». The WARAD is a dynamic atrial refractory period that is automatically triggered after every atrial event (paced or sensed).

  • available in all dual chamber devices
  • objective: discriminate atrial arrhythmia from sinus tachycardia
  • on the marker chains, any atrial event sensed inside the WARAD is marked: Ar
  • any atrial event detected in the WARAD triggers a new WARAD
  • non programmable: the duration of the WARAD period is automatically calculated

 

The calculation of the WARAD is based on the preceding PP interval:

  • 75 % of the current PP interval, if it is less than 750 ms
  • 62.5% of the current PP interval, if it exceeds 750 ms (80 bpm)

 

Ventricular Refractory Periods

Ventricular Refractory Period

The ventricular sense or pace refractory period is triggered after a sensed or paced ventricular event (Vs or Vp).

  • the duration of the sense refractory period is 100 ms (non programmable)
  • the duration of the pace refractory period is 150 ms (non programmable)

 

Ventricular Blanking after atrial pacing

The ventricular blanking is triggered after each atrial pulse (Ap).

  • the post-atrial pacing ventricular blanking is non programmable (30 ms)

 

Ventricular safety window

The safety window follows ventricular blanking and lasts 95 ms minus the programmed length of the ventricular blanking.

  • in DDD mode, when a ventricular signal is sensed, a ventricular pulse is emitted at the end of the safety window
  • in SafeR mode, when a ventricular signal is sensed, no ventricular pulse is emitted at the end of the safety window

PMT response

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
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