Refractory periods and PMT in Abbott pacemakers

In this article
  1. AV delay
  2. Atrial Refractory Periods
  3. Ventricular refractory periods
  4. Pacemaker mediated tachycardia (PMT response)

AV delay

The atrioventricular delay is the interval between an atrial sensed event (sensed AV Delay) or an atrial paced event (paced AV Delay) and the delivery of the ventricular pulse.

  • the nominal value of the sensed AV delay is 150 ms
  • the nominal value of the paced AV delay is 200 ms

 

Rate Responsive AV Delay

The rate responsive AV delay parameter increases or decreases the paced or sensed AV delay in relation to the changes in the sensor-indicated rate (AP) or in the sensed intrinsic atrial rate (AS). In the new platforms, the shortening of the AV delay starts from 60 bpm (rather than 90 bpm for the older devices). The shortening of the AV delay continues until the maximal sensor rate, the maximal tracking rate or the shortest AV delay is reached.

Programmability:

  • low: AV delay is shortened by 0.5 ms per bpm
  • medium: AV delay is shortened by 0.75 ms per bpm
  • high: AV delay is shortened by 1.0 ms per bpm
  • the value of the shortest AV delay is programmable

 

Negative AV Hysteresis

The parameter Negative AV hysteresis allows the device to reduce the paced or sensed AV delay when an R wave is sensed to avoid intrinsic conduction and encourage ventricular pacing.

  • when Negative AV Hysteresis is active, an R wave sensed during the AV delay triggers the subtraction of the programmed Negative AV Hysteresis delta to the measured A-VS interval
  • the shortened AV delay stays active during 32 cycles if no R wave is sensed
  • if no R wave is sensed, the paced/sensed AV delay that is programmed permanently becomes active again after 32 cycles
  • if another R wave is sensed, the shortened paced/sensed AV delay is maintained during 255 cycles before going back to the programmed value

Atrial Refractory Periods

 

 

The atrial refractory period is triggered after a sensed or paced atrial event (AS or AP).

  • during this period, sensing is disabled on the atrial channel
  • events occurring in the atrial sense or pace refractory period are not counted in the filtered atrial rate

 

Post-ventricular atrial blanking (PVAB)

It is an absolute refractory period applied in the atrium after a sensed or paced ventricular event to prevent far-field R waves oversensing from the atrial channel.

  • during PVAB atrial events are blanked
  • events occurring in the PVAB are not counted in the filtered atrial rate
  • its value has to be lower or equal to the PVARP programmed value

 

Post-ventricular atrial refractory period (PVARP)

The PVARP sets the amount of time following the PVAB (after a paced or sensed ventricular event) that the device is unresponsive to signals from the atrial sensing circuit.

  • an atrial event in the PVARP does not start the AV delay timer
  • atrial events in the PVARP update the Filtered Atrial Rate Interval

 

Rate Responsive PVARP

Rate Responsive PVARP (RRPVARP) is designed to optimize the PVARP during activity by shortening PVARP depending on the heart rate to allow P-wave tracking at faster rates.

In the last platforms, the algorithm begins to operate when the intrinsic rate or the pacing rate exceeds 60 bpm or the base rate (whichever rate is higher). When the rate falls below 60 bpm or Base Rate, the algorithm is suspended.

  • a Low setting changes the Post-Paced Refractory Period and PVARP settings to preserve a 10% atrial alert period at the maximum rate (Maximum Sensor Rate or Maximum Tracking Rate)
  • a Medium setting changes the Post-Paced Refractory Period and PVARP settings to preserve a 20% atrial alert period at the maximum rate
  • a High setting changes the Post-Paced Refractory Period and PVARP settings to preserve a 30% atrial alert period at the maximum rate

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).

  • to prevent T-wave oversensing or double counting of wide QRS complexes
  • nominally set to rate-responsive; the value decreases as the rate increases

 

Ventricular Blanking after atrial pacing

The ventricular blanking (VB) is triggered after each atrial pulse (AP). This absolute refractory period is made to prevent crosstalk and the detection of the atrial output by the ventricular sense amplifier that would lead to ventricular inhibition.

  • the ventricular blanking can be set to a fixed value or to Auto

 

Automatic ventricular blanking

The automatic ventricular blanking has been designed to maintain the programmed AV delay even in the presence of crosstalk. When ventricular blanking is set on « Auto », every ventricular event sensed during the 12 ms initial blanking period triggers a succession of new 12 ms blanking periods until sensing of ventricular events stops, without exceeding 52 ms.

  • atrial pacing starts a 12 ms absolute blanking window
  • in the absence of crosstalk, the ventricular blanking period remains short (12 ms)
  • if ventricular activity is sensed, an additional 12 ms relative refractory window begins and can retrigger to a maximum of 52 ms
  • if sensing of ventricular events stops before 52 ms, the crosstalk detection window is not generated and ventricular pacing occurs at the programmed AV delay
  • if ventricular activity lasts more than 52 ms, a cross talk detection window is open from 52 ms to 64 ms

 

Ventricular safety window

The safety window follows ventricular blanking and lasts 64 ms minus the programmed length of the ventricular blanking.

  • programming ventricular safety standby ON starts a crosstalk detection window immediately after the ventricular blanking period
  • a ventricular pulse is delivered 120 ms after the atrial pulse if a ventricular signal is sensed during the crosstalk detection window
  • short AV delay and the VSP marker are characteristic of ventricular safety pacing on the EGM

 

Pacemaker mediated tachycardia (PMT response)

PMT response has 3 different settings:

  • OFF, no PMTs are detected
  • Passive, PMTs are detected and counted in the diagnostics but the device does not interrupt the tachycardia
  • Atrial pace, PMTs are detected and the Atrial Pace response is started to interrupt the tachycardia

 

PMT diagnosis

The PMT detection rate determines at what rate the device becomes alert to the presence of a PMT.

  • programmable between 90 and 180 bpm
  • cannot exceed the maximal tracking rate

If the device detects eight consecutive VP-AS intervals with a rate higher than the PMT detection rate, the device calculates the stability of the eight VP-AS intervals.

  • the device averages the 8 VP-AS intervals and compares that average to each of the individual intervals
  • if any one of the eight intervals varies by >16 ms from the average, the VP-AS intervals are considered unstable and the next 8 beats are monitored
  • if all VP-AS intervals are within 16 ms of the average then the VP-AS intervals are considered stable

If the VP-As are considered stable, the device modifies the AV delay for the ninth interval to confirm the diagnosis of PMT.

  • the sensed AV delay is shortened by 50 ms if the programmed Sensed AV (AS-VP) interval is ≥ 100 ms
  • the sensed AV delay is lengthened by 50 ms if the programmed Sensed AV (AS-VP) interval is < 100 ms

The next VP-AS interval is then analyzed.

  • no effect on VP-AS interval; if the next VP-AS is unchanged, the P-wave is considered retrograde with suspicion of PMT
  • significant effect on VP-AS interval; if the next VP-AS is different by more than 16 ms, the P-wave is not considered retrograde (no PMT); the diagnosis algorithm may be repeated after 256 beats

 

PMT interruption

If there is a suspicion of PMT:

  • the next ventricular pulse (10th cycle) is inhibited
  • the device delivers an atrial pulse 330 ms after the detected retrograde P-wave
  • the atrial pulse is followed by ventricular pacing at the end of the AV delay if needed
  • this atrial pulse is inhibited if a P-wave is sensed (AS) within a 210 ms alert period following the absolute atrial refractory period

 
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