Refractory periods and PMT in Boston Scientific pacemakers

In this article
  1. Atrial Refractory Periods
  2. Atrioventricular delay
  3. Ventricular refractory periods
  4. PMT prevention

Atrial Refractory Periods

Atrial Refractory Period after Atrial-sensed Event

Atrial refractory is the time period after an atrial-sensed event when additional sensed-atrial events do not initiate timing cycles or inhibit atrial pacing.

  • 85 ms (30 ms absolute refractory, 40 ms noise window, 15 ms absolute refractory)
  • absolute refractory events are not sensed or marked
  • noise events are sensed but not marked

 

Atrial Refractory after Atrial-paced Event

Atrial refractory is the time period after an atrial-paced event when additional sensed-atrial events do not initiate timing cycles or inhibit atrial pacing.

  • 150 ms (110 ms absolute refractory, 40 ms noise window)
  • absolute refractory events are not sensed or marked and noise events are marked as [AS]

 

Cross-chamber atrial blanking windows

Each cross-chamber blanking window may be programmed to a fixed value or to SMART.

SMART Blanking is designed to promote sensing by shortening the blanking period, while rejecting cross-chamber events by automatically adjusting the sensitivity in the following manner:

  • shortening the blanked period to an absolute refractory (15 ms for A-blank after V-Sense, and 37.5 ms for A-Blank after V-Pace)
  • eliminating the noise portion of the blanking window
  • increasing the AGC setting to avoid far-field oversensing of the event in the opposite chamber; the AGC is bumped up to 3/8 of the current peak average

 

            A-Blank after V-Pace

 

            A-Blank after RV-Sense

 

Post-Ventricular Atrial Refractory Period (PVARP)

PVARP is a programmable period after a ventricular paced or sensed event in which sensed atrial events do not initiate timing cycles or inhibit atrial pacing. PVARP may prevent the tracking of retrograde atrial events initiated in the ventricle that could trigger PMT in a dual chamber device programmed to a tracking mode.

  • the initial section of PVARP is the cross-chamber blanking period (composed of absolute refractory and noise window) followed by the fixed refractory period
  • events that fall into the absolute refractory window of cross-chamber blanking are not sensed or marked
  • events that fall into the noise window are marked as [AS]; there is no noise window for SMART blanking
  • events in fixed refractory are marked as (AS), (AF), or (PAC)
  • although sensed events in the PVARP fixed refractory window do not initiate timing cycles, they do count toward ATR Entry Counts, AFR, and are included in counters/histograms

 

Dynamic PVARP

The cross chamber blanking period is not shortened by programming dynamic PVARP, only the fixed refractory period. PVARP will decrease linearly between the lower rate limit and maximal tracking rate based on a rolling average of the last four V-V intervals.

Atrioventricular delay

Dynamic AV Delay

  • sensed and paced AV delays are automatically shortened in a linear fashion between the maximum value (applied at the lower rate limit) and the minimum value (applied at the greatest of the maximal sensor fate or maximal tracking rate)
  • the value for the current cycle’s AV delay is based on the previous A-A or V-V interval
  • if the previous cardiac cycle contained an AP, the AV Delay is based on the previous scheduled V-V interval
  • if the previous cardiac cycle contained an AS, the AV Delay is based on the previous non-refractory A-A interval (if there is one; otherwise the V-V will be used)

 

AV Search Plus

  • AV Search Plus elicits and promotes intrinsic AV conduction and minimizes right ventricular pacing by periodically extending the AV Delay to a programmed length

Ventricular refractory periods

RV Refractory after RV-sensed Event

RV refractory is the time period after an RV-sensed event when additional sensed-RV events do not initiate timing cycles or inhibit ventricular pacing and are not included in counters.

  • 135 ms (50 ms absolute refractory, 40 ms noise window, 45 ms absolute refractory)
  • absolute refractory events are not sensed or marked and noise events are sensed

 

RV Refractory after RV-paced Event

RVRP is a programmable period in which right ventricular events which follow RV-paced events do not initiate a timing cycle or inhibit ventricular pacing and are not included in counters.

  • the initial section of RVRP is fixed refractory followed by a 40 ms noise window
  • events in fixed refractory are not sensed or marked and noise events are marked as [RVS]

 

Dynamic RVRP

By using Dynamic RVRP, the sensing window will be maintained since the RVRP will shorten as the ventricular rate increases due to atrial tracking or sensor-driven pacing. RVRP will decrease linearly as the rate increases from the lower rate limit to the maximal tracking rate and is based on the previous V-V interval.

 

RV blanking after A pacing

  • nominal setting: 65 ms (last 40 ms is the noise window)

There is no ventricular cross-chamber blanking for an atrial sensed event.

 

Ventricular noise window after atrial pacing

There is no safety pacing window following ventricular blanking in Boston Scientific devices but there is a retriggerable noise window.

  • when an event is sensed in the noise portion of a blanking window, event is not used for timing or detection
  • event is marked with a marker in brackets [VS] and does not typically have an interval value
  • if the noisy signal is continuous, the noise window will be re-triggered as long as the noisy signal is present (or until the pacing interval times out) to help suppress any further sensing of the noise

PMT prevention

  • PVARP after PVC automatically extends the programmed PVARP for one cardiac cycle following a PVC
  • atrial sensed events occurring during this PVARP will be sensed but will not initiate a timing cycle or inhibit atrial pacing
  • PVC is defined as an RV-sensed event which follows an RV paced or sensed event with no intervening atrial event
  • sensed and paced atrial events as well as refractory atrial events are accepted as an intervening atrial event
  • PVARP after PVC will not occur more frequently than every other cardiac cycle since timing cycles are reset following a PVC

 

PMT Termination algorithm

The PMT algorithm is applied if:

  • 16 consecutive VP-AS cycles occur at the maximum tracking rate (VPMT-AS)
  • the associated V-A intervals do not vary by more than 32 ms; the V-A interval of each event is compared to the first V-A interval of 16 intervals; if any one of next 15 V-A intervals is more than 32 ms different than the first V-A interval, the device resets the count to zero and continues to monitor for PMT

When PMT is declared, PVARP is extended to 500 ms for one cycle following the 16th ventricular pace. An EGM is stored in the Arrhythmia Logbook when PMT is ON and a PMT episode is declared.

  • while there is no true PMT counter, the number of PMT events can be assessed by the number associated with the PMT episodes in the Arrhythmia Logbook
  • the event numbers continuously increment even though events may not remain in memory
  • up to three detailed episodes may be stored with associated EGMs
  • up to five total PMT events may be listed
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