Pacing & Sensing in Boston Scientific pacemakers

In this article
  1. Automatic atrial threshold testing (Right atrial automatic threshold, RAAT)
  2. Automatic ventricular threshold testing (PaceSafe RV Automatic Capture, RVAC)
  3. Programming of the sensitivity

Automatic atrial threshold testing (Right atrial automatic threshold, RAAT)

Right atrial automatic threshold (RAAT)

Right atrial automatic threshold (RAAT):

  • required bipolar atrial lead
  • threshold tests are scheduled every 21 hours
  • evaluation of atrial capture is based on the presence of an atrial evoked response
  • amplitude is automatically adjusted to 2X the highest of the last 7 days with successful threshold tests at a non-programmable pulse width of 0.4 ms
  • the output is limited between 2.0 V and 5.0 V

 

Determining the RA pacing threshold

  • evaluation of atrial capture is based on the presence of an atrial evoked response
  • atrial pacing rate is based on the lower rate limit or the sensor rate and be increased by 10 bpm to ensure atrial pacing
  • voltage decrements every 3 captured beats
  • looks for 2 Loss of Capture (LOC) beats out of 4-beat moving window
  • the second LOC in a 4-beat moving window is defined as “Confirmed LOC” (C-LOC)
  • the threshold is defined as one voltage step higher than where C-LOC occurs

 

 Adjusting the RA pacing output

  • no beat to beat verification of atrial capture
  • safety margin: amplitude 2X (non programmable) the highest of the last 7 measured thresholds
  • non programmable pulse width: 0.4 ms
  • maximal amplitude: 5.0 V
  • minimal amplitude: 2.0 V

Automatic ventricular threshold testing (PaceSafe RV Automatic Capture, RVAC)

PaceSafe RV Automatic Capture (RVAC):

  • RVAC is designed to dynamically adjust the right ventricular pacing output
  • ventricular voltage output is automatically adjusted to 0.5 V above the measured threshold
  • beat-to-beat verification of myocadial capture via sensing of ventricular evoked response
  • delivered output from a minimum of 0.7 V to a maximum of 3.5 V
  • the pulse width is non-programmable (0.4 ms)
  • in case of loss of capture, a back-up pacing pulse is delivered
  • when loss of capture is confirmed for two cycles out of four beats, automatic threshold test is initated to check for the new threshold.

 

Determining the RV pacing threshold

  • an automatic threshold test is systematically performed every 21 hours
  • the test starts with an initialization phase (2 warm-up intervals to calibrate the assessment of the evoked response measurement followed by 12 intervals to initiate the evoked response channel filters)
  • ER (evoked response) sensing is performed on a dedicated unipolar sense channel
  • the ER channel uses lower frequency filters than those of the bipolar sensing channel (3-65 Hz versus 25-85 Hz).
  • to assess the effectiveness of the capture, the maximum amplitude of the positive signal is evaluated in a window of 10 to 70 ms following pacing
  • amplitude starts at 3.5 V and steps down in 0.1 V increments
  • throughout the threshold measurement, a backup pacing (BP) is systematically delivered after the first ventricular pacing (70 ms after the primary VP) at a maximal amplitude of 3.5 to 5 V to prevent any ventricular pause as a result of loss of capture
  • voltage decrements every 3 captured beats
  • looks for 2 Loss of Capture (LOC) beats out of 4-beat moving window
  • the second LOC in a 4-beat moving window is defined as “Confirmed LOC” (C-LOC)
  • the threshold is defined as one voltage step higher than where C-LOC occurs
  • LOC at an amplitude > 3.0 V is considered an unsuccessful test
  • if there is no capture at the initial amplitude or if 0.1 V is reached with no Confirmed LOC, the test fails
  • an EGM is stored for the most recent ambulatory threshold test

 

Adjusting the RV pacing output

  • amplitude is automatically adjusted to 0.5 V (non programmable) above the measured threshold
  • the delivered output ranges from 0.7 to 3.5 V
  • if any LOC occurs, a backup pace is delivered
  • beat-to-beat capture verification using an evoked response channel
  • new threshold measurement with adjustment of the delivered amplitude if the capture is ineffective

Programming of the sensitivity

The sensitivity feature can be programmed either to AGC or Fixed sensing. The device has independent AGC circuits for each chamber and the atrial and ventricular sensitivity values are programmable independently. Selection of the AGC sensing method applies that method to all chambers and the type of sensing method used (AGC versus Fixed) must be the same for the atrial and ventricular channels.

 

Fixed sensitivity

Atrial fixed sensitivity

Ventricular Fixed Sensitivity

Automatic Gain Control

The Automatic Gain Control dynamically adjusts the sensitivity in both the atrium and the ventricle.

  • the programmable AGC value is the minimum sensitivity value that can be reached between one beat and the next one
  • this programmable value is not a fixe value present throughout the cardiac cycle; rather the sensitivity level begins at a higher value and decrements towards the programmed floor
  • AGC typically reaches the programmable floor after pacing or low amplitude signals
  • when moderate or high amplitude signals are sensed AGC will not typically reach the programmed floor

The AGC in each respective chamber processes an electrogram signal via a two-step process to optimize sensing of potentially rapidly changing cardiac signals.

  • the device adjusts the ventricular sensitivity according to a slow or fast AGC
  • the AGC applies a “slow” component to set an interval in search of the next QRS amplitude
  • by calculating an “average” of the peaks of the preceding signals, the “slow” component defines a zone that is likely to contain the following peak, and sets this zone between a minimum and a maximum value

 

First step

AGC uses a rolling average of previous signal peak to calculate a search area where the next peak will likely occur with MAX and MIN limits.

  • if the previous beat is sensed, it is incorporated into the rolling peak average
  • if the previous beat is paced, the peak average is calculated using the rolling average and a paced peak value; the paced peak value depends on the settings: for nominal or more sensitive settings, it is a fixed value (initial value 4.8 mV in the ventricle, 2.4 mV in the atrium); for less sensitive settings, it is higher value calculated using the programmed AGC floor value
  • the peak average is used to bound an area with Min and Max limits

 

Second step

The device senses the amplitude of the intrinsic beat. Full R-wave and P-wave range is available up to 32 mV.

  • the device holds the sensitivity level at the peak (or Max) through the absolute refractory period + 15 ms
  • at the end of the refractory period + 15 ms, the sensitivity setting decreases to 75% of the sensed peak or, if the last ventricular event is paced, to 75% of the average peak
  • the device thereafter decreases the sensitivity setting every 35 ms for the ventricle and 25 ms for the atrium, to 7/8 of the previous setting
  • this step-down continues until whether the MIN or the programmed sensing floor is reached, whichever is reached first
  • after a paced beat, the AGC also steps down to 7/8 of previous value so that the MIN or programmed AGC floor is reached 150 ms from the next scheduled pace

 

 

Programmable parameter

Sensing Method: AGC/Fixed

  • atrial AGC is nominally 0.25 mV
  • right Ventricular ACG is nominally 0.6 mV
Top