Automatic measurement of the atrial pacing threshold (ACap Confirm)

Generalities

ACap Confirm® is an algorithm working in the right atrium that allows an automatic measurement of the atrial pacing threshold, with adaptation of the output amplitude.

  • ACap Confirm® is not a beat-by-beat algorithm
  • ACap Confirm® uses the morphology of the evoked response to determine whether there is capture or not
  • the template is based on loss of capture
  • ACap Confirm® requires a bipolar atrial lead
  • pacing must occur in bipolar configuration
  • evoked response is examined from A-Tip to Can
  • back-up pulses are always delivered 30 ms after the test pulse during threshold searches

 

Setup test and acquisition of the morphology template

The algorithm memorizes loss of capture as a morphology template. If the signal is similar to the template, this corresponds to a loss of capture, otherwise capture is verified. Three pairs of pacing pulses are delivered to test the morphology of the evoked response before every threshold measurement.

Three pairs of pacing pulses are delivered, and Evoked Response (ER) signals are measured:

  • Test 1: 3.875V Test Pulse, 5V Back-up Pulse
  • Test 2: 0.0V Test Pulse, 5V Back-up Pulse
  • Test 3: 0.0V Test Pulse, 5V Back-up Pulse
  • ER’s from Test 1 and Test 2 do not correlate
  • ER’s from Test 2 and Test 3 correlates
  • Tests 1 and 2 show that capture morphology differs from loss of capture
  • Tests 2 and 3 show that loss of capture morphology is consistent

 

The most recently acquired loss of capture signal (Test 3) is combined with the previous 15 templates into one continuously updated running average. The most recent template accounts for  1/16th of the running template.

 

Kendall Tau Correlation is used to compare a measured ER to the loss of capture

template.

  • Kendall Tau Correlation compares pairs of points for “direction” and not amplitude
  • if the pair of ER points are heading in the same direction as the corresponding pair of points on the template, the Kendall Tau score goes up
  • 25 points are sampled, then paired for scoring
  • a score is measured on a scale from 0 to 300
  • high score = high correlation between template and ER; diagnosis = Loss of Capture
  • low score = low correlation between template and ER; diagnosis = Capture
  • a threshold is determined based upon LOC and CAP scores
  • scores below the threshold are CAP
  • scores above the threshold are LOC
  • minimum difference between CAP and LOC scores must be 30

 

 

Threshold test

  • rate must be <120 bpm
  • the threshold test starts 0.5 V above the last measured threshold or 0.5 V below the programmed pulse amplitude if no threshold test has been previously run
  • the ER is always evaluated from a first spike of variable amplitude and a second 5 V spike, 30 ms after the first one
  • this back-up pulse is systematically delivered 30 ms after every test pulse regardless of CAP or LOC
  • every two beats, the amplitude of the first spike is decreased by 0.25 V
  • when two losses of capture occur, the pulse amplitude is increased by 0.125 V every two beats until two captures are verified
  • If the algorithm detects a loss of capture at 3.875 V or a capture at 0.0 V, Acap Confirm is disabled automatically and the pulse amplitude is reprogrammed at 5 V (high output mode); a new threshold test will be run one hour later

 

Adjustment of the pulse amplitude

Once the threshold is found, the device sets a new pulse amplitude by adding a security margin depending on the measured pacing threshold. The same output is used until the next scheduled threshold search.

 

Programmable settings

ACap Confirm® is programmable to three modes: On, Monitor, Off

  • On: periodical threshold search, indexed data and amplitude adjustment
  • Monitor: periodical threshold search, indexed data but no amplitude adjustment
  • Off: no threshold search, no amplitude adjustment
  • Search interval: 8h, 24h

Automatic measurement of the ventricular pacing threshold (Ventricular Autocapture)

Ventricular Autocapture®

Generalities

  • automatic verification of capture beat-to-beat: the device looks for loss of capture on every beat
  • automatic back-up stimulation 5 V (unipolar or bipolar): if loss of capture is present, the device delivers a high output back-up pulse to ensure capture
  • automatic threshold search 8 or 24 hours: the device measures periodically (every 8 or 24 hours) the pacing threshold
  • automatic amplitude adjustment 0.25 V working margin: the device sets automatically the parameter Pulse Amplitude above the measured threshold with a non programmable working margin of 0.25 V
  • automatic fusion avoidance
  • AutoCapture® works whatever the programmed Pulse Width is

 

The AutoCapture® system uses 4 algorithms to monitor capture beat-to-beat:

  • capture verification
  • loss of capture recovery (LOC)
  • fusion avoidance
  • threshold search

 

Capture verification

Capture verification efficiency relies on a measurement of the evoked response (ER) and works by triggering a blanking window (closed) of 14 ms, followed by an open window of 46 ms.

  • if the device detects an ER within this second window, capture is confirmed

 

  • otherwise, it delivers a 5 V backup pulse within the 80 to 100 ms following the initial pulse to ensure capture

 

 

Loss of capture recovery

The Loss of Capture Recovery algorithm is activated when the capture verification confirms two consecutive losses of capture.

  • if two consecutive initial pulses indicate loss of capture the device increases the pacing voltage; the amplitude is increased by 0.25 V for one beat
  • if capture is confirmed, the pacemaker delivers the same amplitude again to re-confirm capture; after capture is confirmed a threshold search is initiated
  • if capture is not confirmed, the amplitude is increased one step at a time by 0.125 V increments until two consecutive captured beats are present; a threshold search is then initiated
  • if Capture is not confirmed by 3.875 V, the device switches to “high output mode” of 5.0 V; after 128 cycles, a threshold search is initiated

 

Fusion avoidance

After the first loss of capture (LOC), the sensed/paced AV delay is extended by 100 ms for the next beat because the algorithm always makes the hypothesis that the first LOC is due to fusion.

  • the 100 ms extension may reveal a possible intrinsic R wave

 

Threshold search

An automatic setup test can be done before every threshold test (compulsory during the first activation) to check if the AutoCapture® pacing system is recommended. AutoCapture® is recommended if:

  • the polarization signal amplitude is lower than 4.0 mV
  • the security margin between the ER signal amplitude and the ER sensitivity setting is 1.8:1 or higher
  • the security margin between the polarization signal amplitude and the ER sensitivity setting is 1.7:1 or higher

 

Threshold Search sequence

  • the pulse amplitude is decreased by  0.25 V two beats at a time until loss of capture occurs for two consecutive beats
  • the pulse amplitude is then increased one step at a time by 0.125 V until two consecutive capture beats are present; this determines the value of the capture threshold
  • the amplitude is then set 0.25 V above threshold; this is the “working” safety margin
  • if the decreasing threshold search fails to find loss of capture at the lowest pulse amplitude of 0 V, the device switches to “high-output mode” and then restarts a threshold search after 128 cycles

 

The threshold test is run in different conditions:

  • sudden increase in pacing threshold (2 consecutive losses of capture)
  • automatically every 8 or 24 h
  • at the removal of a telemetry wand
  • at magnet removal (if magnet has been applied for at least 5 seconds)
  • programmer initiated during follow up (when the operator runs the pacing threshold test via AutoCapture®)

 

The sensed/paced AV Delays is automatically shortened during the Threshold Search to minimize the risk of fusion beats.

  • paced AV delay: 50 ms
  • sensed AV delay: 25 ms

 

Programmable settings

  • AutoCapture: On/Off
  • Backup Pulse Configuration: Unipolar/Bipolar (Unipolar if RV lead set on Unipolar)
  • Search Interval: 8h, 24 h
  • AutoCapture Paced/Sensed AV Delay: 50/25, 100/70, 120/100 ms

Programming of the sensitivity

The sensitivity in both the atrium and the ventricle can be set at a fixed value or can be programmed automatically.

 

Fixed value

Atrium

  • sensitivity: 0.1, …, 1.0, …, 5.0 mV

Ventricle

  • sensitivity: 0.5, …, 2.0, …, 12.5 mV

 

Automatic sensitivity (AutoSense® or SenseAbility®)

Post-sensed sensitivity

The sensitivity adapts automatically to the amplitude of the previous signal.

  • after the sense refractory period, the sensitivity is adapted automatically to a percentage (threshold start) of the highest measured amplitude (in both negative and positive components of the signal)
  • whatever the amplitude of the sensed signal, the threshold start in the ventricle cannot be lower than 1.0 mV or higher than 6.0 mV
  • the threshold start in the atrium cannot be lower than 0.3 mV or higher than 3.0 mV
  • the decay delay determines the amount of time after the sensed or paced refractory period that the threshold remains at the programmed threshold start setting before starting its decay
  • the sensitivity then increases (decreases in value) along the cycle, with a 3.0 mV/s slope in the ventricle and a 1.5 mV/s slope in the atrium
  • the max sensitivity parameter is the highest sensitivity the device can achieve

 

Post-paced sensitivity

In the ventricle, the post-paced decay delay and threshold start can be set on Auto. This mode is used to allow dynamic changes in the Threshold Start and the Decay Delay as the pacing rate changes.

  • when programmed to Auto, the device uses a look-up table to determine the Threshold Start and Decay delay for a given pacing rate
  • pacing rates from 30 to 150 are divided into 10 rate bins
  • each rate bin corresponds to a calculated Threshold Start and Decay Delay

 

Programmable settings

Ventricle

  • AutoSense®: On, Off
  • Max Sensitivity: 0.2, 0.3, 0.4, 0.5, …, 2 mV
  • Post-Sensed Decay Delay: 0, 30, 60, 95, 125, 160, 190, 220 ms
  • Post-Sensed Threshold Start: 50, 62.5, 75, 100 %
  • Post-Paced Decay Delay: Auto, 0, 30, 60, 95, 125, 160, 190, 220 ms
  • Post-Paced Threshold Start: Auto, 0.2, 0.3, …, 3 mV

Atrium

  • AutoSense®: On, Off
  • Max Sensitivity: 0.2, 0.3, 0.4, …, 1 mV
  • Post-Sensed Decay Delay: 0, …, 220 ms
  • Post-Sensed Threshold Start: 50, 62.5, 75, 100 %
  • Post-Paced Decay Delay: Auto, 0, 30, 60, 95, 125, 160, 190, 220 ms
  • Post-Paced Threshold Start: Auto, 0.2, 0.3, …, 3 mV