Automatic atrial capture threshold testing (Atrial Capture Control, ACC)

Feature Description

The Atrial Capture Control (ACC) feature periodically measures the atrial pacing threshold with amplitude adjustment.

  • the standard setting is one threshold measurement per day, but the frequency of measurement is programmable
  • the threshold search is based on the presence or absence of atrial sensing markers generated by the device (indirect evaluation of the atrial capture)
  • the atrium is stimulated at a pacing rate higher than the intrinsic rate to suppress atrial intrinsic events
  • when the pacing output is lower than the atrial threshold, sensed atrial event can be detected either due to the emerging intrinsic rhythm or due to retrograde conducted events
  • the detection of sensed atrial activity is used to discriminate between atrial capture and non-capture

 

The atrial capture control is performed in four steps:

Setup-Phase

  • automatic measurements are allowed if the atrial and ventricular rate is below 110 bpm and no mode switching is active
  • if these conditions are met, the activation of the capture control algorithm causes a mode switch to DDI with an atrial overdrive pacing of +20 % of the actual determined rate
  • the Ap-Vp interval are programmed to 50 ms, to avoid retrograde conduction from the ventricle when the atria are captured

Threshold search

  • the threshold is determined by decreasing the amplitude stepwise at a programmed pulse duration until loss of capture occurs
  • loss of capture for one test amplitude is declared if in a test window of five cardiac cycles (5 Ap-Vp intervals), two or more intrinsic atrial events are sensed (suspicion of unsuccessful pacing)

Confirmation phase

  • the pacing threshold is considered to be confirmed if capture is determined with the first step and loss of capture is confirmed with the second step

Amplitude adjustment

  • the pacing amplitude is defined by adding the programmed safety margin to the determined threshold
  • there is no beat-to-beat capture verification

 

 

Programmable parameters

Capture Control

OFF, ON, ATM

 

Minimum Amplitude

0.5…(0.1)…4.8 V, default 1.0 V

This is the minimum atrial pacing output to which the device can be automatically programmed, regardless of the measured threshold.

 

Threshold Test Start

2.4 V, 3.0 V, 3.6 V, 4.2 V and 4.8 V, default 3.0 V

This is the starting voltage when Capture Control (ON or ATM) is looking to determine the current threshold.

 

Safety Margin

0.5 V…(0.1 V)…1.2 V, default 1.0 V

This is the amount of pacing output added to the measured threshold value to ensure capture.

 

Search Type

Interval / Time of Day

This feature determines when the device performs a Capture Control test to check the current threshold.

Ventricular capture threshold testing (Ventricular Capture Control, VCC)

The Ventricular Capture Control (VCC) feature periodically measures the capture threshold, and automatically adjusts the pacing output (with a programmable safety margin) when programmed ON.

  • the feature continuously assesses ventricular pacing capture on a beat-to-beat basis and responds to any loss of capture with a safety back-up pulse
  • differences in the signal morphology between the polarization artifact and the evoked response signal are used to distinguish capture events from non-capture events
  • after an effective blanking period of 20 ms, the signal is evaluated over the next 60 ms
  • several characteristics of the signal falling into this window are evaluated in order to discriminate the evoked response (capture) from polarization artifact (possible non-capture)

 

Capture Control Parameters

 

Capture Control Parameters

 

Capture Control

OFF, ON, ATM

  • when Ventricular Capture Control is OFF, the output is manually programmed
  • when Ventricular Capture Control is ON, the feature determines the capture threshold, programs the output and provides continuous monitoring to insure capture is present
  • the ATM mode differs from Capture Control ON as the device does not automatically adjust the pacing voltage, but just stores the measured threshold values

 

Threshold Test Start

2.4 V, 3.0 V, 3.6 V, 4.2 V and 4.8 V, default 3.0 V

  • this is the starting voltage when Capture Control (ON or ATM) is looking to determine the current threshold
  • this value should only be changed to a higher value if the patient has high thresholds
  • the higher the output, the greater the polarization artifact which may cause the Signal Quality Check to fail

 

Safety Margin

0.3 V…(0.1 V)…1.2 V; default 0.5 V

  • this is the amount of pacing output added to the threshold value to ensure capture
  • the lowest the output can be programmed is 0.7 V, regardless of the threshold

 

Search Type

interval, time of day

This feature determines when the device performs a Ventricular Capture Control test to check the current threshold.

  • Interval: the Active Threshold Monitoring test is performed at a programmed time from when the previous threshold test was performed; this time may vary due to retesting programming changes or threshold testing done after loss of capture; by default, it is set to once every 24 hours
  • Time of day:  the Active Threshold Monitoring test is performed at a specific time of day; by default the time is set to 00:30 AM; this may be changed if the patient is sensitive to the pacing test occurring at night and may be set to a time when the patient is awake

 

Feature of the Ventricular Capture Control

The feature of the Ventricular Capture Control includes three components: Signal Analysis, Capture Threshold Search and Capture Verification.

Signal Analysis (SA)

A polarization artifact that is too large may disturb the cardiac signal following the pacing pulse and result in misclassification of the event. Conversely, the evoked response signal may be too small or may not meet the capture criteria, which again may lead to misclassification of the event. Therefore, the SA analyzes the evoked response and the amplitude of the polarization artifact. A successful SA must always be completed before the Capture Threshold Search or the activation of Capture Verification.

  • SA is performed in two separate phases; in both phases, the AV delay is shortened to 50 ms after a paced atrial event and to 15 ms after a sensed atrial event to ensure ventricular pacing
  • first, five ventricular pacing pulses are delivered at MaxVCCAmp, which is the programmable maximum voltage setting (2.4, 3.0, 3.6, 4.2, 4.8 V); if non-capture is detected at the maximum voltage setting, the second phase of the SA is aborted and the test is classified as unsuccessful
  • in the next phase, five “double” pacing pulses (one pacing pulse followed by another pacing pulse 100 ms later, in the absolute refractory period) are delivered; the first pulse captures the ventricle and the second one (non capture) is used to verify that the polarization artifact is small enough to distinguish capture from non-capture; if the artifact following the second pacing pulse is higher than a certain limit, then SA is classified as unsuccessful

If the first SA after activating VCC is not completed successfully, VCC is immediately disabled, and the pacing amplitude is programmed to MaxVCCAmp. VCC then requires manual reactivation of the feature with the programmer.

If the first SA after activating VCC is completed successfully, but subsequent SA’s are not completed successfully, then VCC is suspended, and the pacing amplitude is programmed to the last measured threshold plus the safety margin. The SA will be attempted up to three times. After the 3rd failure, VCC is disabled, and the pacing amplitude is programmed to the MaxVCCAmp plus the Safety Margin of 1.2 V. VCC then requires manual reactivation of the feature with the programmer.

 

Automatic Threshold Measurement

The Automatic Threshold Measurement is the component of the VCC feature that measures the ventricular pacing threshold by stepping down the output until non-capture occurs.

The Automatic Threshold Measurement occurs over a series of cardiac cycles and begins at the threshold test start that decreases until capture is lost. AV delay is shortened to 50 ms after a paced atrial event and to 15 ms after a sensed atrial event to ensure ventricular pacing.

  • the pacing amplitude decrements with every paced beat by 0.6 V, until the first non-captured beat
  • the algorithm then decrements by smaller increments of 0.1 V until the first failed capture, at which point it determines that the capture threshold is preceding value
  • if two non-captures are detected when the voltage decrements by 0.6 V, the pacing amplitude is set to the previous amplitude and then the amplitude decrements by 0.1 V until the pacing threshold is determined

The pacing amplitude is then set to the pacing threshold plus a programmable safety margin.

In addition to performing the threshold search after a loss of capture, the search is also conducted at a programmable interval or a specific time during the day to provide an accurate safety margin even with gradual changes in the pacing threshold.

 

Capture Verification (CV)

The Capture Verification function is the component of the VCC feature that provides beat-to-beat capture verification.

  • if VCC determines that capture has been maintained, then the pulse amplitude remains at that current setting and no action is required
  • if VCC determines that non-capture (NC) occurred, then a safety back-up pacing pulse is delivered at an increased energy (same output voltage and pulse width extended to 1.0 ms) within 130 ms after the non-captured pacing pulse
  • if a series of ventricular pacing pulses at varying AV delays result in loss-of-capture, a Signal Analysis (SA) and Automatic Threshold Search are initiated to determine the current pacing threshold

The algorithm is designed to respond appropriately to fusion beats. In order to discriminate non-capture from fusion, a capture confirmation algorithm varies the AV delay after detection of non-capture in the dual chamber pacing modes.

  • first, fusion is diminished by extending the AV delay
  • if a second non-capture is detected, the AV delay is returned to the programmed AV delay
  • if a third consecutive non-capture is detected, loss of capture is confirmed and a Signal Analysis and Automatic Threshold Search are initiated
  • if the first event was truly fusion, the extended AV delay could allow intrinsic conduction; the AV delay will not return to the normal programmed value until ventricular pacing is required

 

Algorithm Suspension

The continuous capture control (CV) is suspended when the following events occur:

  • mode switch is ongoing: if Mode switch is ongoing when an SA/CV is scheduled, the SA/CV will be postponed until reversion back to the programmed mode
  • the presence of a magnet is detected: a scheduled SA/CV is postponed when a magnet (programmer wand) is detected; after removal of the magnet, the SA/CV is started
  • the ventricular rate is higher than 110 bpm: a scheduled SA/CV is postponed when the ventricular rate is higher than 100 bpm; when the ventricular rate drops below 10 bpm, the SA/CV is started

The VCC feature is automatically turned OFF when the following events occur:

  • the initial SA/CV sequence after activating VCC failed
  • three subsequent and consecutive failed SA attempts
  • the occurrence of 25 Losses of Capture between two consecutive days
  • ERI: the device will be set to ERI mode with VCC OFF
  • unipolar lead failure is detected

Programming of the sensitivity

The sensitivity values for the atrial and ventricular channels are independently programmable. The sensitivity can be programmed to Auto (adaptive) or to a fixed value.

 

  • Sensitivity screen for the Atrium

  • Sensitivity screen for the ventricle

 

Automatic Sensitivity Control

The Automatic Sensitivity Control (Auto) feature automatically measures the peak amplitude and adapts the sensing threshold automatically.

  • after every sensed event, the function starts the detection hold-off period (refractory period) and measures the highest peak amplitude
  • after detection hold-off period, the sensitivity is initially reduced to 50% of the measured peak
  • at the end of the phase duration, sensitivity is reduced to 25% of the measured peak and maintained until detection of the next event
  • whatever, the amplitude of the signal, the sensitivity can never go below the minimum threshold
  • the duration of the detection hold-off, the step duration and the minimum threshold are non programmable
  • the maximal sensitivity is non programmable: 2 mV in the ventricle and 0.2 mV (bipolar sensing) or 0.5 mV (unipolar sensing) in the atrium

 

Automatic Post Paced Sensitivity Control

  • following paced events, the Detection Hold-off period is extended slightly in all chambers to prevent oversensing of the paced complex
  • the step duration is twice that of sensed events to prevent T-wave oversensing