Automatic atrial threshold testing (Right Atrial Autothreshold, RAAT)

Right Atrial Autothreshold (RAAT)

The device selects one of the following 2 methods, P test and AR test, to evaluate atrial capture based on the patient’s rhythm at the time of the pacing threshold search

  • RAAT is launched every night at 1:00 am
  • a selection phase is required to identify which test will be used
  • P test runs during normal stable sinus rhythm; atrial threshold is measured by increasing the pacing voltage
  • AR test runs when stable 1:1 AV conduction is observed with atrial pacing; atrial threshold is measured by decreasing the pacing voltage

 

Selection phase

 

P test

The P test is launched if the device has detected 6 PP intervals (7 P waves) over the last 8 cycles and the PP intervals are stable (<16 ms).

  • overdriving of the atrial rate
  • progressive increase of the atrial pacing amplitude (steps of 0.25 V)
  • the diagnosis of atrial capture/non capture is based on the absence/presence of a sensed atrial signal following atrial pacing
  • if there is one As after Ap: no atrial capture
  • if there is no As after Ap: atrial capture

 

 

In order to check if the atrial spike is efficient (capture), the device calculates the PP stability window (PP average +/- 8 ms).

For each tested amplitude:

  • the device senses 3 atrial signals that have to be within the stability window
  • it delivers atrial pacing (spontaneous atrial rate – 200 ms to overdrive sinus rhythm)
  • if an atrial signal is detected within the stability window: diagnosis of non capture and increase of the amplitude by 0.25 V steps
  • if no atrial signal is detected within the stability window: diagnosis of capture; if a second capture occurs at the same amplitude, the test value corresponds to the atrial threshold

 

AR test

The device verifies the presence of a stable 1:1 AV conduction by prolonging the AV delay (450 ms) during 10 cycles (2 first cycles of transition).

  • progressive decrease in the atrial pacing amplitude
  • the diagnosis of atrial capture/non capture is based on the absence/presence of a sensed ventricular signal following atrial pacing
  • if there is one R wave (Vs) after Ap: capture
  • if there is no R wave (Vs) after Ap: no capture (atrial back-up pacing)

 

In order to check if the atrial spike is efficient (capture), the device calculates the Ap-Vs stability window (Ap-Vs average +/- 50 ms).

For each tested amplitude:

  • decrease of the amplitude by 0.25 V steps
  • the device delivers 3 control spikes (at 2 V or 5 V) + 1 test spike (tested amplitude)
  • if a ventricular signal is detected within the stability window: capture
  • if a ventricular signal is not detected within the stability window: non capture; atrial backup pacing at the end of the stability window; second test at the same amplitude; if non efficient: atrial threshold previous tested value

 

Adjustment of the amplitude

  • the minimum atrial amplitude is programmable
  • automatic adjustment of the atrial pacing amplitude: measured atrial threshold + 1 V
  • if the measured threshold is > 3 V, the amplitude is automatically programmed to 5 V and RAAT is switched to OFF
  • if RAAT cannot be measured, the device takes the same value as the day before
  • if RAAT cannot be measured during more than 7 consecutive days, the atrial amplitude is programmed to the Safety atrial amplitude (programmable, nominal 3.5 V)

 

Programmable parameters

  • RAAT: Auto, “Monitor”, Off
  • minimum atrial amplitude: 1.0 – 5 – 2.0 – 2.5 V
  • safety atrial amplitude: 2.5 – 5 – 4.0 – 5.0 V
  • atrial autothreshold max rate: 75 – 80 – 85 – 90 – 95 – 100 – 110 min-1

Automatic ventricular threshold testing (Ventricular Autothreshold)

The Ventricular Autothreshold function is available in all pacing modes, which provide ventricular pacing except in SafeR mode.

  • periodical measurement (6 hours, 4 times a day) of the ventricular pacing threshold
  • automatic adjustment of the ventricular pacing amplitude to provide a 100% safety margin for the next 6 hours
  • no beat to beat verification of capture efficiency
  • a minimum ventricular amplitude is programmable

 

Periodical measurement of the ventricular threshold

The automatic threshold test sequence is automatically launched:

  • every 6 hours
  • after a ventricular manual threshold test
  • after reprogramming of the auto threshold function from OFF to “MONITOR” or AUTO
  • after a magnet test
  • after programming of the V pulse width and V pacing polarity

 

Waiting phase

8 cardiac cycles are delivered at the current pacing rate.

  • ventricular amplitude programmed at 5 V (programmed pulse width)
  • the calibration phase can start if there is efficient ventricular capture at 5V and if the ventricular rate is < 95 bpm

 

Calibration phase

The calibration phase is required to demonstrate an appropriate differentiation of the evoked potential response (EPR) and of the post-pacing polarization of the electrode. The signal is analyzed in a window of 64 ms following ventricular pacing.

  • capture: post-pacing polarization+ evoked response
  • non capture: post-pacing polarization

To reduce the risk of fusion beats during the calibration phase and the threshold test:

  • the AV delay is shortened by 65 ms in DDD or VDD
  • the escape interval is shortened by 65 ms in VVI, DDI and fallback mode switch

During the calibration phase, different pacing amplitudes are delivered:

  • 3 ventricular stimuli at 4 V (programmed pulse width): first stimulus to avoid Wedensky effect; calibration on the 2 following spikes with evaluation of the signal in a 65 ms window; average of the 2 measurements
  • 3 ventricular stimuli at 2 V + safety pulse at 2.5 V/1 ms: first stimulus to avoid Wedensky effect; calibration on the 2 following spikes with evaluation of the signal in a 65 ms window; average of the 2 measurements
  • comparison of the average measurements at 4V and 2V to demonstrate similar evoked response
  • 3 ventricular stimuli at “0” V + safety pulse: to better detect possible fusions beats; if the device detects a signal after a 0 V stimulus, diagnosis of fusion beat; a second calibration is performed

 

The calibration phase fails when:

  • diagnosis of fusion beats (even after the second calibration)
  • excessive polarization (poor ratio evoked response/polarization)
  • pacing threshold higher than 2 V

If the calibration phase fails, the ventricular amplitude is set to 5 V and test restarts 6 hours later.

 

Threshold test

The threshold test is performed at the programmed pulse width and pacing polarity.

  • steps of 0.2 V
  • threshold test stops after a loss of capture or at the amplitude of 0.2 V
  • after a loss of capture, a safety pulse is delivered after 65 ms (amplitude 2.5 V, pulse width 1 ms)

 

Adjustment of the amplitude

  • the minimum ventricular amplitude is 1.5 V (programmable)
  • automatic adjustment of the ventricular pacing amplitude to provide a 100% safety margin for the next 6 hours (measured threshold x 2)

 

Programmable parameters

  • AutoThreshold: Auto, “Monitor”, Off

 

  • minimum ventricular amplitude (Vmin): 1.5 V to 5.0 V (nominal: 2.5V)

 

Programming of the sensitivity

The value of sensitivity in both the atrium and the ventricle can be set at a fixed value or can be programmed on Autosensing. Autosensing is different than the adaptive sensitivity used in implantable defibrillators.

 Autosensing

The atrial and ventricular sensed signals are permanently monitored. The amplitude of the signals is averaged over rolling windows of 8 cycles.

  • average of the 8 previous complexes
  • automatic adaptation of the sensitivity threshold in both cavities
  • sensitivity automatically programmed at approximately 1/3 of the averaged amplitude

 

Atrial Autosensing

A bipolar atrial lead is required for atrial Autosensing.

  • following a premature atrial contraction (atrial detection in the WARAD), the device enters in AA suspicion phase and the atrial sensitivity is immediately set to 0.4 mV to optimize atrial sensing (onset of an atrial arrhythmia with low amplitude signals)
  • if the premature atrial contraction is isolated the atrial sensitivity is progressively adjusted to the reference value
  • atrial sensitivity is also forced to 0.4 mV, when the atrium is paced and during Fallback mode

 

Ventricular Autosensing

  • the ventricular sensitivity is forced to 1.5 mV (bipolar configuration) or to 2.0 mV (unipolar configuration) when the ventricle is paced
  • the values of 1.5 mV (bipolar) and 2.0 mV (unipolar) are not programmable