Lesson

Ventricular sensitivity control

1. Principles

The purpose of the Automatic Ventricular Sensitivity Control function is to improve the detection of Ventricular Fibrillation (VF): a high sensitivity of the device must be applied during VFs since most VF signals are of low amplitude (micro-voltage / amplitude less than 3 mV), and to avoid sensing of the T wave both in sinus rhythm and during arrhythmias.

One of the basic postulates for improving VF detection during VFs is that the signal amplitude is much lower than in sinus rhythm and the difference in amplitude from one beat to the next can reach 50% (+ or -).

Another of the postulates is that, to avoid the sensing of T waves, the amplitude of the filtered T waves is generally less than that of the R waves (the amplitude of the T wave is of the order of 25% of the R amplitude). Pre-clinical studies have shown that the maximum amplitude of these T waves is less than 25% of the R waves after filtering.

 

2. Filtering

The system used in pacemakers and cardiac defibrillators for the selection or non-selection of a signal is based on filtering techniques. All devices use amplifiers that are defined to detect events whose rate varies between an upper limit and a lower limit: the bandwidth. Any event whose rate is outside these limits will not be taken into account by the device. For MicroPort defibrillators, the ICD bandwidth is 25 to 80 Hz (at -3 dB), and is centered at 55 Hz.

Filtering thus allows discriminating T waves, R waves or PACs.

On the other hand, given that the filter limits are not completely vertical, it is possible that the circuit detects events outside its bandwidth, if their amplitude is sufficiently significant. A classic but rare example of this type of oversensing is the sensing and recognition by the defibrillator of the T wave as being an R wave (recycling on the T wave).

 

3. Ventricular detections

For the Automatic Sensitivity Control function, only R waves are filtered and adjusted before being analysed for detection. After filtering, the T waves are attenuated and the signal obtained is rendered positive, in order to detect positive and negative slopes.

At first, a non-programmable absolute refractory period of 95 ms is applied after the sensing of the R-wave.

During the first 64 ms of this period, the implant still measures the amplitude of the R wave. The maximum amplitude of the R waves that the implant can measure is capped at 6 mV for this function.

For the ongoing cycle, the sensitivity will change as a function of the amplitude of the R wave measured in the first 64 ms.  It can never go below the programmed sensitivity.

During the 95-156 ms interval following the sensing of the R wave:

  • If the amplitude of the sensed R wave was greater than 6mV, a sensitivity of 3 mV is applied (3 mV = 50% of 6 mV),
  • If the amplitude of the sensed R wave was less than 1.6mV, a sensitivity of 0.4 mV (at the minimum programmed sensitivity) is applied (0.4 mV = 25% of 1.6 mV),
  • Between 1.6 and 6 mV, the applied sensitivity is between 25 and 50% of the measured R-wave (at the minimum programmed sensitivity). The sensitivity is calculated linearly in this interval: [1.6mV; 6mV] è [25%; 50%].

 

These settings allow avoiding the multiple sensing of wide R waves and to sense the R waves in VF.

In the 156-500 interval ms following the sensing of the R wave, the sensitivity is set at 25% of the amplitude of the measured R wave (at the minimum programmed sensitivity). This setting was implemented to reject the T waves.

The following table illustrates how the device sets the sensitivity value for the 2 thresholds proportionally to the amplitude of the measured R wave. The greater the amplitude of the R wave, the lower the sensitivity of the device (i.e. high detection value).

 

Thus, during much of the cycle (after 156 ms), ventricular sensitivity is comprised between 1.5 mV and the programmed value in the event of a complex measured at an amplitude greater than 6 mV.

After 500 ms following the sensing of the R wave, and until the end of the cycle, the sensitivity returns to the programmed value or to 1mV (not configurable) according to certain conditions.

If all of the following conditions are fulfilled:

  • the programmed ventricular sensitivity value is less than 1 mV, and,
  • the device does not detect a short cycle (VF/Fast VT zone) over the last six cycles, and,
  • the measured amplitude is greater than or equal to 4 mV

then, the V detection threshold is set at 1 mV after the second threshold; i.e. after 500 ms.

In sinus rhythm and in the VT zone, the lowest detection limit will be 1 mV and will cover any background noise (myopotential). During this period, the V sensitivity will mostly be 1 mV.

In case of a short coupling interval or weak detection, the lowest detection threshold will decrease to the programmed sensitivity. During this period, if necessary, the sensitivity will be the programmed sensitivity (0.4 mV nominal).

If all of the following conditions are fulfilled:

  • the device detects at least one short cycle (VF/Fast VT zone) over the last six cycles, or,
  • the measured amplitude is less than 4 mV (not applicable for the 3rd diagram)

then, the V detection threshold is set to the programmed sensitivity after the second threshold; i.e. after 500 ms.

4. Ventricular pacing

After ventricular pacing, there is an “automatic” management of the sensitivity. At first, an absolute non-programmable 220 msec refractory period is applied after the sensing of the R wave. This is the “V margin Post-V pacing”. This is a value added to the programmed ventricular sensitivity for the period between 220 ms and 500 ms post-ventricular pacing. The goal is to “cover” broader T-waves in the case of ventricular pacing in general. After this 500 ms period, ventricular sensitivity automatically changes according to certain conditions.

If all of the following conditions are fulfilled:

  • the programmed ventricular sensitivity value is less than 1 mV, and,
  • there is no short cycle (Fast VT/VF zone) over the last six cycles,

then, the V detection threshold is set at 1 mV after 500 ms.

If there is at least one short cycle (Fast VT/VF zone) over the last six cycles, then the V detection threshold is set to the programmed sensitivity after 500 ms.

5. Programming- CHECK

The Automatic Sensitivity Control is fully automatic. In case of difficulty linked to ventricular detection, the only parameter that can be adjusted is ventricular sensitivity.

There is only one possible setting in the case of ventricular pacing. This is the ” V margin post-V pacing” parameter. See the chapter on automatic management of post-ventricular pacing sensitivity. You will need to open the “expert parameters” and choose “A and V Detection”. The “V Margin post-V pacing” parameter is then accessible. By default, knowing that its value is 0.8 mV and the ventricular sensitivity is 0.4 mV, the applied post-ventricular pacing ventricular sensitivity for the period from 220 ms to 500 ms is 1.2 mV.

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