Stability is nominally programmed Off. To program this feature On, a Stability Interval must be selected. This defines, in milliseconds, the allowable variability of the ventricular rate in order for the device to determine the arrhythmia is stable (of ventricular origin) or unstable (the result of AF with rapid ventricular response).

To program this feature, go to Params -> Detection.

A small Stability value may not allow for normal VT interval variation and may decrease the sensitivity of the device to detect ventricular tachycardia.

Stability is an SVT Discriminator that applies in the VT Zone. The feature begins on the third consecutive VT event. The feature compares the current ventricular interval to each of the previous three VV intervals. The device classifies an interval as unstable if the difference between the current interval and any of the 3 previous intervals is greater than the programmed Stability interval. If the device classifies an interval as unstable, the system marks it as a sensed ventricular event and resets the VT event count to zero.

Considerations:

• The Stability feature operates throughout initial detection and redetection of VT and FVT via VT.
• If a patient has a true ventricular arrhythmia that is polymorphic in nature, the stability criterion may not work properly. In such a case, the device may perceive a polymorphic rhythm – actually a VT – as unstable, and may not apply therapy.

Evaluation of the Stability feature
The Data - Arrhythmia Episodes screen may help you assess the performance of the Stability feature.

1. An episode for which detection is withheld by the Stability feature is indicated by “SVT-Stability.”
2. In the Decision Channel of the EGM display, ventricular events on which detection was withheld by the Stability feature are indicated by “Reset: Stability".

The following example illustrates Stability operation when atrial fibrillation is conducted into the VT zone. No therapy is delivered as a result of Stability resetting the VT Counter.

1. Atrial fibrillation starts and is conducted into the ventricle at a rapid rate.
2. After 3 VT events, the device applies the Stability feature. Because the 360 ms interval differs from the 290 ms interval by more than the programmed Stability interval (50 ms, in this case), the device resets the VT event count.