- the sensed ventricular refractory period; this is not a real blanking period since the device is searching for the maximal amplitude of the signal after the sensing threshold has been reached; this refractory period has to be short (2 programmable values, 125 or 157 ms) to prevent the multiple sensing of a same depolarization signal, while preserving the ability to detect a very rapid tachycardia.
- the threshold start: upon expiration of the sensed refractory period, the sensing threshold automatically adjusts to a percentage of the maximum amplitude of the sensed signal with an absolute maximum value of 6 mV in the ventricle and minimal value of 1 mV. The post-sensed threshold start can be set at 50, 62.5, 75 or 100%. If the amplitude of the ventricular signal is > 6 mV, the threshold start is 6 mV if 100% or 3 mV if 50%. If the amplitude of the signal is < 1 mV, the threshold start is 1 mV.
- the decay delay: this parameter determines the amount of time after the sensed refractory period that the threshold remains at the programmed setting before beginning its decay to the programmed sensitivity level. The decay delay is programmable at 0, 30, 60, 95, 125, 190 or 220 ms.
- the maximum sensitivity
- low frequency attenuation : the advanced settings button enables to turn the Low frequency attenuation filter On or Off.
This figure shows the sensing circuit with a threshold start programmed at 50% and a decay delay at 0 ms.
- Threshold Start
- Decay Delay of 0 ms
- Decay Delay of 60 ms
- LVA on: ventricular sensitivity 0.5 mV, threshold start 50%, decay delay 0 ms
- LVA off: ventricular sensitivity 0.3 mV, threshold start 62.5%, decay delay 60 ms
The choice of ranges of heart rates prompting interventions by a defibrillator is a critical programming step. State-of-the-art defibrillators enable the programming of several zones of arrhythmia detection based on the sensed RR intervals. Each zone has its own rate and duration criteria and can be specifically programmed with a view to identify and treat arrhythmias. The Detection Interval/Rate parameter is the interval/rate that must be exceeded within each rate zone to be counted toward detection of a tachyarrhythmia. Faster rates (in bpm) mean shorter cycle lengths (in milliseconds). The Detection Interval/Rate is independently programmable in each rate zone. Each detection interval must be at least 30 ms longer than the next fastest detection interval.
In patients presenting with complete atrio-ventricular block:
In young and active patients:
In patients presenting with Brugada syndrome:
Counting the cycles
To bin an interval, the ICD needs to know how to classify it (VF, VT-1, VT-2, NSR [normal sinus rhythm]). If the average interval = the current interval, the interval is binned in that zone. If either the average interval or the current interval is classified as NSR, the interval is discarded. If the average interval and the current interval are different (but not NSR), the interval is binned in the higher zone.
- Slow VT counter (VT-1)
- Fast VT counter (VT-2)
- VF counter (VF)
- Return to sinus rhythm counter
The first counter reaching the number of programmed cycles determines the detection, the type of event and, perhaps, the therapy(ies) delivered. These cycles are not necessarily consecutive. The same applies to post-therapy redetection, which has its own, separate counters.
In this example, the device is set up in three zones, all bins set to 12 intervals. The 12th T is circled; that is the point where VT is diagnosed.