SecureSense™ algorithm and timing difference between the 2 channels
Tracing
Manufacturer Abbott
Device ICD
Field Securesence
N° 12
Patient
This 35-year-old man received a Abbott Ellipse™ DR dual chamber defibrillator for the management of an arrhythmogenic RV dysplasia with right bundle branch block and ventricular extrasystoles. Episodes of non-sustained ventricular oversensing were stored in the device memories.
Graph and trace
- the presence of VS2 markers on the discrimination channel indicates that the noise counter was activated and previously incremented;
- ventricular bigeminy accurately sensed on the bipolar and on the discrimination channel; premature ventricular contraction systematically classified (-), as the instantaneous cycle was classified as short in the bipolar channel, whereas the average of 4 cycles fits in the sinus zone (discordance between instantaneous and average intervals); the noise counter increases with each short cycle on the bipolar channel; the tracing does not show the interval between the 2 cycles classified VS2 (between the sinus QRS complex and the PVC). The QRS complex seems to be detected first on the discrimination channel, which is coherent in view of the right bundle branch block (delayed sensing on the bipolar channel); in contrast, the PVC originating from the right ventricle (arrhythmogenic RV dysplasia) is sensed first on the bipolar channel (signal less prominent on the discrimination channel). The interval between these 2 signals is, therefore, shorter on the bipolar than on the discrimination channel;
- the noise counter is at 10 whereas the VF or VT counter is not filled. Non-sustained ventricular oversensing (SNS) was diagnosed.
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This unusual tracing may be observed when the SecureSense™ algorithm is programmed, with different sensing circuits on the 2 channels. In presence of premature ventricular contractions, the noise counter should be reset to 0 each time 2 short cycles have been detected on the discrimination channel. Some sensing delay of the signals may be present, however, between the 2 channels, which may cause a difference of classification, since a cycle may be considered short on the bipolar channel (increment of the noise counter), however long on the discrimination channel (no resetting to 0 of the counter). In this patient, the slower zone of tachycardia was programmed at 420 ms. A bigeminy and a coupling of the premature ventricular contraction were close to that slow zone limit (between 390 and 430 ms) with a (-) marker corresponding to a short cycle when the instantaneous interval was shorter than 420 ms. The limit defining a short cycle on the discrimination channel is 450 ms (420 + 30). The interpretation of the algorithm function is complicated by the absence of indication of instantaneous intervals on the discrimination channel and by presence of a single VS2 marker, which does not allow the differentiation of short from long cycles. The sinus QRS complex is sensed first on the discrimination channel, with a prominent delay on the bipolar channel due to the right bundle branch block (delayed activation of the right ventricle, which contains the lead). Conversely, the premature ventricular contractions originate from the right ventricle and are sensed early on the bipolar channel, and later, on the discrimination channel, with also a difference of a few dozens of ms. Ultimately, the difference in the timing of the sensing of these 2 signals explains that the cycles are classified as short (<420 ms) on the bipolar channel and long (>450 ms) on the discrimination channel leading to the recording of an episode classified false positively as oversensing by the device.