• these first 2 tracings (termination of a disorganized polymorphic arrhythmia by a shock, and termination of an organized ventricular arrhythmia by anti-tachycardia pacing) illustrate the basics of how the Boston ScientificTM defibrillator detection counters work.
• it is possible to program 1 (VF), 2 (VF + VT) or 3 (VF + VT + VT-1) detection zones; counter operation is identical for all 3 zones; initial detection requires a minimum ratio of 8 rapid cycles out of 10; this ratio then decreases to 6 out of 10 during the duration of detection; during post-charge confirmation, for a shock to be delivered, a ratio of 2 rapid cycles out of 3 is required; during re-detection or post-shock, identical counters apply (8/10 then 6/10); therefore during an episode, the same probabilistic counters (X/Y rapid cycles) are applied in both VT and VF zones.
• the first tracing showed an episode of VF; probabilistic counters seem to be most suitable for this type of arrhythmia; ventricular fibrillation is by definition rapid, disorganized and, chaotic, with ventricular signals of low and/or variable amplitude; all these characteristics increase the risk of undersensing due to low amplitude signals below the sensitivity threshold (nominal value of 0.6 mV); the high beat-to-beat variability of the amplitude may fool the sensing circuit, as the defibrillator uses a sensitivity level that adapts to the amplitude of the preceding signal; the first counter requires a minimum of 80% rapid cycles (criterion 8/10); this 80% ratio between fast and slow cycles has been chosen to achieve an optimum balance between correct detection of ventricular fibrillation (requiring tolerance of a certain number of pseudo long cycles generated by undersensing) and the need to avoid filling counters in the presence of T wave, P wave or R wave oversensing (frequently associated with a ratio of 50% short cycles), or sensing of frequent ventricular extrasystoles; this 80% ratio is not programmable, nor is the number of cycles (8 out of 10); once this first stage of detection has been achieved, the duration in the VF zone begins, with a lower requirement of 60% rapid cycles; the device considers that if the first stage criterion has been met (8/10), the probability of a true arrhythmia is high, and the priority is then to avoid being fooled by possible undersensing (hence a lower number of rapid cycles are required).
• the second tracing shows a regular, monomorphic ventricular tachycardia detected in the VT zone; for this type of arrhythmia, the operation of a probabilistic counter is also effective since sensing is generally correct and 100% of cycles are correctly sensed

This figure shows the different counters: initial detection of 8/10 with marker (V-Epsd), then initial duration with rolling window and 6/10 counter.