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Wavelet algorithm
It is based on the principle that rhythms of ventricular origin such as ventricular fibrillation (VF) and VT have normally QRS morphology different from rhythms of supraventricular origin. Wavelet is an electrogram morphology algorithm with simple on-off programming (when it is programmed on, ICD inhibits the identification if a sufficient number of QRS during tachycardia are similar to those of stored sample). It automatically compares the electrogram (EGM) morphology during high rate rhythms to a stored template of normal (baseline) rhythm. Wavelet delivers fewer inappropriate therapies for SVT (compared to EGM width) and has been shown to increase specificity without decreasing sensitivity. Wavelet adapts to changes in electrogram morphology and updates it automatically to adapt to changes in the morphology of normal beats.
Onset algorithm
Onset permits to prevent identification of a sinus tachycardia instead of VT examining the acceleration of ventricular rate. If ventricular rate gradually increases, for instance during sinus tachycardia, the device inhibits the counting of VT events. If the ventricular rate rapidly increases, so as during a true tachyarrhythmias, then the device activates counting of VT events.
Stability algorithm
It permits to discriminate atrial fibrillation with a rapid conduction from VT because during AF the ventricular rate is unstable, in fact there is a high variability of RR interval. An RR interval is considered unstable if the difference between its value and one of three previous intervals is greater than the programmed stability interval. For dual-chamber and biventricular devices utilization of other algorithms (Medtronic - PR Logic; Guidant- Rhythm ID; St’Jude Medical - A-V Rate Branch; Biotronik - Smart Detection)makes possible to discriminate VT from SVT. These algorithms logically integrate rate detection with information about conduction patterns, regularity, and AV dissociation. They are designed to maintain high sensitivity for ventricular tachyarrhythmias while improving discrimination for SVTs. Distinction criteria inhibit the inappropriate ventricular recognition during SVT with rapid conduction. The device analyzes activation patterns and temporization in both chambers analyzing P wave and R wave sequences and rates. These data allows to detect AF, atrial flutter, sinus tachycardia and other SVT with 1:1 conduction. When testing shows presence of one of these rhythms, recognition is inhibited.