The initial detection of an arrhythmia is based on frequency and duration criteria. The notion of frequency is essential but too simplistic to differentiate ventricular rhythm disorders from supraventricular arrhythmias: when considered individually, the sole management of this criterion yields a sensitivity of 100% but a specificity of only 60%. The improvement in sensing is based on discrimination, which is the ability of an ICD to specify the supraventricular or ventricular origin of a tachycardia episode based on the characteristics of the arrhythmia such as the stability of the RR intervals, the onset mode, the state of the atrium-ventricle (AV) association, all of which are programmed separately or in combination. Discrimination algorithms are only used in VT zones where the sensing of an episode must be confirmed by the VT/SVT discriminators in order for the programmed therapies to be delivered. In the VF zone, the safety criterion prevails and the sensing of a high ventricular rate entering the « sensed VF » zone by the ICD triggers the programmed therapies without the episode being « filtered » by the discrimination algorithms. Discrimination algorithms should only be programmed in patients who are likely to have supraventricular tachycardia and should therefore be deactivated in patients with complete and permanent atrioventricular block.

A single-chamber ICD cannot analyze the atrial rate. Up until the very latest generation of Biotronik^TM single-chamber devices, discriminating the origin of the arrhythmias was based on the analysis of the onset (sudden or progressive) of the tachycardia and on the stability analysis of the rhythm (stable or unstable). None of these 2 parameters allows a perfect discrimination of all tachycardias. Specificity improves when the latter are associated. Depending on the setting, each parameter can be rendered more or less sensitive and more or less specific. In this example, the sudden onset and the regularity of the rhythm are suggestive of VT. Similarly, the variation in the morphology of the ventriculograms (not integrated in the discrimination algorithm of the device) is also suggestive of a VT.

The sudden onset criterion prevents the treatment of tachycardias with a progressive acceleration while enabling the discrimination of sinus tachycardias. It is based on the principle that a sinus tachycardia accelerates progressively as opposed to a ventricular tachycardia which begins suddenly. On the other hand, this parameter does not allow the discrimination of atrial fibrillation or flutter from ventricular tachycardia. To search for the presence of a sudden onset, the average of the last four RR intervals is compared with the average of the next 4 RR intervals. The calculation is therefore carried out over 8 sliding intervals. The criterion is fulfilled when the difference between these two averages is greater than the programmed « sudden onset » value.  The programmed default value is 20%. When the sudden onset criterion is verified, it remains valid for the entire episode including during redetection.

The stability criterion theoretically allows discriminating between atrial fibrillation (usually irregular) and ventricular tachycardia (usually regular). On the other hand, this parameter does not discriminate sinus tachycardia, atrial tachycardia or atrial flutter, from ventricular tachycardia. The stability criterion is fulfilled when, for a ventricular cycle, the difference between the RR interval and one of the 3 preceding RR intervals is less than the programmed value: the rhythm is considered as stable. The default value is 24 ms. The stability value can be programmed differentially in the VT1 and VT2 zones. The stability criterion is used during the detection and redetection phases.