Discrimination in a single-chamber device

In a Medtronic^TM single-chamber ICD, various discrimination criteria can be programmed as standalone or in combination: sudden onset, stability of the tachycardia cycle length and analysis of the morphology of ventricular complexes (Wavelet).

The sudden onset criterion applies to the VT and FVT via VT zones but not to the VF or FVT via VF zones, during the initial detection phase but not during the redetection phase. This discrimination criterion is based on the principle that sinus tachycardia begins gradually while a ventricular tachycardia begins abruptly; a gradually accelerating tachycardia (unmet sudden onset criterion) is not considered by the device as a VT and is therefore not treated. In practice, the device compares the average duration of 4 consecutive intervals with the average duration of the 4 previous intervals. A minimum percentage of programmable acceleration between these two series of 4 intervals defines a sudden onset. As long as the onset is considered to be gradual, even if the duration of the interval corresponds to the VT zone, the sensing is delayed, the event is marked VS and the annotation "Sudden onset" appears. There are a number of limitations to this parameter: although the onset of ventricular tachycardia is typically abrupt, ventricular tachycardia can begin after a sinus tachycardia (VT occurring in a catecholinergic context) or accelerate progressively.

The stability criterion applies to VT and FVT via VT zones but not for VF or FVT via VF zones (same as for sudden onset) during the initial detection phase as well as during redetection (difference with the sudden onset criterion). This is the only discrimination criterion that applies to redetection. This discrimination criterion is based on the principle that atrioventricular conduction during an atrial fibrillation episode is generally irregular and generates an unstable ventricular rate, whereas an episode of ventricular tachycardia is most often associated with a stable ventricular rate; an irregular tachycardia (unmet stability criterion) is not considered as a VT by the device and is therefore not treated. In practice, the stability analysis begins when the VT counter reaches at least 3. The device compares the duration of a ventricular interval with the duration of the 3 previous intervals. When the duration of a ventricular interval varies from the 3 previous intervals by a difference greater than the programmed stability interval, the rhythm is classified as unstable, the interval is marked as VS, the VT counter is reset to 0 and the "Reset: Stability" notation appears. There are also a certain number of limitations to this parameter: the regular or irregular nature of the tachycardia is inadequately discriminant since a rapid atrial fibrillation (> 180 beats/minute) is typically regular while some ventricular tachycardias are polymorphic and irregular at the beginning of the episode.

The Wavelet (morphology analysis) algorithm only functions for cardiac rates below the programmed limit of supraventricular tachycardia (SVT) which is programmable independently of the tachycardia zones (can therefore operate in the VF zone if the SVT limit is included in the VF zone) and during initial detection. This discrimination criterion is based on the principle that a ventricular complex conducted during a supraventricular tachycardia most often has the same morphology as the complexes recorded in the absence of tachycardia; a tachycardia with ventricular complexes of similar morphology to that of a reference complex recorded in sinus rhythm is not considered by the device as a VT and is therefore not treated. In practice, the device compares the morphology of the 8 QRS complexes preceding the detection of the tachycardia with that of a reference morphology. If at least 6 of the last 8 complexes differ from the reference morphology, the device diagnoses a VT episode. Conversely, if at least 3 of the last 8 complexes are judged to be similar, the device diagnoses an episode of SVT and the VT counter is reset to 0. The matching threshold is programmable (40, 43, ..., 70, ..., 97%). The device uses EGM2 to define the reference and analyze the morphology during the tachycardia. The sampling of the reference signal is a necessary step for the proper functioning of this parameter and can be carried out at the time of the interrogation but also automatically and repeatedly by the device (Auto collection: ON with sampling every 17 minutes). The device then samples 6 "normal sinus rhythm" complexes and calculates a first average reference complex followed by sampling of an additional complex every 10 seconds over 700 seconds. The quality of the reference signal is confirmed by a comparison between the reference template and a waveform collected every 17 minutes. The comparative analysis of the morphology of the electrograms in sinus rhythm and in tachycardia sometimes allows a diagnosis of quasi-certainty. If there is no difference in the pattern of the ventriculogram between tachycardia and sinus rhythm, it is likely that the tachycardia is of supraventricular origin. Conversely, if the signals differ, it is not possible to assert the diagnosis since it can either be a ventricular tachycardia or a supraventricular tachycardia with aberrant conduction. Wavelet can be programmed initially in «Monitor» and only be activated after verification of its proper functioning.

When the 3 parameters are programmed to ON, the VT diagnosis thus requires a sudden onset (most often validated at the beginning of the episode) and stability of the ventricular intervals (throughout the tachycardia during initial detection and in redetection) and a change in the morphology of the QRS complexes (evaluated over the last 8 cycles before the counter is filled)