Medtronic Discrimination

The discriminating criteria offered by single chamber ICD include sudden onset, stability of the tachycardia cycle length and morphology of the ventricular EGM.

The « sudden onset » criterion withholds therapies for tachycardia of gradual onset, and is typically used to prevent inappropriate therapies for sinus tachycardia. The « stability » criterion limits the delivery of therapy for irregular tachyarrhythmias in the VT zone and is often used to prevent inappropriate therapy for atrial fibrillation with a rapid ventricular rate, which is usually irregular, whereas it is usually regular during VT. “Morphology” is a matching criterion that compares the EGM during tachycardia with that previously recorded during intrinsic rhythm. It is based on the principle that the ventricular EGM should be identical during a rapidly conducted SVT and during sinus rhythm. It compares dynamically ventricular EGM recorded during all episodes of tachycardia in the VT zone with a template previously recorded. If both EGM are classified as similar, the arrhythmia is diagnosed as “supraventricular”.

Onset monitor : the device performs all the calculations associated with “onset” though does not delay the detection.
Note : the onset criterion applies to VT and FVT via VT during the initial detection ONLY, and NOT in redetections.

At onset in the VT zone, the device measures each interval. If the onset is detected as gradual, a) the intervals are classified as VS, b) a “gradual onset” notation appears, and c) the detection is withheld.

Note: the stability criterion applies to VT and FVT via VT during initial detection and redetection.
When the VT event counter reaches 3, the stability criterion is fulfilled.

If the event is classified unstable, a) it is marked as a normal sensed event (VS), b) the VT event counter is reset to 0 and starts ‘Re-Counting’, and c) a "Stability" notation appears.

If ≥6 of the last 8 QRS do not match the stored template, VT is detected; if ≥3 of the last 8 QRS match the stored template, the VT event counter is reset to 0 and therapy is withheld.

Programmable settings : On, Off, Monitor
Match threshold : 40, 43, …, 70, …, 97%
Auto sampling : On (sampling every 17 min), Off

Wavelet rules :

1. Template sampling and conditions
   The template can be automatically sampled:

• Sampling of 6 normal sinus rhythm complexes and creation of a template based on their average. The automatic template is validated by a comparison of the 6 waveforms among each other (at least 4 out of 6 must match)
• Template confirmation : one waveform is sampled every 10 s and compared to the stored template over 700 s. If ≥70 out of 100 waveforms match the template, it is confirmed.
• Regular verification of the template quality : samples a waveform every 17 min (~1000 s) and compares to the stored template.

2. Wavelet sampling trigger
   Wavelet starts to sample if:

• The VT event count = 2
• The VF event count = 3
  Wavelet begins its comparison with the stored template 2 events later.

3. Wavelet interruption
   Episode detection:

•  ≥6 of the last 8 waveforms do not match to the stored template
• High rate time out
  Episode end : rate >VT interval

Wavelet applies only when the median V-V interval is ≥SVT limit. Wavelet uses EGM 2 : HVA to HVB (Can to RV coil) source. If the printed peak-to-peak amplitude of the R wave is <3 mV, a different EGM 2 source must be chosen. If the base-to-peak amplitude is >6 mV, consider increasing the EGM 2 range.