Seventy-two year old man implanted with a single chamber ICD Marquis VR in the context of a non-ischemic dilated cardiomyopathy presented for routine follow-up.
Discriminators are used in both the VT and the FVT « via VT » zones but cannot be used in the VF or the FVT « via VF » zones.
The stability criterion uses the inherent variability of the RR interval in AF to distinguish it from VT, which is often regular. That criterion is used for both the initial detection and the redetection of the arrhythmia.
The stability criterion presents some limitations. Some polymorphic VTs are irregular. Some VTs are irregular at their beginning and then stabilize only after a couple of beats. If the stability criterion is not met, the VT counter is reset to 0, which temporarily delays detection and therapy. Finally, some episodes of atrial fibrillation present with a fast regular ventricular response. In those patients, the ICD might erroneously consider the tachycardia as a VT instead of a SVT. In the present case, the stability is programmed to 30 ms; which is just below the nominal value (40 ms). Programming a lower threshold value will make the ICD more specific.
In the present case, the final diagnosis remains difficult. However, the Ventricular EGM is very similar to the template. As a consequence, we believe this tachycardia is likely a SVT.
3 detection zones are programmed : VF zone, Fast-VT via VT zone, VT zone. Discriminators are used only in the VT and FVT zones. The 3 discriminators are activated. The morphology algorithm indicates that the tachycardia is probably a SVT, 7 of the 8 VEGM analyzed match the template with a degree of similitude >70% (programmed value).
