Seventy-one year old man implanted with a single chamber ICD Marquis VR in the context of an ischemic cardiomyopathy.
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This tracing demonstrates the importance of programming of the detection zones for good discrimination. This tracing presents an episode of atrial fibrillation with rapid ventricular conduction. The discrimination algorithm works correctly at the beginning of the episode. Unfortunately, when the heart rate accelerates, the tachycardia is detected as VF. In the VF zone, no discrimination is made and the therapy is delivered inappropriately.
The morphology algorithm attempts to distinguish VT from SVT. It is based on the comparison of the VEGM of a suspected tachycardia with the stored VEGM of a normally conducted sinus beat. This criterion is used only for the initial detection and not redetection. Indeed, the lead polarizing effect of a shock induces a distortion of the VEGM morphology rendering this criterion inapplicable for redetection. The morphology is active within the SVT zone (which can cross the border of the VF zone).
There are some limitations of the morphology algorithm. To be accurate, the template should be frequently renewed. Classical pitfalls include :
1) template acquisition early after implantation while the lead matures
2) in patients with frequent ventricular extrasystole, the template can be erroneously based on an ectopic beat 3) template acquisition during a slow undetected VT.
Two detection zones are programmed; only the morphology discriminator is activated; the stability and sudden onset criteria are turned off; an episode of VF is diagnosed via the combined counter. As long as the tachycardia rate stays in the VT zone, the morphology algorithm delays the VT detection; when the tachycardia accelerates, the combined counter detects a VF. In this zone, the morphology algorithm is not used and the committed therapy is delivered.
