This tracing allows illustrating one of the limitations of the VT counter and emphasizes the importance of programming the lower limit of the VT zone and the number of intervals necessary for diagnosis. As explained previously, the specificities of the VT counter must be perfectly integrated to allow an optimal programming. The “up and down” counter was specifically developed to effectively detect episodes of monomorphic and regular ventricular tachycardia. An interval classified as VS does not reset the VT1 counter to 0 but decrements the latter by 1 which is very advantageous in the presence of a moderate ventricular undersensing which simply leads to a delay in diagnosis. On the other hand, as in this example, this functioning becomes problematic in patients with repeated episodes of non-sustained VT (when the number of VT1-classified intervals is greater than the VS intervals). The VT1 counter increments progressively, the first interval of the VT1 episode able to occur several tens of seconds before the diagnosis and not be visualized on the tracing (as in this example). This can be accompanied by a significant number of unnecessary therapies with a potentially pro-arrhythmogenic risk. In a patient with many non-sustained VT episodes, it is therefore necessary to increase the number of intervals necessary for diagnosis but, most of all, likely discuss the need to program a VT zone corresponding to the frequency of non-sustained VT episodes.

In the latest recommendations, there are marked differences in terms of advice for the programming of the sensing zone terminals according to the manufacturers. Differences in counter operation explain part of these differences.