This patient presented with VT episodes, the rate of which oscillated around the lower limit of the tachycardia zone. The onset of the tachycardia is not visible, as it was slightly slower than the programmed zone. Consequently, the choice of rate of the VT zone has a direct influence on the quality of discrimination. The sudden onset criterion, designed to discriminate sinus tachycardia, prevents the treatment of tachycardias that accelerate gradually, on the principle that sinus tachycardia accelerates gradually, as opposed to VT that starts suddenly. This criterion is applied in initial classifications, though not in reclassifications. It may be the source of erroneous diagnosis in 2 types of patients: 1) those who develop VT upon exercise, where a clear demarcation between the rate of sinus tachycardia immediately preceding the VT and the VT rate is absent, and 2) those in whom, as illustrated here, the rate of the VT oscillates around the lower limit of the VT zone. These examples illustrate a) the limits of the use of a single criterion to accurately discriminate tachycardias, and b) the need to associate criteria.

In this patient, a change in the lower limit of the VT1 zone (to 400 ms) enabled an accurate classification and discrimination of the episodes and appropriate delivery of therapies.