4 DISCUSSION
Based on the variability of the VT cycle length, the three important findings of this study were as follows: first, ATP therapy produced a significantly higher termination rate in regular than irregular VTs. Second, spontaneous termination after ATP delivery or without therapy occurred significantly more frequently in irregular than in regular VT. Third, reproducibility of RR interval variability of VT was high, both per episode and per individual.
The mechanism and properties of tachycardia, substrate, local electrophysiology, and stimulation site affect the success of ATP attempts in terminating VTs.14 While it may be difficult to capture all of this information in an ICD, focusing on RR interval variability may be important to improve ATP treatment effectiveness. Three types of VT mechanisms are known—reentry, triggered activity, and automaticity. In general, the regular heart rate is found in VT due to the reentry mechanism, and the irregular heart rate is a finding of non-reentrant VT.15 According to previous reports, small changes in the VT cycle length suggested the increase in ATP effectiveness.16 That evaluation used the percentage of variation, which was calculated by dividing the mean difference between each RR interval and the next one by the VT cycle length. Furthermore, ATP therapy is more effective for VTs that have smaller ventricular beat-to-beat morphologic variation on intracardiac recordings than for those that do not.17 We evaluated the VT characteristics using a simple method, different from the one used in previous studies, to purely measure the variability of VT cycle length. In addition, ventricular tachyarrhythmia caused by a triggered activity or automaticity of the mechanism, can be difficult to terminate using programmed electrical stimulation with reproducibility. In contrast, VT caused by the reentrant mechanism can result in successful termination using programmed stimulation, without excluding triggered activity.18
The efficacy of ATP in these VTs may be explained by the fact that the VT mechanisms were based on reentry, and that VTs that demonstrate poor response to ATP therapy result from the lack of organized reentry.