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.