ATP in the VT zone

A priority in the programming of an implantable defibrillator is to minimize the delivery of shocks as much as possible without compromising patient safety. Ideally, this entails terminating the tachycardia with the least aggressive and least painful treatment modality possible. Anti-tachycardia pacing is therefore favored as first-line therapy for organized tachycardias comparatively to electrical shocks, being less painful and lowers battery consumption and wear. Moreover, the deleterious effect of electrical shocks has been clearly demonstrated. The principle behind anti-tachycardia pacing is to capture the arrhythmia and interrupt an organized VT by penetrating its propagation circuit through the ventricles. The ventricle must hence be paced at a faster rate than that of the tachycardia. The efficacy of this type of therapy has been demonstrated for a wide range of VT rates up to 240 beats/minute and has consistently been shown to terminate nearly 90% of slow ventricular tachycardias with a rate of less than 200 beats/minute and a moderate risk (1 to 5%) of acceleration. These observations have repositioned the ICD as a first-line treatment of arrhythmias by rapid pacing with the possibility of defibrillation as "back-up" only as needed. 

Various parameters must be programmed:

1. the type of sequence: in a burst, the duration of the intervals is constant during a sequence (no change in rate from one stimulus to another). This is the type of sequence most commonly used in clinical practice and probably the least aggressive. According to the new guidelines, the burst should be preferred to other types of sequences. In ramp therapy, the interval is reduced from one stimulus to the other by the decrement value which is programmable. In ramp+ mode (specificity of this manufacturer), a pulse is added to each sequence, reducing the length of the intervals for the first 3 intervals and remaining constant thereafter;
2. the number of programmed sequences varies according to the rate of the tachycardia. In a slow VT zone (<150 beats/ minute), it is possible to program a large number of sequences so as to delay the delivery of a shock to a tachycardia that does not generally threaten short-term survival. It is also possible to not program an electrical shock in this slow VT zone. For tachycardias between 150 and 200 beats per minute, it is common to program 3 to 6 successive sequences of anti-tachycardia pacing;
3. the number of pulses per sequence: on average, 5 to 15 consecutive pulses are programmed for each burst. If the number is insufficient, the pacing sequence may not penetrate the tachycardia circuit and the burst is unsuccessful. On the other hand, if the number is too high, the risk is terminating and subsequently re-inducing the tachycardia. An additional stimulus can be systematically added from one sequence to another. According to the new recommendations, a minimum of 8 stimuli per sequence should be programmed;
4. the value of the coupling and pacing intervals: the shorter the coupling intervals, the more aggressive the therapy and the greater the risk of accelerating the tachycardia. According to the new recommendations, for a burst, an 88% coupling relative to the rate of the tachycardia (calculated over the last 4 cycles before diagnosis) must be programmed;
5. the minimum coupling allows limiting the aggressiveness of a pacing sequence; there is a programmable rate limit above which, irrespective of the programming, the device does not deliver pacing. When, for example, during a ramp, the minimum coupling is reached, the subsequent intervals are paced with this minimum coupling without additional decrement;
6. the pacing amplitude and the pulse duration can be programmed independently so as to promote effective capture during the tachycardia;
7. the pacing site(s) can be programmed; the pacing site is necessarily right ventricular in a single or dual-chamber ICD. In a triple-chamber ICD, pacing can be delivered in the RV, LV or biventricular. From a theoretical perspective, biventricular or left ventricular pacing appears to be superior in patients with left ventricular dysfunction, with the majority of tachycardias originating in the left ventricle (less distance between the reentrant/tachycardia circuit and the pacing site);
8. it is also possible to program the Smart Mode to turn off a therapy when it has proven ineffective. This algorithm deactivates an anti-tachycardia pacing sequence that has been unsuccessful for 4 consecutive episodes (varies according to platform). An additional option is the programming of the progressive therapy algorithm which ensures that each therapy delivered for a single episode is at least as aggressive as the previous therapy.