Progression of therapies in the VT zone

Summary

• Episode classified in the VT zone
• 4 bursts + 4 ramps + 1 shock of 11 Joules

• During this episode, the first sequences of anti-tachycardia pacing fail to terminate the arrhythmia; as in the previous trace, anti-tachycardia pacing is then effective, but the tachycardia recurs before the end-of-episode marker, and successive therapies follow until a shock is delivered ; the programming of ramps following bursts is not systematic for this rate range since it is more aggressive with an increased risk of acceleration into a rapid polymorphic tachycardia
• Type of sequence : in a burst, the interval duration is constant during a sequence (no change in frequency from one stimulus to the next); in a ramp, the interval is reduced from one stimulus to the next by the value of the programmable decrement;
• The number of bursts/ramps programmed varies according to the rate of the tachycardia; it is possible to program 2 burst or ramp sequences per VT or VT-1 zone; in a slow VT zone (< 150 beats/minute), it is possible to program a large number of bursts and/or ramps, to delay as long as possible shock delivery to a tachycardia that does not generally threaten short-term survival; for tachycardias between 150 and 200 beats/minute, it is usual to program 2 to 6 bursts/ramps, even if the success rate beyond 3 or 4 bursts is very limited;
• The number of pulses per sequence: on average, 5 to 15 consecutive pacing impulses are programmed in each burst; if the number is too low, the stimulation sequence may not penetrate the tachycardia circuit and the burst is ineffective; on the other hand, if the number is too high, there is a risk of terminating and then re-inducing the tachycardia; an additional stimulus can be added systematically from one sequence to the next; according to the guidelines, a minimum number of 8 stimuli per sequence should be programmed;
• The value of coupling and pacing intervals: the shorter the coupling, the more aggressive the therapy and the greater the risk of accelerating the tachycardia; according to the guidelines, for a burst, a coupling of 88% in relation to the rate of the tachycardia should be programmed; for Boston Scientific defibrillators, the coupling interval (interval between the last tachycardia cycle and the first pacing cycle) and the pacing interval (interval between different paced impulses) are independently programmable; the percentage is calculated in relation to the average of the 4 intervals preceding therapy;
• Minimum coupling limits the aggressiveness of a stimulation sequence; there is a programmable rate limit above which, regardless of programming, the device will not deliver stimulation; when, during a ramp for example, the minimum coupling interval is reached, subsequent cycles are paced with this minimum coupling interval without further decrementing;
• Stimulation amplitude and pulse duration can be programmed to favor effective capture during tachycardia (5 Volts/1 ms);
• Maximum duration for ATP : this defines the time after which ATP sequences will be interrupted to deliver a shock; the idea is not to delay the onset of the first shock by too many ATP sequences.