This tracing is simple and illustrates the basic function of a defibrillator: the termination of a ventricular fibrillation by shock.

The summary of the initial interrogation screen reveals the proper functioning of the defibrillation lead, with a new battery (output voltage of 3.23V). A FVT/VF episode was detected, treated by shock, which was effective. A shock impedance value of 43 Ohm is provided, normal.

The tachogram indicates a progressive acceleration of the slow rhythm, suggesting an exertion or an emotion, followed by the triggering of a tachycardia in the VF zone, diagnosed as VF, which continues to accelerate and culminating in a maximum shock at 42.1 J, and a return to slow rhythm (SR).

The EGM displayed at a scrolling speed of 12.5 mm/sec, offers a complete view of the event, while scrolling at 25mm/sec allows an accurate reading.

1. After 6 cycles in the VF zone (no exclusion of the first cycles in this zone at the beginning of the tachycardia, unlike the VT zone where the first two cycles are excluded from the counting), the VF is diagnosed. A persistence of 10 cycles is triggered, at the end of which the VF rate is 223 BPM. This VF is therefore classified in the FVT section of the VF zone. Normally, it is a Ramp & Scan that is called if the tachycardia is stable (within 30 ms).
2. However, it is a maximum shock that is called and not a Ramp & Scan, since at the end of the persistence,

• the tachycardia rate is well below 240/min (223 bpm),
• however the tachycardia rhythm is unstable given the programmed stability of 30 ms in absolute value, (the last RR cycles being: 265, 250, 281, 273, 281, 296, 257 and 242 ms),
• moreover, among the last 4 cycles (281, 296, 257 and 242 ms), one is in the VF zone (< 250 ms)

3. and in this case, the Ramp & Scan is not applied.
4. The charge of the capacitors begins.
5. During this time, the tachycardia accelerates to 295/min, for which a VF/FVT shock 1 of 36.8J is applied (information obtained from the analysis function), after a charge at 42.1J, illustrating the difference between the charged energy and the applied energy.
6. The return to slow rhythm is assured at the cost of a few premature contractions.
7. The episode is terminated.

Of note: The reading of certain EGMs can sometimes be confusing since calculation of the rates based on these tracings is not easy, due to a 8-ms step of the device’s digital circuit clock, as well as the inaccuracy of the value of the displayed zone rates which is rounded. There is a difference between what is displayed in the EGMs and the actual cycle values used by the device. This is far from facilitating the understanding of its functioning in certain cases! Only the reading of the « Analysis » tables of the EGMs provides an exact reflection of the logic and the functioning of the system with regard to heart rates, in particular when the tachycardias are in the vicinity of the boundaries of the programmed zones.

Comments

1. The counting of events in the VF zone and the applied therapy are exactly the same, regardless of the implanted model, whether single-, dual- or triple-chamber. Only the rate criterion is used for diagnosis, and only a shock is delivered, without prior ATP since the tachycardia rate is within the FVT zone, 190- 240/min, but unstable.
2. In this patient, the defibrillation lead is of the dual coil type, and the shock vector travels from the RV anode to the device case and the SVC electrode. Note that the shock polarity is programmed in alternation. It is reminded that the shocks labelled « Shock 1 » and « Shock 2 », as well as the first of the shocks labelled « Shock 42J » will be of the anode type, while the following shocks will be alternated. It is reminded that, for all shock programs with 42 J energy selected, the first of the shocks will be of the anode type, while the following shocks will be alternated, whatever the label of shock program (Shock 1, Shock 2 or Shock 42J).
3. The programmed therapy parameters are rather unusual with a very narrow VT zone, leaving little room for the selected therapies. Indeed, in primary prevention, current recommendations are to program a VT zone starting at 180, but a VF zone from 220-230/min. The FVT section of the VF zone begins quite low at 190/min, and 3 ramps are programmed to treat the FVT prior to shocks. At this rate level (between 190 and 240/min), the ramps will quickly result in very short ATP cycles, and it is possible that several cycles in each ramp have values ​​equal to the programmed minimum pacing cycle (205 ms by default in the Fast VT Zone). In this instance, the ramps are almost all identical and close to fixed rate bursts of 60000/205 ms =292 bpm. These rates are therefore particularly aggressive and rather than terminating a stable VT, they are likely to degrade the latter into a VF, thereby resulting in shocks in any event. It would have been preferable to program a higher FVT zone, for example at 220/min, and to program only one ATP burst in the FVT zone up to a rate of 255/min. This is the type of programming currently recommended.

Message

In the VF zone, if a FVT section is programmed, the first applied therapy is:

1. ATP as programmed if the tachycardia is stable (with, at the time of the persistence, the last 4 cycles are in the FVT zone),
2. a choc if the tachycardia is unstable

 

In the VF section (above the FVT section): a shock is the first therapy