Medtronic Therapies

Shock therapies

Shock waveform: early defibrillators delivered monophasic shocks. However, all ICDs now deliver biphasic shocks, as they require less energy to defibrillate the heart. The first phase of a biphasic shock is equivalent to that of a monophasic shock, though with a lower critical mass and the second phase returns the membrane potential to as close to zero as possible to prevent the re-induction of VT or VF.

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Shock vector: most state-of-the-art ICDs allow the programming of shock polarity. This may be useful to lower a high defibrillation threshold. Programming of the shock polarity depends on the number of high voltage electrodes available. The defibrillation shock is delivered via a dedicated lead, which may have a single coil positioned in the right ventricle, or a dual coil with a distal component placed in the right ventricle and a more proximal component in the superior vena cava. With a single coil lead, the shock is delivered between the distal anodal coil in the right ventricle and the cathodal can of the pulse generator. A dual coil lead allows delivery of shocks between the distal coil, the proximal coil and the can. The nominal setting uses the 2 transvenous coils as anode and the can of the pulse generator as cathode. The shock vector can be changed by changing the polarity of the electrodes. For example the coils can be programmed as the cathode and the can as the anode. Or the can is disabled and the energy is delivered between the 2 coils of the defibrillation lead, or the proximal coil is disabled and the shock is delivered between the distal end or the lead and the can. Shock polarity may be also be reversed between each shock.

Shock energy: in the VF zone, the first and subsequent shocks are usually programmed to the maximum output of the device. Programming of the shock energy can be guided by the defibrillation threshold, defined as the lowest energy required to defibrillate the heart. In the VT zones, the first shock can be programmed empirically to 5 to 10 J, which saves the battery and shortens the charge of the capacitor, or to higher energies to increase the likelihood of VT termination.

Number of shocks: in the VF zone, up to a maximum of 6 consecutive shocks can be programmed, limiting the risk of an endless delivery of inappropriate shocks.

Formatting the capacitors: the capacitors are regularly formatted to keep the charge time short, since it increases in the absence of charges. However, in order to spare the battery, the automatic formatting process is postponed each time a full charge has occurred. When the charge time exceeds 16 seconds, the device reforms the capacitors every month. If a second charge time exceeds 16 seconds, the device displays an ERI warning.

Anti-tachycardia pacing Medtronic

Medtronic proposes 3 types of sequences: burst, rampe ou rampe+.

Burst 
 

In a burst the number of impulses is similar for every sequence, although one impulse can be added to every sequence, yet the interval time duration remains constant between two impulses within a sequence.

Burst : example

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Ramp
 

In a ramp, one impulse is added to every sequence, the interval time duration reduces by the programmed decrement value from one impulse to another.
Atrial activity sensing allows for continuous and stimultaneous comparison of the atrial rate and the ventricular rate.

Ramp : example

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Ramp +
 

In a ramp+, a pulse is added to each sequence, reducing the length of intervals for the first 3 pulses and then remaining constant thereafter.

Ramp+: example

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