Device: ICD Field: Therapy

1. Basic concepts



Shock therapies
ICD were originally invented to terminate malignant ventricular arrhythmias using DC shocks. Cardioversion is the delivery of a DC shock synchronized to the rising edge of the R wave of the electrocardiogram to terminate a supraventricular or a ventricular tachyarrhythmia, whereas defibrillation is the delivery of an unsynchronized DC shock to terminate VF. During VF, the instability and low voltage of the ventricular EGM sometimes precludes the synchronization of ICD shocks. Medtronic ICDs systematically seek to synchronize the shock delivery, including in the VF zone. Several shock characteristics are programmable, including vector, amplitude and number of shocks delivered.

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.



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.


A priority of ICD programming is to limit the number of shocks without compromising patient safety. One must apply the least forceful and least painful method to terminate arrhythmias. Antitachycardia pacing (ATP) captures the arrhythmia, and interrupts an organized VT by penetrating a re-entrant circuit. This is achieved by pacing the ventricle at a rate faster than the VT. ATP is painless, lowers the consumption of energy, and thus spares the battery. It should, therefore, be the preferred first-line treatment for organized ventricular arrhythmias, even when very rapid. The effectiveness of ATP has been confirmed in a wide range of VT, from slow to as rapid as 240 bpm. Thus in the fast VT or even the VF zone, it is common to program a burst of ATP, during or before the charge of the capacitors, unless ATP is known to be ineffective or pro-arrhythmic.

  • number of pulses in a sequence: in general, 5 to 15 consecutive stimuli are programmed in each ATP sequence. If the number is insufficient, the sequence does not penetrate the tachycardia circuit and remains ineffective. If the number is too high, ATP might reduce and re-induce the tachycardia.
  • cycle length: the cycle length of ATP is expressed as a percent of the tachycardia cycle length, and is usually set between 80 and 90% of the sensed interval. The lower the value, the faster the pacing and the higher the risk of accelerating the VT. The shortest interval below which the device will not stimulate, regardless of other programming, is programmable, and has a nominal value of 200 ms.
  •  number of sequences: the number of ATP sequences depends on the rate of the VT. In a slow VT zone, several sequences can be programmed to postpone the delivery of a shock as long as possible, since a slow VT is usually not life-threatening. It may also be preferable not to program the delivery of shocks in a slow VT zone. For rates between 150 and 200 bpm, 3 to 6 sequences of ATP are often programmed. For faster VT, which may be hemodynamically unstable, a single sequence of ATP is programmed in order to avoid delay of delivery of a first shock and preserve patient safety.

Smart Mode is a programmable algorithm that turns ‘off’ ATP after 4 consecutive, ineffective sequences. Another option is the progressive therapy algorithm which guarantees that each sequence of ATP delivered for the same VT episode is at least as aggressive as the previous.

2. Specificities by company

Boston Scientific

3. Take home messages

1. A priority of programming is to reduce shocks without compromising patient safety. Antitachycardia pacing is effective for a wide range of VTs, from slow to fast VTs, is painless, reduces energy consumption and prolongs battery longevity.

2. In case of programming a slow VT zone (<150 beats / minute), one must 1) promote the possible spontaneous VT termination by increasing the number of cycles during detection 2) prefer anti-tachycardia pacing by programming a greater number of sequences of stimulation and a greater number of pulses in each sequence 3) program in such a way to avoid shock delivery.

3. In the traditional VT zone (> 150 beats / minute), a series of bursts, then a series of ramps, and a series of shocks are programmed. The amplitude of the first shock can be set to a high value 1) to increase the probability of quickly terminating a VT episode and to minimize the number of shocks, 2) to terminate AF if the shock is inappropriate and 3) to reduce the likelihood of inducing AF (shock amplitude above the upper limit of atrial vulnerability). In contrast, the amplitude of this first shock can be set lower (5 to 10 Joules) to minimize energy consumption and the myocardial injury associated with the shocks.

4. In the VF zone, a significant number of tachyarrhythmias are organized and monomorphic VTs are likely to be terminated by anti tachycardia pacing. The burst during capacitor charging avoids a shock, but if it becomes necessary because the burst has been inefficient, the shock is delivered without delay. However, capacitor charging results in energy consumption and increases wear of the batteries. The burst before charging avoids the shock and the capacitor charging at the cost of a slight delay in the shock delivery if the burst is ineffective.

5. In the VF zone, the strength of the first and subsequent shocks is usually programmed at the highest value the device is able to deliver. Programming of the defibrillation shock amplitude can be guided by the defibrillation threshold, defined as the least amount of energy that converts VF to sinus rhythm.