Implantable defibrillators have been historically developed to prevent the risk of sudden death and reduce malignant ventricular arrhythmia by electric shock. This tracing corresponds to the normal operation of an ICD. Indeed, an episode diagnosed in the VF zone is accurately detected and effectively treated by an electric shock. At the outset, the arrhythmia is extremely rapid, polymorphous and disorganized. Any attempt to reduce this type of arrhythmia by antitachycardia pacing is seemingly doomed to failure and electric shock remains the therapy of reference in this setting. No discrimination of the origin of the arrhythmia is achieved in this rate range even if the device provides the stability and sudden onset values. It should be noted however that the stability analysis is used in the decision to deliver a one-shot ATP.

In VF zone, the number of shocks is limited to 8 for the same episode, thus avoiding the risk of an endless series of shocks delivered in case of inappropriate therapies. The amplitude of the first 2 shocks is programmable, the next 6 shocks are delivered at maximum energy (40 joules). The amplitude of the first shock can either be programmed to the maximum capacities of the device or to a lower value of 10 joules (25 joules) or to a lower amplitude tested during an induction procedure. Programming a first shock of average amplitude (between 15 and 20 joules) allows reducing the charge time and the delay between the onset of the arrhythmia and the delivery of the electric shock and can in certain specific cases reduce the risk of loss of consciousness (an important issue for a driver, for example). The choice of the amplitude of the first shock in VF zone thus represents a compromise: an average energy may suffice to stop the VF after a short charge time, but in the event of failure, the second maximum energy shock occurs over a long total VF time; a high starting energy is more efficient on the VF, but at the cost of a longer initial loading time.

When shock confirmation is programmed to ON, if the ICD senses 3 slow intervals out of 4 (intervals classified in the sinus zone) during the charging of the capacitors, the device stops its charge. If the charge is not interrupted, at the end of the charge, the device delivers the shock 30 ms after the sensing of a short interval. If, at the end of charge, the device does not sense a short interval and if 3 long intervals are sensed, the charge is discontinued with a progressive discharge of the capacitors. This dissipation of energy can take approximately ten minutes. During this period, if the device resenses an episode, the charge time is shorter (use of the energy already charged). If a shock is delivered, the next shock is also confirmed. If a charge is interrupted, the following shock is not confirmed. Two consecutive charges cannot be interrupted, which could be problematic in the event of VF undersensing. If the shock confirmation is turned OFF, the charge cannot be interrupted once it has initiated. At the end of the charge, the device attempts to synchronize itself, however if it does not sense an R wave, it delivers a non-synchronized shock 2 seconds after the end of the charge. At the end of the charge, there is a 50 ms ventricular blanking period where no sensing is possible.

A 1-second blanking period without sensing or pacing follows the delivery of an electric shock. Following this blanking period, begins the post-shock pacing whose duration is programmable (from OFF to 10 minutes with a default value of 10 seconds). The post-shock mode is DDI for the DDD(R), DDI(R) or AAI(R) modes, VVI for the VVI(R) mode and VDI for VDD(R) or VDI(R) modes.