The defibrillation threshold does not correspond to a fixed value. The first maximum energy shock is inefficient, while the second with the same amplitude allows restoring a worrisome situation.

Erreur de traductionIn addition to the amplitude delivered, various parameters can or cannot be modified to optimize the effectiveness of the shocks delivered:

1. the number of phases; historically monophasic, the shock wave has become biphasic in modern ICDs, which allows lowering the defibrillation thresholds. The first phase of a biphasic shock is equivalent to that of a monophasic hock but with a lower critical mass; the second phase reduces the membrane potential as closely to zero as possible, which significantly reduces the defibrillation thresholds and the risk of re-induction.
2. the shape of the shock wave with 2 programming possibilities; for a voltage-controlled shock, the charged voltage is 100%, the voltage tilt of the first phase is 40%, which means that 60% of the initial voltage is delivered during the first phase (fixed tilt at 60); the cut-off voltage of the second phase is 20%, which means that 50% of the remaining voltage (40%/2) is delivered during the second phase (tilt at 50); it is therefore a two-phase voltage-controlled shock with fixed 60/50 tilt; the delivered voltage is constant, the pulse duration of each phase varies as a function of the impedance of the shock, the pulse duration being longer with higher impedance. There is a second programming possibility for the shock wave (biphasic II, controlled voltage/pulse duration); the charged voltage is 100%, the voltage tilt of the first phase is also 40%; the cut-off of the second phase occurs automatically after a fixed pulse duration of 2 ms regardless of the voltage delivered; this option can be programmed in patients with a high defibrillation threshold especially when the patient is receiving amiodarone therapy known to raise the threshold.
3. the polarity of the shock; modern ICDs allow programming the polarity and shock vector; this functionality can be useful in the event of a high defibrillation threshold by allowing to chose the shock vector offering the best efficiency ratio; the polarity of the shock can be programmed to normal, reverse or alternate; for a single-coil lead, the normal polarity for Biotronik^TM ICDs reflects the fact that the shock is delivered between the pulse generator which is the anode for the first phase and the right ventricular coil which is the cathode; it is therefore called a cathodic shock; when the polarity is reversed, the right ventricular coil becomes the anode during the first phase; this reverses the two phases of a biphasic shock (first phase is negative, second phase is positive); it is therefore an anodic shock; in the case of alternating polarity, the first shocks are delivered with normal polarity followed by alternation between normal polarity and inverted polarity when the first maximum energy shock has been delivered; it is therefore possible to alternate the polarities of the shocks (cathodic or anodic) starting from the first full-energy shock.
4. the shock vector; the programming of this parameter is contingent on the number of available shock electrodes; the defibrillation shock is transmitted by a dedicated lead which can be single-coil (a single defibrillation electrode or coil placed in the right ventricle) or double-coil (a distal defibrillation electrode placed in the right ventricle, a defibrillation electrode placed more proximally, at the level of the superior vena cava). The single-coil shock is delivered between the distal coil of the right ventricular lead and the pulse generator; the double-coil shock is delivered between three structures: the distal coil, the proximal coil and the pulse generator. The shock vector is programmable with the possibility of programming or deprogramming the proximal electrode in the superior vena cava for a double-coil lead (single-coil shock) and deprogram the pulse generator (cold can); deprogramming a double-coil shock in the presence of a high defibrillation threshold allows excluding the superior vena cava coil when it is positioned too low, floating in the atrium, and part of the energy delivered is dissipated in the atrium.