Parameters characterizing implantable defibrillator output: a proposal

Recently, a discussion was carried out in Heart Rhythm on the specifications that could characterize implantable defibrillators. It is the intention of this paper to participate in this discussion on defibrillation characteristics and to give recommendations on how this problem could be solved. Theoretical considerations and results There are different defibrillation theories, all finding that the defibrillation's efficacy depends on the time constant RC which is output capacitance C times load resistance R. Efficacy decreases with increasing RC. This means that (i) the knowledge of C is of paramount importance, (ii) the energy is 'devalued' with increasing RC and that those parameter settings such as tilt or pulse duration should be adjusted to the time constant, and (iii) the energy values given without further specification are not meaningful. As there is always a voltage drop across an internal resistance within the ICD, the measured voltage across the output differs from the capacitor voltage and is reduced which determines the efficiency of the device. From the data given by Thammanomai et al., one can determine the parameters maximum voltage, capacitance, internal resistance, and tilt. These parameters are adequate and necessary to describe an ICD device and to derive the effective energy for device comparison. Discussion The 'high output devices' with their high nominal energy are reduced in their effective energies to a degree that they are comparable to the best 'standard output devices'. They do not offer that superiority which is promised by the nominal energy. Moreover, if the tilt is fixed and larger than optimal, the energy requirements are still higher or the effective energy will further drop. The term 'delivered energy' is not used by us because the delivered energy increases with increasing tilt. However, today's tilts are too large as judged by theories, which means that high delivered energies can be worse than lower ones. The delivered energy is, therefore, not a meaningful parameter in judging ICDs.

ICD devices should be characterized by: (i) voltage, (ii) capacitance, (iii) tilt or pulse duration (if not programmable), and (iv) internal resistance. All other parameters can be derived from them by simple calculations. Introduction of a 'devaluation factor' characterizes the decreasing efficacy with increasing time constant and renders the output characteristics transparent and comparable.