Does reducing capacitance have potential for further miniaturisation of implantable defibrillators?

Objective - To determine whether considerably smaller capacitors could replace 125 μF capacitors as the standard for use in implantable defibrillators. Methods - Measured energy, impedance, voltage, and current delivered were compared at defibrillation threshold in 10 mongrel dogs for defibrillation using 75 μF and 125 μF capacitors alternated randomly. Defibrillation was attempted with biphasic shocks of comparable tilt between an endocardial lead in the right ventricular apex and a 'dummy' active can of an experimental implantable device placed in the subpectoral position. Results - A reduction of capacitor size of 40% was associated with an increase in voltage of 21% and in current of 22%. With a 65% tilt, no significant differences were found between the two capacitances with respect to the impedance or energy required for defibrillation. Conclusions - Multiple advances in electrode material, electrode configuration, shock morphology, and shock polarity have reduced defibrillation energy requirements. Smaller capacitors could be used in implantable cardioverter/defibrillators without a major decrease in effectiveness.