Smaller capacitors improve the biphasic waveform

Current implantable cardioverter defibrillators (ICDs) use relatively large capacitance values. Theoretical considerations suggest, however, that improved defibrillation energy requirements may be obtained with smaller capacitance values.

We compared the energy requirement for defibrillation in a porcine model using a biphasic waveform generated from two capacitance values of 140 microF and 85 microF. Phase 1 reversal of the shock waveform occurred at 65% tilt. Phase 2 pulse width was equal to phase 1. Shocks were delivered through epicardial patch electrodes after 10 seconds of induced ventricular fibrillation. The defibrillation threshold (DFT) was determined by a "down-up" technique requiring three reversals of defibrillation success or failure. The DFT was defined as the average of the values obtained with all trials starting from the successful shock prior to the first failure to defibrillate to the last successful defibrillation. In eight experiments, the measured parameters at DFT were as follows. The average stored and delivered DFT energies for the 85 microF capacitor were 6.1 +/- 2.1 and 6.0 +/- 2.0 J, respectively, compared to 7.5 +/- 1.3 and 7.4 +/- 1.3 J for the 140 microF capacitor (P < 0.04). The phase 1 pulse widths were significantly shorter for the 85 microF capacitor (5.1 +/- 0.8 msec vs 9.2 +/- 1.3 msec) and the impedances were lower (54.4 +/- 5.8 omega vs 59.9 +/- 6.3 omega). The mean leading edge voltage was trending higher for the 85 microF capacitor, but this difference did not reach statistical significance (374 +/- 63 V vs 326 +/- 30 V; P = 0.055).

Smaller capacitance values do result in lower energy requirements for the biphasic waveform, at a possibly higher leading edge voltage and a much shorter pulse width. Smaller capacitance values could represent a significant enhancement of well-established benefits demonstrated with the biphasic waveform.