EFFECT OF CAPACITOR SIZE AND PATHWAY RESISTANCE ON DEFIBRILLATION THRESHOLD FOR IMPLANTABLE DEFIBRILLATORS

Background The time constant of truncated exponential pulses used with implantable defibrillators is determined by the output capacitor size and defibrillation pathway resistance. The optimal capacitor size is unknown. Methods and Results This study compared defibrillation thresh old (DFT) for standard 120-mu F capacitors (DFT120) and smaller 60-mu F capacitors (DFT60) at implantation of cardioverter-defibrillators in 67 patients using epicardial electrodes (15 patients) or one of four transvenous electrode configurations (52 patients). Paired comparisons of DFT60 and DFT120 were made for 44 defibrillation pathways using mo nophasic pulses and for 53 pathways using biphasic pulses. Truncated e xponential pulses with 65% tilt were used. Pooled data from all electr ode configurations showed a significant inverse correlation between pa thway resistance and the ratio of stored energy DFT60 to DFT120 (monop hasic pulses: r=.75, P=.0001; biphasic pulses: r=.68, P=.0001). Data f rom all electrode configurations formed a continuum with 120-mu F capa citors superior for low-resistance pathways and 60-mu F capacitors sup erior for high-resistance pathways. For pathways with resistance less than or equal to 40 Ohm, the modest advantage of 120-mu F capacitors a pplied primarily to pathways with low DFTs: 8.2+/-6.1 versus 9.6+/-5.4 J (P=.001) for monophasic pulses and 4.1+/-2.8 versus 5.1+/-3.1 J (P< .02) for biphasic pulses. The greater advantage of 60-mu F capacitors for pathways with resistance greater than or equal to 61 Ohm applied t o pathways with higher DFTs: 12.4+/-4.3 versus 23.1+/-6.4 J (P=.0001) for monophasic pulses and 8.5+/-4.9 versus 12.5+/-6.4 J (P=.0001) for biphasic pulses. For pathways with resistance greater than or equal to 61 Ohm, the DFT was less than or equal to 15 J for 19% of pathways us ing monophasic 120-mu F pulses versus 95% for 60-mu F pulses. Similarl y, the DFT was less than or equal to 10 J for 48% of pathways using bi phasic 120-mu F capacitors versus 83% for 60-mu F pulses. Conclusions In comparison with conventional 120-mu F capacitors, 60-mu F capacitor s had clinically insignificant higher DFTs for low-resistance pathways and clinically important lower DFTs for high-resistance pathways. Opt imal capacitance is inversely related to pathway resistance for clinic al defibrillation pathways and waveforms.