APPLICATION OF MODELS OF DEFIBRILLATION TO HUMAN DEFIBRILLATION DATA - IMPLICATIONS FOR OPTIMIZING IMPLANTABLE DEFIBRILLATOR CAPACITANCE

Background Theoretical models predict that optimal capacitance for imp lantable cardioverter-defibrillators (ICDs) is proportional to the tim e-dependent parameter of the strength-duration relationship. The hyper bolic model gives this relationship for average current in terms of th e chronaxie (t(c)). The exponential model gives the relationship for l eading-edge current in terms of the membrane time constant (tau(m)). W e hypothesized that these models predict results of clinical studies o f ICD capacitance if human time constants are used. Methods and Result s We studied 12 patients with epicardial ICDs and 15 patients with tra nsvenous ICDs. Defibrillation threshold (DFT) was determined for 120-m u F monophasic capacitive-discharge pulses at pulse widths of 1.5, 3.0 , 7.5, and 15 ms. To compare the predictions of the average-current ve rsus leading-edge-current methods, we derived a new exponential averag e-current model. We then calculated individual patient time parameters for each model. Model predictions were validated by retrospective com parison with clinical crossover studies of small-capacitor and standar d-capacitor waveforms. All three models provided a good fit to the dat a (r(2)=.88 to .97, P<.001). Time constants were lower for transvenous pathways (53+/-7 Ohm) than epicardial pathways (36+/-6 n) (t(c), P<.0 01; average-current tau(m) P=.002; leading-edge-current tau(m), P<.06) . For epicardial pathways, optimal capacitance was greater for either average-current model than for the leading-edge-current model (P<.001) . For transvenous pathways, optimal capacitance differed for all three models (P<.001). All models provided a good correlation with the effe ct of capacitance on DFT in previous clinical studies: r(2)=.75 to .84 , P<.003. For 90-mu F, 120-mu F, and 150-mu F capacitors, predicted st ored-energy DFTs were 3% to 8%, 8% to 16%, and 14% to 26% above that f or the optimal capacitance. Conclusions Model predictions based on mea sured human cardiac-muscle time parameter have a good correlation with clinical studies of ICD capacitance. Most of the predicted reduction in DFT can be achieved with approximate to 90-mu F capacitors.