Defibrillation Efficacy of Different Electrode Placements in a Human Thorax Model

Abstract

The objective of this study was to measure the defibrillation threshold (DFT) associated with different electrode placements using a three-dimensional anatomically realistic finite element model of the human thorax. Coil electrodes (Endotak DSP, model 125, Guidant/CPI) were placed in the RV apex along the lateral wall (RV), withdrawn 10 mm away from the RV apex along the lateral wall (RVprox), in the RV apex along the anterior septum (RVseptal), and in the SVC. An active pulse generator (can) was placed in the subcutaneous prepectoral space. Five electrode configurations were studied: RV-->SVC, RVprox-->SVC, RVSEPTAL-->SVC, RV-->Can, and RV-->SVC + Can. DFTs are defined as the energy required to produce a potential gradient of at least 5 V/cm in 95% of the ventricular myocardium. DFTs for RV-->SVC, RVprox-->SVC, RVseptal-->SVC, RV-->Can, and RV-->SVC + Can were 10, 16, 7, 9, and 6 J, respectively. The DFTs measured at each configuration fell within one standard deviation of the mean DFTs reported in clinical studies using the Endotak leads. The relative changes in DFT among electrode configurations also compared favorably. This computer model allows measurements of DFT or other defibrillation parameters with several different electrode configurations saving time and cost of clinical studies.

A computer modeling tool for comparing novel ICD electrode orientations in children and adults

“…13,14 Finite element modeling of defibrillation has been shown to correlate well with clinically observed defibrillation thresholds in laboriously constructed conductivity models of the adult torso. [15][16][17][18][19][20][21][22][23] These studies have validated the use of realistic models for accurate prediction of intrathoracic electric fields, allowing estimation of the defibrillation threshold voltages, currents, and impedances that would be associated with such fields. Extension of these studies to allow modeling of different electrode orientations, within variable body sizes and habitus and under anatomically variable conditions, requires simulation systems that can facilitate rapid model creation, interactive electrode placement, and clinically relevant visualization of the results.…”

“…The predicted DFT in this model was 8.3J, conforming closely to both previously simulated and clinically observed results. 15,17,29,30 …”

“…Aguel and DeJongh both utilized FEM to compare transvenous electrode orientations with results closely approximating those published clinically. 15,17,29,30 Although these comparisons are reassuring, neither the current nor prior models incorporate many factors (particularly myocardial) known to affect defibrillation (see below). Furthermore, the threshold predicted by the critical mass hypothesis, while supported by considerable experimental evidence, is a generalization.…”

“…Conductivities for the individual tissues were based on values derived from the literature as follows (all in siemens/meter): bowel gas 0.002, connective tissue 0.220, liver 0.150, kidney 0.070, skeletal muscle 0.250, fat 0.050, bone 0.006, lung 0.067, blood 0.700, myocardium 0.250. 15,17,19,22,25 The resulting finite element model incorporated a set of equations similar to those used in previous defibrillation studies. In this implementation, we assumed a linear and isotropic volume conductor model, with negligible capacitance and inductance, and applied the Galerkin finite element formulation with tri-linear interpolation.…”

“…The critical mass hypothesis was then used to define metrics for intrapatient comparison of defibrillation "success" in different electrode configurations. 15,17,26 Note that this approach is not intended to calculate the actual DFT which would be recorded in clinical study, but to establish a yardstick by which the intrathoracic field strength over the myocardium can be compared given differing electrode configurations. The critical mass hypothesis proposes that a defibrillation shock will be successful if it produces a threshold voltage gradient over a "large" fraction of the myocardial mass, rendering it transiently inexcitable.…”