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

Abstract

Background: Use of implantable cardiac defibrillators (ICDs) in children and patients with congenital heart disease is complicated by body size and anatomy. A variety of creative implantation techniques has been used empirically in these groups on an ad hoc basis.

Objective: To rationalize ICD placement in special populations, we used subject-specific, image-based finite element models (FEMs) to compare electric fields and expected defibrillation thresholds (DFTs) using standard and novel electrode configurations.

Methods: FEMs were created by segmenting normal torso computed tomography scans of subjects ages 2, 10, and 29 years and 1 adult with congenital heart disease into tissue compartments, meshing, and assigning tissue conductivities. The FEMs were modified by interactive placement of ICD electrode models in clinically relevant electrode configurations, and metrics of relative defibrillation safety and efficacy were calculated.

Results: Predicted DFTs for standard transvenous configurations were comparable with published results. Although transvenous systems generally predicted lower DFTs, a variety of extracardiac orientations were also predicted to be comparably effective in children and adults. Significant trend effects on DFTs were associated with body size and electrode length. In many situations, small alterations in electrode placement and patient anatomy resulted in significant variation of predicted DFT. We also show patient-specific use of this technique for optimization of electrode placement.

Conclusion: Image-based FEMs allow predictive modeling of defibrillation scenarios and predict large changes in DFTs with clinically relevant variations of electrode placement. Extracardiac ICDs are predicted to be effective in both children and adults. This approach may aid both ICD development and patient-specific optimization of electrode placement. Further development and validation are needed for clinical or industrial utilization.

Figure 1. A. Examples of nonstandard subcutaneous, epicardial and transvenous electrode orientations

Left: Infraclavicular can with single subcutaneous electrode, Middle: Abdominal can with epicardial lead, Right: Infraclavicular can with SVC and RV transvenous electrode as well as left subcutaneous electrode. B. User interface for electrode placement. A subcutaneous electrode (red) extending left posterior with right abdominal can (green) is shown in two year old torso shown in two views used while placing electrodes in finite element model. Moveable cutting planes allow the user to examine anatomical detail during electrode placement. The blue spheres on the red electrode and bounding cage on the can indicate handles for user interaction. Electrodes can be placed with similar precision in epicardial and transvenous orientations.

Figure 2. Example of calculation of defibrillation metrics and visualization

Results from exemplary model of subcutaneous right posterior electrode coil and left abdominal can in 29 year old torso. Upper left: Voltage and defibrillation metrics for myocardial elements. Upper right: Projection of voltage gradient onto portion of myocardial compartment. Lower left: Visualization of voltage gradients using interactive cutting plane. Lower right: Visualization of current density using cutting plane.

Figure 3. Effect of varying position of 25cm subcutaneous electrode with right abdominal can

This figure also demonstrates the observed trend for increased DFT with torso size.

Figure 4. Relation between electrode placement class and distribution of myocardial voltage gradient

Analysis of configurations in 75 kg torso employing a single electrode coil and can shows that placement of electrodes further from heart (subcutaneous coil) results in more homogenous distribution of myocardial electrode field and smaller fraction of myocardial compartment >30V/cm when the energy applied meets criterion for defibrillation, i.e., elevation of exactly 95% of myocardial elements to a voltage gradient ≥ 5V/cm. Median, upper and lower quartiles, and range are presented.

Figure 5. Optimization of epicardial coil and can electrode placement in 75 kg torso

Coils are shown as colored lines overlying heart silhouette in following locations: red – inferoposterior, blue – apical, green – anterosuperior.

Figure 6

Effect of electrode length on predicted DFT and distribution of myocardial voltage gradient for left lateral epicardial electrode with right abdominal can

Figure 7. Patient specific modeling in patient with congenital heart disease

Top: Post-implantation chest X-ray and corresponding finite element model showing can and epicardial coil electrode placement. Middle: Epicardial voltage gradient distribution for three alternative shock vectors tested using model. Bottom: Predicted and observed defibrillation energies.