Original-experimentalA computer modeling tool for comparing novel ICD electrode orientations in children and adults
Introduction
Implantable cardiac defibrillators (ICD) have become the standard of care for patients at risk of fatal cardiac arrhythmias, and indications for their use continue to expand.1, 2, 3, 4 Although ICD systems are routinely implanted in adult patients using a transvenous approach, there is a growing population of pediatric and adult patients in whom transvenous ICD systems cannot or should not be implanted.5 These include patients of very small size and those with intracardiac shunts or anatomical obstruction to lead placement.6, 7, 8, 9
In these patient populations, a variety of innovative approaches to ICD implantation have been reported, including subcutaneous, epicardial, and caval electrode placements and/or abdominal can implants (Figure 1).7, 8, 9, 10, 11, 12 Using ad hoc adaptations of existing ICD components, these approaches attempt to minimize system invasiveness, incorporate patient-specific options to adapt to complex anatomy, and achieve low defibrillation thresholds (DFTs). Assumptions of efficacy are based on extrapolation of data from the use of subcutaneous arrays in adults, limited animal research, and postimplantation assessment of DFTs.7, 9 Although defibrillation research has elucidated predictive relationships between distribution of myocardial voltage gradient and both defibrillation efficacy and myocardial injury, no information currently exists that describes the effects of interactions among variations in body size, habitus, and novel ICD geometries on these fields.13, 14
Finite element modeling of defibrillation has been shown to correlate well with clinically observed DFTs 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 DFT 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 desire to make such tools part of the repertoire of the defibrillation research and clinical communities motivates the use of open-source tools for this purpose, so that the underlying computer code is available to the community to be improved and altered for a variety of purposes.
In this proof-of-concept study, we describe the results of subject-specific, image-based finite element modeling of standard and nonstandard ICD electrode placement using a novel, interactive, open-source computing environment. The driving hypothesis of this research was that alterations in electrode placement, reflecting realistic variations of surgical practice, would result in clinically significant changes in the electric fields predicted in the myocardium and thus support the goal of determining optimal electrode placements for special populations of adults and children.
Section snippets
Image acquisition and segmentation
Anatomically realistic torso models and a computer modeling environment were created in which the effect of varying ICD electrode placement on myocardial voltage gradients could be assessed as a proxy predictor of effective defibrillation. Models were constructed by segmenting 64-detector computed tomography (CT) scans (1.25-mm slices) from normal or trivially abnormal subjects obtained from a radiology trauma database with appropriate internal review board approvals. Three scans were selected
Comparison with prior modeling and clinically observed results
We carried out simulations on a reference model of a standard implant in the 75-kg adult torso (left pectoral can to superior vena cava [SVC] and right ventricular [RV] transvenous orientation, shock vector AX > B). The predicted DFT in this model was 8.3 J, conforming closely to both previously simulated and clinically observed results.15, 17, 29, 30
Evaluation of electrode configurations
Four basic classes of electrode placement variants were simulated in each torso: can site, transvenous coils, epicardial coils, and subcutaneous
Discussion
Use of ICD therapy in pediatric and congenital heart populations has increased as the numbers of patients who may benefit from defibrillator therapy have increased and the apparent risks of the procedure have decreased. Transvenous implantation often cannot be performed in children because of patient size, lack of vascular access, and increased risk of embolic phenomena due to intracardiac shunts.5, 31 Children with ICDs have high rates of both lead failure and vascular occlusion, and also have
Conclusion
We developed and used an interactive computational and visualization tool to compare relative efficiency of standard and nonstandard ICD electrode placement in torso models of various sizes, showing significant differences in myocardial voltage gradients associated with different strategies. In patients with contraindications to standard approaches to ICD implantation, the ability to interactively assess the relative efficacy of different electrode orientations would provide insight into which
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This work was supported in part by National Institutes of Health grant P41 RR12557 (Scientific Computing Institute) and National Institutes of Health grant P41 RR13218 (Surgical Planning Laboratory). Dr. Jolley was supported by National Institutes of Health grant T32 HL07572 and a Fast-Forward Award from the Center for Integration of Medicine and Innovative Technology (CIMIT).