**6. Electrode configurations for subcutaneous ICDs**

**Figure 2.** Single-chamber single-coil ICD system with the lead placed in a mid-septal location (arrow). Postero-anterior

**Figure 3.** Single-chamber dual-coil ICD system with the lead placed at the base of the right ventricular outflow tract

(arrow). Postero-anterior (PA) and lateral (LAT) radiographic views.

(PA) and lateral (LAT) radiographic views.

36 Cardiac Defibrillation

Total subcutaneous implantable subcutaneous defibrillators (S-ICDs) have been developed as alternative ICD strategies allowing more widespread application of ICD therapy for the pri‐ mary prevention of sudden death. The optimal device and electrode configurations for S-ICDs are not well known. Image-based defibrillation finite element models have been used to predict the myocardial electric field generated during defibrillation shocks in a variety of subcutaneous electrode positions, in order to determine factors affecting optimal lead positions for subcutane‐ ous ICDs (S-ICD), and ultimately to improve the efficacy of these defibrillation systems.

Jolley et al.[47] used image-based finite element models (FEM) to predict the myocardial electric field generated during defibrillation shocks (pseudo-DFT) in a wide variety of subcutaneous electrode positions to determine factors affecting optimal lead positions for subcutaneous implantable cardioverter-defibrillators (S-ICD). An image-based FEM of an adult man was used to predict pseudo-DFTs across a wide range of technically feasible S-ICD electrode placements. Generator location, lead location, length, geometry and orientation, and spatial relation of electrodes to ventricular mass were systematically varied. Best electrode configurations were determined, and spatial factors contributing to low pseudo-DFTs were identified using regression and general linear models.[47] One previously published and validated S-ICD configuration[48] was selected as the base case for normalization of the predicted DFTs of all tested configurations. This is the system proposed by Lieberman et al[48], which uses a low, medial pectoral position of an active generator and a 25-cm posterolateral electrode extending around the back of the left thorax between the 6th and 10th intercostal space, extending the tip as close to the spine as possible.

ICD design and for current subcutaneous arrays used to augment transvenous systems with unacceptably high DFTs that a simple, single-electrode system is likely to offer as much benefit

Defibrillation and Cardiac Geometry http://dx.doi.org/10.5772/55120 39

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as an array, which is more difficult to implant and may be more prone to failure.

and Craig A. McPherson2

1 Massachusetts General Hospital - Harvard Medical School, USA

2 Bridgeport Hospital – Yale University School of Medicine, USA

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**Author details**

, Razvan Dadu2

Dan Blendea1

**References**

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The study by Jolley and colleagues revealed that a wide variety of conceivable electrode orientations, some of them quite unusual and not previously reported, were predicted to be as effective or more effective than the base case (pseudo-DFT ratio <=1).[47] Univariate modeling results suggested that a variety of anatomical factors affecting the geometry of system configuration influenced pseudo-DFT. Placement of the generator in the parasternal position was more efficient than more lateral and remote positions (mid-clavicular, anterioraxillary, abdominal). Anterolateral and posterior electrode positions were better than para‐ sternal, and anterolateral better than anterior. Right-sided generators were more efficient than left-sided generators, whereas the converse was true for electrode laterality. Although some of these alternatives represent simple modifications of the previously proposed system, many involve changes in lead design and implant technique that are substantial; in particular, the contralateral placement of generator and lead.

Multivariate modeling using linear regression models showed that favorable alignment of shock vector with ventricular myocardium, increased lead length, can horizontal position, contralateral lead-generator position, and distance of can from the heart independently predicted pseudo-DFTs. The relative positions of the generator, the lead(s), and the ventricular myocardium accounted for nearly half of the predicted variability in the pseudo-DFT. This reflects the intuitive observation that electrodes should be positioned to place the heart as nearly between them as possible. This multivariate analysis revealed important principles that may guide the design of subcutaneous ICDs. Placement of the electrodes to align the interelectrode shock vector as closely as possible to the center of mass of the ventricular myocar‐ dium, and use of longer electrode coil lengths are associated with lower DFTs and account for the majority of variability of this parameter. Manipulation of electrode length contributed almost 25% of the variability in pseudo-DFTs, with decreases in pseudo-DFTs predicted with extension of coil length from 5 to 10 cm and longer. Neither of these factors has previously been quantified for subcutaneous electrode placement and may prove useful in determining optimal orientations. Notably, although electrode arrays were often identified as useful in many efficient configurations, the use of an array was generally not necessarily more efficient than a single electrode of equal length similarly positioned. This finding implies both for S- ICD design and for current subcutaneous arrays used to augment transvenous systems with unacceptably high DFTs that a simple, single-electrode system is likely to offer as much benefit as an array, which is more difficult to implant and may be more prone to failure.
