Adding subcutaneous coil electrodes to right-ventricular active-can defibrillator reduces defibrillation threshold and shock impedance

A finite difference computer model of a thorax with main anatomical structures was used to calculate DFT and impedance changes due to addition of 1, 2 and 3 subcutaneous 25 cm coil electrodes (SQ) to the Active Can (AC) of a defibrillator system using a 5 cm long right ventricular electrode (RV) and AC with 37 cm² projected area. The efficiency of a configuration was based on the 90 percentile of myocardial current density (J₉₀). Effects of natural variations in anatomy were simulated by variations of electrode and can positions. Clinical results from RV-AC systems (DFT=13 J, impedance 50.4 Ω) were used to derive model constants for DFT calculation. DFT, impedance and percentage AC current were calculated for 3 subpectoral AC positions, 12 RV positions and 1, 2 or 3 SQ at 3 heights and 3 distal tip positions at 0, 5 and 10 cm distance from dorsal mid-line. Adding 1, 2 or 3 SQ, combined for all positions of RV, AC and SQ, resulted in a DFT reduction from 13.0±4.6 to 6.7±1.9, 5.5±1.4 and 5.0±1.2 J, respectively. Shock impedance dropped from 50.4±1.5 to 43.0±2.4, 40.5±2.3 and 39.2±2.2 Ω, respectively. The DFT was dependent on SQ tip distance to dorsal mid-line, combined for 1, 2 and 3 SQ and all RV and AC positions, as follows. 5.7±1.5 J at 0 cm, 5.8±1.8 J at 5 cm and 6.4±1.9 J at 10 cm. Thus addition of 25 cm SQ to an RV-AC defibrillation system reduces DFT with approximately half of the energy, the greatest DFT reduction is obtained with a single SQ; variation in SQ implant (distance of SQ tip to dorsal mid-line) has minor impact on DFT and impedance