DFT changes in a RV active can (AC) defibrillator due to AC position variations and smaller AC size are reduced by adding a subcutaneous coil electrode

A finite difference computer model was used to simulate defibrillation threshold (DFT) and impedance for an Active Can (AC) defibrillator with a 5 cm long right ventricular (RV) electrode. The effects of adding a single subcutaneous (SQ) 25 cm coil electrode to AC were calculated. The efficiency of a configuration was based on the 90 percentile of myocardial current density. Effects of natural variations simulated by variations of RV electrode positions. Clinical results from RV-AC systems (DFT=13J, Z=50.4 /spl Omega/) were used to derive model constants for calculation of DFT. In a factorial experiment DFT, impedance and percentage AC current were calculated for 15 sub-pectoral AC, 12 RV; and 6 SQ positions, resulting in a total of 1260 combinations. Reduction of projected AC size from 37 to 22 and 13 cm/sup 2/, combined for all positions of RV and AC without SQ, resulted in a DFT increase from 13.0/spl plusmn/4.6 to 14.7/spl plusmn/5.3 and 16.5/spl plusmn/5.8 J, and a shock impedance rise from 50.4/spl plusmn/1.5 to 59.0/spl plusmn/2.0 and 68.4/spl plusmn/1.7 /spl Omega/, respectively. Adding a single SQ at 9 different positions, resulted in the following DFT and impedance values: 6.8/spl plusmn/1.9, 6.5/spl plusmn/1.7 and 6.4/spl plusmn/1.5 J; 43.1/spl plusmn/2.5, 46.5/spl plusmn/2.4 and 49.3/spl plusmn/2.4 /spl Omega/, for AC sizes of 37, 22, and 13 cm/sup 2/, respectively. For 5 different AC positions, the DFT without SQ ranged from 10.7/spl plusmn/2.8 J in the most lateral AC positions to 32.1/spl plusmn/10.7 J in the most mid line AC position, and with one SQ combined at 9 positions from 5.8/spl plusmn/1.3 J to 8.4/spl plusmn/2.2 J respectively. Thus addition of one 25 cm SQ to AC of an RV-AC defibrillation system abolishes DFT rise due to AC size reduction and achieves the largest DFT reduction for the most unfavorable AC position.