Surface coverage effects on defibrillation impedance for transvenous electrodes

Transvenous defibrillation electrodes are constructed by wrapping conductive elements around an insulating base. However, these conductive elements do not cover the entire area of the base. The effects of varying the surface area coverage on the defibrillation impedance (DZ) are unknown. To understand the effects, four transvenous right ventricular test leads were specially fabricated. A ring design was used with 3 mm diameter cylinders equally spaced along a 5 cm length, ending 11 mm from the pacing tip. Three leads consisted of 4, 8, and 15 rings each of length approximately 2.4 mm so that the coverages were 20%, 40%, and 70%, respectively. The fourth lead used 8 rings of length approximately 1.2 mm each and had a coverage of 20%. DZ for each lead was obtained using three methods: (i) computer simulation; (ii) in vitro measurement in a tank; and (iii) in vivo measurement in nine dogs during defibrillation testing. The DZs from either of the first two methods correlated very well (r = 0.98) with the mean DZs from the third method, indicating that in vivo DZs can be predicted from computer and in vitro models. The study shows that: (i) at the same ring length, DZ decreases as coverage (number of rings) increases; (ii) at the same coverage, DZ decreases as ring length decreases; and (iii) in vivo, a statistically significant difference was observed in DZ between the leads with 20% coverage and the leads with higher coverages. No statistically significant difference was observed between leads with coverages > 40%.