Random nature of fibrillation leads to the probabilistic nature of defibrillation.
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Biomedical subjects
R Pendekanti
Publications and source records attributed to R Pendekanti.
Importance of electrode conductive surface area and edge effects on ventricular defibrillation efficacy.
INTRODUCTION: The role of edge effects and electrode surface area of the right ventricular (RV) transvenous lead (TVL) on defibrillation efficacy is unknown. METHODS AND RESULTS: Defibrillation threshold (DFT) testing was conducted randomly in 12 dogs using ring electrode leads in an RV/SVC (superior vena cava) or RV/SVC/patch system. The leads (RV-4, RV-8t, RV-8, RV-15) had electrode surface areas of 20%, 20%, 40%, and 70%, respectively. A computer model predicted the magnitude of electrode surface current (RV-8t > RV-4 > RV-8 > RV-15) and the potential distribution (PD) at four sites: electrode surface (site a) and at 2 mm (b), 4 mm (c), and 8 mm (d) away from the surface. Despite different near-field PDs (sites a, b, c), PDs were nearly identical at site d. Resistance decreased as the surface area increased. DFT energy for the RV-15 lead was lower than the RV-4 and RV-8t. There was no difference between energy requirements for the RV-15 and RV-8 leads. No difference was found in DFT current for each lead. Comparison of the RV-8t and RV-4 leads showed no difference in DFT energy despite a lower resistance and a greater number of edges. CONCLUSIONS: Increasing the RV TVL surface area lowered the resistance. However, surface area coverages > or = 40% did not lower DFT energy. No significant change in DFT current occurred despite different predicted near-field current densities. PDs were nearly identical 8 mm from the electrode surface. Thus, the far-field current density appears to play a more important role in determining defibrillation success.
Animals↗
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%.
Animals↗
Spatial potential and current distributions along transvenous defibrillation electrodes: variation of electrode characteristics.
The therapeutic efficacy of an endocardial defibrillation lead system can be improved by controlling the profile of current delivery through a suitable choice of electrode characteristics, which include the length, radius, number of conductor elements, electrode resistance, and point of connection to the voltage source. Such control will minimize tissue and lead damage during long-term use. In this study, a semianalytical model was developed to study cylindrical electrodes of different constructions in an idealized electrolytic medium. Simulations were performed to investigate the effects of varying the electrode characteristics on the spatial voltage and current distributions and interelectrode resistance for cylindrical electrodes of different constructions. The results show that, for transvenous electrodes of realistic dimensions, the current distributions are determined largely by the edge effects. The edge effects increase as the aspect ratio of the electrode (length/radius) decrease. The multiple edges resulting from wrapping conductor elements over a nonconducting base are found to increase the nonuniformity and the current density over the conductor-covered surface. The model is used to demonstrate two techniques of controlling the current distribution. The first method involve modifying the electrode resistivity profile and point of connection. In the second approach, the electrode surface is covered with a thin film having a model-computed resistance profile. By using either methods to produce isocurrent electrodes, the interelectrode resistance is found to increase.
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