Iridium Oxide–Coated Defibrillation Electrode
Reduced Shock Polarization and Improved Defibrillation Efficacy
Background Transvenous implantable cardioverter-defibrillator (ICD) leads are designed to deliver electric shocks to the heart for termination of ventricular dysrhythmias. However, the efficiency of different lead materials has not been well studied. This study compares an ICD lead coated with iridium oxide (IROX), a material that reduces shock-induced polarization, with an otherwise identical, uncoated lead.
Methods and Results The defibrillation threshold (DFT) was determined in 13 swine with both IROX-coated and uncoated ICD leads paired with an uncoated “can” electrode. The leads were exchanged through a Teflon sheath to reproduce the intracardiac position. The delivered energy DFT of the IROX-coated lead was 15.9±5.4 J and was significantly lower than the delivered energy DFT of the uncoated lead (19.1±5.1 J; P<.006). The initial lead impedance was equivalent in both leads (IROX, 41.7±5.8 Ω; uncoated, 41.3±4.7 Ω; P=NS) at DFT. However, the impedance rose by 7.3±2.0 Ω during the first phase and by 3.7±2 Ω during the second phase with the uncoated lead, whereas the corresponding impedance change was 1.0±0.3 Ω during phase 1 and 1.6±0.5 Ω during phase 2 (P<.01 each phase) when the IROX-coated lead was used.
Conclusions This study shows that an IROX coating of this lead system significantly lowers the DFT energy in the swine model. The blunting of the impedance rise by the IROX coating that is seen is consistent with a reduction in electrode polarization.
The use of nonthoracotomy defibrillation leads has reduced the morbidity and mortality associated with the ICD.1 2 The desire for wider use of the ICD and easier implantation has driven the development of smaller ICD generators. These devices can be implanted with minimal anesthesia in the prepectoral fascia, in a manner similar to pacemaker implantation. This reduction has been facilitated, in large part, by the development of more effective biphasic shock waveforms that have reduced the DFT energy by 10% to 15% compared with monophasic shocks.3 4 The use of an active pectoral “can” electrode has also improved defibrillator shock efficacy.5 Transvenous ICD electrodes are used to deliver the defibrillating energy pulse to the heart, and a number of authors have studied the effect of varying the number or position of electrodes on defibrillation efficacy.6 7 8 Few studies, however, have examined the effect of transvenous electrode materials on shock impedance and the efficacy of defibrillation.
Much of the research on electrode materials has been performed with pacemaker electrodes at pacing stimulus amplitudes. This is not surprising, given the relatively short period of time that nonthoracotomy defibrillator leads have been available compared with pacemaker leads. Studies conducted on pacing electrodes that use a coating of the low-polarization material IROX have demonstrated a reduction of electrode polarization and a reduction of pacing thresholds.9 10 A transvenous lead with an IROX-coated defibrillation electrode has been developed and is now in clinical trials. This study examines the in vivo electrode characteristics and defibrillation efficacy of the IROX-coated, transvenous defibrillation electrode placed in the right ventricle, paired to an uncoated, pectoral can electrode. The lead characteristics as well as defibrillation energy requirements (DFT) of this lead were compared with an otherwise identical system using an uncoated transvenous lead in an anesthetized swine model.
The experimental protocol for studying defibrillation efficacy with the IROX-coated electrode was reviewed and approved by the animal research committee at the Cleveland Clinic Foundation. Thirteen swine were anesthetized with sodium pentobarbital (30 mg/kg IV), ketamine (20 mg/kg IM), and morphine (2 mg/kg IM). Oxygenation was maintained by endotracheal intubation with a cuffed endotracheal tube and ventilation with room air through a Harvard respirator (Harvard Apparatus Co). Intermittent arterial blood gas measurements were taken and ventilator adjustments were made to keep the oxygen saturation >90% and the Pco2 normal. Standard ECG leads and femoral arterial blood pressure were monitored throughout the experimental protocol. These data, as well as delivered shock energy, voltage, and current measurements, were stored digitally at 0.1-ms intervals on a personal computer for later analysis.
Endocardial Lead Positioning and Exchange
The order in which the uncoated and IROX-coated electrodes were tested was randomized for each experiment. The active fixation transvenous defibrillator lead with IROX coating (model 497-20, Sulzer Intermedics Inc) or without IROX coating (model 497-06, Sulzer Intermedics Inc) was inserted through the right internal jugular vein, and the active fixation tip was positioned in the right ventricular apex under fluoroscopic guidance. The shock electrode incorporated into this 11F lead consists of a titanium coil (with or without IROX coating) 5 cm in length, with a surface area of 4.4 cm2 located 1 cm from the distal tip. The defibrillation path consisted of the transvenous shock electrode coil (±IROX coating) paired to a titanium can electrode (surface area, 92 cm2) implanted subcutaneously in the left prepectoral fascia. Defibrillation testing of the lead was then carried out (see description below).
Once testing with the initial endocardial lead was completed, that lead was exchanged as follows. First, a Teflon sheath (Cook Pacemaker Corp) was advanced over the lead, under fluoroscopic guidance, until the sheath reached the right ventricular endocardium (Fig 1⇓). Then, the lead was withdrawn through the sheath, which was left in place to maintain the endocardial position. The second lead was immediately advanced through the sheath and fixed to the endocardium. The sheath was then removed, leaving the new lead in the same endocardial location as the original lead. An equivalent amount of lead “slack” was adjusted fluoroscopically. Defibrillation testing of the new lead was then performed. The pectoral can electrode was not moved during the study.
Testing of each lead/can combination was performed with the endocardial coil configured as the cathode for the first phase of a biphasic waveform and the pectoral can as the anode. The defibrillation waveform used for both leads was generated by an experimental defibrillator (Angeion Corp) programmed to deliver a 135-μF biphasic waveform with fixed 65% tilt (each phase). This device sampled delivered voltage and current at 0.1-ms intervals throughout each phase. Delivered energy and lead impedance were calculated from these data. Stored energy was calculated from the peak voltage and capacitance values.
Defibrillation efficacy was established with a down-up, down-up protocol that has been described in previous studies and shown to approximate the probabilistic method of determining the ED50.11 This protocol has been detailed elsewhere and consists of a series of fibrillation-defibrillation episodes (steps) in which the test shock intensity is decremented (down) or incremented (up) by 1 J. This protocol was followed for both uncoated and IROX-coated leads. At each step, ventricular fibrillation is induced with a 4-second alternating current burst of 10 to 20 V delivered through the high-energy electrodes. Once ventricular fibrillation was confirmed by ECG and loss of arterial pressure pulses and a total duration of 10 seconds had elapsed (including ventricular fibrillation induction time), a test shock was delivered. Success or failure of the test shock was judged by the ECG and by the return of arterial pressure pulsation. If the test shock failed, a second monophasic shock of 40- to 50-J intensity was delivered through the electrodes. This second rescue shock succeeded in all animals. A 3- to 5-minute stabilization period was observed between each step. Subsequent test shocks were either decremented by 1-J intensity after a successful test shock or incremented by 1 J after a failed test shock. All shock parameters (energy, voltage, current, and impedance) were averaged, beginning with the last successful shock during the initial down sequence and up to and including the final shock observed during the second up series. The average values for each parameter were defined as the DFT values.
In Vivo Lead Performance
To compare the impedance characteristics of the two transvenous leads, the energy, current, and impedance values obtained for shocks of similar intensity (peak voltage of 575±50 V) were compared, regardless of defibrillation success or failure. A least-squares regression of impedance versus time (Ω/ms) was obtained in each pig for both leads to obtain the rate of impedance rise (slope). This slope was used to quantify the polarization effect present in both electrodes.
The distributions of all DFT values were found to have similar variances. Comparison of these values and those for the in vivo comparison of lead impedance for both noncoated and IROX-coated leads was performed with the paired t test. Analysis of the lead voltage, current, and impedance values and slopes of the impedance regression lines during the in vivo lead performance testing were analyzed by the nonparametric Wilcoxon signed-rank test because of the unequal variance seen. A value of P<.05 was required to denote statistical significance of endocardial lead characteristics between groups.
As shown in Table 1⇓, the mean delivered energy at DFT was lower (15.9±5.4 J) when the transvenous lead was IROX-coated compared with the noncoated lead (19.1±5.1 J; P<.006). Stored energy was also lower with the IROX-coated lead, and both stored and delivered energies at DFT were reduced by 16% to 17% when the IROX-coated electrode was used compared with the uncoated electrode. Peak and final phase 1 voltages were also significantly reduced with the IROX-coated lead compared with the noncoated lead at DFT. The corresponding current values were not significantly different, however. Phase 2 voltage and current values were also not significantly different between the IROX-coated and noncoated leads.
Interestingly, the duration of the second phase was longer when the uncoated lead was used (8.4±1.1 versus 6.4±1.4 ms, P<.01) compared with the IROX-coated lead. The biphasic waveform used in this study was of variable duration because of the requirement for a fixed 65% tilt. The first phase duration, although slightly longer when the uncoated electrode was used compared with the IROX-coated electrode, was not significantly different.
In Vivo Lead Performance
At DFT intensity, the impedance changes (ie, initial and final) during each phase were obtained, and the results of both leads were compared (Table 1⇑). The initial impedance of the uncoated titanium coil and titanium can combination (41.3±4.7 Ω) was equivalent to the initial impedance of the IROX-coated lead system (41.7±5.8 Ω). However, the impedance rises during the two phases were significantly greater during shocks delivered through the uncoated lead (7.3±2.0 and 3.7±2.0 Ω, phases 1 and 2, respectively) than during shocks delivered through the IROX-coated lead (1.0±0.3 and 1.6±0.5 Ω, respectively; P<.01 both phases).
Because of concern that the impedance rise may be voltage dependent, the rates of impedance rise of both leads (ie, slopes of impedance versus pulse duration) during phase 1 were compared at equivalent peak voltage shocks (≈575 V; range, 523 to 616 V). Fig 2⇓ shows the lead impedance measurements throughout both phases obtained from one experimental preparation for shocks delivered through the noncoated and the IROX-coated leads. As can be seen, the impedance rose more slowly when the IROX-coated electrode was used compared with the uncoated lead. The mean slope of the regression line for the impedance-versus-time curve obtained for the uncoated lead during phase 1 (1.0±0.3 Ω/ms) was significantly greater than that obtained for shocks delivered through the IROX-coated lead (0.4±0.3 Ω/ms, P<.005; Table 2⇓).
The experimental results demonstrate that the IROX-coated leads behave in vivo as predicted from their previously described polarization behavior in vitro.9 Specifically, both leads demonstrated similar initial impedance values, but the impedance increased at a faster rate during shocks delivered through the uncoated electrode. The IROX coating, therefore, significantly improves current flow through the heart for shocks of similar voltage. Because current has been shown to more accurately reflect defibrillation requirements, shocks delivered through the IROX-coated electrode would be expected to be more effective at lower peak voltages and delivered energies.12 Our data support this hypothesis. The DFT energy and voltage values were significantly lower for the IROX-coated lead system compared with the uncoated lead system, by 16% to 17%. This reduction in DFT energy requirement approaches the reduction seen when biphasic waveforms were introduced.3 4
Polarization is an electrochemical process that occurs at the stimulating electrode/electrolyte interface during a stimulation impulse. It is manifested by an impedance rise during the delivery of the energy pulse and as a transient afterpotential (of opposite polarity) that appears immediately after the delivered pulse.13 The polarization effect of defibrillator electrodes has been recognized in a previous study by Alt et al14 in which the effectiveness of a carbon-braid electrode (a low-polarization material) was compared with two clinically available leads: the CPI 0062 Endotak and the Medtronic 6966 Transvene. The carbon-braid electrode exhibited significantly lower DFTs compared with the clinical leads.
Although the superiority of the carbon-braid electrode was demonstrated, the mechanism underlying the differences seen was not elucidated. There are several differences between this study and that of the carbon-braid electrode study. Positioning of the three electrodes in the study by Alt et al was visually approximated with fluoroscopy, whereas the electrodes used in our study were carefully exchanged to maintain a constant endocardial position. Our concern over the electrode exchange is due to the results of prior studies that have shown that small differences in endocardial lead positioning produce significant differences in defibrillation efficacy.6 7 We believe that the sheath exchange method closely reproduces the endocardial position between electrodes and reduces this source of variance. In addition, the subcutaneous can electrode position was constant throughout our study, and this type of electrode is more clinically relevant to present ICD systems than the subcutaneous array electrode used in the study by Alt et al. Another difference between these studies is that DFTs in the Alt study were obtained by a modified step-down technique, in which the lowest-intensity successful shock in a decremental series of shocks is defined as the DFT. The DFT method in the present study is an average based on a minimum of five shocks, and the reproducibility of this technique, as well as its correlation to the probabilistic model of defibrillation efficacy, has been documented previously.11
The leads used in this study are identical except for the IROX coating, in that the coil length and conductor materials were exactly alike. The three electrodes compared in the carbon electrode study, however, were of various lengths and constructions.15 Such differences may have affected current distribution and consequently, defibrillation efficacy. Therefore, the differences in DFT seen among the three leads used in the Alt study cannot be attributed entirely to the carbon-braid material and its nonpolarizing properties.
The results of the present study indicate a small difference between the defibrillating waveforms delivered through the IROX-coated and uncoated leads. Specifically, the mean duration of the second phase, delivered through the uncoated lead, was ≈2 ms longer than the first phase, whereas shocks delivered through the IROX-coated lead yielded a mean second phase only 0.5 ms longer than the first phase (not statistically different). This difference is due to the nature of the defibrillating waveform used in this study and the impedance changes related to the respective leads, noted above. The biphasic waveform used in this study is a clinically available, biphasic truncated exponential waveform with a fixed tilt (65%) for both phases. Because only the tilt was used to determine the waveform, duration would vary to fulfill the tilt criterion. The impedance differences shown between the two leads used in this study are the most likely explanation for the differences in phase duration.
Previous authors had reported that certain biphasic waveforms with longer second phases produced higher DFT energies than waveforms with second phases less than or equal to the first phase.16 17 However, those earlier studies were performed in dogs with epicardial patches, and the DFT energies reported were <10% of the DFT energies seen in the present study. A more recent study using a swine model and endocardial electrodes, similar to the present study, had somewhat different results.18
In this recent study, the DFT of a biphasic waveform similar to that produced in the present experiment (first phase, 6 ms) was tested with various second-phase durations.18 There was no significant effect on DFT, measured as energy or peak voltage, for a longer second phase of up to 10-ms duration, when the polarity of the first phase was negative (cathodal), as in our study. This finding supports our conclusion that the IROX coating is responsible for the reduction in DFT seen in our results.
Limitations of the Study
The present study was performed over a relatively narrow range of shock intensity in each animal, so that the in vivo lead performance was not assessed over the entire range of clinically relevant peak voltages. For example, it is possible that the magnitude of the polarization effect may differ at lower shock intensities, such as those used for cardioversion of ventricular tachy-cardia. In addition, a fixed 65% tilt biphasic waveform was used in this study. Other shock waveforms may exhibit different effects on DFT when delivered through an electrode coated with IROX. The experimental design also did not allow for measurement of a polarization-induced afterpotential, which may have effects on defibrillation, postshock dysrhythmias, and possibly postshock sensing.
In summary, this study demonstrates that a transvenous defibrillation electrode coated with IROX significantly reduces defibrillation energy requirements compared with an otherwise identical, uncoated electrode. The major difference seen between the electrodes was the reduction in the time-dependent rise in impedance seen during the shock pulse, theoretically due to electrode polarization. As a result, current was reduced as the impedance rose, necessitating a higher peak voltage and consequently, a higher delivered energy to achieve defibrillation. It is noteworthy that the magnitude of the reduction in energy requirement afforded by the IROX coating approaches the difference seen between biphasic and monophasic waveforms. The inclusion of an IROX-coated electrode with present ICD technology, which incorporates a biphasic waveform and an active pectoral can electrode, may further reduce clinical DFTs. This combination could allow for additional ICD size and energy output reduction.
Selected Abbreviations and Acronyms
This work was supported by a grant from Sulzer Intermedics Inc, Angleton, Tex. We are grateful to Donald Hills for his expert technical assistance and Marianne Jarosiak for her secretarial support.
- Received April 14, 1997.
- Revision received June 20, 1997.
- Accepted July 15, 1997.
- Copyright © 1997 by American Heart Association
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