John E. Heil

Implantable cardioverter defibrillator lead technology : improved performance and lower defibrillation thresholds

The performance of an ICD system depends, in part, on the effectiveness with which the lead system functions. Engineering trade‐offs are made during the design of a lead to optimize one or more performance characteristics: e.g., lead handling, fatigue life, size, and optimized therapy delivery. To assure low defibrillation thresholds, careful attention must be taken during the design process to prevent these trade‐offs from hampering the leads therapy effectiveness. Four basic design rules are described that capture many of the engineering concepts that will enhance a leads efficacy: (1) minimize electrode pullback, (2) deliver current to the apex, (3) minimize energy loss in the lead, and (4) use large, efficient electrodes. These rules speak to optimizing delivery of current to the heart and efficiency of the lead and electrode interface. When the lead performs its function well, the complete ICD system of the heart, lead, and implantable pulse generator will provide optimal safety margins for device implant and an increased number of patients that can be implanted with a single‐lead system.

Effect of Shock Timing on Defibrillation Success

The goal of this study was to determine whether delivering transvenous defibrillation shocks, coordinated with the up/down‐slope VF waveform patterns in the shocking lead, would improve the probability of successful defibrillation. Anesthetized swine (32–38 kg, n = 8) were implanted with an RV → SVC + SQArray transvenous system to measure VF waveform patterns and to deliver shocks. The shocks were generated by a Cardiac Pacemakers Inc. biphasic waveform generator. Energy required for 50% success probability (E50) was determined using the multishock up‐down protocol. VF was repeatedly induced and defibrillation shocks at E50 were given after 10 seconds. The defibrillation outcome, delivered energy (Ed), peak voltage (V), peak current (I), system impedance (Z) and VF waveform pattern at the time of shock were recorded and measured. Out of a total of 685 shocks, 324 (47%) succeeded and 361 (53%) failed. The Ed, V, I, and Z were similar for the two defibrillation outcome groups (success or failure). VF patterns were classified as high or low amplitude at the time of the shock based on the peak‐to‐peak amplitude of signals recorded between shocking electrodes. Shocks that coincided with high amplitude VF patterns were further divided into shocks that occurred on the up‐slope or on the down‐slope. The probability of success when the E50 shocks were coincident with high or low amplitude fibrillation did not differ significantly (Students t‐test: 46% vs 48%, P = NS). However, during high amplitude fibrillation, shocks delivered on the up‐slope were significantly more successful than those delivered on the down‐slope (Chi‐square: 67% vs 39%; P < 0.001). These results suggest that delivering defibrillation shocks during the up‐slope of the high amplitude signal in the shocking lead may improve the probability of successful defibrillation of ICDs.

Large Capacitor Defibrillation Waveform Reduces Peak Voltages Without Increasing Energies

This study tested the hypothesis that increasing capacitance would allow a reduction in ICD size without reducing the deliverable energy. For example, the volume of a single 450 μF capacitor (390 V peak) is 1/3 less than that of two 250 μF capacitors (780 V), but it can store equivalent amounts of energy.Methods: Endocardial defibrillation electrodes (3.4 cm) were positioned in the RV apex and at the RA/ SVC junction in six mixed‐breed, isoflurane anesthetized pigs (41 ± 3 kg). Three 17‐cm ribbon wires were positioned subcutaneously on the left lateral chest (SQArray). Two CPI VENTAK ECDs were equipped to deliver 60/40 biphasic waveforms using either 125 μF (STD) or 500 μF (LD) of capacitance. A 15 shock up/down protocol was used to determine the 50% probability of success levels for each waveform in each animal. Shocks were delivered from RV(‐)→SVC + SQArray(+) in random order. Results were compared using paired Studentsf‐tests and are reported as mean ± SE. Results: The 500 μF long duration waveform reduced peak voltage 41% (374 ± 18 V [STD] vs 219 ± 14 V (LD], P < 0.001) and reduced peak current 38% (11.0 ± 1.1 A [STD] vs 6.8 ± 0.6 A [LD], P < 0.001) but did not significantly change the delivered energy(12.4 ± 1.3 J [STD] vs 13.4 ± 1.0 J [LD]). Durations increased from 10.0 ± 0.2 to 17.6 ± 0.5 msec (P < 0.001).Conclusions: Defibrillation with a 500 μF, long duration biphasic defibrillation waveform received similar amounts of energy but significantly reduced peak voltage and current compared to a waveform produced from 125 μF. A single large capacitor could be used to reduce the physical size of an ICD compared to the standard two capacitor system.

A comparison of standard vs up/down methods for determining the deftbrillation probability curve

Computer simulations showed that the combination of a sequential up/down test protocol and maximum likelihood curve fitting produced better estimates of defibrillation probability curves than a batch method. Specifically, for equal numbers of test shocks the up/down method gave 22% better estimates of p50 than the standard 5 bin method. Alternatively. the up/down method required only half the number of shocks for similar accuracy.

Differences in Intersequence System Impedance Trends For Nonthoracotomy Defibrillation Electrode Configurations In Pigs

System impedance (Z) impacts Automatic Implantable Cardioverter Defibrillator (AICDTM) design and therapy effectiveness. During multi-shock defibrillation studies, Z changes. Therefore, we investigated the impact of electrode configuration on Z. Two electrode configurations employing endocardial catheter electrodes and a subcutaneous mesh patch (P) were studied in six pigs. Plots of Z and current division over 15 shock test sequences suggest that the impedance contribution from P (I) dominates Z when the catheter electrodes are connected to form a common cathode (parallel configuration) but (2) has minimal influence on Z when the proximal catheter electrode is connected with P to form a common anode (orthogonal configuration). These findings agree with a simple theoretical circuit model and may have important clinical implications for defibrillation systems utilizing constant duration waveforms.