Stephen J. Hahn

A New Algorithm to Reduce Inappropriate Therapy in the S-ICD System

The subcutaneous ICD system (S‐ICD) has been shown to be a safe and effective treatment for patients at risk for sudden cardiac death. This device reliably detects ventricular tachyarrhythmias with a low incidence of inappropriate shocks for supraventricular arrhythmias. However, T‐wave oversensing (TWOS) is more common with the S‐ICD compared with transvenous systems. We developed a novel discrimination algorithm to reduce TWOS without compromising tachyarrhythmia discrimination.

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.

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.

Evaluation of the current distribution from implantable defibrillation electrodes using image analysis of bubble formation in a homogeneous tank

Typical electrodes used for internal and external defibrillation are known to have nonuniform current distributions which are suspected to reduce their defibrillation efficacy. The authors have developed a system based on the image analysis of the bubble formation on the surface of electrodes to qualitatively assess the the current distribution on a defibrillation electrode. Results show that with typical patch electrodes most of the current emanates from the corners. Similarly, with catheter electrodes most of the current emanates from the ends. The system was used to optimize the segment spacing of a new array electrode to achieve better field uniformity.<<ETX>>

A flexible, interactive finite difference system for modeling defibrillation shock fields

A system has been developed for modeling potential and current density in an arbitrary geometry using a finite difference approach and has been applied to the study of current flow from defibrillation electrodes in either homogeneous media or a physically accurate torso model. The torso models can be used to estimate the percent energy delivered to the heart from an implanted lead system and to identify regions of high or low gradient in the heart. Realistic torso models were generated from magnetic resonance imaging or computerized tomography scans which have been automatically tissue typed and converted to a conductivity array. Results demonstrate the pathways current follows through the thorax when using a lead system involving various endocardial and subcutaneous electrodes.<<ETX>>

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.