David K. Swanson

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>>

Strength-duration relationship for biphasic defibrillation in dogs

The relationship between biphasic shock duration and defibrillation efficacy was measured in dogs with a catheter/patch lead system to better define defibrillation mechanisms with this waveform. The defibrillation energy, voltage, and current at 50% defibrillation success were measured for short (6 ms) and long (12 ms) biphasic shocks with similar fixed tilts and pulsewidth ratios (60:40). The short biphasic waveform had comparable energy requirements, but higher voltage (42%) and currents (39%) than the long biphasic shock (p<0.01). For these pulse widths, biphasic defibrillation in the dog follows a strength-duration relationship, with constant defibrillation energy requirements.<<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.

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.