Field topography and waveform efficacy

This study examines whether electric field topography as created by transvenous catheter electrodes influences the shape of the probability of defibrillation (PD) curves for monophasic (MP) and biphasic (BP) shock waveforms. Langendorff isolated rabbit hearts were immersed in the middle of a shock bath filled with an isotonic, isoresistive bath solution. To create a relatively uniform field, patch electrodes were placed one at either end of the bath. A catheter field topography was produced by a vertical catheter electrode positioned 5 mm from the heart. Equal duration, equal time constant monophasic (5 ms) and biphasic (3 ms:2 ms) waveforms were tested under one field topography for each heart. Shock voltages corresponding to 50% (VSO) and 80% (V80) PD were determined. V50 for the MP and BP waveforms were 316±72 V and 231±51 V (↓27%, p=0.001) for the uniform field and 292±76 V and 200±54 V (↓31%, p=0.001) for the catheter field. V 80 was 375±85 V and 298±66 V (↓20%, p=0.005) and 350±100 V and 236±66 V (↓32%, p=0.001) respectively. The relative efficacy of BP over MP was enhanced under the catheter field compared to the uniform field and this enhancement increased with increasing shock strength. This suggests that waveform efficacy is influenced by field topography