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This chapter summarizes the current understanding of fibrillation and defibrillation mechanisms, and also explores one of the leading hypotheses of defibrillation, the virtual electrode polarization (VEP) hypothesis. The VEP hypothesis of defibrillation presents the most comprehensive explanation of defibrillation. It is based on solid experimental evidence and theoretical foundation. The chapter explores the VEP mechanism in more detail, especially in the context of successful and failed defibrillation mechanisms using optical mapping technology. The virtual electrode hypothesis of defibrillation, along with optical mapping techniques has made great strides toward explaining many experimentally and clinically observed phenomena, with implications for safe and efficient defibrillation. However, clinical advances are still to be gained from this knowledge. The authors believe further improvement of the VEP hypothesis of defibrillation may result in development of damage- and pain-free, effective and safe therapy that could realistically be translated to humans.

Magnetic resonance imaging (MRI) allows visualization of location and extent of radiofrequency (RF) ablation lesion, myocardial scar formation, and real-time (RT) assessment of lesion formation. In this study, we report a novel 3-Tesla RT -RI based porcine RF ablation model and visualization of lesion formation in the atrium during RF energy delivery.The purpose of this study was to develop a 3-Tesla RT MRI-based catheter ablation and lesion visualization system.RF energy was delivered to six pigs under RT MRI guidance. A novel MRI-compatible mapping and ablation catheter was used. Under RT MRI, this catheter was safely guided and positioned within either the left or right atrium. Unipolar and bipolar electrograms were recorded. The catheter tip-tissue interface was visualized with a T1-weighted gradient echo sequence. RF energy was then delivered in a power-controlled fashion. Myocardial changes and lesion formation were visualized with a T2-weighted (T2W) half Fourier acquisition with single-shot turbo spin echo (HASTE) sequence during ablation.RT visualization of lesion formation was achieved in 30% of the ablations performed. In the other cases, either the lesion was formed outside the imaged region (25%) or the lesion was not created (45%) presumably due to poor tissue-catheter tip contact. The presence of lesions was confirmed by late gadolinium enhancement MRI and macroscopic tissue examination.MRI-compatible catheters can be navigated and RF energy safely delivered under 3-Tesla RT MRI guidance. Recording electrograms during RT imaging also is feasible. RT visualization of lesion as it forms during RF energy delivery is possible and was demonstrated using T2W HASTE imaging.