Abstract:
It is widely argued that shock induced virtual electrode polarizations (VEPs) are key contributors to successful termination of reentry (cardioversion). To date, however, neither experimental nor modeling studies have been able to confirm the role of VEPs formed by myocardial laminar microstructure in cardioversion. The objective of this study was to elucidate the extent to which laminar discontinuities could contribute to the success or failure of cardioversion using a biophysically based model of myocardial microstructure, sufficiently large enough to support reentry. Spiral wave reentry was generated in large domain (~15cm2) 2D models derived from high resolution images of midwall porcine left ventricular myocardium. A modified LRd cell model incorporating an asymmetric response to electric shock and electroporation was used. Electric shocks of varying duration and waveform with uniform voltage gradients were applied at a range of strengths. Action potentials from regions adjacent to the pre-shock wavefront were reconstructed. The defibrillation threshold (DFT) for models with representation of discontinuous laminar microstructure was compared to that for models with continuously varying description of myofibre orientation. Virtual electrodes were generated distal to shock electrodes along laminar discontinuities. They mediated cardioversion by eliminating available excitable tissue and extending the action potential duration (APD) of tissue directly in the path of the pre-shock wavefront. Weak shocks and shocks of short duration failed to extend APD sufficiently, leading to failure of cardioversion by persistence of pre-shock reentry or generation of reentry with a different morphology. The defibrillation threshold (DFT) with a monophasic shock 10ms in duration was substantially lower for the discontinuous model (2.1 V/cm) than the control continuous model (>14 V/cm). The DFT in the discontinuous model is close to the range of shock strengths used clinically. This study shows for the first time that virtual electrodes generated by laminar discontinuities derived from real myocardium are a key contributor to effective cardioversion.