Aim: Cardiac surface stimulation aims to terminate ventricular fibrillation with less pain and damage than strong electrical shocks administered to the chest and body. This study used computational cardiac electrophysiology to determine the minimal surface stimulus strength necessary to defibrillate human ventricles.
Methods: Ventricular defibrillation was studied in a large 10cmx10cmx1cm finite element model (100µm discretization with hexahedral elements) of left ventricular (LV) tissue with human electrophysiology simulated with CARP (https://carp.medunigraz.at/). This LV model was initially paced until steady-state at 1.3 Hz with S1 stimuli (LV apex) at twice threshold. A single S2 stimulus (apical posterior corner) was delivered at twice threshold and 300 ms after the last S1 to initiate reentry in the middle of the LV model. To terminate reentry, S3 stimuli (entire epicardial or endocardial surface) were applied 2 seconds after S2. Starting at the capture threshold, S3 strength was continuously doubled until reentry terminated. All stimuli were delivered as transmembrane current.
Results: Minimum endocardial S3 strength to terminate reentry was 1.6 amp at 64 times threshold, compared to epicardial S3 of 3.2 amp at 128 times threshold. This difference was due to transmural heterogeneity in action potential duration and conduction velocity resulting in different wavelengths for reentry (6.7cm epicardium < 9.0cm endocardium). For failed S3, the scroll wave region nearest the stimulus was slowed or halted, but eventually resulted in more complex reentry from loss of synchronicity with the opposing surface.
Conclusions: Endocardial surface stimulation at 64 times threshold is optimal for defibrillating human ventricles. To help guide future VF electrotherapies, optical imaging of this study repeated in swine LV wedge preparations is underway to determine if these stimulus strengths are feasible in living cardiac tissue, as well as below both the pain and damage thresholds for electrical shocks.