Near-threshold field stimulation: intramural versus surface activation.

OBJECTIVE The mechanism by which an electric field terminates arrhythmias continues to puzzle investigators. Existing experimental methods provide information about epicardial manifestations of electrical cardioversion, yet little is known about field effects deep inside the myocardium. Here we combine specially designed optical mapping experiments and computer modeling to separate the intra-myocardial and surface field effects. METHODS We used isolated coronary perfused and superfused slabs of pig right ventricular wall (n=6) stained with di-4-ANNEPS. A uniform transmural field was produced via two parallel planar (5 x 5 cm) transparent mesh electrodes aligned with the endocardial and epicardial surfaces. Low-intensity shocks (< or =3.3 V/cm) were applied during diastole. The electrical activity under both electrodes was recorded simultaneously using two CCD cameras at 800 frames/s. Shock responses were also simulated using a bidomain Luo-Rudy model. RESULTS We discovered that during the near-threshold diastolic field stimulation, when surface polarization should be dominant, the early activation occurs not at the cathodal surface, as might be expected, but deep inside the myocardium. Comparison of epi- and endocardial activation delays suggests that the sites of early activation are located closer to the endocardium. Our experimental observations could be reproduced computationally by assuming large resistive heterogeneities inside the myocardial wall. CONCLUSIONS Surface polarization plays a minor role during field stimulation. Intramural virtual electrodes produced even by weak fields are sufficiently strong to initiate intra-myocardial excitation. Significant heterogeneities in tissue resistivity may explain the strength of the virtual electrodes.