Optical Mapping of Intramural Virtual Electrodes in Porcine Left Ventricular Wall
Intramural virtual electrodes (IVE) are believed to play an important role in defibrillation, however their nature is still debatable and experimentally unproved. We used optical mapping technique and an isolated coronary perfused preparation of the porcine left ventricular wall to measure defibrillation shock-induced changes in cardiac cell transmembrane potential (∆Vm) during action potential (AP) plateau and diastole. The uniform electrical field was applied across ventricular wall in an epicardial-toendocardial direction. Primary study of this project (Fast et al., Circulation, 2004) performed measurements of shock-induced ∆Vm from the cut transmural surface of the LV preparation during the AP plateau. This study revealed the widespread distribution IVE across the entire LV wall. It was hypothesized that these IVE are due to multiple microscopic structural tissue discontinuities normally present in myocardial tissue due to collagen septa and blood vessels. This hypothesis could explain why in this study strong shocks applied during the AP plateau induced only negative ∆Vm. According to this concept, shocks produce microscopic polarizations of both signs but, because of spatial averaging and nonlinear membrane response, ∆Vm of only one sign is measured by macroscopic optical mapping. This prediction was successfully tested in the same LV preparations using microscopic and macroscopic optical mapping techniques during application of plateau and diastolic shocks. However, the extrapolation of these findings to the intact myocardium is limited by the differences between electrical properties of the cut transmural surface in wedge preparations and the intact LV wall. We hypothesized that due to integration of optical signals from the different tissue depths, IVE can be detected in optical recordings of ∆Vm measured from the intact LV epicardium. Therefore, we conducted studies where optical signals from the intact LV epicardium were recorded through the transparent mesh shock electrode. In these studies, the LV wedges were stained with a Vm-sensitive dye using two methods that allowed collecting the fluorescent light from different tissue depths. The obtained difference in shockinduced ∆Vm provides evidence for the existence of sub-epicardial IVE in the intact LV wall and confirms their critical role in shock-induced rapid activation of the myocardial tissue.
Research Assistant Professor Department of Biomedical Engineering University of Alabama at Birmingham Birmingham, Alabama