Do intramural virtual electrodes facilitate successful defibrillation? Model-based analysis of experimental evidence.
BACKGROUND
Recent computer model and experimental studies have suggested that microscopic intramural collagenous planes may facilitate successful defibrillation through the generation of shock-induced virtual electrodes deep within the ventricular wall. Evidence supporting the existence of intramural virtual electrodes has been drawn from several recent studies, which map shock-induced membrane potential (Vm) over the cut transmural surface of dissected segments of porcine left ventricle (LV). The artificially created transmural boundary in these experiments is impermeable to intracellular current. It is not known how this constraint limits the interpretation of these experiments in terms of the shock response of the intact ventricle.


RESULTS
This study uses a realistic 3D computer model of LV myocardium to aid experimental interpretation. The model incorporates a microstructural description of intramural cleavage plane discontinuities measured by confocal microscopy of rat LV. Electrical shocks are applied across the model tissue, with and without introduced transmural boundaries. Shocks of varying strength (4-40 V/cm) are also applied to the model and the response analyzed. Results show that shock-induced Vm changes (deltaVm) on a transmural tissue boundary are significantly different to deltaVm of the intact ventricle, and the extent of difference depends on boundary orientation. However, the presence and qualitative behavior of intramural virtual electrodes is preserved irrespective of boundary placement. The model also confirms experimental observations that most rapid transmural activation occurs for shocks of strength 5-10 V/cm. Two distinct mechanisms suppress virtual electrode propagation, and hence slow tissue activation, outside of this optimal shock strength range.


CONCLUSIONS
This study supports the hypothesis that distributed microscopic intramural virtual electrodes contribute to rapid activation of the ventricular wall during defibrillation.
Hooks
Darren A
DA
Bioengineering Institute, University of Auckland, New Zealand. d.hooks@auckland.ac.nz
Trew
Mark L
ML
Smaill
Bruce H
BH
Pullan
Andrew J
AJ
D000818:Animals; D003198:Computer Simulation; D004554:Electric Countershock / Q000295:instrumentation*; D004566:Electrodes; D021621:Imaging, Three-Dimensional; D008564:Membrane Potentials; D018613:Microscopy, Confocal; D051381:Rats; D014693:Ventricular Fibrillation / Q000503:physiopathology / Q000628:therapy; D016276:Ventricular Function