Intramural virtual electrodes during defibrillation shocks in left ventricular wall assessed by optical mapping of membrane potential.
BACKGROUND
It is believed that defibrillation is due to shock-induced changes of transmembrane potential (DeltaV(m)) in the bulk of ventricular myocardium (so-called virtual electrodes), but experimental proof of this hypothesis is absent. Here, intramural shock-induced DeltaV(m) were measured for the first time in isolated preparations of left ventricle (LV) by an optical mapping technique.


RESULTS
LV preparations were excised from porcine hearts (n=9) and perfused through a coronary artery. Rectangular shocks (duration 10 ms, field strength E approximately 2 to 50 V/cm) were applied across the wall during the action potential plateau by 2 large electrodes. Shock-induced DeltaV(m) were measured on the transmural wall surface with a 16x16 photodiode array (resolution 1.2 mm/diode). Whereas weak shocks (E approximately 2 V/cm) induced negligible DeltaV(m) in the wall middle, stronger shocks produced intramural DeltaV(m) of 2 types. (1) Shocks with E>4 V/cm produced both positive and negative intramural DeltaV(m) that changed their sign on changing shock polarity, possibly reflecting large-scale nonuniformities in the tissue structure; the DeltaV(m) patterns were asymmetrical, with DeltaV-(m)>DeltaV+(m). (2) Shocks with E>34 V/cm produced predominantly negative DeltaV(m) across the whole transmural surface, independent of the shock polarity. These relatively uniform polarizations could be a result of microscopic discontinuities in tissue structure.


CONCLUSIONS
Strong defibrillation shocks induce DeltaV(m) in the intramural layers of LV. During action potential plateau, intramural DeltaV(m) are typically asymmetrical (DeltaV-(m)>DeltaV+(m)) and become globally negative during very strong shocks.
Fast
Vladimir G
VG
Department of Biomedical Engineering, University of Alabama at Birmingham, Birmingham, Ala 35294, USA. fast@crml.uab.edu
Sharifov
Oleg F
OF
Cheek
Eric R
ER
Newton
Jonathan C
JC
Ideker
Raymond E
RE
D000200:Action Potentials; D000818:Animals; D046508:Culture Techniques; D004554:Electric Countershock; D004566:Electrodes; D005260:Female; D005456:Fluorescent Dyes / Q000737:chemistry; D006352:Heart Ventricles / Q000033:anatomy & histology; D007700:Kinetics; D008297:Male; D008564:Membrane Potentials; D013552:Swine; D016276:Ventricular Function
D005456:Fluorescent Dyes