Extracting intramural wavefront orientation from optical upstroke shapes in whole hearts

Information about intramural propagation of electrical excitation is crucial to understanding arrhythmia mechanisms in thick ventricular muscle. There is currently a controversy over whether it is possible to extract such information from the shape of the upstroke in optical mapping recordings. We show that even in the complex geometry of a whole guinea pig heart, optical upstroke morphology reveals the 3D wavefront orientation near the surface. To characterize the upstroke morphology, we use V(F)(*), the fractional level at which voltage-sensitive fluorescence, V(F), has maximal time derivative. Low values of V(F)(*)( approximately 0.2) indicate a wavefront moving away from the surface, high values of V(F)(*) ( approximately 0.6) a wavefront moving toward the surface, and intermediate values of V(F)(*) ( approximately 0.4) a wavefront moving parallel to the surface. We further performed computer simulations using Luo-Rudy II electrophysiology and a simplified 3D geometry. The simulated V(F)(*) maps for free wall and apical stimulations as well as for sinus rhythm are in good quantitative agreement with the averaged experimental results. Furthermore, computer simulations show that the effect of the curvature of the heart on wave propagation is negligible.