Optical action potential upstroke morphology reveals near-surface transmural propagation direction

The analysis of surface-activation patterns and measurements of conduction velocity in ventricular myocardium is complicated by the fact that the electrical wavefront has a complex 3D shape and can approach the heart surface at various angles. Recent theoretical studies suggest that the optical upstroke is sensitive to the subsurface orientation of the wavefront. Our goal here was to (1) establish the quantitative relationship between optical upstroke morphology and subsurface wavefront orientation using computer modeling and (2) test theoretical predictions experimentally in isolated coronary-perfused swine right ventricular preparations. We show in numerical simulations that by suitable placement of linear epicardial stimulating electrodes, the angle phi of wavefronts with respect to the heart surface can be controlled. Using this method, we developed theoretical predictions of the optical upstroke shape dependence on phi. We determined that the level VF* at which the rate of rise of the optical upstroke reaches the maximum linearly depends on phi. A similar relationship was found in simulations with epicardial point stimulation. The optical mapping data were in good agreement with theory. Plane waves propagating parallel to myocardial fibers produced upstrokes with VF*<0.5, consistent with theoretical predictions for phi>0. Similarly, we obtained good agreement with theory for plane waves propagating in a direction perpendicular to fibers (VF*>0.5 when phi<0). Finally, during epicardial point stimulation, we discovered characteristic saddle-shaped VF* maps that were in excellent agreement with theoretically predicted changes in phi during wavefront expansion. Our findings should allow for improved interpretation of the results of optical mapping of intact heart preparations.