It is important to recognize that neither of the measurement methods described here interrogates just from a single cell. One can interpret each recording obtained from hearts optically or with an extracellular electrode as a weighted summation of contributions from many cells within an interrogated volume. In hearts, the optically interrogated volume is relatively large (approximate millimeter scale) because significant scatter of light allows some photons that originate from regions surrounding the center of the recording site to get into the light detector. Enhanced volume of interrogation due to scattering is not specific to laser scanner methods. Monte Carlo models also indicate that the interrogated width exceeds the imaged surface area due to scattering when the broad-field illumination and photodiode array-based or camera-based optical mapping methods are used.37

Due to optical summation in the heart studies, it would not have been possible to observe shock-induced changes in transmembrane potentials on a cellular size scale. If microscopic membrane polarization does exist in hearts, this might be a mechanism for electric field stimulation. The anomalous activation observed in experiments might be produced by microscopic membrane polarizations. From a theoretical perspective, small structures including capillaries, connective tissues, and individual cells or cell bundles, are capable of producing membrane polarization by the local redistribution of current.

The conclusion that the interrogated volume for the optical method extends deeper than that for the electrical methods opens possibilities for more detailed study of surface and subsurface activation in the heart. These two mapping methods used together provide limited information on three-dimensional distributions of activation near the surface.

Future research may achieve subcellular optical mapping in hearts. Multiphoton excitation, which is capable of microscopic resolution, has been used to excite transmembrane potential dependent fluorescence in hearts.38'39 This suggests possibilities to examine subcellular membrane polarizations during shocks. Also optical measurements with cellular resolution in hearts may enable a more complete understanding of the activations during arrhythmias.

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