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The maximal upstroke of transmembrane voltage (dVm/dtmax) has been used as an indirect measure of sodium current INa upon activation in cardiac myocytes. However, sodium influx generates not only the upstroke of Vm, but also the downstroke of the extracellular potentials V including epicardial surface potentials Ves. The purpose of this study was to evaluate the magnitude of the maximal downstroke of Ves (|dVes/dtmin|) as a global index of electrical activation, based on the relationship of dVm/dtmax to INa. To fulfill this purpose, we examined |dVes/dtmin| experimentally using isolated perfused mouse hearts and computationally using a 3-D cardiac tissue bidomain model. In experimental studies, a custom-made cylindrical “cage” array with 64 electrodes was slipped over mouse hearts to measure Ves during hyperkalemia, ischemia, and hypoxia, which are conditions that decrease INa. Values of |dVes/dtmin| from each electrode were normalized (|dVes/dtmin|n) and averaged (|dVes/dtmin|na). Results showed that |dVes/dtmin|na decreased during hyperkalemia by 28, 59, and 79% at 8, 10, and 12 mM [K+]o, respectively. |dVes/dtmin| also decreased by 54 and 84% 20 min after the onset of ischemia and hypoxia, respectively. In computational studies, |dVes/dtmin| was compared to dVm/dtmax at different levels of the maximum sodium conductance GNa, extracellular potassium ion concentration [K+]o, and intracellular sodium ion concentration [Na+]i, which all influence levels of INa. Changes in |dVes/dtmin|n were similar to dVm/dtma- during alterations of GNa , [K+]o, and [Na+]i. Our results demonstrate that |dVes/dtmin|na is a robust global index of electrical activation for use in mouse hearts and, similar to dVm/dtmax, can be used to probe electrophysiological alterations reliably. The index can be readily measured and evaluated, which makes it attractive for characterization of, for instance, genetically modified mouse hearts and drug effects on cardiac tissue.