1. A method of deriving spatial and/or temporal distributions of action potentials across a heart, comprising: determining a T-wave alternan waveform by (a) differencing a plurality of temporally adjacent T-wave segments to obtain preliminary alternan waveforms, (b) compensating for disturbances and/or ectopic beats in the preliminary alternan waveforms, and (c) computing a final alternan waveform; and decomposing the final alternan waveform to provide information relating to changes in cardiac action potentials of the subject.