1. A method of quantifying risk of cardiac arrhythmia, comprising: receiving a first set of electrocardiogram (ECG) signals from spatially separated leads; generating a baseline beat template associated with the morphology of each ECG signal of the first set of ECG signals; receiving a second set of ECG signals from spatially separated leads; calculating, for each lead, a difference between each ECG signal in the second set of ECG signals and the corresponding baseline beat template to produce a residuum signal for each lead; averaging the residuum signals across the leads to produce an averaged residuum signal; and quantifying a spatio-temporal heterogeneity of the second set of ECG signals based on the residuum signals and, the averaged residuum signal, wherein the spatio-temporal heterogeneity is associated with arrhythmia risk.