1. A method of quantifying risk of cardiac arrhythmia, comprising: receiving a first set of electrocardiogram (ECG) signals from spatially separated leads; generating, for each ECG signal in the first set of ECG signals, a median beat associated with the morphology of each respective ECG signal of the first set of ECG signals; receiving a second set of ECG signals from spatially separated leads; generating, for each ECG signal in the second set of ECG signals, a second median beat associated with the morphology of each respective ECG signal of the second set of ECG signals, wherein each second median beat corresponds to a respective one of the first median beats; calculating, for each ECG signal in the first set of ECG signals and a corresponding ECG signal in the second set of ECG signals, a residuum signal based on the first median beat for each ECG signal in the first set of ECG signals and the corresponding second median beat for each ECG signal in the second set of ECG signals; averaging the residuum signals 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.