Abstract: Known methods of heart function analysis, while useful, suffer from a number of limitations. These include the fact that these methods are inherently static, in that they consider an individual electrocardiogram (ECG) that can only provide a picture of heart function at a particular moment in time. Furthermore, the assessment depends upon the expertise of specialists performing a visual analysis of ECG traces, which may be subjective, and of limited sensitivity. Accordingly, a method of processing ECG signals comprises the steps of extracting data from at least one ECG signal, determining a time derivative of the data and generating a normalised index value representative of the energy in the time derivative. The index value is compared with corresponding historical data of the patient.