Abstract: A frequency domain signal processing and analysis method and apparatus displays, plots and makes measurements upon electrocardiographic signals (ECG) recorded from the body surface. Both the graphic plots and the numeric parameters measured reveal abnormalities of electrical conduction within the heart thought to be important in the identification of patients at risk of serious disturbances of heart rhythm including sudden death. The invention employs Fourier analysis of short overlapping segments of ECG signal to create a three dimensional map ("spectrocardiogram") whose axes are time, fequency and power thus disclosing changes in the frequency content of the ECG signal over short intervals of time.