A method of determining whether events in a heart's electrical activity result in corresponding blood flow events comprises simultaneously taking an electrocardiograph (ECG) and an impedance cardiograph (ICG) of the heart. Successive periods of the ECG are examined to detect successive occurrences of at least one periodic waveform feature indicative of electrical activity of the heart (14), and a period of time (search period) is defined following each such detection. The ICG is examined in each search period to detect the occurrence of a waveform feature (16) indicative of blood flow resulting from the detected occurrence in the ECG, and a signal is generated indicative of the degree of concordance (20) of the ICG with the ECG as a function of measurements made in respect of said detected waveform features.
