A method of analysing changes in the electrical activity of a patients heart between a reference state and a test state, the method using a reference data set of electrophysiological data captured from the patient in the reference state and at least one test data set of electrophysiological data captured from the patient in the test state, each data set defining a plurality of electrograms for a respective plurality of spatial locations relative to the heart, the method comprising processing the electrophysiological data by, matching each electrogram in the reference data set to a corresponding electrogram in the at least one test data set to create a pair of electrograms for each of the plurality of spatial locations, and deriving a time delay for each spatial location by calculating the time delay between the electrograms of the pair of matched electrograms for that spatial location.
