A method and system for determining a source of a cardiac rhythm disorder includes collecting cardiac signals at a plurality of locations during a cardiac arrhythmia and identifying activation times within the cardiac signals. After computing the time-dependent phase of each region of tissue, a level of phase synchrony is determined between each pair of locations to assign a synchronization number for each pair of locations. Using the synchronization number, a spatial synchrony map is generated to identify one or more asynchronous tissue regions surrounded by regions of synchrony in the patient's heart.
