An example system and method of determining regularity associated with a rhythm disorder of a heart are disclosed. In accordance with the method, a derivative of a first cardiac signal at a plurality of first time points is processed against a derivative of a second cardiac signal at a plurality of second time points to define a plurality of coordinate pairs of the first cardiac signal against the second cardial signal. Thereafter, an index of regularity that exceeds a threshold is determined. The index of regularity indicates an approximate congruence of the plurality of coordinate pairs among the first cardiac signal and the second cardiac signal.
