The present invention relates to a system for automatically distinguishing atrial flutter from atrial fibrillation based on a sequence of R-R-intervals of ECG data. The system of the invention comprises means adapted to calculate or simulate an R-R-pattern from a model which is based on a periodic or almost periodic signal representing peaks of an atrial flutter signal, a first AV-block scheme blocking respective ones of the atrial excitations while transmitting the others with a predetermined transmittal delay and at least one more AV-block scheme blocking respective ones of the excitations transmitted by the previous AV-block scheme and transmitting the others with a predetermined transmittal delay. The excitations transmitted by the last AV-block scheme constitute the calculated R-R-pattern. The system further comprises consistency checking means adapted to automatically check whether said sequence of R-R-intervals of the ECG-data is to some predetermined degree consistent with said calculated or simulated R-R-pattern. The system is configured to identify the ECG-data to be based on atrial flutter if said predetermined degree of consistency is found.
