1. A method for classifying atrial tachyarrhythmia comprising: obtaining pre-defined numbers N, X and Y with N≦X<Y; monitoring atrial intervals and comparing a length of each atrial interval with a pre-defined value IL; storing a length of atrial interval into a stored length if the length is shorter than IL; evaluating the most recent up to N atrial intervals which have a length shorter than IL, in case X among a most recent Y number of atrial intervals are found to have length shorter than IL, and performing for each of the stored length a test with respect to the following criteria: an absolute value of a difference between the stored length and its preceding stored length does not exceed a pre-defined value SL, an absolute value of a difference between twice the stored length and one time of the preceding stored length does not exceed double of the value SL, and an absolute value of a difference between the stored length and twice the preceding stored length does not exceed double of the value SL; classifying an atrial tachyarrhythmia into a classification as stable if all tested lengths pass at least one of the criteria; and, controlling a cardiac device depending on the classification.