A medical device for determining an atrial arrhythmia event, comprising: a sensor to sense a signal comprising ventricular cycle length information in a patient; and a processor (54, 60) to receive the signal and configured to determine RR intervals defining ventricular cycle lengths during an established time interval, and characterized by the processor being configured and programmed to detect signal noise during the time interval, said detecting comprising comparing each RR interval with a noise interval threshold, and determining an RR interval to be indicative of signal noise if it is less than or equal to the noise interval threshold; reject a ventricular cycle length determined to be indicative of signal noise, determine a plurality of cycle length differences from the cycle lengths not rejected during the time interval, and determine the atrial arrhythmia event in response to the determined cycle length differences; wherein determining the atrial arrhythmia event comprises: populating bins of a two dimensional histogram for the time interval with data points each having an x-coordinate and a y-coordinate and each representing three consecutive non-rejected cardiac cycle length differences of the determined plurality of cycle length differences, to provide a representation of ventricular cycle length variability; wherein populating the histogram comprises skipping up to four possible data points involving the rejected cardiac cycle length, and determining the atrial arrhythmia event from the populated histogram and based on the number of occupied bins. a sensor to sense a signal comprising ventricular cycle length information in a patient; and a processor (54, 60) to receive the signal and configured to determine RR intervals defining ventricular cycle lengths during an established time interval, and characterized by the processor being configured and programmed to detect signal noise during the time interval, said detecting comprising comparing each RR interval with a noise interval threshold, and determining an RR interval to be indicative of signal noise if it is less than or equal to the noise interval threshold; reject a ventricular cycle length determined to be indicative of signal noise, determine a plurality of cycle length differences from the cycle lengths not rejected during the time interval, and determine the atrial arrhythmia event in response to the determined cycle length differences; wherein determining the atrial arrhythmia event comprises: populating bins of a two dimensional histogram for the time interval with data points each having an x-coordinate and a y-coordinate and each representing three consecutive non-rejected cardiac cycle length differences of the determined plurality of cycle length differences, to provide a representation of ventricular cycle length variability; wherein populating the histogram comprises skipping up to four possible data points involving the rejected cardiac cycle length, and determining the atrial arrhythmia event from the populated histogram and based on the number of occupied bins. populating bins of a two dimensional histogram for the time interval with data points each having an x-coordinate and a y-coordinate and each representing three consecutive non-rejected cardiac cycle length differences of the determined plurality of cycle length differences, to provide a representation of ventricular cycle length variability; wherein populating the histogram comprises skipping up to four possible data points involving the rejected cardiac cycle length, and determining the atrial arrhythmia event from the populated histogram and based on the number of occupied bins.