A method and medical device for determining a cardiac event that includes sensing a cardiac signal, determining a predetermined number of sensed cardiac events in response to the sensed cardiac signal, determining a plurality of sensed event windows in response to the predetermined number of the sensed cardiac events, determining, for each of the plurality of sensed event windows, whether a number of paced events is less than a paced event threshold, determining whether intervals within the sensed event windows having a number of paced events less than the paced event threshold are greater than an interval threshold, determining an interval difference factor for each of the plurality of windows having intervals less than the interval threshold, and determining the cardiac event in response to the interval difference factors determined for each of the plurality of windows.