A cardiac device configured to terminate atrial fibrillation detection within a heart signal obtained from a heart comprising at least two sensing electrodes; at least one input channel configured to provide an input heart signal; a detector configured to detect QRS events, and to classify said QRS events as either sense events or noise events; a noise counter; an interval counter; a stable packet counter; and a processor, wherein said processor is configured to - increment and reset the noise counter; - increment, decrement and reset the interval counter; - increment and decrement the stable packet counter; wherein the cardiac device is configured to monitor ventricular signals defined by two consecutive sense events so as to determine whether an interval is unstable; and in that the processor is further configured - for a noise event, to decrement the interval counter and to increment the noise counter - to reset the noise counter and the interval counter and to increment the stable packet counter when the noise counter reaches a predetermined limit, - to increment the interval counter for two consecutive sense events, - to reset the noise counter and the interval counter and to increment the stable packet counter when the interval counter reaches a defined packet size and the number of unstable intervals is below a predetermined threshold, and - to terminate the detection of atrial fibrillation when the stable packet counter reaches a predetermined limit. at least two sensing electrodes; at least one input channel configured to provide an input heart signal; a detector configured to detect QRS events, and to classify said QRS events as either sense events or noise events; a noise counter; an interval counter; a stable packet counter; and a processor, wherein said processor is configured to - increment and reset the noise counter; - increment, decrement and reset the interval counter; - increment and decrement the stable packet counter; wherein the cardiac device is configured to monitor ventricular signals defined by two consecutive sense events so as to determine whether an interval is unstable; and in that the processor is further configured - for a noise event, to decrement the interval counter and to increment the noise counter - to reset the noise counter and the interval counter and to increment the stable packet counter when the noise counter reaches a predetermined limit, - to increment the interval counter for two consecutive sense events, - to reset the noise counter and the interval counter and to increment the stable packet counter when the interval counter reaches a defined packet size and the number of unstable intervals is below a predetermined threshold, and - to terminate the detection of atrial fibrillation when the stable packet counter reaches a predetermined limit. - increment and reset the noise counter; - increment, decrement and reset the interval counter; - increment and decrement the stable packet counter; wherein the cardiac device is configured to monitor ventricular signals defined by two consecutive sense events so as to determine whether an interval is unstable; and in that the processor is further configured - for a noise event, to decrement the interval counter and to increment the noise counter - to reset the noise counter and the interval counter and to increment the stable packet counter when the noise counter reaches a predetermined limit, - to increment the interval counter for two consecutive sense events, - to reset the noise counter and the interval counter and to increment the stable packet counter when the interval counter reaches a defined packet size and the number of unstable intervals is below a predetermined threshold, and - to terminate the detection of atrial fibrillation when the stable packet counter reaches a predetermined limit.