Detects external noise using a motion sensor signal for example to increase the specificity of arrhythmia detections based on active muscle noise detection. Whenever a motion signal is present that is below or above a certain frequency, for example 5 Hz, or within a certain frequency range, for example 1 to 10 Hz, and/or above a certain amplitude, for example greater than 1 mg, or close to a known motion pattern, then the detection of fast ventricular arrhythmia is suspended. For the detection of slow arrhythmia, for example asystole or syncope, an episode is confirmed when a short lasting motion sensor signal occurs. Uses a motion sensor based signal, for example as obtained from an accelerometer on an implantable electrode lead and/or implantable device.