An implantable medical device and associated method monitor a physiological signal for sensing physiological events and detecting a physiological condition in response to the sensed physiological events. The device senses a first event from the physiological signal, senses a noise signal in the physiological signal and senses a next event from the physiological signal wherein the first event and the next event define a signal interval. The signal interval is declared as a noisy interval in response to the sensed noise signal.