A medical device includes a housing configured for implantation within a body of a patient, and detection circuitry disposed in the housing and coupled to an electrode arrangement. The detection circuitry is configured to sense cardiac signals from the patient. A processor is coupled to the detection circuitry. The processor is configured to compare the cardiac signals to an initial detection threshold, automatically generate an additional detection threshold in response to a predetermined number of the cardiac signals meeting or exceeding the initial detection threshold or a previously generated detection threshold, count each occurrence of a cardiac signal meeting or exceeding each of the respective detection thresholds, and record cardiac signal data only for a cardiac signal that meets or exceeds the highest of the detection thresholds.