Patent ID: 8744556

Claim:
An implantable or other ambulatory medical device comprising: a primary cardiac signal sensing circuit configured to sense at least a first cardiac signal associated with cardiac depolarization, the primary cardiac signal sensing circuit configured to be electrically coupled to at least first and second implantable or wearable electrodes; a secondary cardiac signal sensing circuit configured to sense a secondary cardiac signal, the secondary cardiac signal sensing circuit to be coupled to at least a third electrode different from the first and second electrode; and a control circuit, communicatively coupled to the primary and secondary cardiac signal sensing circuits, the control circuit including a noise detection circuit that includes: an alignment circuit configured to align a segment of the sensed first cardiac signal with a segment of the sensed secondary cardiac signal; and a turn counter configured to: determine a number of turns in the first cardiac signal segment; and determine a number of turns in the secondary cardiac signal segment; wherein the noise detection circuit is configured to identify a signal turn as a change in direction of the cardiac signal from a positive direction to a negative direction or from a negative direction to a positive direction; increment a turn count when identifying a signal turn within a specified turn detection timing window; compare a number of turns identified in a detection timing window specified for the first cardiac signal segment to a first specified turn count threshold; compare a number of turns in a detection timing window specified for the secondary cardiac signal segment to a second specified turn count threshold; and generate the indication of noise when the number of turns in the first cardiac signal detection timing window and the number of turns in the secondary cardiac signal detection timing window both satisfy their respective turn count threshold; and provide the indication of noise to a user or process.