a method of detecting heart beat coincidence includes receiving first and second signals from first and second heart beat sources . the method further includes detecting first heart beat occurrences on the first signal , each first heart beat occurrence having a respective time associated therewith and detecting second heart beat occurrences on the second signal , each second heart beat occurrence having a respective time associated therewith . the method further includes comparing the times of the first and second heart beat occurrences to detect coincidence .