An apparatus comprises first and second sensing circuits, a template generator circuit, and a correlation circuit. The correlation circuit is configured to identify a first fiducial position in a third cardiac signal sensed using the first sensing circuit during a detected rhythm with elevated ventricular rate, align the template correlation features and the correlation features of a fourth cardiac signal using the first fiducial position, calculate a correlation using the correlation features of the template and the correlation features of the fourth cardiac signal, and iteratively searching for a replacement to the first fiducial position in the third cardiac signal according to the calculated correlation.