The Circadia pacemaker was one of the very few devices that had a lead-borne thermistor to measure cental venous temperature (CVT) as a sensor for rate-response.
A unique feature of this pacemaker was an iridium-oxide (IrOx)-coated button welded to the can. It was believed that this button would improve unipolar IEGM sensing and reduce unipolar pacing thresholds (it didn’t).
Click here for more detail CVT-sensing rate-responsive pacemakers.
A very detailed engineering discussion on CVT-sensing pacemakers is available at Mayotte MJ. “Rate adaptation by temperature.” In: Webster JG, ed. Design of Cardiac Pacemakers. IEEE Press, Piscataway, NJ, 1995: 335–368.