This is an intra-atrial tracing recorded via the telemetry channel of a Medtronic Model 7077 pacemaker. The pacemaker was implanted in a patient who had a heart rate of 75/min. when the pacer was set to the DDD mode. For this tracing the pacer was programmed to VVI mode at a rate of 70/min. Why did the pacer pace at 75/min in the DDD mode and why is it not now pacing at 70/min ?