Alternating duration of ventricular paced cycles during automatic mode switching of a DDDR pacemaker

This report describes the response of a Vitatron Diamond II DDDR pacemaker to atrial flutter in a patient who developed alternating duration of the ventricular paced cycles during automatic mode switching to the DDIR mode. This device activates mode switching on a beat-to-beat basis and exhibits "ventricular hysteresis" in the DDIR mode when an atrial sensed event is detected during the period corresponding with the AV delay prior to emission of the ventricular stimulus. Such unusual behavior must not be interpreted as pacemaker malfunction.