A heart pacemaker comprising means (3,5) for sensing electrical events in the atrium and ventricle of a heart; means (11, 13) for pacing the ventricle in response to sensed atrial events; and means for selecting a value of a ventricular rate limit which defines the maximum rate at which the ventricle can be paced comprises means (11) for selecting a value for a VA time period and for timing the period from a sensed or paced ventricular event; means (15, 17) for pacing the atrium if an atrial event is not sensed within said VA time period; and means for increasing the VA time period by a predetermined VA amount when the ventricle is paced at said ventricular rate limit.
The pacemaker enhances the synchronization of ventricular pacing to natural atrial events by defining an extended VA interval when the ventricle is paced at the ventricular rate limit.