UPPER RATE LOCKOUT - AN UNEXPECTED FEATURE OF RATE-ADAPTIVE ATRIOVENTRICULAR DELAY

We present a case in which use of rate adaptive AV delay resulted in u nexpected pacemaker 2:1 AV block when the patient's atrial rate exceed ed the pacemaker maximum tracking rate but was below the predicted mul tiblock rate. 'Lockout'' of normal upper rate behavior was accompanied with th e requirement of a slower atrial rate for reassociation than loss of atrial tracking, a form of upper rate hysteresis. The mechanis m of upper rate lockout is discussed, along with potential ways to avo id the problem. The use of software based pacemakers with an extended range of programmable options allows the most flexibility in optimizin g pacemaker performance in an individual patient.