Paradoxical AV delay shortening of a pacemaker.

Paradoxical shortening of the paced AV delay (atrial paced-ventricular paced or Ap-Vp interval) was observed at rest in the DDD and DDDR modes in three patients with implanted CPI Vigor DR pacemakers programmed with a long AV delay and a relatively narrow difference between the lower and upper rates. This behavior is related to the VA extension algorithm designed to prevent the sensor-driven atrial pacing rate from exceeding the programmed upper rate whenever a sensed conducted QRS complex continually follows an atrial stimulus. We found that this algorithm also becomes manifest at rest and may cause shortening of the Ap-Vp intervals. The VA extension algorithm is best conceptualized in terms of a separate atrial upper rate that functions on exercise and at rest. The atrial and ventricular upper rates are equal but the atrial upper rate is initiated by an atrial-paced or sensed event and the ventricular upper rate is initiated by a paced or sensed ventricular event. Under certain circumstances delay in the release of the atrial stimulus Ap to conform to the atrial upper rate interval produces variable abbreviation of the paced AV (Ap-Vp) delays with resultant variation in the duration of the atrial escape intervals despite fundamental ventricular-based lower rate timing.