Prediction of Wenckebach behavior and block response in DDD pacemakers

Voltage dip in pacemaker battery supply: a new cause of pacemaker mediated tachycardia

A new cause of pacemaker mediated tachycardia was observed in a patient equipped with a Vitatron Quintech 931 DDD pacemaker. In this type of pacemaker the microprocessor is switched off during the atrial refractory period. Beyond the recommended replacement time, the internal resistance of the battery may increase to such an extent that switching on and off the microprocessor may cause voltage dips. Those voltage dips are erroneously interpreted as P waves by the atrial sensing amplifier, which may cause self-triggering of the pacemaker and initiate a pacemaker mediated tachycardia.

Quantitative analysis of Wenckebach behavior in DDD pacemakers

Wenckebach-type behavior in P-synchronous pacing modes allows a gradual transition into 2:1 block. As this behavior is dependent on precise timing intervals, it can be quantitated with mathematical equations. In this report, equations for quantitating the Wenckebach escape rate and the (N + 1)/N block points have been derived. These equations were then used to produce plots of Wenckebach escape rates for given AV intervals, atrial refractory periods, and atrial rates. These equations were validated utilizing a DDD pulse generator interfaced with a patient simulation device. The predicted values for the Wenckebach escape rate and ratio (n = 33) correlated highly with the observed values (r = 0.99, P less than 0.0001, and r = 0.97, P less than 0.0001, respectively) thereby validating the derived equations. This quantitative analysis has allowed a close look at the pacing behavior between the maximum tracking rate and the 2:1 block point. These equations may be clinically useful for assessing whether a nontriggering P wave resulted from Wenckebach type block or atrial undersensing and for the more complicated interval analysis in DDDR pacing with rate responsive AV delays and postventricular atrial refractory periods.

Sensor-driven rate smoothing in a DDDR pacemaker

DDD pacemakers may have large cycle-to-cycle variations in rate at the upper rate limit because of 2:1 block or Wenckebach-type block. Rate smoothing was introduced as an option to eliminate these large variations. Now, DDDR pacemakers can produce similar electrocardiographic displays through a different mechanism that uses an activity sensor. This is termed "sensor-driven rate smoothing" because it occurs only when the activity sensor is driving the pacemaker. In the case described, as the atrial rate exceeded the maximum tracking rate and reverted to Wenckebach-type block, the RR interval varied only from 600 msec to 680 msec (13.3% rate-smoothing value) because of sensor-driven pacing. Maximal sensor-driven rate smoothing requires optimal programming of the rate response indicators. This sensor-driven rate-smoothing effect is an electrocardiographic manifestation that will undoubtedly be seen more frequently as DDDR devices come into widespread clinical use.