The addition of an independent sensor to DDDR pacemakers allows for a conditional ventricular tracking limit, preventing inappropriate tracking of non-physiological high atrial rates.
A limitation of current DDD and DDDR pacemakers is the inability to distinguish between inappropriate high rate atrial sensed events that are physiologically appropriate to track (e.g., elevated sinus rates resulting from exercise, emotional responses, etc.) and those that are physiologically inappropriate to track (e.g., paroxysmal atrial dysrhythmias, myopotentials, retrograde conduction, etc.). The sophistication of sensing circuitry is not yet sufficiently advanced to permit a pacemaker to distinguish appropriate atrial events by morphology. The addition of an independent sensor to a DDD pacemaker (i.e., DDDR) gives more information about the patient's condition, especially with respect to exercise. This information can be used to judge the appropriateness of a high sensed atrial rate, and to modulate the pacemaker's response. If the sensor input is below a specified level, indicating lack of exercise, the DDDR can track sensed atrial events only to a tolerably low limit-the conditional ventricular tracking limit (CVTL). Wenckebach-type behavior ensues at the CVTL until the sensor input increases, indicating that exercise is occurring, or until the sensed atrial rate decreases. If the sensor input indicates exercise, the DDDR pacemaker can track up to the programmed maximum rate. Two DDDR systems have been developed that incorporate this feature; one based on temperature, the other on activity. Currently the CVTL is set at a value about 30 ppm above the pacing rate, as a compromise to support emotional needs not seen by the sensor. Improved sensors could cause the decision to raise the tracking limit (i.e., recognition of physiological need for higher rates) to be more accurate, perhaps making the CVTL proportional to the sensor signal.
Lee et al. (1990) studied this question.