Measurement of retrograde conduction during pacemaker implant has demonstrated that 45% of patients who require pacemaker implantation for any indication--ventricular or supraventricular tachycardia, sinus node dysfunction or AV block--have retrograde conduction at some paced rate if paced from the ventricle. Sixty-seven percent of patients paced for sinus node dysfunction have retrograde conduction, and 14% of those with fixed complete antegrade heart block have retrograde conduction. Even patients who have had AV block for many years may retain retrograde conduction. The mean retrograde conduction time from the ventricular stimulus to the atrial intrinsic deflection is 235.+-.50 ms and the range is 110 ms to 450 ms. [See Furman and Holmes, A Practice of Cardiac Pacing, Futura Publishing Co., New York 1989, pp. 66-67.] During pacemaker implants and follow-ups, the measurement of retrograde conduction intervals could allow for the setting of the appropriate postventricular atrial refractory period (PVARP) to avoid retrograde conduction causing the onset of pacemaker mediated tachycardia (PMT).
The existence of retrograde conduction via the natural pathway, and the antegrade conduction via the implanted dual-chamber pacemaker, provide a reentry circuit that mimics the natural situation in which an accessory pathway allows a circus movement tachycardia. This teentry tachycardia, PMT, may occur when a P-wave displaced from its natural position before the QRS complex is tracked in the ventricle. The P-wave can be displaced by a ventricular premature contraction or ventricular stimulus with retrograde conduction. If the displaced P-wave falls within the atrial refractory period, it will not be tracked and no further event will occur. On the other hand, if the displaced P-wave falls outside the atrial refractory period, it will begin an AV interval and be tracked in the ventricle. If retrograde conduction exists, another P-wave will occur following the ventricular stimulus, and the reentry loop will be sustained.
If retrograde conduction exists, PMT can be prevented by programming the PVARP longer than the retrograde conduction interval. The retrograde conduction time begins with a ventricular event. The retrograde conduction time ends when a retrograde P-wave occurs (assuming that the retrograde conduction results in an atrial event). The PVARP interval similarly begins with the ventricular event. Thus, PMT can be prevented by making the PVARP interval longer than the retrograde conduction interval because the atrial event will not be tracked (although it can be sensed) if it occurs during the PVARP. On the other hand, higher tracking rates can be programmed if the PVARP is shortened. Therefore, to provide an optimal trade-off between these two factors, the retrograde conduction interval should be measured if it exists.
At the present time, retrograde conduction intervals are determined manually by reprogramming the pacemaker to some special parameter settings. For example, the pacemaker can be programmed to operate in the VDD mode, with a long PVARP to prevent the occurrence of PMT. During the test, there is certainly no desire to trigger PMT so a relatively long PVARP is programmed. The mode is set to VDD because ventricular pacing (V) is desired since each retrograde conduction interval to be measured begins with a ventricular event, but both chambers are sensed (the first D) because atrial sensing is required in order to determine if a retrograde P-wave is present and in order to measure the retrograde conduction time; there is no tracking of the sensed retrograde P-wave because the retrograde P-wave occurs during the PVARP and thus there is no triggering of an AV delay. Ventricular overdrive pacing (a fast pacing rate) is employed to facilitate retrograde conduction if it exists. (Early ventricular beats occur when the atria are not likely to be refractory so that retrograde conduction is possible. It is for a similar reason that retrograde conduction, which can give rise to PMT, usually occurs after a premature ventricular contraction.) Typically, retrograde conduction intervals can be measured manually by observing main timing events (MTEs) and/or a surface ECG and/or the telemetered intracardiac ECG on the programmer display. The manual determination of retrograde conduction intervals in current pacemaker systems is often quite time consuming. Not surprisingly, retrograde conduction intervals are rarely determined at most clinical centers. Patient safety is potentially compromised if the PVARP parameter is not programmed appropriately.