Pacemaker Interactions

This is an interesting case where a patient with a single chamber pacer on which he is dependent is upgraded to a dual chamber device. This requires the placement of an atrial lead and may also require the replacement of the ventricular lead. Unfortunately replacement of the ventricular lead can place the patient at risk of a lead displacement that occurs rarely but in the case of a dependent patient, with serious consequences.

One technique is to implant the new system from a different site (in this case the VVI.R was on the left and the new system was placed on the right side) and leave the both systems operational with the VVI pacer set a low pacing rate. This is in the hope that if the new pacer fails to capture that the old one will not be inhibited and provide back-up pacing, or that in the emergency room a magnet could be placed over the old pacer to begin pacing (this has been of use in one such patient). In this case the VVI pacer (which still captured the ventricle) was set to 30/min.

First why so many spikes? The answer lies in the fact that the VVI pacer (Medtronic Activitrax) at end of life disregards the programmed rate and mode and reverts to VVI pacing at 65/min. In the first strip one sees a mixture of three spikes, the atrial and ventricular spikes of the DDD pacer and the ventricular spike of the VVI pacer. This becomes clearer as one sees all three spikes together in the lower tracing. Note that the larger QRS complexes are from the VVI pacer and the smaller ones at the end of the bottom tracing are from the DDD (seen as a result of the magnet being placed over the DDD pacer and speeding it to a DOO mode at 100/min.).

Interestingly the VVI pacer senses the ventricular pacing of the DDD and the VVI pacer is not seen with the magnet applied to the DDD and the rate of the ventricular pacer is slowed in the first few complexes by the DDD's spike which does not capture the ventricle. The bottom strip illustrates that the Ventricular pacer does not sense the atrial spikes of the DDD and that the DDD paces the ventricle in a committed mode in spite of capture of the ventricle by the old Ventricular pacemaker.

The patient did well and noticed a marked improvement of his functional class with the new DDD pacemaker. The old pacer was removed one week later without incident.

Dr. M. Rosengarten