A month earlier the patient had been implanted with a VIGOR® DR Model 1230 pacemaker. The pacemaker was initially programmed bipolar, to the DDD mode, with a lower rate limit of 55 ppm, an upper rate limit of 110 ppm and a maximum sensor rate of 120 ppm. The atrial amplitude was set at 2.5 V at 0.6 ms pulse width, with an atrial sensitivity of 0.5 mV. The ventricular amplitude was set at 2.5 V at 0.3 ms pulse width, with a ventricular sensitivity of 1.5 mV. The dynamic atrioventricular (AV) delay was programmed on, with the maximum AV delay at 220 ms and the minimum AV delay at 100 ms. Atrial tachy response mode switching was also enabled.
Upon evaluating his pacemaker function, I noted that the electrogram (EGM) tracing revealed closely paired atrial and ventricular activity. However, there was an unusual presentation to the event markers, since atrial and ventricular activity did not always appear to be appropriately notated. When the ventricular activity occurred prior to atrial activity, the event markers noted the ventricular event as a premature ventricular contraction (PVC); there was no marker for the atrial activity. But in a few instances when the atrial activity preceded the ventricular activity, both atrial and ventricular sensing were appropriately noted.
When the pacing rate was increased above the intrinsic rate, both atrial and ventricular capture were normal. Magnet testing also demonstrated appropriate AV pacing. Pacing thresholds were within normal limits, demonstrating that the leads were intact and capturing appropriately.
I concluded that the patient’s underlying rhythm was an accelerated junctional rhythm. Because the electrical impulses were originating in the AV node, both atrial and ventricular activation were occurring in rapid succession. Often ventricular activation was detected prior to atrial activation, and in these cases – because there was no preceding atrial activity – the ventricular events were classified as PVCs. The atrial activity that immediately followed was not sensed because a ventricular sensed event sets up an 80-ms atrial blanking window. Any atrial activity during that 80-ms interval is not sensed by the pacing system.
On occasion, when atrial activity occurred prior to ventricular activity, both the atrial and the ventricular events were sensed by the system and noted by the event markers (Figure 2: As, Vs). (Following an atrial sensed event, there is no blanking interval precluding ventricular sensing.)
The EGMs in this device helped confirm that the pacemaker was functioning appropriately, given this patient’s accelerated junctional rhythm. The patient is not symptomatic, and he continues to do well.
Henry D. Storch, M.D., is a cardiologist with the Olean Medical Group in Olean, N.Y.