Case background: An 82-year-old male was referred for outpatient device implantation for symptomatic junctional bradycardia. A dual chamber Boston Scientific generator was implanted with active fixation leads secured in the right atrial appendage and right ventricular septum. The procedure was without complication and the device was programmed DDD 50–130 bpm.
Implant parameters: R wave 9.8 Mv and no P wave at 30 ppm. Both thresholds 0.5 V at 0.4 ms. Two hours later, the patient was recovering well post-procedure, but reported some transient postural dizziness. Blood pressure was stable at 139/68 mmHg (supine) and 130/60 mmHg (sitting); however, a telemetry strip showed a slow, irregular rhythm with clear atrio-ventricular dissociation (Figure 1A). A follow-up device check was performed to review the rhythm and device function. Figure 1B shows the presenting EGM and a lead I telemetry strip.
Electrogram interpretation: On initial review of our presenting AEGM (Figure 1B) we can see an AP marker with local atrial capture. On the VEGM, we can see a sharp signal around 160 ms post-AP, in line with our versus marker. It would not be unreasonable, given the implant indication, to assume correct device function based on this atrial paced rhythm with consistent atrio-ventricular conduction. However, on closer inspection of the VEGM, we can see irregular, low amplitude signals which do not correlate with our marker channel. The signals bear no relation to our VS events, and are therefore unlikely to be associated with T-wave oversensing. Out of context, and in the absence of symptoms or ECG abnormalities, these additional signals may be ignored.
In this case, we have a telemetry strip (Figure 1A) that shows regular bipolar pacing spikes and an irregular narrow complex ventricular rhythm, warranting further investigation given the patient’s symptoms. Looking at the bigger picture (Figure 1B), we have conflicting information from our intra-cardiac EGM and surface ECG.
In this case, we present paced P-wave oversensing on the VEGM, in the context of latent atrial capture. Each VS signal occurs around 160 ms post-AP event, suggesting the two events are related. Our surface QRS complexes are not sensed on our VEGM, therefore our VS signals must be atrial in origin. Looking back, the patient initially presented with a junctional bradycardia; therefore, we were not able to comment on AV conduction time. On review of Figure 1B, it is clear this patient has variable AV conduction; reflected in the irregularity of the ventricular rhythm on the surface ECG. Given the evidence of P-wave oversensing and the likelihood of acute lead displacement post-implant, a chest X-ray was performed to confirm the aetiology of the issue (Figure 1C).
Case summary: Figure 1C shows the right ventricular lead has displaced from the ventricular septum and is freely moving within the right ventricle, close to the tricuspid valve. The proximity of the displaced ventricular lead to the atria explains the paced P-wave oversensing. This case highlights the importance of a live ECG trace during all routine device checks. Although EGM signals show us what a lead is sensing, this is only relevant if we know where that lead is positioned. Our pacing programmer provides us with a unique opportunity to simultaneously review surface ECG and intra-cardiac EGMs, allowing for a comprehensive review of rate, rhythm and lead position. ❑
Figure 1
