Commentary:
The electrograms in Fig 1B suggested a possible lead noise (Fig 1B) resulting in oversensing. She was promptly managed by making the device into VOO mode temporarily and she regained consciousness immediately. Based on the above findings, lead malfunction was suspected in spite of normal lead impedance and threshold trends. It is well known that lead noise is the earliest to manifest during lead failure even when impedance is normal. Therefore, after a discussion with patient relatives she underwent ventricular lead replacement. The device was also upgraded to a dual chamber pacemaker [DDDR Assurity MRI]. Unexpectedly, in the immediate post-operative period she became unresponsive requiring prompt resuscitation when she was being transferred from the Cath lab to recovery area. Monitor suggested frequent short runs of polymorphic VT which correlated with the recorded and real time HVR episode in the pacemaker (Fig 2). There was a drop in invasive BP during the PMVT runs. The PMVT got suppressed with high rate obligatory pacing (DDDR 110 bpm -Ap-Vp). Similar runs of VT resurfaced immediately as the rate was lowered below 90 and became very frequent at lower rate below 80. All these indicated some triggered activity related to PMVT. The lower rate was kept at 110 bpm for next 48 hours. She was started on intravenous lignocaine and propranolol. Her blood biochemistry revealed K of 4.3 and Mg of 2.2 . Her ionized Ca was normal. As she recovered well without any further recurrence, the lower rate was brought down gradually. She was discharged home with a lower rate of 90 bpm. Over the next 2 months the LRL was brought down to 70 in a stepwise manner every 15 days. Her cardiac PET-CT done on follow up did not reveal any cardiac uptake.
A retrospective analysis of her initial electrogram suggested that it was PMVT and there was no lead noise at all (Fig 3). Neither they were reflected in the marker channel. All ‘VS’ in the marker channel were true QRS complexes from the run of PMVT (Fig 3). Being a unipolar lead, those noise-mimicker were possibly some external artefacts. Retrospectively, it also appears that she has some underlying pathology to produce PMVT and we should have implanted an ICD instead of dual PPI. However, she is doing well for the last one and half years on oral propranolol. Mexiletine stopped 6 months ago. Oral Mg supplementation was given empirically targeting Mg level above 2.5. It also stopped 6 months ago. Since her discharge, there are no more episodes of NSVT on device and Holter monitoring despite lowering the rate to 70 bpm. She was offered an ICD upgrade but the patient is not willing as she is doing well and does not want to undergo another invasive procedure. She also did not want to undergo any genetic testing for channelopathies. In essence, this patient developed a PMVT late after a VVI pacemaker implant. The artefacts during device checkup were misdiagnosed as lead noises. The reason for her syncope was PMVT which is relative bradycardia dependent. The cause for her PMVT is although unclear. The QTc does not seem to be prolonged in old and recent ECGs (Appendix).  The relative hypomagnesemia could be contributory but additional underlying long QTc is a strong possibility.
We wondered whether the PMVT was related to long term VVI pacing. There are studies suggesting the same [1]. VDD pacemakers have a higher tendency for PMVT due to intermittent bradycardia associated with atrial undersensing [2,3]. Dual chamber upgradation can reduce the arrhythmogenicity in them [3]. Similar finding was observed in our case too as she can tolerate a rate < 80-90 after a few days of her dual chamber upgradation. The immediate postoperative episode (PMVT) might suggest residual arrhythmogenicity which abated over time. There is another case reported by our group where symptomatic VT happened late after a VVI pacemaker and the stored EGM revealed VT [4]. However, in that case, a long-short sequence was noted at the onset of VT. The long cycles probably resulted from hysteresis [4]. In this index case, the episodes happened without any active hysteresis. However, the immediate appearance of VT in device clinic (when the LR was set to 45 bpm) points to a preponderance of the VT linked to a relative bradycardia. The same was noted in the postoperative period. Putting these together, it appears that long term VVI pacing itself might be arrhythmogenic with an aggravation during long cycles.
To summarize, this case highlights one of the rarer causes of arrhythmic syncope in pacemaker patients despite a normally functioning pacemaker circuit. It also highlights the importance for careful interpretation of stored / real time electrograms to preclude any inappropriate intervention. The exact cause for her polymorphic VT is still not clear. Relative hypomagnesemia, late presenting long QT syndrome and arrhythmogenicity of VVI pacing in isolation or combination remain the probable offenders.