Dr Farhana Ara, MBBS MSc MRCP(UK), Miss Sandra Silva, BSc CCDS
Royal Papworth NHS Trust, Cambridge, UK
Disclosure: The authors have no conflicts of interest to declare and would like to thank Dr Greg Mellor, Consultant Cardiologist and Electrophysiologist, Royal Papworth NHS Trust, Cambridge, UK.
An 81-year-old male with previous atrial fibrillation underwent a VVI permanent pacemaker (PPM), (Medtronic Vivatron G20A2 SR), implantation for complete heart block. At the post-implant check and six-week in-house device interrogation, the right ventricular (RV) threshold was 0.5 V at 0.4 ms.
The first remote transmission (five months post-implant), showed a gradual increase in threshold from the day of implant.
The RV threshold was 2.5 V at 0.4 ms and the output was automatically set to 5 V at 1.0 ms.
Lower rate: 60 ppm
Upper sensor rate: 130 ppm
Pace/sense polarity: Bipolar/bipolar
Output management: Adaptive
Figure 1: RV lead threshold
What would be next step in management?
A: Continue monitoring
C: In-house device interrogation
D: List for lead revision
D: List for lead revision
The patient was listed for a lead revision. Fluoroscopy of the lead and device interrogation on the morning prior to procedure showed no lead noise or displacement, with a stable lead impedance (Figure 2).
Figure 2: RV lead impedance trends
A manual threshold test was then performed (Figure 3), which showed loss of capture at 0.25 V at 0.4 ms, giving a manual threshold of 0.5 V at 0.4 ms.
Figure 3: Manual threshold test
An automated threshold test was also performed (Figure 4).
Figure 4: Automated threshold test
What is the next step in management?
A: Add an atrial lead
B: List for lead revision
D: Switch off autocapture
D: Switch off autocapture
Autocapture adaptive pacing was switched off. There was a consistently normal RV threshold (0.5 V at 0.4 ms). Inappropriate capture management was noted, and the impression was of inappropriate evoked response due to hyperpolarization.
Lead revision was cancelled, and the patient was well at the three-month follow-up appointment.
Ventricular capture management is an automatic pacing threshold adjustment algorithm, that automatically measures pacing threshold through detection of the evoked response after a pacing stimulus.
This algorithm then automatically updates the pacing output to the optimal value. This function avoids unnecessary high output pacing and is considered to prolong battery longevity.
Pacing threshold search is initiated according to the schedule and frequency programmed by the clinician in pacemakers (nominally set to ‘day-at-rest’).
After implant detection is complete, the device initiates the first pacing threshold search 12 hours after.
If a search cannot be completed, the device retries after 30-minutes.
A ‘high threshold condition’ warning is issued if the amplitude threshold is greater than 2.5 V. The pacemaker responds, by adapting to an amplitude of 5.0 V and a pulse width of 1.0 ms. It will not give you an exact RV threshold measurement (Figure 1).
Figure 5: Ventricular capture management
Figure 5 shows ventricular capture management with a pacing stimulus or ‘test pulse’ being delivered to the right ventricle. When there is no evoked response detected during the pacing search, within a timed interval, back-up pulse is delivered, followed by three further pacing pulses or support events.
Capture verification efficiency relies on the measurement of the evoked response and works by triggering a blanking window (closed), followed by an open window.
If the device detects an evoked response (ER) within this second window, capture is confirmed (Figure 6). Otherwise, it will deliver a 5 V back-up pulse to ensure capture (Figure 7).
Figure 6: Capture verification
Figure 7: Capture verification with back-up pulse
Figure 8: Ventricular capture management in patient
Figure 8 shows that the device is allowing three support cycles as the programmed rate and output followed by a faster test pace. The test pace automatically has a back-up pace at programmed voltage at 1.0 ms, that occurs 110 ms after the test pace. These back-up pulses are not seen on this strip.
This cycle repeats, looking for the 2/3 capture before moving on, or 3/3 capture to confirm measurement. Looking at the timing of the rhythm then, it seems that all test paces capture as the ventricular event does not shift 110 ms later. However, the evoked response signal is not consistently detected by the device and so it sometimes labels this as loss of capture (LOC).
Evoked response detection and limitations: in rare instances, the pacemaker may not detect the electronic waveform created by the contracting myocardium immediately following a pacing pulse. High thresholds may be appropriate due to lead dislodgement, or incomplete connection of the lead in the header block. Acute effects of lead maturation can also cause failure to detect the evoked response. Hyperpolarization of the EGM leading to signal saturation or changes in lead tip and muscle interface (i.e. tissue fibrosis) may be responsible for this.
In our patient, the phenomenon was seen following the lead maturation period (typically within 4 to 12 weeks), when fibrosis between tip to tissue interface is expected. The inflammatory reaction and the resulting fibrous tissue that occurs after implantation may act as an insulating shield around the electrode, effectively raising the threshold for stimulation.Remote transmission has increased during the COVID-19 pandemic. This case highlights the importance of in patient/face-to-face review and interpretation of automated diagnostics.
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6. Murgatroyd FD, Helming E, Lemke B, et al. Manual vs. automatic capture management in implantable cardioverter defibrillators and cardiac resynchronization therapy defibrillators. Europace 2010;12:811–816. doi:10.1093/europace/euq053.
Co-published with thanks to the the British Heart Rhythm Society.