Unsuccessful far field sensing
Noise detected on the RV near field sensing circuit, is subsequently sensed and binned as VF (marked as an F and counted towards intervals to detection). At the point of detection (in this case 12 binned VF intervals), the Secure Sense RV Lead noise algorithm (designed to detect RV lead noise) was not applied as it had switched to passive function, due to a lack of sensing on the far-field channel. This is indicated on the trace by a single VS2 (shown by arrow in Figure 2b) marker during the detection period. The device perceives this single sensed beat as indicative of poor or inaccurate far-field sensing and therefore switches to passive. This is to reduce the chance of withholding therapy for a genuine ventricular arrhythmia. Therefore, the noise is detected as VF and results in delivery of inappropriate ATP and an aborted shock.
The history and recorded episode were highly suspicious of RV lead failure. As there was a risk of further inappropriate therapies and pacing inhibition the patient was immediately contacted and urgently admitted to our coronary care unit.
ICD therapies were disabled and re-programming attempted to avoid pacing inhibition. R wave sensitivity settings were assessed but it was not possible to avoid oversensing of noise by any reprogramming of the sensitivity threshold. Programming to DOO was considered however there was a high burden of ventricular ectopy and concerns of triggering an R on T episode. LV only pacing was also not an option for this device.
The best short term fix was to programme the mode to DDT to limit the inhibition during episodes of ventricular lead noise and the max trigger limit was decreased to 100 bpm to avoid pacing at excessively high rates with the patient monitored on telemetry overnight.
The next morning the patient was taken to the lab for RV lead replacement. Unfortunately, there was no left sided venous access with an occlusion at the left axillary vein therefore the procedure was abandoned. A decision was made to perform a full left sided system extraction and to implant a new CRT-D system on the right side. The extraction proved complex but was successful. Examination of the extracted RV lead found conductor externalisation proximal to the RV coil which was clearly the cause of the patients’ problems. A week later the patient had an uncomplicated right sided CRT-D implant with normalisation of the ECG. Follow up has been uneventful since implantation of the new device.
In conclusion, this case demonstrates the benefit of remote monitoring in the early identification of potential lead issues and adverse events for patients. In addition, the case was uncovered by the On-Call Physiologist demonstrating the merits of On-Call provision in centres dealing with complex device patients.
Despite a thorough in person review we were unable to reproduce noise episodes with manoeuvres reminding us that the location of the lead failure is a key determinant in whether noise can be reproduced.
As demonstrated in this case study, discrimination algorithms for noise can be complex and may not always be successful, knowledge of how these algorithms work is essential to understand device behaviour.
A final thought perhaps was that fluoroscopy may have been useful at uncovering the externalised conductor compared to a standard X-ray, although this is not often routine practice and based on the information available at the time, it is unlikely that we would have managed this patient differently.
1. St Jude Medical, User Manual – Bradycardia and Tachycardia – Secure Sense Algorithm 2018