The application of remote monitoring will cause an imminent change in the clinical practice of cardiac rhythm management. The aim of this article is to present the role of remote patient monitoring in ICD follow-up, its potential benefits and its barriers to more general application.

Implantable Cardioverter–Defibrillator Follow-up
Routine technical follow-up visits of patients with ICDs are usually performed at short intervals due to safety concerns. With the advances in device technology, it has become theoretically possible to increase the follow-up intervals. However, the disadvantage of longer intervals is the lack of information on system integrity and the clinical status of the patient. A close follow-up schedule is mandatory over the first three months after implantation as clustering of system- and procedure-related complications occur most commonly during this period.^5–8 However, more than half of all system-related complications, e.g. lead failure, may occur at any time during long-term follow-up. These complications are unpredictable and any extension of the follow-up interval increases the risk of delayed detection of late system-related complications, even though these happen infrequently.

Monitoring of System Integrity
The integrity of the implanted system is essential for appropriate device therapy. Unfortunately, a significant proportion of ICD recipients experience system-related complications. The majority of these complications are lead-related and usually occur within three months of implantation.^6,9–12 Causes of ICD lead failure are an insulation defect or conductor disruption. These can affect both the high-voltage and the pace-sense circuit of the system. Potential complications of ICD lead failure include oversensing of noise, undersensing of ventricular arrhythmias, inappropriate therapy and lethal proarrhythmia. However, more than half of all system-related complications may occur at any time during long-term follow-up.^11–14 For example, the annual failure rate of defibrillation leads increases progressively over time after implantation and reaches 20% in 10-year-old leads.¹⁴ Early detection of these complications is desirable to ensure patient safety. Current ICDs provide lead impedance monitoring for the detection of lead failure, which is based on daily measurement of lead impedance. When the measurement for lead impedance monitoring occurs once daily (this is usually nocturnal), it is very unlikely that this measurement will reveal abnormal impedance values. Alert features producing an acoustic warning signal have been implemented in ICDs for early detection of system-related complications. However, these features are of limited use because of their low sensitivity in detecting lead failures.^15,16 In a recently published study, Vollman et al.¹⁶ reported on the reliability of acoustic alerts to detect lead failures. These acoustic alerts have a low sensitivity and warn of only 30% of lead failures. Acoustic patient-alert features are a useful additional tool, facilitating early detection of lead-related complications or battery depletion.

Structural lead defects can be discreet at first, and may present with loss of electrical integrity for brief moments (e.g. arm movement may be momentarily affected). To increase the sensitivity to lead failures, the ‘short interval counter’ or ‘sensing integrity counter’ (SIC) has been developed to keep track of the number of short non-paced intervals (<140ms).¹⁷ This method allows the detection of noise oversensing that can be caused by clinically silent lead failures. The combination of the abnormal lead impedance trend and an increase in SIC yield a higher sensitivity for the detection of lead failure.^16,17 However, the potential limitation of SIC is the likelihood of false-positive detections when applied in patients with integrated bipolar sensing electrodes.¹⁶ Integrated bipolar leads are susceptible to oversensing intracardiac signals¹⁸ and diaphragmatic myopotentials.¹⁹ Remote monitoring has the potential for early detection of lead-related complications, such as an insulation defect, lead fracture and faulty connections with the header, by continuous monitoring of lead and high-voltage impedance, sensing value and detection of episodes caused by oversensing.^20–23 In an overall analysis of a worldwide database, the proportion of events related to abnormal device status was 3%. The proportion of events related to system configuration was 11%.²⁴

Recent advisories and recalls concerning the implanted defibrillation system have given a new perspective on remote onitoring.^25,26 Potential device or lead failures often occur in unpredictable ways and put patients at risk.^27–29 Immediate solutions such as systematic ICD generator replacement are associated with a number of complications, including death.²⁵ ‘Field actions’ and device advisories usually result in a recommendation of close follow-up of the outpatient, with reprogramming and frequent visits at short intervals. These recommendations further increase the workload of ICD clinics. Continuous surveillance of defibrillation systems at risk is the most attractive alternative and provides an immediate detection of device or lead failure, as shown in Figure 1. In addition, remote monitoring can eliminate unnecessary replacements and obviate the increased follow-up of large patient populations affected by these advisories.