Abstract and Introduction
Implantable cardiac monitors (ICMs) continuously monitor the patient's electrocardiogram and perform real-time analysis of the heart rhythm, for up to 36 months. The current clinical use of ICMs involves the evaluation of transitory symptoms of possible arrhythmic origin, such as unexplained syncope and palpitations. Moreover, ICMs can also be used for the evaluation of difficult cases of epilepsy and unexplained falls, though current indications for their application in these sectors are less clearly defined. Finally, the ability of new-generation ICMs to automatically record arrhythmic episodes suggests that these devices could also be used to study asymptomatic arrhythmias, and thus could be proposed for the long-term evaluation of the total (symptomatic and asymptomatic) arrhythmic burden in patients at risk of arrhythmic events. In particular, ICMs may have an emerging role in the management of patients with atrial fibrillation and in those at risk of ventricular arrhythmias.
Long-term ambulatory electrocardiogram (AECG) monitoring in cardiovascular disease has been hampered by the lack of suitable equipment and inadequate patient compliance. Indeed, the use of external recorders over extended periods of time has been limited by patient discomfort and complexity.[1,2] In principle, implantable cardiac monitors (ICMs) do not have these disadvantages. Smaller than a pacemaker, once implanted and correctly programmed, these devices can continuously monitor the patient's ECG and perform long-term continuous analysis and classification of the heart rhythm.
ICMs are equipped with a memory loop and, once activated by the patient at the time of symptoms by means of an external activator, store a one-lead ECG tracing, both retrospectively and prospectively, for several minutes. The loop memory enables the device to be activated even after symptoms have resolved; this means that such devices can be used even in the presence of incapacitating symptoms, which normally prevent the activation of other ECG monitoring devices that do not have this feature.
By implementing dedicated algorithms and sensing parameters similar to those of implanted cardioverter defibrillators (ICDs) and pacemakers, the new-generation ICMs are also able to automatically detect (i.e. without any active intervention by the patient) any kind of arrhythmic event (Fig. 1): from bradycardia to asystole, and from atrial fibrillation (AF) to ventricular tachycardia.[3–5] Finally, the new-generation devices now have a monitoring life of at least 36 months.
Arrhythmic events automatically detected by ICMs: (Panel A) asystole; (panel B) atrial fibrillation; (panel C) ventricular tachycardia.
The implantation of an ICM is a simple and minimally invasive procedure. The standard method follows few fundamental steps: external ECG mapping to determine the optimal implantation site and device position, corresponding to a sufficient R-wave amplitude; insertion of the ICM through a small skin incision into a subcutaneous pocket; anchoring of the device to the muscular plane to avoid mechanical instability, displacement, and migration; and checking of ECG trace recorded by the device with the external programmer, at the end of the procedure.
The typical location of an ICM is in the left parasternal area of the chest (Fig. 2). This position guarantees good R-wave amplitude and a clearly analyzable P, QRS, and T waves on the stored ECG[3–5] and, in our experience, it makes mapping not always strictly necessary. Other locations have been suggested, especially in younger patients, to minimize the aesthetic and psychological impact of the surgical scar in the anterior chest region, without impairing the performance of the device. One of these is the so-called left axillary location; a small incision is performed at the fourth intercostal space at the level of the left anterior axillary line, and the ICM is inserted into a submuscular pocket parallel to the intercostal space. The inframammary location has been proposed in young girls; the ICM is implanted through a 2-cm transverse incision at the inferior and medial border of the left or right breast. Recently, a new location has been proposed in the left upper chest area, midway between the supraclavicular notch and the left breast area. Finally, ICMs are easily explanted once the diagnosis has been made or the battery is depleted.
All the stored data are analyzed by interrogating the device. The follow-up of the ICMs can be performed periodically in-office, by means of the external programmer similar to that of pacemakers, or remotely through an automatic transmitter and Web-based software. Furthermore, in case of a symptomatic event (e.g. syncope or palpitations), the patient is asked to come as soon as possible to the hospital, for an in-office check of the device, or to transmit the data stored in the ICM via the remote monitoring system. This last feature is very promising, in that it can significantly reduce both costs and the time to diagnosis and therapy, and increase patient compliance. However, it must be underlined that up-to-now ICMs are not yet able to provide automatic instant feedback, and that they still need to be interrogated.
Several companies started designing implantable diagnostic devices for syncope and general cardiac arrhythmia monitoring,[9–12] demonstrating a growing interest about this application not only by researchers and physicians, but also by the manufacturers.
The Reveal XT™ (Medtronic Inc., Minneapolis, MN, USA) can monitor brady and tachyarrhythmias through manually activated ECG storage or automatic detection (Fig. 3). The device has a specific algorithm to detect the presence of AF, and to monitor the amount of time a patient is in AF. Currently, this is the only device with a validated algorithm for AF monitoring. The Reveal XT detects the occurrence of AF from variations in the ventricular rhythm. AF episodes are detected using an automatic algorithm based on the pattern of R-wave interval variability within 2-minute periods. The differences between consecutive R-wave intervals are plotted in a Lorenz plot. The follow-up of the device can be performed also remotely, through an automatic transmitter and Web-based software.
The Confirm™ (St. Jude Medical, St. Paul, MN, USA) offers data storage options that include manual (patient-triggered) and programmable automatic (asystole, bradycardia, tachycardia) activations for ECG storage. Recently, the Confirm™ has a specific algorithm to detect AF, even though, up until now, there is really no data about its performance. AF detection is based on evaluation of the ventricular response to atrial events during AF. Algorithm uses statistical techniques to assess the transitional behavior of one R-R interval to the next and compare these to the known interval transitions during AF and non-AF episodes. Algorithm also uses additional rhythm discrimination criteria to reduce the frequency of false-positive AF detections that could result from other types of irregular rhythms. The follow-up of the device can be performed also remotely, through a telephone line (Fig. 4).
In current clinical practice, ICMs are used as diagnostic tools to evaluate transitory symptoms of possible arrhythmic origin, such as unexplained syncope and palpitations. Moreover, ICMs can also be used in the field of difficult cases of epilepsy and unexplained falls, though current indications for their application in these sectors are less clearly defined. The ability of new-generation ICMs to automatically record arrhythmic episodes suggests that these devices could also be used to study asymptomatic arrhythmias, and thus could be proposed for the long-term evaluation of the total (symptomatic and asymptomatic) arrhythmic burden in patients at risk of cardiac arrhythmic events; however, the clinical indications have not yet been established. In the present review, we will analyze both the current and possible future indications of ICMs (Table I), according to the European Heart Rhythm Association guidelines on the use of implantable and external ECG loop recorders, their limitations, and their desirable future development.
Pacing Clin Electrophysiol. 2012;35(9):1169-1178. © 2012 Blackwell Publishing