Overview
The implantable loop recorder (ILR) or insertable cardiac monitor (ICM) is a subcutaneous, single-lead, electrocardiographic (ECG) monitoring device used for diagnosis in patients with recurrent unexplained episodes of palpitations or syncope, for long-term monitoring in patients at risk for or with documented atrial fibrillation (AF), and for risk stratification in patients who have sustained a myocardial infarction (MI) and those who have certain genetic disorders. [1, 2, 3, 4]
The device is typically implanted in the left parasternal region and is capable of storing ECG data automatically in response to a significant bradyarrhythmia or tachyarrhythmia or in response to patient activation. It is particularly useful either when symptoms are infrequent (and thus not amenable to diagnosis using short-term external ECG recording techniques) or when aggregate long-term data (eg, burden of AF) are required.
Indications, Contraindications, and Disadvantages
Indications
Patients with syncope
Syncope is defined as a transient loss of consciousness due to global cerebral hypoperfusion and is characterized by rapid onset, short duration, and spontaneous complete recovery. It is an extremely common clinical problem and can have an underlying cardiovascular etiology.
The most common cardiovascular cause of syncope is a disorder of autonomic function, such as neurally-mediated syncope (vasovagal syncope, carotid sinus hypersensitivity, situational syncope), chronic orthostatic intolerance (ie, postural orthostatic tachycardia syndrome [POTS]), and orthostatic hypotension (secondary to volume depletion, systemic illness, use of a vasoactive drug or pure autonomic failure/multiple system atrophy).
Another cause for syncope is obstruction to blood flow (eg, aortic stenosis, aortic dissection, pulmonary embolism, pericardial tamponade, pulmonary hypertension, etc); these can readily be identified by echocardiography. The most concerning cause is an arrhythmia (bradyarrhythmia [5] or tachyarrhythmia), which can sometimes be difficult to differentiate from a disorder of autonomic function. [1] 2 Independent predictive factors for pacemaker implantation in patients with unexplained syncope who undergo implantable loop recorder (ILR) insertion appear to include obesity, right bundle branch block, and chronic renal failure. [5]
The initial evaluation of syncope consists of a comprehensive history, physical examination (including orthostatic blood pressure measurements), and an electrocardiogram (ECG). This initial evaluation dictates the decision on whether hospitalization is necessary for further evaluation as well as the need for additional diagnostic testing. An important additional test is the echocardiogram, which can identify patients at high risk for ventricular arrhythmias, such as patients with hypertrophic cardiomyopathy and severe left ventricular dysfunction.
In one study, Sayed and colleagues used ILRs to characterize the nature of cardiac arrhythmias in 20 consecutive patients with newly diagnosed, severe cardiac light chain (AL) amyloidosis and symptoms of syncope or presyncope. [6] The investigators’ results supported a study of prophylactic pacemaker insertion in this patient population.
In patients at low risk for a lethal ventricular arrhythmia, the need for additional evaluation depends on the frequency of syncope and its impact on quality of life. In patients with a single episode of syncope, an ILR may be useful in patients with underlying bundle branch block.
A prior study evaluating 52 patients with syncope and a bundle branch block who underwent ILR implantation after an unremarkable initial evaluation (including an electrophysiological study found that syncope recurred in 42% of these patients. [7] The ILR showed that recurrent syncope was commonly attributable to paroxysmal AV block.
In patients with frequent recurrences of unexplained syncope, the 2009 European Society of Cardiology (ESC) guidelines now advocate the “early” use of ILRs.
Because recurrent syncope occurs sporadically and because ECG documentation at time of recurrent syncope is an extremely important diagnostic modality, ILRs have a significantly greater diagnostic yield than 24-hour Holter, 30-day event, or 30-day mobile cardiovascular telemetry monitoring. The diagnostic yield of any ECG monitoring strategy increases as the monitoring period is increased [8] ; ILRs (with their near 3-year battery life) offer the best opportunity for diagnosis. [9, 10, 11]
In a retrospective record review of patients with syncope, Onuki et al found that 43% of patients with unexplained syncope received an ILR. The cumulative ILR diagnostic rate was 47% at 1 year post-ILR implantation and 65% at 2 years post-ILR implantation. The estimated ILR diagnostic rates were significantly greater than the diagnostic rates for conventional testing without the use of an ILR. [12]
The PICTURE registry, a prospective, multicenter, observational study that followed 570 patients with recurrent unexplained presyncope or syncope who received an ILR, showed that these patients were evaluated on average by three different specialists and underwent a median of 13 nondiagnostic tests (range, 9-20). [13] Within the first year, syncope recurred in a third of the patients; the ILR provided a diagnosis in 78% of the patients, most commonly a cardiac etiology.
In patients with recurrent vasovagal syncope, in contrast to tilt table testing, the ILR provides information about the characteristics of the heart rhythm during recurrent syncope [14, 15] and may help guide decisions regarding specific therapy. [1, 15, 16] For example, studies are currently under way to determine whether patients with ILR-documented bradycardia can benefit from a permanent pacemaker.
Patients with palpitations
ILRs also have an important role in the evaluation of patients with recurrent unexplained episodes of palpitations. In patients with infrequent episodes, short-term ambulatory ECG monitoring often fails to establish a diagnosis. The use of ILRs in these types of patients is safe and cost-effective and has a high diagnostic yield. [17]
Risk stratification post MI
There may be a role for ILRs in the risk stratification of post-MI patients. The CARISMA trial followed 1393 patients who received an ILR within 11±5 days of an acute MI, which resulted in a left ventricular ejection fraction £ 40%. [18] A significant bradyarrhythmia or tachyarrhythmia was documented in 46% of patients. The development of intermittent high-degree atrioventricular (AV) block was the most potent predictor of mortality.
Data from the VEST/PREDICTS trial (Vest prevention of Early Sudden death Trial)/(PREDiction of ICD Therapies Study) showed that a wearable defibrillator used for the 3 months after infarction can attenuate early mortality. [19]
Patients with or at risk for AF
ILRs can provide long-term ECG monitoring in patients at risk for or with documented AF. [20, 21] In the CARISMA (Cardiac Arrhythmias and Risk Stratification After Myocardial Infarction) trial, a 28% incidence of new-onset AF was observed. [18] ILRs also play an important role in cryptogenic stroke patients. The CRYSTAL-AF (Continuous Cardiac Monitoring to Assess Atrial Fibrillation After Cryptogenic Stroke) study demostrated that ILRs improve the diagnostic yield over short-term ambulatory external ECG recording techniques in this patient population. [22] Over a 36-month monitoring period, 29% of patients with cryptogenic stroke were found to have clinically significant AF. Similary, a 2020 retrospective study (2015-207) of data from 178 patients who presented with cryptogenic stroke and received an ILR found nearly 20% (19.6%; n = 36) had AF (median follow-up: 365 days). [23] Although the sole independent predictor was increased p wave dispersion, other predictive variables included advanced age, diastolic dysfunction, and ECG features of premature atrial contractions and p wave dispersion.
Finally, multiple trials are currently underway to assess the clinical utility of ILRs in patients who have undergone ablation of AF. It is hoped that the ILR will be able to provide information about AF burden; this type of information is currently not possible to obtain in the absence of an implanted pacemaker or defibrillator.
Contraindications
The presence of an active infection or a bleeding diathesis may preclude implantation.
Disadvantages
Disadvantages include the need for a minor surgical procedure, the difficulty of always being able to differentiate supraventricular from ventricular arrhythmias, the presence of under- or over-sensing that may exhaust the memory of the ILR, and the cost of the device. The ILR has a high initial cost; however, it may actually be more cost-effective than a strategy incorporating multiple nondiagnostic, noninvasive investigations. [24, 25, 26, 27]
Preparation
Equipment
Implantable loop recorders (ILRs) are small leadless boxes that have evolved from 6.5-9 mL in volume to 1.2 -1.4mL with two self-contained electrodes that are implanted subcutaneously under local anesthesia, typically in a left parasternal location. Current ILR devices have a battery life of 2-4 years.
ILRs have a solid-state loop memory capable of recording and storing bipolar ECG recordings when either activated by the patient or a bystander in response to a symptomatic episode of palpitations or syncope [28, 29] or automatically activated upon occurrence of predefined bradyarrhythmia or tachyarrhythmia. [30, 31, 32] These devices can transmit data transtelephonically to a physician’s office for review (see the table below).
Implantable Loop Recorder. Feature comparison of insertable cardiac monitors (ICMs) by manufacturer. AF = atrial fibrillation; AT = atrial flutter; EGM = electrogram; ICM = insertable cardiac monitor; RF = radiofrequency.
AT = atrial tachycardia; FVT = fast ventricular tachycardia; HR = heart rate; HRV = heart rate variability; MRI = magnetic resonance imaging; VT = ventricular tachycardia.
Technique
Placement of an implantable loop recorder (ILR) can be performed in any standard procedure suite. ILRs have traditionally been placed in the electrophysiology (EP)/catherization laboratory hospital environment. However, ICM implantation can be pefformed safely in catherization laboratory holding areas and office settings. [33, 34, 35, 36]
The device is typically inserted in the left parasternal region. Prophylactic antibiotics can be administered intravenously prior to the incision. [37] A mild anxiolytic and/or analgesic can also be administered to enhance patient comfort. Under sterile conditions, the creation of a small subcutaneous pocket using local anesthesia is required. Vector mapping to maximize the R-to-T can be considered; however, this is rarely necessary.
Postprocedure Complications and Outcomes
Complications
The most common problems associated with the implantable loop recorder (ILR) are pain at the implant site and a local pocket infection that may necessitate removal of the implanted hardware. Poor R-wave sensing necessitating relocation of the device is also a rare possibility.
Outcomes
In general ILRs are effective achieving symptom-rhythm correlation, with a change in management in 47% patients with syncope, presyncope, unexplained palpitations, or cryptogenic stroke in one study. [38] These devices also appear to be safe and effective in the management of syncope in the geriatric population (age: ≥80 years). [39]
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Implantable Loop Recorder. Medtronic Reveal® DX. Courtesy of Medtronic, Inc.
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Implantable Loop Recorder. Medtronic Reveal® XT. Courtesy of Medtronic, Inc.
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Implantable Loop Recorder. St. Jude Medical Confirm. Courtesy of St. Jude Medical.
