Implantable Loop Recorder

Updated: Sep 04, 2014
  • Author: Suneet Mittal, MD, FACC, FHRS; Chief Editor: Eric H Yang, MD  more...
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The implantable loop recorder (ILR) 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.


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 or tachyarrhythmia), which can sometimes be difficult to differentiate from a disorder of autonomic function. [1, 2]

The initial evaluation of syncope consists of a comprehensive history, physical examination (including orthostatic blood pressure measurements), and an 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 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. [5] 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.

Since 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 [6] ; ILRs (with their near 3-year battery life) offer the best opportunity for diagnosis. [7, 8, 9]

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). [10] 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 [11, 12] and may help guide decisions regarding specific therapy. [1, 12, 13] 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. [14]

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%. [15] 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.

The ongoing VEST/PREDICTS trial (Vest prevention of Early Sudden death Trial)/(PREDiction of ICD Therapies Study) is seeking to determine whether a wearable defibrillator used for the 2 months after infarction can attenuate early mortality; additionally, it is hoped that methods for risk stratification can be developed.

Patients with or at risk for AF

ILRs can provide long-term ECG monitoring in patients at risk for or with documented AF. [16, 17] In the CARISMA trial, a 28% incidence of new-onset AF was observed. [15] ILRs may come to play an important role in cryptogenic stroke patients; the CRYSTAL-AF study is currently prospectively investigating whether ILRs improve the diagnostic yield over short-term ambulatory external ECG recording techniques in this patient population.

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.


The presence of an active infection or a bleeding diathesis may preclude implantation.


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 investigations. [18, 19, 20, 21]




Implantable loop recorders (ILRs) are small leadless boxes, about 6.5-9 mL in volume (the size of a standard memory stick), with 2 self-contained electrodes that are implanted subcutaneously under local anesthesia, typically in a left parasternal location. Current ILR devices have a battery life of up to 3 years.

Medtronic Reveal® DX. Courtesy of Medtronic, Inc. Medtronic Reveal® DX. Courtesy of Medtronic, Inc.
Medtronic Reveal® XT. Courtesy of Medtronic, Inc. Medtronic Reveal® XT. Courtesy of Medtronic, Inc.

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 [22, 23] or automatically activated upon occurrence of predefined bradyarrhythmia or tachyarrhythmia. [24, 25, 26] These devices can transmit data transtelephonically to a physician’s office for review (see the table below).

Table 1. Implantable Loop Recorders (Open Table in a new window)

  Medtronic Reveal® DX Medtronic Reveal® XT St. Jude Confirm
Size (Dimension) 62 x 19 x 8 mm 62 x 19 x 8 mm 56.3 x 18.5 x 8 mm
Size (Volume) 9 mL 9 mL 6.5 mL
Weight 15 g 15 g 12 g
Arrhythmia Detection Asystole, bradyarrhythmia VT/FVT Asystole, bradyarrhythmia VT/FVT, AT/AF Asystole, bradyarrhythmia VT/FVT
R-wave Sensing Tracking threshold, noise reversion, over range detector Tracking threshold, noise reversion, over range detector Tracking threshold, noise inhibition -Sense Ability
Extended Diagnostics  


AT/AF burden, HRV, Patient Activity, Day/Night HR, Cardiac Compass  


Longevity 3 years 3 years 3 years
Event Storage 49.5 minutes 49.5 minutes 48 minutes
Patient Assistant Marking symptoms Marking symptoms and query Marking symptoms
MRI Labeling MR conditional MR conditional MR conditional

AT = atrial tachycardia; FVT = fast ventricular tachycardia; HR = heart rate; HRV = heart rate variability; MRI = magnetic resonance imaging; VT = ventricular tachycardia.



Placement of an implantable loop recorder (ILR) can be performed in any standard procedure suite. The device is typically inserted in the left parasternal region. Prophylactic antibiotics are administered intravenously prior to the incision. Under sterile conditions the creation of a small subcutaneous pocket using local anesthesia is required. A mild anxiolytic and/or analgesic can also be administered to enhance patient comfort. Vector mapping to maximize the R-to-T should be considered.




The most common problems associated with the device 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.