Implantable Cardiac Monitors

Introduction

• ICMs, also referred to as insertable cardiac monitors or insertable looper recorders, are small subcutaneous devices designed for continuous cardiac rhythm monitoring. Their minimally invasive implantation, multi-year battery life, and automated recording capabilities make them useful diagnostic tools when short-term monitors are insufficient.^1,2

Indications

• Unlike short-term external monitoring devices such as a Holter monitor (24-48 hours) or an event monitor (2-4 weeks), ICMs provide continuous, long-term cardiac rhythm monitoring over several years. Their extended monitoring capabilities provide a cost-effective means of correlating infrequent and intermittent symptoms with underlying arrhythmias that would otherwise go undetected.²
• ICMs play an important diagnostic role in patients with recurrent, unexplained syncope, particularly when initial testing, such as electrocardiography (ECG), echocardiography, and short-term ambulatory ECG monitoring, fails to establish a diagnosis. Arrhythmogenic syncope is a significant risk factor for underlying heart disease; therefore, long-term rhythm surveillance has high diagnostic value.^1,3
• The 2018 European Society of Cardiology Syncope Guidelines recommend the use of ICMs in patients with syncope in the setting of primary cardiomyopathies or inherited arrhythmogenic disorders.⁴
• The Randomized Assessment of Syncope Trial study aimed to assess the efficacy of prolonged monitoring in patients with unexplained syncope. Sixty patients with unexplained syncope were randomized to either a conventional diagnosis strategy, including 2-4 weeks with an external loop recorder and a provocative tilt and electrophysiological test, or an ICM for 1 year. A diagnosis was made in 52% of patients with ICM, compared with 20% of those undergoing conventional strategies.⁵
• The ICM’s improved diagnostic performance was reflected in its greater ability to detect spontaneous arrhythmias, particularly bradyarrhythmias. Provocation testing and short-term monitoring may fail to diagnose patients with infrequent symptoms because of their brief monitoring window. However, the continuous long-term surveillance of ICMs makes it a reliable tool for symptom-rhythm correlation, meriting it as an effective initial strategy in patients with unexplained syncope.^3,5
• ICMs can provide long-term surveillance for patients with recurrent palpitations, particularly when abnormalities are not detected on short-term monitoring. The identification of arrhythmias can provide prognostic and therapeutic value. For example, detection of supraventricular tachycardia in a patient with palpitations may identify them as an appropriate candidate for catheter ablation. ICMs also allow detection of atrial fibrillation and other supraventricular or ventricular arrhythmias that would otherwise be missed with short-duration monitoring.¹
• In patients with an ischemic stroke of an unknown etiology, ICMs demonstrate substantial value in detecting paroxysmal, asymptomatic atrial fibrillation compared to conventional follow-up. Detection of occult atrial fibrillation has significant implications for secondary stroke prevention, including the initiation of oral anticoagulation therapy.⁶
• Additional clinical uses of ICMs include assessing the average heart rate and rate control in patients with atrial fibrillation. They can also establish the clinical need for permanent pacemaker implantation by evaluating bradycardic episodes. These devices can be useful for detecting asymptomatic ventricular premature beats or nonsustained ventricular tachycardia in individuals with congenital or acquired cardiac syndromes. Furthermore, they can provide valuable prognostic information for patients at increased risk of acute coronary events.^1,2

Implantation Process

• ICM implantation is a minimally invasive procedure. The small size of modern ICMs allows for subcutaneous insertion under local anesthesia, with or without light sedation. These procedures are commonly performed in cardiac catheterization and electrophysiology laboratories. Procedural simplification over time has contributed to their shift into outpatient settings.^7-9
• Anesthesia and Preparation: The procedure is typically well-tolerated with minimal sedation and local anesthetic infiltration.^7-9
• Insertion Technique: A small incision, approximately 1cm, is made in the left parasternal region, most often in the 4th intercostal space, 2-3 cm lateral to the sternum. Modern ICMs use proprietary injection tools to subcutaneously implant the device. The device is loaded into the injector and then inserted parallel to the skin. It is then rotated approximately 180 degrees to create a subcutaneous pocket. The ICM is then deployed beneath the dermis.^7-9

• Device Programming: After insertion, the device is verified to ensure proper sensing and is then programmed according to the patient’s clinical indication. Once the incision is closed, patients are typically discharged the same day.^7-9

Types of ICMs

• ICMs have evolved significantly over the past several decades and advances have been made to optimize device size, battery, monitoring capability, and implantation technique. They are generally classified as first- or second-generation devices.⁹
• First-generation devices were larger and required the creation of a subcutaneous pocket through a surgical incision. These devices were reliable for rhythm monitoring but involved a more invasive implantation process. Examples include the Reveal™ (Medtronic Inc.), which measures approximately 8 cc, and the Confirm™ (Abbott Laboratories), which measures approximately 6.5 cc. These models offered up to three years of battery life and provided up to 18 months of atrial fibrillation and arrhythmia data, including stored event timestamps and episodic durations.^7,9
• Second-generation ICMs are smaller, injectable devices with proprietary insertion tools. These devices only require a 1cm incision and a brief subcutaneous injection. For example, the Reveal LINQ™ (Medtronic Inc.) is approximately 1 cc in volume – roughly one third the size of a AAA battery – providing up to 3 years of battery life. These devices incorporate improved sensing algorithms and more accurate long-term monitoring than first-generation models. The insertion process typically only takes a few minutes.^7,9

Anesthetic Considerations

• Patients with ICMs have a different perioperative risk profile than those with a therapeutic cardiac implantable electronic device (CIED), such as a pacemaker, implantable cardioverter-defibrillator (ICD), or cardiac resynchronization therapy device.¹⁰
• Since ICMs do not provide pacing or defibrillation, they do not require perioperative programming. There are no pacing-dependent or brachytherapy considerations during anesthesia. Correct identification of the patient’s CIED is important to avoid unnecessary pacemaker/ICD workflows. If the surgical field is near the subcutaneous pocket, care should be taken to prevent mechanical damage to the device. Exposure to electromagnetic interference may result in recorded artifacts that resemble tachyarrhythmias; however, these have no hemodynamic consequences because the device lacks therapeutic output.¹⁰
• Overall, ICMs do not meaningfully change anesthetic management. Perioperative planning should instead focus on the patient’s underlying cardiac pathology for which the monitor was implanted and the patient’s overall cardiovascular risk profile.¹⁰