This retrospective cohort study was conducted at a single pediatric cardiology center from January 2016 to December 2023. This study was conducted in accordance with the Declaration of Helsinki and the ethical standards of Bristol Royal Hospital for Children’s Institutional Review Board, including ethics approval. The study aimed to evaluate the effectiveness and safety of the Reveal LINQ II ILR in diagnosing and managing undiagnosed cardiac symptoms in pediatric patients. The study included patients under 18 years of age with unexplained symptoms such as palpitations, syncope, or seizure-like episodes, or those with a high risk of arrhythmias that had a Reveal LINQ II ILR implantation and had a follow-up of at least 12 months post-implantation. Most of these patients had undergone initial non-invasive monitoring, including a 24-h Holter monitoring, 7–10-day patch monitoring, and wearable rhythm monitoring (e.g., Kardia app or Apple Watch) without achieving a conclusive diagnosis, which were considered for ILR implantation. Exclusion criteria targeted patients who were lost to follow-up before 12 months post-implantation, as this would limit the comprehensive assessment of the ILR’s diagnostic yield and outcomes.

Pre-implantation management

The pre-implantation cardiac screening approach was tailored to each case by the primary healthcare provider, following an individualized assessment. As a baseline, all patients underwent a minimum screening comprising a 12-lead ECG, a 24-h Holter monitoring, and echocardiography. For certain cases, additional investigations were conducted based on presenting symptoms, with options including an exercise stress test (EST), cardiac magnetic resonance imaging (CMRI), electrophysiology study (EPS), and/or electroencephalogram (EEG). The decision to pursue more extensive investigations was made on an individual basis.

Indications for implantable loop recorder (ILR)

ILR indications encompassed a spectrum ranging from the diagnosis of infrequent or unexplained cardiac symptoms such as palpitations, chest pain, or seizure-like episodes to the follow-up of patients with previously recorded cardiac arrhythmias or those at high risk, such as those with ICCs.

Essential features of ILR implantation technique

Reveal LINQ II ILRs (Medtronic Inc., Minneapolis, MN, USA) were implanted following standard sterile precautions in a dedicated cardiac catheterization laboratory. The procedures were performed under general or local anesthesia, depending on the patient’s age, cooperation, and preference. The implantation site was subcutaneous in the left parasternal region for all patients, except for one case where an axillary region implantation was chosen for aesthetic reasons. The devices were typically inserted at a 45-degree angle to the sternum, which is an approach chosen to enhance device stability and patient comfort. After sterile field preparation, the skin was infiltrated with local anesthetic, and the device was injected into the subcutaneous layer. The skin wound was closed with a resorbable single suture. Antibiotics were not routinely administered.

Device programming and standardized follow-up

Following implantation, patient-specific programming of tachycardia and bradycardia detection zones was conducted, taking into consideration the patient’s age, underlying heart disease, medication, and clinical history. In selected patients with clinical evidence of non-sustained VT, the settings were adapted to enhance sensitivity and arrhythmia detection by reducing the rates to 180 bpm and the number of beats to 8 when necessary. Furthermore, patients and their families were trained to manually activate recordings during symptomatic episodes, allowing the capture of non-sustained VT that did not meet automatic detection criteria.

After recovering from anesthesia, patients and their family members were instructed on how to activate the monitor and transmit data, and were advised to contact the cardiology department in the event of symptoms. Patients were typically discharged on the same day following a 3- to 4-h observation period.

All patients performed a test download at home to ensure connectivity to the CareLink website. Test downloads were not included in the analysis. Transmissions were reviewed by dedicated pediatric cardiac rhythm physiologists, who forwarded them to pediatric electrophysiologists for evaluation in cases of potentially abnormal recordings.

The transmissions were subsequently subclassified into two subgroups: [1] diagnostic transmissions that reveal arrhythmias directly related to the primary reason for the ILR implantation and [2] relevant transmissions, including all significant findings, whether or not they are directly related to the primary reason for the ILR implantation. While such findings may not directly relate to the initial clinical suspicion, they still contribute valuable information to the overall clinical picture and patient management.

Elective follow-up was scheduled annually for patients with non-pathological transmissions, or more frequently as needed for those with positive or pathological findings.

For the purposes of this manuscript, relevant arrhythmias are defined as pauses of 3 s or longer, high-degree (type IIb or type III) atrioventricular (AV) block, and evidence of sinus node dysfunction (SND) based on age-related definitions, as well as supraventricular tachycardia (SVT), ventricular tachycardia (VT), or inappropriate sinus tachycardia (IST) [9, 10]. Non-relevant arrhythmias are defined as sinus bradycardia (SB), sinus pauses of less than 3 s, sinus tachycardia (ST), supraventricular ectopy (SVE), and ventricular ectopy (VE).

Data collection

A retrospective chart review was conducted at our center following institutional review board approval. All patients under 18 years of age who underwent ILR implantation from January 2016 to December 2023 were included. Medical records were reviewed, and data were collected on patient demographics, family history, presence of congenital heart disease or genetic diagnosis, treatment, and ILR implantation indications. Device-related data collected included patient-activated and device-activated transmissions, recorded rhythm abnormalities, time from implantation to diagnostic transmission, patient management changes, and complications. For the purpose of this study, major complications were defined as those requiring significant medical intervention, prolonged hospitalization, or causing lasting harm; while minor complications included issues which could be resolved with minimal intervention.

Statistical analysis

The results of this study are predominantly descriptive and are expressed as percentages, with mean values and standard deviations (SD) for continuous variables. A p-value of less than 0.05 was considered to indicate statistical significance. All statistical analyses were performed using the IBM SPSS Statistics 25.0 software (SPSS Inc., Chicago, IL, USA).

Differences between groups were evaluated using the chi-squared test and Fisher’s exact test for categorical variables. For continuous variables with a normal distribution, an unpaired Student’s t-test was used. For continuous variables without a normal distribution, the Mann–Whitney U test was applied. The Wilcoxon rank-sum test was employed for skewed continuous variables, and the ANOVA was used for comparisons among more than two groups.