A graphical summary of the mechanism underlying the underestimation of a prolonged pause episode by an implantable cardiac monitor (ICM). This case illustrates that ICM episode summaries may underestimate clinically significant pauses, underscoring the importance of electrogram review and remote monitoring for accurate diagnosis. Implantable cardiac monitors (ICMs) have become essential tools in evaluating unexplained syncope by enabling continuous, long-term rhythm surveillance and detecting clinically significant arrhythmias, including bradycardia 1. Automated arrhythmia detection algorithms have substantially improved current-generation ICMs' diagnostic yield compared with conventional monitoring modalities 2. Nevertheless, important limitations persist, particularly when arrhythmic events fall outside programmed detection thresholds or are influenced by signal quality and sensing performance 3. Here, we report a case in which an ICM implanted for recurrent syncope failed to capture the full duration of a prolonged pause episode. The event's true extent was identified only through careful review of the stored subcutaneous electrograms (EGMs) recorded by the ICM, rather than event counters alone. A 79-year-old man with a history of rheumatic mitral stenosis with mild regurgitation underwent percutaneous transvenous mitral commissurotomy at 70 years of age. He experienced persistent atrial fibrillation (AF) at 75 years, and warfarin therapy was initiated. One year later, the patient experienced a syncopal episode resulting in facial trauma. Comprehensive evaluation, including blood tests, 12-lead electrocardiography (ECG), transthoracic echocardiography, brain magnetic resonance imaging, and electroencephalography, failed to identify a definitive cause. After a second syncopal episode 2 months later, an ICM (Reveal LINQ, Medtronic Inc., Minneapolis, MN, USA) was implanted for long-term rhythm monitoring (Figure 1). It was appropriately positioned, and adequate QRS sensing during AF was confirmed. Subsequently, the patient remained syncope-free for 3 years. However, he experienced another fall with facial trauma on the scheduled day of ICM removal due to battery depletion. Following this index syncopal recurrence, the patient was admitted for continuous inpatient monitoring. A Holter ECG recording was performed during hospitalization to further evaluate bradyarrhythmia; however, it was obtained under controlled inpatient observation and did not represent a separate spontaneous syncopal episode. The ICM automatically detected a 5-s pause. However, a detailed review of the corresponding EGM revealed that the actual pause duration was considerably longer (Figure 2A). The Reveal LINQ defines a pause as the absence of sensed ventricular activity beyond a predefined threshold and terminates the episode after detecting 12 subsequent sensed signals, regardless of whether these signals represent a true QRS complex. In other words, after the AD marker, the Reveal LINQ only evaluates the end criteria for the episode, which does not affect the duration of the pause. In the present case, approximately 10 s post-event onset, AF waves were misclassified as QRS complexes, resulting in a false ventricular sense (VS). This was followed by three fast ventricular tachycardia zone sensing (FS) markers. The episode was terminated after an AF wave was misinterpreted as a VS marker. Consequently, oversensing caused premature episode termination and an underestimation of the true pause duration, which became apparent only upon review of the stored EGM. Immediate pacemaker implantation was not feasible owing to institutional constraints; therefore, continuous inpatient rhythm monitoring was performed using a Holter ECG, which was automatically stored in the electronic medical record. This additional recording confirmed prolonged bradyarrhythmic AF with a maximum pause of 20.8 s, without evidence of f-wave oversensing (Figure 2B). Importantly, this Holter ECG did not represent a separate syncopal event but was performed after the index event under inpatient monitoring to corroborate the ICM findings and guide clinical decision-making regarding permanent pacemaker implantation. Device interrogation revealed 1952 pause episodes recorded in the event counter. However, remote monitoring was unavailable; therefore, the true duration and frequency of prolonged pauses may have been substantially underestimated. Bradyarrhythmic AF diagnosis was ultimately established based on the combined findings from the ICM and inpatient Holter ECG. The clinically significant prolonged pause remained underrecognized until a systematic review of stored EGMs following the syncopal recurrence, underscoring the importance of prompt and meticulous device interrogation post-symptom recurrence on incidental review. ICMs are the gold standard for associating clinical symptoms with arrhythmic events in patients with unexplained syncope 1, 2, 4. In the present case, the device appropriately detected the pause; however, the episode was prematurely terminated due to f-wave oversensing, which is a nonventricular signal. Without careful EGM confirmation, such oversensing may lead to clinically relevant bradyarrhythmic burden underestimation. Oversensing is a known limitation of ICMs and may result from myopotentials, electromagnetic interference, or atrial activity interpretation as VS. In this patient, despite appropriate exclusion of AF, the absence of true QRS complexes during the prolonged pause episode may have increased device sensitivity, resulting in f-wave misclassification 5. Although this case involved the Reveal LINQ, sensing algorithms, pause detection logic, and episode storage strategies vary among manufacturers. Because most ICM systems rely on automated detection and limited EGM storage capacity, similar risks of event misclassification or underestimation may exist across platforms. This case highlights the importance of a comprehensive assessment integrating clinical presentation, pause episode counters, and stored EGMs. Reliance on pause episode counts alone may substantially underestimate the true bradyarrhythmic burden in patients with recurrent syncope or a high pretest probability of arrhythmia. A normal or inconclusive ICM report should not preclude further diagnostic evaluation. Moreover, the Reveal LINQ stores only a limited number of pause episode EGMs, and frequent events may overwrite clinically significant recordings when remote monitoring is not implemented. Accordingly, routine review of stored EGMs and remote monitoring implementation post-symptom recurrence are essential for accurate diagnosis and timely intervention. Although this case involves the Reveal LINQ, similar detection and termination algorithms are employed in the Reveal LINQ II, suggesting the existence of comparable limitations. In conclusion, this case illustrates a critical limitation of ICMs: clinically significant pause episode underestimation due to oversensing and restricted storage algorithms. Despite appropriate implantation and programming, the ICM failed to capture the full duration of a prolonged pause, identified only through systematic EGM review post-syncope. Clinical interpretation should extend beyond automated episode summaries in high-risk patients with AF and recurrent syncope to include detailed EGM analysis. Regular follow-up and remote monitoring are essential to prevent data loss and ensure timely diagnosis and management. The authors have nothing to report. The authors have nothing to report. Ethical approval was waived because this was a single-patient case report, and the patient provided informed consent for publication. Shinya Kowase has received honoraria from Abbott, Biotronik, Boston Scientific, Japan Lifeline, and Medtronic. The other authors have no conflicts of interest to declare. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.
Takahashi et al. (Fri,) studied this question.
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