Coexistence of R-Wave Oversensing and Undersensing in an Implantable Loop Recorder: The Issue of Multiple Sensing in Implantable Loop Recorders

Introduction

Unexplained syncope and palpitations present significant diagnostic challenges in clinical practice, often requiring extended cardiac monitoring to identify underlying arrhythmias. Traditional methods, such as Holter monitors and event recorders, offer limited diagnostic yield because of their short monitoring duration [1]. Implantable loop recorders (ILRs) have emerged as a valuable tool in the prolonged evaluation of patients with unexplained syncope, particularly when initial noninvasive testing remains inconclusive. According to the 2017 American College of Cardiology/American Heart Association/Heart Rhythm Society guideline for the evaluation and management of syncope, ILRs are recommended for patients with recurrent, unexplained episodes when an arrhythmic etiology is suspected but not confirmed through conventional monitoring [2]. Studies have demonstrated that ILRs significantly increase diagnostic yield compared with standard cardiac monitoring. One study showed that ILRs detected an arrhythmic cause in 73% of patients with frequent unexplained palpitations, leading to definitive treatment decisions [3].

Despite their advantages, ILRs are not without limitations. A key challenge is signal sensing, which can lead to misinterpretation of cardiac events and inappropriate arrhythmia detection. Oversensing can occur due to myopotentials, electromagnetic interference, or misclassification of physiologic signals such as prominent P or T waves. Our case highlights an instance of ILR oversensing leading to erroneous atrial fibrillation (AF) detection, emphasizing the need for individualized device programming and careful parameter adjustments to optimize diagnostic accuracy.

Case Report

Our patient was a 73-year-old woman with a history of hypertension and hyperlipidemia, who has been experiencing unexplained episodes of lightheadedness, palpitations, and hypotension for the past several months. Despite multiple evaluations, including repeated electrocardiograms (Figure 1) and ambulatory Holter monitoring, no definitive arrhythmia was identified. Echocardiography revealed no structural heart disease, and brain imaging findings were unremarkable. Due to persistent symptoms without a clear etiology, a Biotronik Biomonitor III ILR was implanted in the right parasternal region, for extended rhythm monitoring.

Initial parameters of the ILR included a heart rate above 150 bpm or below 30 bpm, as well as patient-initiated triggers. The sensing filter was set to a standard 10 Hz, while the identification of sudden rate drops was omitted from the detection criteria. AF sensitivity was set at a low threshold, with a detection/termination window of 16/24, detection intervals at 11, a count of 2 detection windows, RR variability limited to 12%, a confirmation time of 1 min, and use of an aggressive bigeminy rejection mode.

Three months after implantation, ILR interrogation revealed 215 episodes of AF and several episodes flagged as ventricular fibrillation, despite the patient remaining asymptomatic during these events. The cumulative AF burden was approximated at 2%. Upon further review, signal artifacts suggested erroneous oversensing of P waves (Figure 2A, 2B) and T waves, with under-sensing of QRS complexes, leading to false-positive arrhythmia detections (Figure 2C, 2D). Adjusting the sensing filter of the ILR from 10 Hz to 24 Hz successfully mitigated the oversensing issue (Figure 2E), preventing erroneous arrhythmia diagnoses.

After the sensing filter of the ILR was reprogrammed, the patient continued regular follow-up visits for monitoring and rhythm assessment. After the adjustment, the ILR did not detected any further false-positive episodes, and no additional intervention was deemed necessary. No other arrythmic events were recorded.

Discussion

Our case is particularly unique in that it demonstrates how standard signal sensing settings on ILRs can lead to erroneous arrhythmia detection because of simultaneous oversensing of P waves and T waves, and at times, undersensing of QRS complexes.

ILRs have revolutionized the evaluation of unexplained syncope by enabling continuous, long-term cardiac monitoring, thereby significantly improving diagnostic yield. The PICTURE registry found that ILRs identified an arrhythmic cause in 78% of patients with unexplained syncope, often prompting a change in clinical management [1]. In alignment with this, the 2017 American College of Cardiology/American Heart Association/Heart Rhythm Society guidelines recommend ILR use in patients with recurrent, infrequent unexplained syncope when an arrhythmic etiology is suspected but not confirmed by conventional testing, regardless of structural heart disease status [2].

ILRs have also shown utility in evaluating recurrent, unexplained palpitations, particularly when traditional testing fails to capture arrhythmic events. The Recurrent Unexplained Palpitations (RUP) study reported that ILRs identified a causal arrhythmia in 73% of such patients, enabling tailored treatment strategies [3].

Despite their diagnostic benefits, ILRs have known limitations. Chief among them is oversensing, which can result in misinterpretation of cardiac signals and false arrhythmia alerts. Oversensing can occur due to myopotentials, electromagnetic interference, or improper device positioning [4].

Multiple case reports have documented instances of oversensing and undersensing with ILRs. One case described P-wave oversensing that masked clinically significant pauses during syncope episodes [5]. Another documented false-positive asystole episodes [6], while a third reported T-wave oversensing that was resolved by repositioning the device [7]. Given that ILRs are subcutaneous devices, such limitations are not unexpected. Supporting this, a multicenter cohort study found that 59.8% of all ILR-generated alerts were false positives, with even higher rates reported for AF and atrial tachycardia [8].

What makes our case distinctive is the right parasternal implantation site and the occurrence of R wave oversensing of P waves and T waves, with undersensing of QRS complexes. Although nontraditional, this right parasternal location was selected to enhance visualization of atrial activity, particularly P waves.

Regarding the optimal implantation technique, previous studies have recommended placement at the midclavicular line of the third intercostal space or the left sternal border of the fourth intercostal space, citing more reliable R-wave amplitudes and minimal impact from insertion angle [9]. These locations are widely accepted and well supported in the literature [10–12]. However, in our case, the choice of a right parasternal site, while beneficial for atrial signal clarity, may have contributed to inappropriate multiple sensing. This case also illustrates that an early post-implant review of remote transmissions with rapid reprogramming prevented the subsequent false-alert burden [13,14].

The cause of syncope in our patient still remains elusive despite exhaustive workup. She experienced no recurrence of syncope, and 2.5 years of monitoring revealed no episodes of tachychardia or bradyarrhythmia.

Conclusions

This case highlights how unconventional implantation sites, such as the right parasternal region, can yield distinct sensing profiles that necessitate careful device programming to prevent inappropriate arrhythmia detection. This also highlights the eloquent solution in reprogramming the ILR as opposed to repositioning it, as is done in some left parasternal implantation locations.