A LINQ to the Diagnostic Link: Use of Implantable Loop Recorders in EP

Introduction

The patient is a 50-year-old male farmer who takes no medications and reports no medical problems. He admits to “dizziness” and even frank fainting spells on occasion. A complete work-up for these spells is negative, including a stress test, head CT, labs, 24-hour Holter monitor, and echo. A 12-lead EKG shows only a left bundle branch block. He refuses an electrophysiology study (EP study). What additional work-up could be done to evaluate the patient?

Unexplained syncope is potentially a bad diagnosis to have. The patient may live in fear while waiting for another syncopal episode to strike, causing disability or in rare events, death. It also can become a liability for the physician, since it may be unclear when patients can safely resume driving.

This review article will examine the usefulness of the implantable loop recorder (ILR) in the evaluation of syncope. Additionally, we will go over the implantation procedure and data derived from the ILR. Finally, we will cover novel uses and future directions for the device. 

Background

For years, the Reveal LINQ Insertable Cardiac Monitor (Medtronic) and SJM Confirm Implantable Cardiac Monitor (St. Jude Medical) paved the way for long-term implantable ECG monitoring. The size of a flash drive, both monitors provided up to 3 years of one-lead ECG data. Limitations included occasional oversensing of arrhythmic events. However, both allowed for patient activation of events as well as auto activation to record pauses or arrhythmias in which patients might be unaware.

A large number of well-designed prospective studies have documented the utility of ILRs to evaluate patients with unexplained syncope. Krahn et al looked at 85 such patients who received an ILR. An arrhythmia was detected in 42% of the patients in ten months of follow-up, with most being bradycardia.¹ The authors later studied 60 syncopal patients who were randomized to ILR usage versus “conventional monitoring,” which included a 2- to 4-week Holter monitor, tilt table testing, and EP study.² The results were striking: by one year of follow-up, 55% of those in the ILR group versus only 19% in the conventional group had a diagnosis. The majority of diagnostic yields led to implantation of a pacemaker. 

Other multicenter ILR studies followed. The ISSUE trial enrolled 35 patients with syncope who had a negative echo, EP study, carotid ultrasound, and EKG.³ All received an ILR as work-up for the syncope. Ultimately, 11% of the patients received a pacemaker or defibrillator to treat their events, based on ILR data, and 14% more were treated with medical management. 

The PICTURE trial enrolled 570 patients who received a Reveal Plus, DX, or XT ILR.⁴ In the end, 38% had recurrent syncope by one-year follow-up, which matches that found by other such studies. Of 218 events within the study, the ILR guided the diagnosis in 170 patients (78%), with the majority again being bradycardia. These large-scale, prospective studies highlight the importance of utilizing ILR technology in patients with unexplained syncope.

Use of ILRs has been proposed as Class I and Class IIA indications (meaning that they are indicated or are very useful) in the evaluation of “patients with recurrent syncope of uncertain origin” or with traumatic syncope, among other indications.⁵ The yield for ILRs can be higher than that of EEGs or EP studies in certain patient populations.^6,7

Implantation Procedure

We implanted a Reveal LINQ Insertable Cardiac Monitor in the patient described above with recurrent, unexplained syncopal/dizzy spells. Incredibly, a 17-second period of intermittent complete heart block with asystole and syncope was recorded (Figure 1). We removed the Reveal LINQ ICM and placed a pacemaker, which successfully treated his spells.

The “new kid” on the ILR block is the Reveal LINQ ICM, which was approved by the FDA for use in 2014. Tiny and powerful, the Reveal LINQ ICM is only 2.5 grams. It can store up to 30 minutes of patient-activated episodes or 27 minutes of automatic activation downloads. The data can be stored remotely and wirelessly with the online CareLink Network (Medtronic). It provides excellent, one-lead EKG monitoring with a sophisticated atrial fibrillation (AF) algorithm, and it is also MRI-compatible.

Implantation of the Reveal LINQ ICM is easy and can be done in an EP/cath lab or (theoretically) in the office, although most implanting physicians still choose to use a dedicated lab for the procedure. The Reveal LINQ ICM is injected under the skin and closed with Dermabond or a couple of stitches. The battery lasts up to 3 years.

Novel Uses and Future Directions

An emerging use for the Reveal LINQ ICM is for the evaluation of patients with cryptogenic (unexplained) stroke.⁸ A fascinating randomized, prospective study of 441 patients with unexplained stroke or TIA were randomized to receive either a Medtronic ICM or “conventional follow-up” with the CRYSTAL AF study published in 2014.⁸ The study group was relatively young, with an average age of only 61 years old. At one year of follow-up, 12% of patients in the ICM group versus 2% in the control group had been diagnosed with AF. By three years of follow-up, this had expanded to 30% diagnosed with AF through ICM usage versus only 3% in the control group. Being diagnosed with AF would naturally lead to more aggressive medical treatment with drugs like dabigatran, coumadin, etc., as opposed to aspirin, which could in turn lead to reduction of recurrent stroke events in those patients.

The ILR is also a powerful tool in the armamentarium to diagnose cryptogenic stroke. For example, we recently treated a 75-year-old female with well-controlled hypertension who suffered a massive, unexplained stroke. A 24-hour Holter monitor, EKG, echo, labs, and transesophageal echo were unrevealing as to its etiology. She was placed on aspirin to prevent future strokes. A Reveal LINQ ICM was placed as work-up. Several months later, feeling good and without symptoms, the patient had her Reveal LINQ ICM interrogated (Figure 2). The tracings demonstrated paroxysmal atrial fibrillation. Coumadin was substituted for aspirin to more fully treat the AF, which was now recognized to be the cause of her stroke. 

Another developing use of the ICM/ILR is to evaluate the efficacy of pulmonary vein catheter ablation to eradicate atrial fibrillation.⁹ Its excellent AF algorithm and easy implantation make it ideal to see if AF ablation has been successful and possibly guide discontinuance of anticoagulation in the right patient. 

Finally, the pediatric EP community has utilized ILRs to assist in the management of children with inherited arrhythmia syndromes such as long QT or Brugada syndrome.¹⁰ It can be very challenging in these patients to know if a defibrillator should be implanted, and ILRs can provide needed data in this complex patient population.

Conclusion

Implantable loop recorders represent a high-tech, quantum leap forward in providing real-time ECG data for those patients with unexplained syncope, palpitations, or even stroke. It is likely that the utility of the ILR will only increase over time as we gain more knowledge into its usefulness.

Disclosures: The author has no conflicts of interest to report regarding the content herein.   

References

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