Ambulatory ECG monitoring in the age of smartphones

Implantable loop recorders

Implantable loop recorders are small devices that contain a pair of sensing electrodes housed within an outer shell (Figure 1). They are implanted subcutaneously, usually in the left parasternal region, using local anesthesia. The subcutaneous location eliminates many of the drawbacks of the skin-electrode interface of external loop recorders.

Similar to the external loop recorder, this device monitors continuously and can be activated to record either by the patient by pressing a button on a separate device, or automatically when an arrhythmia is detected using a preprogrammed algorithm.

In contrast to external devices, many internal loop recorders have a battery life and monitoring capability of up to 3 years. This extended monitoring period has been shown to increase the likelihood of diagnosing syncope or infrequent palpitations.^11,12 Given that paroxysmal atrial fibrillation can be sporadic and reveal itself months after a stroke, internal loop recorders may also have a role in evaluating cryptogenic stroke.^13,14

The most important drawbacks of internal loop recorders are the surgical procedure for insertion, their limited memory storage, and high upfront cost.¹⁵ Furthermore, even though they allow for extended monitoring, there may be diminishing returns for prolonged observation.

What is their best use? For patients with palpitations, intermittent event monitoring has been shown to be cost-effective for the first 2 weeks, but after 3 weeks, the cost per diagnosis increases dramatically.¹⁶ As a result, internal loop recorders are reserved primarily for scenarios in which prolonged external monitoring has not revealed a source of arrhythmia despite a high degree of suspicion.

Mobile cardiac telemetry

Mobile cardiac telemetry builds on other ECG monitoring systems by adding real-time communication and technician evaluation.

Physically, these devices resemble either hand-held event records, with a single-channel sensing unit embedded in the case, or a traditional Holter monitor, with 3 channels, wires, and electrodes  (Figure 1).

The sensor wirelessly communicates with a nearby portable monitor, which continuously observes and analyzes the patient’s heart rhythm. When an abnormal rhythm is detected or when the patient marks the presence of symptoms, data are recorded and sent in real time via a cellular network to a monitoring center; the newest monitors can send data via any Wi-Fi system. The rhythm is then either evaluated by a trained technician or relayed to a physician. If necessary, the patient can be contacted immediately.

Mobile cardiac telemetry is typically used for up to 30 days, which  allows for evaluation of less-frequent symptoms. As a result, it may have a higher diagnostic yield for palpitations, syncope, and presyncope than the 24-hour Holter monitor.¹⁷

Further, perhaps because mobile cardiac telemetry relies less on stored information and requires less patient-device interaction than external loop recorders, it is more effective at symptom evaluation.¹⁸

Mobile cardiac telemetry also has a diagnostic role in evaluating patients with cryptogenic stroke. This is based on studies showing it has a high rate of atrial fibrillation detection in this patient population and is more effective at determining overall atrial fibrillation burden than loop recorders.^18,19

What is its best use? The key advantage of mobile cardiac telemetry is its ability to make rhythm assessments and communicate with technicians in real time. This allows high-risk patients to be immediately alerted to a life-threatening arrhythmia. It also gives providers an opportunity to initiate anticoagulation or titrate antiarrhythmic therapy in the outpatient setting without a delay in obtaining information. This intensive monitoring, however, requires significant manpower, which translates to higher cost, averaging 3 times that of other standard external monitors.¹⁵

Patch monitors

These ultraportable devices are a relatively unobtrusive and easy-to-use alternative for short-term ambulatory ECG monitoring. They monitor continuously with full disclosure, outpatient telemetry, and post-event recording features.

Patch monitors are small, leadless, wireless, and water-resistant (Figure 1). They are affixed to the left pectoral region with a waterproof adhesive and can be worn for 14 to 28 days. Recording is usually done continuously; however, these devices have an event marker button that can be pressed when the user experiences symptoms. They acquire a single channel of data, and each manufacturer has a proprietary algorithm for automated rhythm detection and analysis.²⁰

Several manufacturers produce ECG patch monitors. Two notable devices are the Zio patch (iRhythm Technologies, San Francisco, CA) and the Mobile Cardiac Outpatient Telemetry patch (BioTelemetry, Inc, Malvern, PA).

The Zio patch is a continuous external monitor with full disclosure. It is comparable to the Holter monitor, but has a longer recording period. After completing a 2-week monitoring period, the device is returned for comprehensive rhythm analysis. A typical Zio report contains information on atrial fibrillation burden, ectopic rhythm burden, symptom and rhythm correlation, heart rate trends, and relevant rhythm strips.

The Mobile Cardiac Outpatient Telemetry patch collects data continuously and communicates wirelessly by Bluetooth to send its ECG data to a monitoring center for evaluation.

A principal advantage of patch monitors—and a major selling point for manufacturers—is their low-profile, ergonomic, and patient-friendly design. Patients do not have to manage wires or batteries and are able to shower with their devices. Studies show that these features increase patient satisfaction and compliance, resulting in increased diagnostic yield.^21,22 Additionally, patch monitors have the advantage of a longer continuous monitoring period than traditional Holter devices (2 weeks vs 1 or 2 days), affording an opportunity to capture events that occur less frequently.

Validation studies have reinforced their efficacy and utility in clinical scenarios.^22,23 In large part because of the extended monitoring period, patch monitors have been shown to have greater diagnostic yield than the 24-hour Holter monitor in symptomatic patients undergoing workup for suspected arrhythmia.

The role of patch monitors in evaluating atrial fibrillation is also being established. For patients with cryptogenic stroke, patch monitors have shown better atrial fibrillation detection than the 24-hour Holter monitor.²⁴ Compared with traditional loop monitors, patch monitors have the added advantage of assessing total atrial fibrillation burden. Further, although screening for atrial fibrillation with a traditional 12-lead ECG monitor has not been shown to be effective, clinical studies have found that the patch monitor may be a useful screening tool for high-risk patients.^25,26

Nevertheless, patch monitors have drawbacks. They are not capable of long-term monitoring, owing to battery and adhesive limitations.²⁰ More important, they have  been able to offer only single-channel acquisition, which makes it more difficult to detect an arrhythmia that is characterized by a change in QRS axis or change in QRS width, or to distinguish an arrhythmia from an artifact. This appears to be changing, however, as several manufacturers have recently developed multilead ECG patch monitors or attachments and are attempting to merge this technology with fully capable remote telemetry.