Ambulatory ECG monitoring in the age of smartphones

AMBULATORY ECG DEVICES

Numerous ambulatory ECG devices are available, each with various features (Table 2). Which features are most important depends on the severity and frequency of the symptoms, the suspected diagnosis, and the risk that the patient will not adhere to recording instructions.

Continuous external monitoring: The Holter monitor

The traditional ambulatory ECG device is the Holter monitor, named after its inventor. This light, portable, battery-operated recorder can be worn around the neck or clipped to the belt (Figure 1). The recorder connects via flexible cables to gel electrodes attached to the patient’s chest. The monitor may have 2, 3, or 12 channels.

Recording is typically done continuously for 24 to 48 hours, although some newer devices can record for longer. Patients can press a button to note when they are experiencing symptoms, allowing for potential correlation with ECG abnormalities. The data are stored on a flash drive that can be uploaded for analysis after recording is complete.

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What is its best use? Given its relatively short duration of monitoring, the Holter device is typically used to evaluate symptoms that occur daily or nearly daily. An advantage of the Holter monitor is its ability to record continuously, without requiring the patient to interact with the device. This feature provides “full disclosure,” which is the ability to see arrhythmia data from the entire recording period.

These features make Holter monitoring useful to identify suspected frequently occurring silent arrhythmias or to assess the overall arrhythmia burden. A typical Holter report can contain information on the heart rate (maximum, minimum, and average), ectopic beats, and tachy- and bradyarrhythmias, as well as representative samples.

The Holter device is familiar to most practitioners and remains an effective choice for ambulatory ECG monitoring. However, its use has largely been replaced by newer devices that overcome the Holter’s drawbacks, particularly its short duration of monitoring and the need for postmonitoring analysis. Additionally, although newer Holter devices are more ergonomic, some patients find the wires and gel electrodes uncomfortable or inconvenient.

Intermittent monitoring: Event recorders

Unlike the continuous monitors, intermittent recording devices (also called event recorders), capture and store tracings only during an event.

Intermittent recording monitors are of 2 general types: post-event recorders and loop recorders. These devices can extend the overall duration of observation, which can be especially useful for those whose symptoms and arrhythmias are infrequent.

Post-event recorders are small and self-contained, not requiring electrodes (Figure 1). The device is carried by the patient but not worn continuously. When the patient experiences symptoms, he or she places the device against the chest and presses a button to begin recording. These tracings are stored on the device and can be transmitted by telephone to a data center for analysis. Although post-event recorders allow for monitoring periods typically up to 30 days, they are limited by requiring the patient to act to record an event.

What is its best use? These devices are best used in patients who have infrequent symptoms and are at low risk. Transient or debilitating symptoms, including syncope, can limit the possibility of capturing an event.

Intermittent monitoring: Loop recorders

Loop recorders monitor continuously but record only intermittently. The name refers to the device’s looping memory: ie, to extend how long it can be used and make the most of its limited storage, the device records over previously captured data, saving only the most important data. The device saves the data whenever it detects an abnormal rhythm or the patient experiences symptoms and pushes a button. Data are recorded for a specified time before and after the activation, typically 30 seconds.

Loop recorders come in 2 types: external and implantable.

External loop recorders

External loop recorders look like Holter monitors (Figure 1), but they have the advantage of a much longer observation period—typically up to 1 month. The newest devices have even greater storage capacity and can provide “backward” memory, saving data that were captured just before the patient pushed the button.

In studies of patients with palpitations, presyncope, or syncope, external loop recorders had greater diagnostic yield than traditional 24-hour Holter monitors.^7,8 This finding was supported by a clinical trial that found 30-day monitoring with an external loop recorder led to a 5-fold increase in detecting atrial fibrillation in patients with cryptogenic stroke.⁹

Disadvantages of external loop recorders are limited memory storage, a considerable reliance on patient activation of the device, and wires and electrodes that need to be worn continuously.

What is their best use? External loop recorders are most effective when used to detect an arrhythmia or to correlate infrequent symptoms with an arrhythmia. They are most appropriately used in patients whose symptoms occur more often than every 4 weeks. They are less useful in assessing very infrequent symptoms, overall arrhythmia burden, or responsiveness to therapy.¹⁰