Ambulatory ECG monitoring in the age of smartphones

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Table 3. Ambulatory electrocardiography devices
The available ECG monitoring devices have distinct features, indications, advantages, and disadvantages (Table 3). The Holter monitor, for example, provides full-disclosure recording, but it can store only 24 to 48 hours of data. To extend its recording length, this feature would have to be abandoned in favor of looping memory.

Recent improvements in battery life, memory, detection algorithms, wireless transmission, cellular communication, and adhesives have enabled multiple features to be combined into a single device. Patch monitors, for example, are small devices that now offer full-disclosure recording, extended monitoring, and telemetry transmitting. Automated arrhythmia recognition that triggers recording is central to all modern devices, regardless of type.

As a result of these trends, the traditional features used to differentiate devices may become less applicable. The classic Holter monitor may become obsolete as its advantages (full disclosure, continuous recording) are being incorporated into smaller devices that can record longer. Similarly, external monitors that have the capacity for full disclosure and continuous recording are no longer loop recorders in that they do not record into a circular memory.

It may be preferable to describe all non-Holter devices as event monitors or ambulatory monitors, with the main distinguishing features being the ability to transmit data (telemetry), full disclosure vs patient- or arrhythmia-activated recording, and single-channel or multichannel recording (single-lead or 3-lead ECG).

The following are the main distinguishing features that should influence the choice of device for a given clinical context.

Real-time data evaluation provided by mobile telemetry makes this feature ideal to monitor patients with suspected high-risk arrhythmias and their response to antiarrhythmic therapy.

Full-disclosure recording is necessary to assess the overall burden of an arrhythmia, which is frequently important in making treatment decisions, risk-stratifying, and assessing response to therapy. In contrast, patient- or arrhythmia-activated devices are best used when the goal is simply to establish the presence of an arrhythmia.

Multichannel recording may be better than single-channel recording, as it is needed to determine the anatomic origin of an arrhythmia, as might be the case in risk-stratification in a patient with a ventricular tachycardia.

Long duration. The clinician must have a reasonable estimate of how often the symptoms or arrhythmia occur to determine which device will offer a monitoring duration sufficient to detect an arrhythmia.


The newest ambulatory ECG devices build on the foundational concepts of the older ones. However, with miniaturized electronic circuits, Bluetooth, Wi-Fi, and smartphones, these new devices can capture ECG tracings and diagnose offending arrhythmias on more consumer-friendly devices.

Smartphones and smartwatches have become increasingly powerful. Some have the ability to capture, display, and record the cardiac waveform. One manufacturer to capitalize on these technologies, AliveCor (Mountain View, CA), has developed 2 products capable of generating a single-lead ECG recording using either a smartphone (KardiaMobile) or an Apple watch (KardiaBand).

KardiaMobile has a 2-electrode band that can be carried in a pocket or attached to the back of a smartphone (Figure 1). The user places 1 or 2 fingers from each hand on the electrodes, and the device sends an ultrasound signal that is picked up by the smartphone’s microphone. The signal is digitized to produce a 30-second ECG tracing on the phone’s screen. A proprietary algorithm analyzes the rhythm and generates a description of “normal” or “possible atrial fibrillation.” The ECG is then uploaded to a cloud-based storage system for later access or transmission. KardiaMobile is compatible with both iOS and Android devices.

The KardiaBand is a specialized Apple watch band that has an electrode embedded in it. The user places a thumb on the electrode for 30 seconds, and an ECG tracing is displayed on the watch screen.

The Kardia devices were developed (and advertised) predominantly to assess atrial fibrillation. Studies have validated the accuracy of their algorithm. One study showed that, compared with physician-interpreted ECGs, the algorithm had a 96.6% sensitivity and 94.1% specificity for detecting atrial fibrillation.27 They have been found useful for detecting and evaluating atrial fibrillation in several clinical scenarios, including discharge monitoring in patients after ablation or cardiac surgery.28,29 In a longer study of patients at risk of stroke, twice-weekly ECG screening using a Kardia device for 1 year was more likely to detect incident atrial fibrillation than routine care alone.30

Also, the Kardia devices can effectively function as post-event recorders when activated by patients when they experience symptoms. In a small study of outpatients with palpitations and a prior nondiagnostic workup, the KardiaMobile device was found to be noninferior to external loop recorders for detecting arrhythmias.31 Additional studies are assessing Kardia’s utility in other scenarios, including the evaluation of ST-segment elevation myocardial infarction32,33 and QT interval for patients receiving antiarrhythmic therapy.34

Cardiio Inc. (Cambridge, MA) has developed technology to screen for atrial fibrillation using an app that requires no additional external hardware. Instead, the app uses a smartphone’s camera and flashlight to perform photo­plethysmography to detect pulsatile changes in blood volume and generate a waveform. Based on waveform variability, a proprietary algorithm attempts to determine whether the user is in atrial fibrillation. It does not produce an ECG tracing. Initial studies suggest it has good diagnostic accuracy and potential utility as a population-based screening tool,35,36 but it has not been fully validated.

Recently, Apple entered the arena of ambulatory cardiac monitoring with the release of its fourth-generation watch (Apple Watch Series 4 model). This watch has built-in electrodes that can generate a single-lead ECG on the watch screen. Its algorithm can discriminate between atrial fibrillation and sinus rhythm, but it has not been assessed for its ability to evaluate other arrhythmias. Even though it has been “cleared” by the US Food and Drug Administration, it is approved only for informational use, not to make a medical diagnosis.

Integration of ambulatory ECG technology with smartphone and watch technology is an exciting new wearable option for arrhythmia detection. The patient-centered and controlled nature of these devices have the potential to help patients with palpitations or other symptoms determine if their cardiac rhythms are normal.

This technology, however, is still in its infancy and has many limitations. For example, even though these devices can function as post-event recorders, they depend on user-device interactions. Plus, they cannot yet perform continuous arrhythmia monitoring like modern loop recorders.

Additionally, automated analysis has largely been limited to distinguishing atrial fibrillation from normal sinus rhythm. It is uncertain how effective the devices may be in evaluating other arrhythmias. Single-lead ECG recordings, as discussed, have limited interpretability and value. And even though studies have shown utility in certain clinical scenarios, large-scale validation studies are lacking. This technology will likely continue to be developed and its clinical value improved; however, its clinical use requires careful consideration and collaborative physician-patient decision-making.

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