Medical papers

Ambulatory electrocardiography:indications and devices

The article “Ambulatory electrocardiography: indications and devices”, published in the British Journal of Cardiac Nursing in March 2019 [1], provides an overview of devices used for ambulatory ECG monitoring in contemporary cardiology practice. The article covers both external and implantable devices. The different classes of ECG monitoring devices are described in detail. 
Continuous recorders
The first class of devices described in the article is continuous recorders. This includes Holter monitors and patch electrode monitors. Continuous recorders record each heartbeat and cardiac monitoring with such devices provides valuable information about arrhythmia burden, number of ectopic beats, and the heart rate profile. The limitation of continuous recorders is the short recording time.
Holter Monitors
Historically, this is the first class of ambulatory ECG monitoring devices that were invented in the 1950s. The electrical signal is recorded using electrodes that are attached to the skin, while a recording device is worn by the patient on the shoulder or on a belt. The main limitation of Holter monitors is the limited memory space, and that’s why most of the available models are capable of recording no more than 2-3 days. Despite the existence of models with the ability to record up to 7 days, they are much less common. Also, most Holter monitors register a signal via 2-3 channels only.
According to the authors, Holter monitoring with conventional devices has several limitations:
  • The need for the patient to keep a diary of symptoms (otherwise the interpretation of the results can be difficult)
  • Short recording time, leading to the inability to capture low-frequency events
  • Poor patient comfort (including the inability to shower or bathe while wearing the device)
  • The delay in obtaining results and making a diagnosis, since the decryption of the recorded data requires a cardiac technician or physiologist and can take from 1-2 days to several weeks.

Patch electrode monitors
Patch electrode monitors (PEMs) are small disposable recorders that are attached to the skin (usually in the left pectoral region) with a self-adhesive patch. These devices register an electrical signal through 2 channels and are capable of working for 7-14 days.
Such devices provide up to 14 days of continuous recording. Patients are still encouraged to keep a symptoms diary (on paper or in a smartphone app.
Compared to Holter monitors, these devices are more compact and comfortable to use for patients (including waterproof). According to a study by Barrett et al [2], due to the longer recording period, patch monitors may allow detection of twice as many rhythm disturbances as with the standard 24-hour Holter.
Intermittent recorders
The second class of devices that are used for ambulatory ECG monitoring are intermittent recorders. This includes event recorders, external loop recorders, implantable loop recorders, and cardiac implantable electronic devices. Intermittent recorders allow recording an ECG signal for a long period of time - up to 4 weeks for external recorders, and up to 3-4 years for implantable devices.
The authors of the article divide this group of devices into 2 subgroups - event recorders and continuous recorders.
Event Recorders
Event recorders are activated by the patient and record an ECG only during symptoms. Unlike Holter, the continuous wearing of the device is not required - the patient should press the device to the chest only during symptoms. The recorded date can be transferred to the hospital over the telephone line, which reduces time for analysis and interpretation compared to conventional Holter.
Limitations of event recorders:
  • Only short ECGs during symptoms are recorded
  • Inability to register asymptomatic events
  • Inability to record an ECG before the onset of symptoms (due to a delay in activating the device)
  • Low diagnostic yield in the evaluation of syncope
External Loop Recorders
ELRs record heart rate continuously, and recording can be activated either by the patient or by internal auto-trigger function. ELRs are designed similarly to traditional Holter monitors - the signal is recorded using electrodes on the chest surface, and the device itself is attached to a belt. The device continuously monitors the electrical activity of the heart and is able to independently activate the recording when it detects rhythm disturbances. Also, the activation of the recording is possible by the dedicated button. Upon activation, the device not only records the ECG during symptoms but also saves the ECG immediately prior to activation, allowing the onset of symptoms to be recorded (unlike event recorders).
Limitations of ELR:
  • Difficulties in data transferring to the processing center
  • Low patient’s compliance
Implantable Loop Recorders
Implantable loop recorders are implanted subcutaneously in the pectoral region and allow ambulatory ECG monitoring for a long period of time - from 1 to 4 years. According to the authors of the article, this is an optimal solution for patients with rare symptoms (once a month or less), in whom other methods of ambulatory ECG monitoring aren't effective.
The most significant device in this category is Medtronic LINQ. This device is the size of a USB flash drive, which is implanted subcutaneously using a minimally invasive approach and is designed to work for 3 years. The recording is activated by the patient or by the device itself via the auto-trigger function. The records are read by means of a special programmer, as is the case with other implantable devices. Due to the long observation period, the diagnostic accuracy is very high - 80-90% for palpitations, 30-50% for syncope, and 15-20% for cryptogenic stroke [3]. The device doesn't interfere with the patient's daily activity in any way, although its removal is possible only in a hospital setting.
Medtronic LINQ limitations:
  • high cost (over £ 2,000 per patient at the time of writing)
  • infection of the pocket in 2,4% of patients [4]
Cardiac implantable electronic devices
If the patient has a pacemaker or other implantable device (ICD, CRT), it can also be used for ambulatory ECG monitoring. The authors point out that the use of other methods of ambulatory ECG monitoring in such patients is inappropriate.
Patient-led monitoring
KardiaMobile
A small device, about the size of a credit card. Works only in pair with a smartphone with the KardiaMobile application installed. The device has 2 buttons with electrodes. By pressing the buttons with two different fingers patient can obtain a single-channel ECG record. The device has basic algorithms for arrhythmia detection and is able to distinguish between sinus rhythm and atrial fibrillation. According to William et al. [5], the sensitivity of KardiaMobile for AF is 96.6%, and specificity is 94.1%. The KardiaMobile device has a number of limitations: the inability to detect atrial flutter, signal distortion up to the impossibility of interpretation when recording at a great distance from the smartphone or in a noisy room. 
Apple Watch
The authors describe the capabilities of the 8th generation Apple Watch, which was the last at the time of this writing. Like the KardiaMobile, Apple Watch 8th gen has 2 electrodes - one on the bottom of the case and one on the digital crown. To record the I lead, the patient should activate the application and touch the crown with a finger. Apple Watch has a basic algorithm for diagnosing rhythm disturbances, which categorizes all recordings into normal rhythm, atrial fibrillation, and unclassified. Overall, the functionality of Apple Watch 8th gen in ECG monitoring is very limited and comparable with KardiaMobile.
Conclusion
In summary, the authors draw several conclusions:
  • The limitations and disadvantages of traditional Holter monitors have been corrected in the new generation of devices (patch electrode monitors). 
  • For recording of infrequent events, implantable monitors are the most effective solution. 
  • Patient-led monitoring is a promising direction, but currently available devices have extremely limited functionality.


References:
  1. Sampson, M. (2019). Ambulatory electrocardiography: indications and devices. British Journal of Cardiac Nursing14(3), 114-121. 
  2. Barrett, P. M., Komatireddy, R., Haaser, S., Topol, S., Sheard, J., Encinas, J., ... & Topol, E. J. (2014). Comparison of 24-hour Holter monitoring with 14-day novel adhesive patch electrocardiographic monitoring. The American journal of medicine127(1), 95-e11.
  3. Steinberg, C., Bennett, M. T., & Krahn, A. D. (2017). Extended ECG Monitoring. In Cardiac Arrhythmias, Pacing and Sudden Death (pp. 49-60). Springer, Cham. 
  4. Ziegler, P. D., Rogers, J. D., Ferreira, S. W., Nichols, A. J., Sarkar, S., Koehler, J. L., ... & Richards, M. (2015). Real-world experience with insertable cardiac monitors to find atrial fibrillation in cryptogenic stroke. Cerebrovascular Diseases40(3-4), 175-181.
  5. William, A. D., Kanbour, M., Callahan, T., Bhargava, M., Varma, N., Rickard, J., ... & Tarakji, K. G. (2018). Assessing the accuracy of an automated atrial fibrillation detection algorithm using smartphone technology: The iREAD Study. Heart rhythm15(10), 1561-1565.