2 The diagnostic tests

Clinical need and practice

Atrial fibrillation

2.1 Atrial fibrillation is a type of arrhythmia that causes an irregular or abnormally fast heart rate. It is the most common arrhythmia. When someone has atrial fibrillation, the upper chambers of their heart (the atria) beat irregularly, which makes the heart less effective at moving blood into the ventricles. This can cause clots to form in the blood, which may cause a stroke. The abnormal electrical impulses in the heart muscle that cause atrial fibrillation can be persistent, permanent or intermittent. Paroxysmal atrial fibrillation involves intermittent episodes that usually last less than 2 days and stop without treatment.

Cryptogenic stroke

2.2 Cryptogenic strokes (including transient ischaemic attack [TIA]) have no identified probable cause after diagnostic assessment, and account for around 15% to 40% of ischaemic strokes. When people have treatment for stroke, they are tested for atrial fibrillation. However, if they have paroxysmal atrial fibrillation, it may not happen during the initial assessment, or during subsequent diagnostic tests. Further longer-term testing can potentially detect it, for example by using heart rhythm monitors that can be worn, or implanted. These continuously monitor the heart's electrical activity while a person goes about their daily routine.

The interventions

2.3 Implantable cardiac monitors are also known as implantable loop recorders or insertable cardiac monitors. They monitor heart rhythm for longer than heart rhythm monitors that are worn externally (for example, Holter monitors) and can therefore be used for long-term monitoring (potentially over years, rather than days) for suspected atrial fibrillation. Implantable cardiac monitors can identify atrial fibrillation and could be particularly helpful for identifying paroxysmal atrial fibrillation in people who have had a cryptogenic stroke. If people are diagnosed with atrial fibrillation, they can then be offered anticoagulant therapy to reduce the risk of having another stroke or TIA.

2.4 The monitors are implanted under the skin of the person's chest using a small incision under local anaesthetic. They can continuously monitor heart rhythm for several years, and they record information if the device detects an arrhythmia. The devices use algorithms based on electrocardiogram (ECG) features to detect potential atrial fibrillation. The algorithm parameters can be varied to adjust the ECG features identified and flagged as potential atrial fibrillation. Recorded ECGs are remotely transmitted to clinicians, who determine if the person has had atrial fibrillation. They then decide to either continue to monitor or to treat.

BioMonitor 2-AF (Biotronik SE & Co KG)

2.5 The BioMonitor 2‑AF system consists of:

  • a BioMonitor 2‑AF insertable cardiac monitor (dimensions 88 mm × 15 mm × 6 mm)

  • an optional Remote Assistant for patient-activated recordings

  • a remote monitoring system (a CardioMessenger Smart transmitter), which sends data to the Biotronik Home Monitoring Service Centre through a cellular phone network

  • a Renamic programmer for the insertable cardiac monitor.

2.6 BioMonitor 2‑AF is implanted using a Fast Insertion Tool (FIT) accessory kit. First an incision of at least 1.5 cm is made. Then the FIT 1 tool is used to form a pocket for the device under the skin, and the FIT 2 is used to implant and position the device. The battery life of the device is estimated as 4 years, assuming data are sent from the device once a day. The company says that clinicians should decide whether to remove the device once it is no longer in use.

2.7 The BioMonitor 2‑AF continuously monitors heart rhythm and ECGs are automatically recorded when atrial fibrillation is detected. Atrial fibrillation is detected based on irregular RR intervals (the interval between heartbeats), absence of P waves and atrial rate greater than 300 beats per minute. Parameters for sensing settings can be adjusted to vary the sensitivity of atrial fibrillation detection. There are also standard settings for parameters, such as atrial fibrillation sensitivity (described in the product manual). BioMonitor 2‑AF can also detect other cardiac arrhythmias such as high ventricular rate, asystole, bradycardia, and sudden ventricular rate drop.

2.8 Recordings made by BioMonitor 2‑AF are automatically and wirelessly sent to a transmitter unit every day. Data are encrypted and sent anonymously to the Biotronik Home Monitoring Service Centre over mobile phone networks. Data are stored in Germany and can be accessed by clinicians through an online platform. Automatic alerts are sent to clinicians when a reading is received that meets pre-defined criteria. Clinicians review readings to make a final diagnosis.


2.9 During the assessment phase for this guidance, the manufacturer of BioMonitor 2‑AF launched BIOMONITOR III, a new implantable cardiac monitor device, which supersedes the existing version. The new device uses the same algorithm to detect atrial fibrillation as the BioMonitor 2‑AF. The predicted battery life and cost are also the same. The device is smaller and lighter than the BioMonitor 2‑AF and can be used with a patient app.

Confirm Rx Insertable Cardiac Monitor (Abbott Medical UK)

2.10 The Confirm Rx system consists of:

  • a Confirm Rx Insertable Cardiac Monitor (dimensions 49.0 mm × 9.4 mm × 3.1 mm)

  • a remote monitoring system (the myMerlin application installed on a smartphone or tablet) and the Merlin.net Patient Care Network (PCN)

  • a Merlin Patient Care System and a magnet (to interrogate and program the insertable cardiac monitor).

2.11 Confirm Rx is implanted using proprietary insertion and incision tools. The device is implanted under local anaesthetic through a small cut made using the incision tool. The insertion tool is then used to implant the device under the skin. The battery life of the device is estimated as 2 years, assuming an average of 1 automatically detected episode a day and 1 patient-activated episode a month.

2.12 Heart rhythm is continuously monitored by Confirm Rx, and ECGs are automatically recorded when atrial fibrillation is detected. Confirm Rx assesses 3 aspects of ECG trace to identify potential atrial fibrillation: regularity of rhythm pattern, variance of RR intervals and how sudden the onset of arrhythmia is. All 3 tests must indicate atrial fibrillation to trigger episode recording. The settings used by the device to detect atrial fibrillation can be varied; for example, to set the length of episode needed to trigger a recording. Confirm Rx also detects bradyarrhythmias, tachyarrhythmias and pauses.

2.13 Recordings made by Confirm Rx are transmitted using Bluetooth to a smartphone or tablet with the myMerlin app. The app can be downloaded from the company's website. The company says that the app automatically reads data from the implanted device and sends the data to a database using a cellular or Wi-Fi network during the night. It recommends that a smartphone or tablet with the app installed is kept by the person's bedside at night to allow data transmission. Confirm Rx encrypts its wireless communications and only transmits to a single authenticated and paired myMerlin app at any given time. Emails or SMS notifications can be sent to alert that a recording has been sent. Clinicians can then access transmitted ECG recordings on the Merlin.net PCN by logging on with a User ID and password. Access to the Merlin.net PCN is restricted to authorised users set by the clinic administrator.

Reveal LINQ Insertable Cardiac Monitoring System (Medtronic Limited)

2.14 The Reveal LINQ Insertable Cardiac Monitoring System consists of:

  • a Reveal LINQ Insertable Cardiac Monitor device (dimensions 45 mm × 7 mm × 4 mm)

  • an optional Reveal Patient Assistant handheld device, which is held over the implanted Reveal LINQ monitor by the user to start an ECG recording or mark an event on the ECG record

  • a remote monitoring system (a bedside MyCareLink Patient Monitor), which sends data to the MyCareLink network cloud storage facility

  • a MyCareLink Programmer, which is a portable computer system used by a healthcare professional to program the devices.

2.15 Reveal LINQ is implanted using proprietary incision and insertion tools. The incision tool makes a small opening in the skin (less than 1 cm) and the insertion tool is used to make a small pocket for the device and to implant it under the skin.

2.16 Reveal LINQ continuously monitors heart rhythm and identifies potential atrial fibrillation episodes from the person's ECG trace using an algorithm. An ECG trace is assessed in 2‑minute windows which are considered positive if atrial fibrillation is present for longer than a programmable threshold. If the algorithm detects a potential episode of atrial fibrillation, the ECG trace is stored. The device can also be programmed to only store episodes that persist for a set period of time (6, 10, 20, 30 or 60 minutes). Total atrial fibrillation is also calculated, consisting of all 2‑minute windows in which atrial fibrillation was present for longer than the threshold value. Reveal LINQ can also detect tachyarrhythmia, bradyarrhythmia or pause episodes. The device contains an accelerometer to allow changes in patient activity over time to be monitored.

2.17 Rhythm abnormalities recorded by Reveal LINQ are wirelessly transmitted to the MyCareLink Patient Monitor and then sent to a CareLink server in the Netherlands. Transmitted and stored data are encrypted. A care alert is sent to clinicians when the device detects a rhythm abnormality. They can access the data through the CareLink website using a password protected log-in. Alternatively, daily notifications of cardiac activity can be sent. The device also sends alerts if the battery is low. If the device is unable to communicate with CareLink it registers as disconnected.

2.18 The company also offers a triage and monitoring service (FOCUSON) to review ECG recordings made by Reveal LINQ. ECGs are reviewed by cardiologists and ECG technicians at a Monitoring and Triaging Service Centre. Any clinically relevant cases requiring clinical action or escalation are notified to the NHS clinician by phone or email. Detected episodes are categorised by colour (red, amber or green). The company says that red events are notified on the same working day from when the transmission reaches the CareLink Network service. Amber events are notified by email by the next working day, and green events are aggregated and notified in a weekly email.

The comparator

No further testing after outpatient external ambulatory ECG monitoring

2.19 The clinical experts said that if no atrial fibrillation is detected by an external ambulatory ECG monitor, the person is unlikely to have any further monitoring for atrial fibrillation, unless an implantable cardiac monitor is available. Undetected atrial fibrillation may be later identified if it causes symptoms (for example, palpitations), incidentally when someone's pulse is checked (for example, when blood pressure is taken), or on investigation after a recurrent stroke or TIA. Therefore the comparator is no further monitoring.

  • National Institute for Health and Care Excellence (NICE)