Guide to Ambulatory Monitoring

Ambulatory monitoring in evaluating heart rhythm problems

Thierry Dosogne

If your doctor suspects you might be having episodes of a cardiac arrhythmia, but has been unable to prove it, he or she might prescribe a period of ambulatory monitoring for you. (Being “unable to prove it” means, essentially, that no arrhythmia has been found during the 12 seconds it takes to record an ECG in the office.)

With ambulatory monitoring, you will have your electrocardiogram recorded over a prolonged period of time, while you go about your daily life.

There are three four main types of ambulatory cardiac monitors - Holter monitors, event recorders, mobile cardiac telemetry (MCT), and implantable loop recorders (ILR). With all four types the purpose is the same - to look for episodes of transient cardiac arrhythmias that come and go unpredictably, and that may be quite infrequent.

How Is Ambulatory Monitoring Performed?

With the Holter monitor, electrode leads are applied to the skin (similar to the leads used in recording a standard ECG), and attached to a recorder. The patient is sent home and resumes normal activities while the recorder stores a continuous ECG tracing for 24 or 48 hours. The Holter equipment is then removed, and the entire recording is analyzed. Any arrhythmias that may have occurred during the recording period will be found in this way.

Event recorders today are used infrequently because of the availability of MCT. Event recorders can be used for weeks at a time.

They continuously store the most recent 30 seconds (approximately) of the heart rhythm a patient's heart rhythm. When a patient experiences a symptom - such as palpitations or dizziness - he or she presses a button that freezes the recording. The "frozen" 30 seconds is then transmitted to an interpreting center.

Event recorders are designed to capture episodes of symptoms, rather than capture each and every heart beat.

Mobile cardiac telemetry - MCT. MCT systems can be worn for several weeks at a time, and like event recorders they can be used to capture an episode of symptoms whenever they occur. But MCT systems also have Holter-like features. Some can record every heart beat during the entire time of the study, which can then be "played back" at the end of the study. Other MCT systems use sophisticated signal processing to automatically detect cardiac arrhythmias as they occur - and then they transmit the detected arrhythmia wirelessly to a monitoring center, often within a few minutes.  Studies have shown that MCT systems can be quite useful in detecting arrhythmias that have been missed by either Holter monitoring or event recorders. 

Implantable Loop Recorders - ILR - are small devices, implanted under the skin, that monitor the heart rhythm for up to three years. These devices, like event recorders, do not store every heart beat forever. Rather, they continuously store the previous 20 minutes or so of the heart rhythm in their memory. If an event occurs (palpitations or or an episode of syncope), the patient can freeze the recording, and transmit it to a monitoring center for analysis.

These devices can be useful for patients who are suspected to have significant cardiac arrhythmias that occur very infrequently, so that even MCT monitoring has been unable to capture an event.

What Information Can Be Gained From Ambulatory Monitors?

Ambulatory monitoring is useful for diagnosing cardiac arrhythmias that occur infrequently and unpredictably. People who have sporadic episodes of palpitations, dizziness, or even syncope, where an arrhythmia is suspected to be the cause, are good candidates for ambulatory monitoring.

If symptoms pretty reliably occur each day or so, Holter monitoring is usually the best way to start, since it is the most readily available method - and the cheapest - of ambulatory monitoring.

If symptoms are not captured with a Holter monitor, then moving to MCT is usually the next step. ILR devices are only rarely necessary, and are reserved for people whose symptoms are serious enough to demand a diagnosis, but are very infrequent.

A relatively new reason for performing ambulatory monitoring is to look for episodes of atrial fibrillation in patients who have had cryptogenic strokes (strokes whose cause is unknown). It has now become apparent that a substantial proportion of patients with cryptogenic stroke turn out to have episodes of atrial fibrillation, an arrhythmia that often leads to embolic strokes. By making the correct diagnosis, treatment with anticoagulant drugs can be used to prevent further strokes.

If you have symptoms that might be due to an undiagnosed cardiac arrhythmia, you should talk with your doctor about the ambulatory monitoring, and which of these ambulatory systems might be best for making the diagnosis in your case.

Disclaimer: From 2007 - 2014 Dr. Fogoros was the Chief Medical Officer of Corventis, a start-up company that developed and marketed a novel MCT device. Corventis and its products were subsequently acquired by Medtronic, Inc. Dr. Fogoros has no affiliation with Medtronic.


Crawford MH, Bernstein SJ, Deedwania PC, et al. ACC/AHA guidelines for ambulatory electrocardiography: executive summary and recommendations. A report of the American College of Cardiology/American Heart Association task force on practice guidelines (committee to revise the guidelines for ambulatory electrocardiography). Circulation 1999; 100:886.

Olson JA, Fouts AM, Padanilam BJ, Prystowsky EN. Utility of mobile cardiac outpatient telemetry for the diagnosis of palpitations, presyncope, syncope, and the assessment of therapy efficacy. J Cardiovasc Electrophysiol 2007; 18:473.

Mittal S, Sanders P, Pokushalov E, et al. Safety Profile of a Miniaturized Insertable Cardiac Monitor: Results from Two Prospective Trials. Pacing Clin Electrophysiol 2015.

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