Ambulatory monitoring (Holter monitoring, event recorders, and mobile cardiac telemetry, or MCT) is done to study a person's heart rhythm for a prolonged period of time, on an outpatient basis. The purpose of ambulatory monitoring is to look for evidence of transient cardiac arrhythmias - that is, to look for heart rhythm problems that come and go sporadically, and that are not apparent when a standard ECG is performed.
How Is Ambulatory Monitoring Performed?
There are three general types of ambulatory monitoring - the Holter monitor, the event recorder, and MCT.
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.
In contrast, event recorders do not record every heart beat. Instead, event recorders use a circular tape that stores the most recent 30 seconds (approximately) of a patient's heart rhythm. Event recorders are useful in correlating the heart rhythm with transient symptoms the patient may experience - such as palpitations or dizziness. When the patient experiences the symptom of interest, he/she presses a button that freezes the recording. The "frozen" 30 seconds - which contains the ECG that was present when the patient was experiencing symptoms - is then transmitted to an interpreting center. The major advantage of event recorders is that they can be used for up to 30 - 60 days, until the transient symptom being looked for finally occurs. So event recorders are quite useful in cases where symptoms occur infrequently.
Mobile cardiac telemetry - MCT - is a newer technology that combines features of both the Holter and the event recorder. Several MCT systems are available, and all work in somewhat different ways. But essentially, MCT systems can be worn for several weeks at a time, and (like event recorders) can be used to capture symptoms, such as palpitations, 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 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.
What Information Can Be Gained From Ambulatory Monitors?
The Holter monitor will reveal each and every one of the patient's heart beats for a continuous 24 - 48 hour period. If any abnormal beats or heart arrhythmias occur during that time, they will be identified. So any arrhythmia that occurs during the monitoring period will be detected.
Event recorders are excellent at correlating a patient's heart rhythm with that patient's symptoms. If a patient's symptoms are caused by a transient cardiac arrhythmia, event recorders are often the best way to make the diagnosis. Further, if the patient's symptoms (when they occur) routinely last more than 5 - 10 minutes, it may not be necessary for the patient to have the recorder "hooked up" 24 hours a day. (Wearing these devices 24 hours a day can be uncomfortable.) Instead, the patient may be able to quickly attach the recorder at the onset of symptoms. However, with event recorders, any cardiac arrhythmias that do not cause symptoms will be entirely missed.
MCT systems can detect any arrhythmias - both symptomatic and asymptomatic - whenever they occur, over the entire monitoring period. The chief disadvantage of MCT systems are, first, that because of the advanced technology they use, they tend to be more expensive than the older methods; and second, many MCT systems can be uncomfortable to wear 24 hours a day - and they cannot detect arrhythmias that appear when they are not being worn.
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.
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.