How is ambulatory monitoring performed?

There are two general types of ambulatory monitoring - the Holter monitor, and the event recorder.

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 tape recorder. The patient is sent home and resumes normal activities while the tape recorder records a continuous ECG tracing for 24 or 48 hours. The Holter equipment is then removed, and the tape is analyzed.

In contrast, event recorders do not record every heart beat on a tape. Instead, event recorders use a circular tape that stores only approximately 30 seconds of a patient's heart rhythm. That is, at any given time while a patient is wearing them, event recorders will have the most recent 30 seconds of the patient's ECG. When the patient experiences the symptom of interest, he/she presses a button that freezes the recording, which is then transmitted by telephone to an interpreting center. A major advantage of event recorders is that they can be used for up to 30 - 60 days, until the transient symptom being looked for occurs. Another advantage is that, as long as the symptom of interest typically lasts for more than a minute or two, the ECG electrodes do not actually need to be attached all the time. Since attaching the electrodes takes less than a minute, they can simply be attached as needed.

What information can be gained from ambulatory monitors?

The Holter monitor will show the doctor every one of the patient's heart beat for a continuous 24 - 48 hour period. If any abnormal beats or heart arrhythmias occur during that time, they will be identified. In addition, the Holter monitor can give information about changes in the ST segment of the ECG during the monitoring period. Some patients who have cardiac ischemia will not actually experience symptoms during their ischemic episodes. (This condition is called "silent ischemia.") By analyzing changes in the ST segment, silent ischemia can often be diagnosed by Holter monitoring.

Event monitors are excellent at correlating a patients heart rhythm with a patient's symptoms. If a patient's symptoms are caused by a transient cardiac arrhythmia, event monitors are often the best way to make the diagnosis.

What are the limitations of ambulatory monitoring?

The Holter monitor often fails to provide the diagnosis if the condition being sought occurs less frequently than every day or every other day. In other words, continuous monitoring for 24 - 48 hours may miss an arrhythmia that occurs on average of once a month. Also, since the Holter records every heart beat whether there are symptoms or not, it is difficult to correlate any arrhythmias that are seen with any symptoms the patient may be having.

The event recorder is not useful in diagnosing silent ischemia, since, by definition, silent ischemia is not symptomatic (and so patients will not know to transmit the ECG recorded during any silent ischemia).