What Is a Holter Monitor?

The Holter monitor is a type of  electrocardiogram (ECG or EKG) used to monitor the ECG tracing continuously for a period of 24 hours or longer. An ECG is one of the simplest and fastest procedures used to evaluate the heart.

Electrodes (small, plastic patches) are placed at certain locations on the chest, arms, and legs. When the electrodes are connected to an ECG machine by lead wires, the electrical activity of the heart is measured, interpreted, and printed out for the physician's information and further interpretation.

When symptoms such as dizziness, fainting, low blood pressure, prolonged fatigue, and palpitations continue to occur without a definitive diagnosis obtained with a resting ECG, an exercise ECG, or a signal-averaged ECG, your physician may request an ECG tracing to be run over a long period of time, using the Holter monitor.

Certain dysrhythmias/arrhythmias (abnormal heart rhythms), which can cause the symptoms noted above, may occur only intermittently, or may occur only under certain conditions, such as stress.

Dysrhythmias of this type are difficult to obtain on an ECG tracing that only runs for a few minutes. Thus, the physician will request a Holter monitor to allow a better opportunity to capture any abnormal beats or rhythms that may be causing the symptoms.

The Holter monitor records continuously for the entire period of 24 to 48 hours. Some Holter monitors may record continuously but also have an event monitor feature that you activate when symptoms begin to occur.

You will receive instructions regarding how long you will need to wear the recorder (usually 24 to 48 hours), how to keep a diary of your activities and symptoms during the test, and personal care/activity instructions.

What is an event monitor?

Event monitoring is very similar to Holter monitoring, and is often ordered for the same reasons. With an event monitor, you wear ECG electrode patches on your chest, and the electrodes are connected by wire leads to a recording device.

Unlike the Holter monitor, however, which records continuously throughout the testing period of 24 to 48 hours, the event monitor does not record until you feel symptoms and trigger the monitor to record your ECG tracing at that time.

When you feel one or more symptoms, such as chest pain, dizziness, or palpitations, you push a button on the event monitor recorder. Some monitors have a feature (memory loop recorder) which captures a short period of time prior to the moment you triggered the recording and afterwards.

This feature can help your physician determine more details about the possible change in your ECG at the time the symptoms started, and what was happening with your ECG just before you triggered the recorder. Other monitors, called "post-event recorders," simply start recording your ECG from the moment you trigger it.

Post-event recorders are quite small - some may even be worn on the wrist (similar to a wristwatch). Memory-loop recorders are about the size of a pager.

After you experience symptoms and record them, you will send the recording of the event to your physician or to a central monitoring center. This transmission is done over the telephone. You will be instructed regarding how to do this on the recorder.

You will also keep a diary of your symptoms and corresponding activities (as done during the Holter monitoring procedure).