An ambulatory electrocardiogram (EKG or ECG) records the electrical activity of your heart while you do your usual activities. Ambulatory monitors are referred to by several names, including ambulatory electrocardiogram, ambulatory EKG, Holter monitoring, 24-hour EKG, or cardiac event monitoring.

Many heart problems become noticeable only during activity, such as exercise, eating, sex, stress, bowel movements, or even sleeping. A continuous 24-hour recording is more likely to detect any abnormal heartbeats that occur during these activities.

Many people have irregular heartbeats (arrhythmias) from time to time. The importance of irregular heartbeats depends on the type of pattern they produce, how often they occur, how long they last, and whether they occur at the same time you have symptoms. Because arrhythmias can occur off and on, it may be difficult to record an arrhythmia while you are in the doctor's office.

There are several different types of ambulatory monitors.

Continuous recorders

The most common type is the continuous recorder (such as the Holter monitor). It provides a 24- to 72-hour record of the electrical signals from your heart. A standard EKG monitors only 40 to 50 heartbeats during the brief period you are attached to the machine. A continuous recorder monitors about 100,000 heartbeats in 24 hours and is likely to find any heart problems that happen with activity.

Another type of continuous recorder can be implanted under the skin of the chest. This recorder can be kept in your chest for more than a year to record the electrical signals from your heart.

Intermittent recorders

Another kind of ambulatory EKG monitoring is the intermittent recorder, which is used when symptoms of an abnormal heart rhythm do not occur very often. An intermittent recorder can be used for a longer time than a continuous recorder. The information collected by an intermittent recorder can often be sent over the phone to a doctor's office, clinic, or hospital.

Two types of intermittent recorders are available:

• Loop recorders. A loop recorder constantly records your heartbeats. When you have symptoms, you press a button on the monitor to record your heart rhythm. Loop recorders also save a small amount of information about how your heart was beating when you pressed the recording button (presymptom recording). This feature is especially useful for people who lose consciousness when their heart problems occur and can press the button only after they wake up.
• Event monitor. This small device is used only when symptoms of the heart problem occur. You are not attached to the machine. One type is worn on the wrist like a watch. When symptoms occur, you press a button to start the EKG recording. The other type is a device that you carry where you can reach it easily, such as in your purse or pocket. When symptoms occur, you press the back of the device against your chest and then press a button to start the recording. The back of the device has small metal discs that work like electrodes. These handheld monitors can be very small (some are about the size and shape of a credit card).