Holter / Event Monitor

What Are Holter and Event Monitors?

Holter and event monitors are medical devices that record the heart’s electrical activity. Doctors most often use these monitors to diagnose arrhythmias (ah-RITH-me-ahs).

Arrhythmias are problems with the rate or rhythm of the heartbeat. During an arrhythmia, the heart can beat too fast, too slow, or with an irregular rhythm.

Holter and event monitors also are used to detect silent myocardial ischemia (is-KE-me-ah). In this condition, not enough oxygen-rich blood reaches the heart muscle. “Silent” means that no symptoms occur.

The monitors also can check whether treatments for an arrhythmia or silent myocardial ischemia are working.

This article focuses on using Holter and event monitors to diagnose problems with the heart’s rate or rhythm.


Holter and event monitors are similar to an EKG (electrocardiogram). An EKG is a simple test that detects and records the heart’s electrical activity. It’s a common test for diagnosing heart rhythm problems.

However, a standard EKG only records the heartbeat for a few seconds. It won’t detect heart rhythm problems that don’t occur during the test.

Holter and event monitors are small, portable devices. You can wear one while you do your normal daily activities. This allows the monitor to record your heart for a longer time than an EKG.

Some people have heart rhythm problems that occur only during certain activities, such as sleeping or physical exertion. Using a Holter or event monitor increases the chance of recording these problems.

Although similar, Holter and event monitors aren’t the same. A Holter monitor records your heart’s electrical activity the entire time you’re wearing it. An event monitor records your heart’s electrical activity only at certain times while you’re wearing it.

Holter Monitors

Holter monitors sometimes are called continuous EKGs (electrocardiograms). This is because Holter monitors record your heart rhythm continuously for 24 to 48 hours.

A Holter monitor is about the size of a large deck of cards. You can clip it to a belt or carry it in a pocket. Wires connect the device to sensors (called electrodes) that are stuck to your chest using sticky patches. These sensors detect your heart’s electrical signals, and the monitor records your heart rhythm.

Event Monitors

Event monitors are similar to Holter monitors. You wear one while you do your normal daily activities. Most event monitors have wires that connect the device to sensors. The sensors are stuck to your chest using sticky patches.

Unlike Holter monitors, event monitors don’t continuously record your heart’s electrical activity. They only record during symptoms. For many event monitors, you need to start the device when you feel symptoms. Some event monitors start automatically if they detect abnormal heart rhythms.

Event monitors tend to be smaller than Holter monitors because they don’t need to store as much data.

Different types of event monitors work in slightly different ways. Your doctor will explain how to use the monitor before you start wearing it.