A Holter monitor is a small, wearable device that records the heart’s rhythm. It’s used to detect or determine the risk of irregular heartbeats (arrhythmias). A Holter monitor test may be done if a traditional electrocardiogram (ECG or EKG) doesn’t provide enough details about the heart’s condition. The Holter monitor is a type of portable electrocardiogram (ECG). It records the electrical activity of the heart continuously over 24 hours or longer while you are away from the doctor’s office. A standard or “resting” ECG is one of the simplest and fastest tests used to evaluate the heart.