A Holter monitor is a small, wearable device that records your heart rhythm. You usually wear a Holter monitor for 12-24 hours. A Holter monitor test is performed after a traditional test if your electrocardiogram (ECG) hasn't provided your doctor with enough information on your heart. Your GP may also order a Holter monitor if you have a heart condition that increases your risk of an abnormal heart rhythm such as hypertrophic cardiomyopathy.