A Holter monitor is a portable electrocardiogram (EKG) that monitors the electrical activity of an ambulatory (freely moving) patient’s heart while the person goes about daily activities. There are a few reasons a physician might request that a patient wear a Holter monitor, but the most common is to diagnose an abnormal heart rhythm (arrhythmia). They are also sometimes used to monitor existing pacemakers, diagnose a lack of oxygen-rich blood flowing to the heart (cardiac ischemia) and measuring variability in the heart rate.

There is no special preparation for this painless test. Wires (electrodes) from the Holter monitor are taped to thoroughly cleansed areas on the patient’s skin, and the patient is sent home to go about his or her regular daily activities (except for showering/bathing). Patients will be asked to keep a diary of their activities and any symptoms they may experience. The test will last about 24 hours or up to five days, depending on the kind of Holter monitor and the patient's symptoms. Depending on the results output from the Holter monitor, the physician may order additional tests or treatment (e.g., antiarrhythmics).