Holter monitoring involves a continuous electrocardiographic (ECG) recording of the heart's rhythm. The patient presents on day one for electrode placement on his chest with leads attached to a small recorder. He keeps a diary of his activities and symptoms for a specified number of hours or days, and returns at the end of that time to have the monitor removed. Typically, cardiologists use the monitors to diagnose arrhythmias that only present themselves sporadically, making them difficult to capture with standard ECG assessment. While ECG assessment looks for a period of 30 to 60 seconds, Holter monitoring typically involves continuous monitoring for 24 hours. Patients who have had a heart attack or who start a new heart medication also may require the test. You also may see the service referred to as "continuous ECG," "continuous ambulatory ECG," "dynamic electrocardiography," and "long-term ECG recording."