Heart monitor. Portable electrocardiograph on a man's belt. The wires lead to electrodes taped to his chest. An electrocardiograph monitors the electrical activity of the heart, allowing doctors to diagnose abnormal heart rhythms (arrhythmias). This portable version, called a Holter monitor, records the heart's activity for 24 hours or more while the patient is free to continue normal activities. The recording is then analyzed by computer. This technique is useful for diagnosing sporadic heartbeat disturbances that could be missed by a short examination with a fixed monitor.