A Holter monitor (often simply "Holter" or occasionally ambulatory electrocardiography device) is a portable device for continuously monitoring various electrical activity of the cardiovascular system for at least 24 hours (often for two weeks at a time). The Holter's most common use is for monitoring heart activity (electrocardiography or ECG), but it can also be used for monitoring brain activity (electroencephalography or EEG) or arterial pressure. Its extended recording period is useful for observing occasional cardiac arrhythmias or epileptic events which would be difficult to identify in a shorter period of time. For patients having more transient symptoms, a cardiac event monitor which can be worn for a month or more can be used. The Holter monitor is named after physicist Norman J. Holter, who invented telemetric cardiac monitoring in 1949. Clinical use started in the early 1960s.