Template:Infobox laboratory equipment
Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
In medicine, a Holter monitor (also called an ambulatory electrocardiography device), named after its inventor, Dr. Norman J. Holter, is a portable device for continuously monitoring the electrical activity of the heart for 24 hours or more. Its extended recording period is sometime useful for observing occasional cardiac arrhythmias that 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.
Much like standard electrocardiography (ECG), the Holter monitor records electrical signals from the heart via a series of electrodes attached to the chest. The number and position of electrodes varies by model, but most Holter monitors employ from three to eight. These electrodes are connected to a small piece of equipment that is attached to the patient's belt, and is responsible for keeping a log of the heart's electrical activity throughout the recording period.
Old devices used to use reel to reel or a standard C60 or C90 audio cassette and ran at a very slow speed to record the data. More modern units record onto digital flash memory devices. The data are uploaded into a computer which then automatically analyzes the input, counting ECG complexes, calculating summary statistics such as average heart rate, minimum and maximum heart rate, and finding candidate areas in the recording worthy of further study by the technician.
Electrodes should be placed over bones to minimize artifacts from muscular activity.
Both 24 hr and 30 day event recorders are available.
In addition to wearing the device, most patients are asked to keep a diary of activities such as running and sleeping, symptoms, and times that their symptoms occur. This information is used by doctors and technicians to rapidly pinpoint problem areas in the vast amount of data recorded during the monitoring period.
- Holter monitor - MedLine Plus
- Holter monitor - Texas Heart Institute
- Holter Monitor information from Children's Hospital Heart Center, Seattle.
de:Langzeit-EKG sk:Holterovo monitorovanie