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 C120 audio cassette and ran at a 2mm/second to record the data. Once the recording has been made, it is played back and analysed at 60x the speed so 24 hours of recording can be analysed in 24 minutes. 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 24h and 30 day event recorders are available.
Generally, the monitors cannot be worn in the shower.