Holter monitoring has been named after it's discoverer, physicist Norman J. Holter (and therefore it is written with a capital H). It is a portable ECG recorder (sometimes called a portable loop recorder) which usually records every heart beat during 24 hours. Holter recordings are often used in patients with paroxysmal arrhythmias, in whom registration on a single ECG has not succeeded. A 24 hours registration shows the frequency and type of arrhythmias. The patient is asked to make notes in a diary if symptoms occur, so these can be matched with the recording.