Holter and ambulatory monitors are small recorders that a patient wears for a day or more while going about normal life, to record the heart's rhythm or the blood pressure. A Holter monitor records the ECG, the heart's electrical signals. An ambulatory blood pressure monitor measures the blood pressure at regular times through the day and night. Doctors use them when a problem comes and goes, so it may not appear during a short test in the clinic.
A Holter monitor is a recorder about the size of a matchbox, clipped to a belt or hung around the neck. Wires connect it to between three and twelve stickers on the chest. It records every heartbeat, which comes to about one lakh beats a day. The patient keeps a diary of symptoms, or presses a button on the recorder when they feel something. Newer versions are a single patch stuck on the chest, which can record for up to two weeks.
The hard part is reading the recording. No one can look at one lakh beats by eye. So analysis software reads the whole recording. It finds each beat, measures the gap between beats, and sorts the beats into normal and abnormal kinds. It flags pauses, stretches that are very fast or very slow, and extra beats, and matches them to the diary. A technician reviews the flagged parts, and a cardiologist signs the report. More and more often, an AI model does the first reading, as described in AI ECG analysis.
Imagine a 47-year-old bus conductor in Guwahati who has dizzy spells a few times a week. He wears a Holter monitor for 24 hours, and it records about 1,03,000 beats. The software flags three pauses in his heartbeat that last longer than 2.5 seconds. One is at 4:10 p.m., when his diary says "felt dizzy at the depot". The cardiologist now has a recording of the rhythm at the moment of the symptom, and decides on his treatment.
An ambulatory blood pressure monitor works on the same idea. A cuff on the arm inflates by itself, for example every 30 minutes by day and every 60 minutes at night. The software then calculates the average pressure for the day, for the night and for the whole 24 hours.
How is this different from a bedside patient monitor? The bedside monitor watches a patient live and sounds alarms. A Holter monitor records quietly, and the recording is read afterwards. A smart ring also measures the pulse all day, but it is a consumer product and is not usually approved for diagnosis. Wearing a recorder at home in this way is one kind of remote patient monitoring.