If a patient has signs or symptoms of a heart problem, such as an irregular heartbeat (arrhythmia) or unexplained fainting, their doctor may order a test called an electrocardiogram. An electrocardiogram is a brief, non-invasive test that uses electrodes taped to the chest to check the heart's rhythm. However, sometimes an electrocardiogram doesn't detect any irregularities in the heart rhythm because the patient is connected to the ECG machine for only a short time. If their signs and symptoms suggest that an occasionally irregular heart rhythm may be causing their condition, the doctor may recommend the wearing of a Holter monitor for up to 48 hours. Over that time, the Holter monitor may be able to detect irregularities in the heart rhythm that an electrocardiogram couldn't detect. The doctor may also order a Holter monitor if you the patient has a heart condition that increases your risk of an abnormal heart rhythm.
The Holter monitor was developed at the Holter Research Laboratory in Helena Montana by experimental physicists Norman J. Holter and Bill Glasscock, who started work on radio telemetry in 1949. Inspired by a suggestion from cardiologist Paul Dudley White in the early 1950s, they redirected their efforts toward development of a wearable cardiac monitoring device. The Holter monitor was released for commercial production in 1962.
A 24 hour Holter monitor is a battery-operated portable device that measures and records your heart’s ECG activity continuously for 24 to 48 hours (or even longer) depending on the type of monitoring used. The device is usually about the size of a small camera.
24-hour ECG recording, also called Holter monitoring or ambulatory ECG monitoring, involves continuously recording the heart’s electrical activity. The size of the recorder differs depending on the manufacturer of the device. The average dimensions of today’s 24 hour Holter monitors are about 110x70x30mm but some are only 61x46x20 mm and weigh 99 grams. Most of the devices operate with two AA batteries. In case the batteries are depleted, some Holters allow their replacement even during monitoring. Most of the Holters monitor the ECG via only two or three channels (Note: depending on manufacturer, different counts of leads and lead systems are used). Today’s trend is to minimize the number of leads to ensure the patient’s comfort during recording. Modern Holter units record via an EDF-file onto digital flash memory devices. The data is 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.
The Holter has an extended recording period, as compared to a standard ECG machine, that is can be useful for observing occasional cardiac arrhythmias that would be difficult to identify in a shorter period. This device can help diagnose symptoms (such as palpitations) which don’t happen all the time. It has wires with small electrodes that attach to the patients skin. 24 hour Holter monitors, and other devices that continuously record ECG activity as the patient goes about their daily activities, are often referred to as ambulatory electrocardiograms.
When used to study the heart, much like standard electrocardiography, the Holter monitor records electrical signals from the heart via a series of electrodes attached to the chest. Electrodes are placed over bones to minimize artifacts from muscular activity. The number and position of electrodes varies by model, but most Holter monitors employ between three and eight. These electrodes are connected to a small piece of equipment that is attached to the patient's belt or hung around the neck, recording the heart's electrical activity throughout the period it is worn.
12 lead Holter systems are also available when precise ECG signal information is required to analyse the exact nature and origin of the rhythm signal.
These devices are normally used for people having experienced symptoms such as palpitations, dizziness, fainting, chest pain or shortness of breath, and where a routine resting 12-lead electrocardiogram (ECG) examination has not identified any problems. The patient does not need to stay in hospital, but is expected to go on with their normal daily activities during the test period.
When they return their monitor to the hospital, a cardiac physiologist will analyse the data and produce a report for their doctor to review. The Holter test provides the doctor with much more information about the patients heart rhythm on which to base any medical decisions. The test is completely painless and the monitor is worn continuously throughout the day and night.
12 channel Holters
Although two/three channel recording has been used for a long time in the Holter monitoring history, 12 channel Holters have recently appeared. These systems use the classic Mason-Likar lead system, i.e. producing a signal in the same format as during the common rest ECG and/or stress test measurement. These Holters can occasionally provide information similar to that of an ECG stress test examination. They are also suitable when analyzing patients after myocardial infarction.
Recordings from these 12-lead monitors are of a significantly lower resolution than those from a standard 12-lead ECG and in some cases have been shown to provide misleading ST segment representation, even though some devices allow setting the sampling frequency up to 1000 Hz for special-purpose exams such as detection of "late potential".
Another innovation is the inclusion of a triaxial movement sensor, which records the patient's physical activity, and on examination and software processing, extracts three movement statuses: sleeping, standing up, or walking. Some modern devices also have the ability to record a vocal patient diary entry that can be later listened to by the doctor. These data help the cardiologist to better identify events in relation to the patient's activity and diary.
When the recording of an ECG signal is finished (usually after 24 or 48 hours), it is up to the cardiologist to perform the signal analysis. Since it would be extremely time demanding to browse through such a long signal, there is an integrated automatic analysis process in the software of each Holter device which automatically determines different sorts of heart beats, rhythms, etc. However the success of the automatic analysis is very closely associated with the signal quality. The quality itself mainly depends on the attachment of the electrodes to the patient body. If these are not properly attached, electromagnetic disturbance can influence the ECG signal resulting in a very noisy record. If the patient moves rapidly, the distortion will be even bigger. Such recordings are then very difficult to process. Besides the attachment and quality of electrodes, there are other factors affecting the signal quality, such as muscle tremors, sampling rate and resolution of the digitized signal (high quality devices offer higher sampling frequency).
The automatic analysis commonly provides the cardiologist with information about heart beat morphology, beat interval measurement, heart rate variability, rhythm overview and patient diary (moments when the patient pressed the patient button). Advanced systems can also perform spectral analysis, ischemic burden evaluation, and graphs of the patient’s activity or PQ segment analysis. Another requirement is the ability of pacemaker impulses detection and analysis. Such ability may be useful when the physician desires to check for correct basic pacemaker function.
There are no significant risks involved in wearing a Holter monitor other than possible discomfort or skin irritation where the electrodes were placed. However, the Holter monitor must not get wet, or it may become damaged. Patient’s are expected to wear these devices continuously during the recording, and are not allowed to swim or bathe for the entire time they are wearing the Holter monitor. Holter monitors aren't usually affected by other electrical appliances. But it is recommended to avoid metal detectors, magnets, microwave ovens, electric blankets, electric razors and electric toothbrushes whilst wearing one because these devices have been known to interrupt the signal from the electrodes to the Holter monitor. Also, it is recommended to keep mobile phones and portable music players at least 6 inches from the Holter monitor for the same reason.
Edited by John Sandham