In medicine, a Holter monitor is a portable device for continuously monitoring various
electrical activity of the cardiovascular system for at least 24 hours.
The Holter's most common use is for monitoring heart activity, but it can also be used for
monitoring brain activity or arterial pressure. Its extended recording period is sometimes
useful for observing occasional cardiac arrhythmias or epileptic events which 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.
The Holter monitor is named after physicist Norman J. Holter, who invented telemetric
cardiac monitoring in 1949. Clinical use started in the early 1960s.
When used for the heart, 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, and is responsible for keeping a log of the heart's electrical activity
throughout the recording period.
Data storage Older devices used reel to reel tapes or a
standard C90 or C120 audio cassette and ran at a 1.7 mm/s or 2 mm/s speed to record
the data. Once a recording was made, it could be played back and analyzed at 60x speed so
24 hours of recording could be analyzed in 24 minutes. More modern units record 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. Components
Each Holter system consists of two basic parts – the hardware for recording the signal,
and software for review and analysis of the record. Advanced Holter recorders are able
to display the signal, which is very useful for checking the signal quality. Very often
there is also a “patient button” located on the front site allowing the patient to
press it in specific cases such as sickness, going to bed, taking pills…. A special mark
will be then placed into the record so that the doctors or technicians can quickly pinpoint
these areas when analyzing the signal. Recorder
The size of the recorder differs depending on the manufacturer of the device. The average
dimensions of today’s Holter monitors are about 110x70x30 mm but some are only 61x46x20
mm and weigh 99 g. 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 just in two or three channels. Depending on the model,
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. Although 2/3 channel
recording has been used for a long time in the Holter monitoring history, recently 12
channel Holters have appeared. These systems use the classic Mason-Likar lead system, thus
producing the signal in the same representation as during the common rest ECG and/or stress
test measurement. These Holters then allow to substitute stress test examination in cases
the stress test is not possible for the current patient. 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 like the late potential.
Another interesting innovation is the presence of a triaxial movement sensor, which records
the patient physical activity, and later shows in the software three different statuses:
sleeping, standing up, or walking. This helps the cardiologist to better analyze the recorded
events belonging to the patient activity and diary. Holter monitoring is a very useful
part of an ECG. Some modern devices also have the ability
to record a vocal patient diary entry that can be later listened to by the doctor.
Analyzing software
When the recording of ECG signal is finished, it is up to the physician 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 each Holter software 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 record
is 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. The automatic analysis commonly provides the
physician with information about heart beat morphology, beat interval measurement, heart
rate variability, rhythm overview and patient diary. Advanced systems also perform spectral
analysis, ischemic burden evaluation, graph of patient’s activity or PQ segment analysis.
Another requirement is the ability of pacemaker detection and analysis. Such ability is useful
when one wants to check the correct pacemaker function.
History The cardiac event monitor has been used for
over twenty years. When it first started it was not portable and had to be used only in
hospital buildings. When inventors made an advance it became a bit smaller but was still
only used in hospitals for twenty four to forty eight hours. Soon researchers made a
huge advance they made a portable event monitor. The monitor though was the size of computer
monitor and about thirty pounds. After five more years of research it became only ten
pounds and the size of a sheet of paper. At last the monitor became merely one pound and
very small making it much easier to wear for longer periods. Modern technology has made
it much easier for patients using the heart monitor.
Procedure Although some patients may feel uncomfortable
about a Holter examination, there is nothing to worry about. No hazards are involved, and
it should have little effect on one's normal daily life.
The recording device can be worn in a case on a belt or on a strap across the chest.
The device may be visible under light clothing, and those wearing a Holter monitor may wish
to avoid shirts with a low neckline. Persons being monitored should not limit normal
daily activities,since its purpose is to record how a heart works under various actual conditions
over an extended period. It is an electrical device, however, and should be kept dry; showering
or swimming should probably be avoided. Monitors can be removed for a few minutes without invalidating
collected data, but proper reattachment is critical to avoid degradation of its signals.
Beyond changing batteries, one should leave its handling to trained personnel.
Gallery
References
External links Holter monitor - MedLine Plus