A standard electrocardiograph (ECG) records the heart's electrical activity for only a few seconds. This can detect abnormalities that are constant; however, sometimes abnormal heart rhythms and inadequate blood flow to the heart muscle occur only briefly or unpredictably. To detect such problems, doctors may use continuous ambulatory electrocardiography (ECG), in which the ECG is recorded continuously for 24 to 48 hours while the person engages in normal daily activities. Doctors may also use event-based electrocardiography (ECG), in which the ECG is continuously monitored for up to a month but only recorded for a pre-determined time-frame (usually a few seconds to minutes) just before and after the person triggers the monitor or the device detects an abnormal heart rhythm. (See also Electrocardiography.)
A Holter monitor is a small monitor that is attached to a strap that the person wears over the shoulder or neck or around the waist. Electrodes attached to the person's chest continuously record the activity of the heart. This type of monitor is painless and is worn for 24 to 48 hours.
The monitor detects the heart's electrical activity through electrodes attached to the chest and records the ECG. While wearing the monitor, the person notes in a diary the time and type of any symptoms. Subsequently, the ECG is run through a computer, which analyzes the rate and rhythm of the heart, looks for changes in electrical activity that could indicate inadequate blood flow to the heart muscle, and produces a record of every heartbeat during the monitoring period. Symptoms recorded in the diary can then be checked against any changes in the ECG to determine whether an abnormal heart rhythm is the cause of the symptoms.
If necessary, the ECG can be transmitted by telephone to a computer at the hospital or doctor's office for an immediate reading as soon as symptoms occur.
A newer option for continuous monitoring is a small disposable wireless adhesive patch that is worn on the chest for up to 2 weeks. Because it is much smaller than a regular Holter monitor and does not require multiple wire leads to be placed on the chest, people may find it easier to wear for longer periods of time. However, the device is more expensive.
An event monitor is used when a person must be monitored longer than 48 hours. It is similar to a Holter monitor, but it records only when the user activates it—that is, when symptoms occur—by pressing a button on the device or when the device detects an abnormal heart rhythm.
If symptoms occur so rarely that they cannot be captured during the initial monitoring period, an event monitor may be placed under the skin. A small magnet is used to activate this monitor. These types of implantable event monitors are called implantable loop recorders and can last several years before the battery needs to be replaced.