A Holter monitor is a portable EKG that monitors the electrical activity of a freely moving person's heart, generally for one to two days, 24 hours a day. It is most often used when the doctor suspects an abnormal heart rhythm or ischemia, which means not enough blood flowing to the heart muscle. 

It is a painless test – electrodes from the monitor are taped to the skin. Once the monitor is in place, you can go home and perform all of your normal activities, except showering. You will be asked to keep a diary of your activities, any symptoms you experience and when they occur.

What is an event monitor?
If your symptoms are infrequent, your doctor may suggest an event monitor. This is a device that, when you push a button, will record and store the heart's electrical activity for a few minutes. Each time you develop symptoms you should try to get a reading on the monitor. They are used for weeks to months, typically one month. This information can later be transmitted by telephone to the doctor for interpretation.

What is a Holter scan?
After the patient wears the Holter monitor for a 24-hour period, he or she brings the monitor back and a technician scans through more than a hundred thousand beats looking for any irregular rhythms or rates. The technician interprets the findings, creates a written report and has it reviewed by an electrophysiologist. Finally, the report is sent to the referring physician.

The Cardiac Event Monitor is small device, about the size of an iPod, that is worn on the chest and is used to detect abnormal heart rhythms. When patients feel an “event,” such as dizziness, weakness or palpitations, they push a button on the device. The monitor stores the event and it is later sent over a telephone line to the cardiologist for review. 

This test allows for on-demand heart monitoring outside the office setting as patients go about their normal routine. The monitor can be worn for up to 30 days and is only removed for bathing.