An Ambulatory ECG  can be performed with a Holter monitor which monitors your heart for rhythm abnormalities during normal activity for an uninterrupted 24-hour period. During the test, electrodes attached to your chest are connected to a portable recorder - about the size of a paperback book - that's attached to your belt or hung from a shoulder strap.

Another form of ambulatory ECG test is an Event recorder which covers 1-2 weeks.  You wear a monitor (much smaller than a Holter monitor) and if you have any symptoms, such as dizziness, you press a button on a recording device which saves the recording of your heart rhythm made in the minutes leading up to and during your symptoms.  Because you can wear this for a longer period of time it has a higher rate of catching your abnormal rhythm.