Cardiac arrhythmias are abnormal heart rhythms that can cause palpitations, weakness, dizziness, fainting, blood clots, or death. A variety of treatments have been developed for arrhythmias; however, selection of the appropriate treatment requires an accurate diagnosis, which may be difficult since arrhythmias can occur infrequently and unpredictably and may not cause obvious symptoms. To detect infrequent arrhythmias, patients can undergo 24 to 48 hours of continuous outpatient electrocardiographic (ECG) recording with a Holter monitor. A shortcoming of this device is that repeated monitoring sessions may be necessary if an arrhythmia does not occur during the first 1 or 2 days. Another method for detection of infrequent arrhythmias is the use of an event recorder, which stores 1 to 2 minutes of ECG data as soon as the patient experiences symptoms and presses a button to activate the device. Although this technique enables a much longer period of monitoring, some arrhythmias do not cause obvious symptoms and some symptomatic patients fail to turn on the recorder at the right time.