For these reasons, the use of Holter monitors is not universal in medium and high-risk syncope patients. In one UK ED study, only 158 of 540 (29%) admitted syncope patients underwent 24 hour monitoring (which in the majority comprised ward telemetry rather than Holter). There are other devices available to the Cardiologist to investigate syncope patients who are classified as European Society of Cardiology (ESC) medium and high-risk and whose Holter investigation is unrevealing. Event recorders do not record a continuous ECG but require patient activation at the time of symptoms and must be applied to the chest wall at the time of the event and must be activated by the patient. A brief, typically 90-second, single lead ECG recording is captured and stored. Because of limited data storage capability, data must be transmitted to a monitoring centre for validation and analysis. Event recorders can be used for cardiac monitoring over longer periods of time but the big drawback is that they must be activated following symptom onset, which may be difficult to achieve if the patient has suffered syncope or an injury related to the event. Finally, these devices cannot be used to document asymptomatic arrhythmias.