Loop recorders are event recorders with a ‘loop memory’: they continually analyse the ECG and retain information pertaining to relevant arrhythmias that are automatically detected thanks to predefined algorithms and the registration of the ECG a few minutes before the onset of the arrhythmia. According to their specific functions they can be divided into two categories: loop recorders, which include external loop recorders (ELRs) and implantable loop recorders (ILRs) and post-event recorders (non-looping recorders). Newer generation monitors are the so-called event recorders. Despite their advantages, ambulatory Holter monitors also have many limits: a relatively brief duration of monitoring, the impossibility of transmitting real-time data to the attending cardiac unit and the need for close collaboration between the patient and health professionals. The quantification of arrhythmic events may aid the clinician in making a therapeutic decision, especially for those arrhythmias that occur frequently and those that have disabling symptoms. There are two main advantages of a continuous Holter monitoring system: the possibility of quantifying the real burden of an arrhythmia, and the detection of rhythm disturbances outside the limits set by an algorithm or memory. Two-week Holter monitoring is preferred when diagnosing or excluding AF in cryptogenic stroke, but loop recorders and outpatient telemetry have even higher detection rates. They can be short- (24–48 hours) or long-duration (1–2 weeks) devices. Traditional ambulatory Holter monitors are simple devices that usually have three leads and continuously register the ECG. In other circumstances the detection of atrial or ventricular arrhythmias may alert the cardiologist, even if they occur asymptomatically, thus prompting a specific therapeutic decision such as starting antiarrhythmic or anticoagulant drugs or implanting a pacemaker or a cardioverter defibrillator. When interpreting the results of the ECG, the cardiologist has to determine whether symptoms reported by the patient could be linked to significant disturbances in heart rhythm. This need for a prolonged ECG monitoring has been addressed by event recorders, which can monitor patients for up to 3 years, storing the ECG obtained a few minutes before and after the onset of an arrhythmia in its memory and transmitting data to the cardiac unit. Extending the time of ECG registration will increase the diagnostic yield of Holter monitoring, especially for those rhythm disturbances that are infrequent but recurrent. The most common monitoring systems allow the continuous registration of three or more leads for 24–48 hours newer Holter monitors allow continuous electrocardiogram (ECG) registration for 2 weeks. Since the 1960s, Holter monitoring has been a cornerstone for diagnosing suspected arrhythmias in patients of all ages.
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