The goals of this ECG 101 article are to define ambulatory electrocardiographic (ECG) monitoring, identify clinical scenarios where ambulatory monitoring has proven useful, and discuss the different types of monitoring available. We will highlight the three major categories of devices: Holter monitors, event recorders, and implantable loop recorders. Although implanted pacemakers and defibrillators also have monitoring capabilities, they are beyond the scope of this discussion. By the end of this article, the reader will be aware of the types of monitoring devices available and be able to select the appropriate device for a given clinical scenario. What is Ambulatory ECG Monitoring? Ambulatory ECG monitoring is the continuous recording of ECG signals from patients while they are at home and engaged in routine daily activities. These are non-invasive or minimally invasive diagnostic exams that have been shown to be very efficacious and cost-effective in the right clinical scenario. From the pioneering work of Dr. Norman Holter, who published his landmark article in Science1 (1961) discussing a new technique for continuous portable electrocardiography, ambulatory ECG monitoring has evolved to include myriad device designs that provide a wide array of recording options. The type of patient and clinical indication will dictate which device and recording options are most appropriate. What are Some Scenarios Where ECG Ambulatory Monitoring has Proven Useful? The most common indication for ambulatory ECG monitoring is to evaluate patients with symptoms (such as palpitations, atypical angina, or syncope) that suggest a possible arrhythmia or other electrocardiographic abnormality. The correlation of symptoms with the time of a recorded arrhythmia can often firmly establish or eliminate an etiology. The method of initiation for an arrhythmia is also often captured and can help identify the arrhythmia and guide therapy. ECG monitoring can also identify suspected abnormalities even when the patient remains asymptomatic. Another important use of ambulatory ECG monitoring is to monitor efficacy of antiarrhythmic therapy — both pharmaceutical and procedural. Many patients can be asymptomatic with rhythm disturbances (such as atrial fibrillation or ventricular tachycardia), and ambulatory monitoring may be necessary to identify the existing burden of the arrhythmia. This can help the clinician identify the success of the current therapy and guide any potential changes or additions to management. Ambulatory ECG monitoring has also been used to prognosticate patients with certain underlying cardiac conditions such as ischemic2 or nonischemic cardiomyopathy and congenital abnormalities.3 The presence of ventricular arrhythmias, abnormal heart rate variability, late potentials, and T wave alternans can all help identify patients at risk for potentially harmful arrhythmias. Silent ischemia can also be elucidated through ST segment changes found on monitoring. Despite this potential to effectively identify such disorders, it is important to remember that ambulatory monitoring is not indicated for patients who are clinically unstable. Patients with active and significant hemodynamic, ischemic, or arrhythmic compromise should be admitted directly for further evaluation and treatment. Given the large number of clinical uses for ambulatory monitoring, it is important to select the right test for the right clinical scenario. Next, we will discuss the three major categories of ambulatory monitoring: Holter monitors, event recorders, and implantable loop recorders. Holter Monitors Holter monitors are typically a continuous recording of several ECG channels for 24 to 48 hours. Any number of standard ECG leads are adhered to a patient’s chest and connected by wire to a digital or analog recording device. Patients are instructed to wear loose-fitting clothes over the leads and perform their usual activities, provided they keep the device dry. They are also asked to keep a log of their symptoms, noting both the nature of the symptoms and their time of occurrence. This log is then used to correlate the symptoms with the ECG recordings at that time. While the device is worn, Holter monitors provide a continuous and complete recording similar to full disclosure on inpatient telemetry. From the recordings, algorithms identify rhythm strips that are automatically selected and displayed to the reading clinician. Typically, highlighted rhythms include significant bradycardia or tachycardia outside of set heart rate limits, or significant heart rate variability within a short period of time. Other abnormalities such as heart block or rhythm irregularity can also be automatically identified. Also generated is an electronic statistical summary that includes heart rate ranges as well as frequencies of premature atrial and ventricular complexes. Importantly, the rhythms recorded during patient-reported symptoms are selected and displayed. This helps the clinician to identify whether the symptoms may be related to a change in rate or rhythm. Clinically, a Holter monitor is most useful in evaluating a patient who is having frequent or daily symptoms that may be due to an abnormal cardiac rhythm. It can also be helpful in identifying the frequency of ventricular ectopy in a patient with ischemic or nonischemic cardiomyopathy. It is important, however, to remember that many of the findings from Holter monitors, such as premature atrial or ventricular complexes, may not be clinically significant in an otherwise healthy patient, so clinical correlation is necessary for proper evaluation. Event Recorders For many patients, symptoms are not frequent enough to be caught on a Holter monitor that is worn for only one or two days. An event recorder can provide ECG readings for up to an entire month. ECG leads are again placed on a patient’s chest and then connected by wire to a recorder that is typically the size of a pager. The device will continuously monitor data from the ECG tracings, but only maintains data from a specified period of time, typically the last 30 to 60 seconds. This data is continuously updated in a loop-like fashion. Data is selected to be saved by one of two mechanisms — patient-triggered or automatically triggered events. When patients experience symptoms, they are instructed to push a button that will trigger the device to store the current rhythm as well as data from the short period before and after the event. Alternatively, when the ECG meets programmed criteria in the device (such as a rapid or slow heart rate), the device will be automatically triggered to store the current data. Once data is stored, it can be uploaded to a monitoring center by telephone. This allows the timely identification of potentially dangerous rhythm disturbances before the end of the entire monitoring period. The main advantage of an event recorder over a Holter monitor is that it provides monitoring for a longer period of time, which can significantly improve diagnostic yield. It remains non-invasive and is a lightweight apparatus. Unlike a Holter, however, all data is not permanently stored unless triggered by the patient or the device’s preset parameters. If a patient is unable or forgets to activate the device following an event, the data will not be stored unless it meets the programmed criteria for automatic triggering. Therefore, patients being evaluated for syncope are asked to recognize their presyncopal symptoms and promptly activate their device prior to losing consciousness. Since the device is to be worn for a longer time, the number of ECG vectors is typically limited to one or two, and the signal quality can be less consistent than with a Holter. The ideal scenario for an event recorder is a patient who is symptomatic but not on a daily basis, is willing and capable of wearing the monitoring device for an extended period of time, and is able to activate the device with the onset of symptoms. In these situations, the diagnostic yield of event recorders is quite high with minimal risk to the patient. Implantable Loop Recorders Implantable loop recorders are ECG recording devices about the size of a small pack of chewing gum that are implanted just under the skin in the chest. These devices can provide recordings for an extensive period of time, often one to two years. The data is recorded in a loop-like fashion similar to event recorders, and data is again permanently stored as a result of patient or automatic triggering. Typically, a patient can activate the device to store data by passing a small magnetic activator over it. With an implantable loop recorder, usually a much greater preceding period of time (often up to a half hour) is continuously maintained and stored when triggered by the patient. Therefore, patients who have a syncopal event could activate the device even after waking and still capture the culprit rhythm. In the office, a programmer is used to retrieve the stored event data. The main advantage of an implantable loop recorder is that it provides monitoring for such an extended duration and allows the patient a longer period to activate the device. Although it is surgically implanted, it remains minimally invasive with very low complication rates. The signal quality is tested prior to implantation, and tends to be more consistent than with external leads. Overall, it has significantly increased the diagnostic efficacy of ambulatory monitoring. After the diagnosis is made, the device can be explanted on an outpatient basis. Summary Ambulatory ECG monitoring is a safe and effective means of monitoring a patient’s heart rhythm on an outpatient basis. There are several different methods of monitoring available, and proper selection is dependent on the clinical scenario being evaluated. With the development of longer term monitoring techniques, the diagnostic yield to determine arrhythmic etiologies (or lack thereof) has significantly improved. Proper selection and use of these devices has led to a significant improvement in our overall ability to identify and treat heart rhythm disturbances.