7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate Official reprint from UpToDate www.uptodate.com 2023 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Ambulatory ECG monitoring : Christopher Madias, MD : Peter J Zimetbaum, MD : Nisha Parikh, MD, MPH All topics are updated as new evidence becomes available and our peer review process is complete. Literature review current through: Jun 2023. This topic last updated: Nov 29, 2022. INTRODUCTION In contrast to the standard electrocardiogram (ECG), which provides a brief sample of cardiac electrical activity over 10 seconds, ambulatory ECG monitoring provides a view of ECG data over an extended period of time, thereby permitting evaluation of dynamic and transient cardiac electrical phenomena. The most common ambulatory ECG application is in the diagnosis and assessment of cardiac arrhythmias or conduction abnormalities (symptomatic or asymptomatic) or the presence of potential arrhythmias (such as in patients with syncope or presyncope); however, ambulatory ECG also has a role in stratification of certain cardiomyopathies, in assessing the effectiveness of arrhythmia therapy, and in the evaluation of silent ischemia. (See "Ventricular arrhythmias during acute myocardial infarction: Incidence, mechanisms, and clinical features" and "Silent myocardial ischemia: Epidemiology, diagnosis, treatment, and prognosis" and "Evaluation of heart rate variability".) Ambulatory ECG monitoring, which can be performed using a variety of techniques for as short as 24 to 48 hours and for as long as months to years ( table 1), offers the opportunity to review cardiac ECG data during routine activity, as well as during periods of physical and psychological stress. Ambulatory ECG monitoring for longer periods (when compared with standard ECG for a 10-second time period) is more sensitive for detecting spontaneous, often highly variable cardiac arrhythmias or conduction abnormalities [1,2]. Ambulatory monitoring, in conjunction with clinical and ECG findings, can be a useful component in the evaluation of the patient with unexplained syncope, presyncope, or palpitations. A detailed discussion of the evaluation of syncope and palpitations, including the https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 1/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate role of ambulatory monitors, is presented separately. (See "Syncope in adults: Clinical manifestations and initial diagnostic evaluation" and "Evaluation of palpitations in adults".) INDICATIONS Ambulatory ECG monitoring is the most widely employed technology for the evaluation of a patient with symptoms suggestive of cardiac arrhythmia or conduction abnormality. Professional society guidelines on ambulatory ECG recommend ambulatory ECG monitoring for the symptomatic patient with [3,4]: Unexplained syncope, near syncope, or episodic dizziness (see "Syncope in adults: Risk assessment and additional diagnostic evaluation", section on 'Introduction') Unexplained recurrent palpitations (see "Evaluation of palpitations in adults") The choice of initial ambulatory ECG monitoring for the symptomatic patient depends somewhat on the frequency and severity of symptoms. Continuous ECG (Holter) monitoring for 24 to 48 hours is most practical as the initial monitor for patients with daily or near daily symptoms, while those with less frequent symptoms are more likely to benefit from extended monitoring with an event monitor or an insertable cardiac monitor (sometimes referred to as an implantable cardiac monitor or an implantable loop recorder). (See 'Types of ambulatory ECG monitoring' below and 'Our approach to choosing an ambulatory ECG monitoring strategy' below.) In addition to being used for the evaluation of patients with unexplained symptoms suggestive of cardiac arrhythmia, ambulatory ECG monitors may also be used in risk stratification, in assessing the effectiveness of arrhythmia therapy, and in the evaluation of silent ischemia. As examples: To assess the average heart rate and adequacy of rate control in the patient with atrial fibrillation (AF). (See "Atrial fibrillation: Overview and management of new-onset atrial fibrillation", section on 'Additional cardiac testing'.) To evaluate for occult AF as a potential cause of cardioembolism in patients with cryptogenic stroke [5]. (See "Overview of the evaluation of stroke", section on 'Monitoring for subclinical atrial fibrillation'.) To screen for asymptomatic ventricular premature beats or nonsustained ventricular tachycardia in a patient with hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, long QT syndrome, dilated or restrictive cardiomyopathy, https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 2/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate congenital heart disease, or Brugada syndrome. (See "Hypertrophic cardiomyopathy: Clinical manifestations, diagnosis, and evaluation", section on 'Ambulatory ECG monitoring' and "Congenital long QT syndrome: Diagnosis", section on 'Ambulatory ECG monitoring' and "Arrhythmogenic right ventricular cardiomyopathy: Diagnostic evaluation and diagnosis", section on 'Ambulatory monitoring'.) To evaluate prognosis following acute coronary syndrome. (See "Incidence of and risk stratification for sudden cardiac death after myocardial infarction".) To assess for silent myocardial ischemia in a patient with known or suspected coronary artery disease. (See "Silent myocardial ischemia: Epidemiology, diagnosis, treatment, and prognosis", section on 'Ambulatory monitoring'.) OUR APPROACH TO CHOOSING AN AMBULATORY ECG MONITORING STRATEGY The approach to choosing an ambulatory ECG monitoring strategy depends on the indication for the test in addition to the frequency and duration of symptoms ( table 1). When to choose continuous ECG (Holter) monitoring A Holter monitor is the preferred ambulatory ECG monitoring test for patients with daily or near daily symptoms, and for patients in whom a comprehensive/continuous assessment of all cardiac activity is required. Clinical scenarios in which a Holter monitor is a good choice include ( table 1): Patients with syncope, near syncope, or dizziness occurring on a daily or near daily basis. (See "Syncope in adults: Risk assessment and additional diagnostic evaluation", section on 'Ambulatory ECG monitoring'.) Patients with palpitations occurring on a daily or near daily basis. (See "Evaluation of palpitations in adults".) Patients with atrial fibrillation (AF) for assessment of ventricular rate control. (See "Atrial fibrillation: Overview and management of new-onset atrial fibrillation", section on 'Additional cardiac testing'.) Patients with frequent ectopy (either ventricular premature beats [VPBs or premature atrial complexes [PAC; also referred to a premature atrial beat, premature supraventricular complex, or premature supraventricular beat]) requiring quantification of the ectopy burden. (See "Arrhythmia-induced cardiomyopathy".) https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 3/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate Patients with recent acute coronary syndrome (especially non-ST-segment elevation myocardial infarction [NSTEMI] or STEMI), hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, long QT syndrome, dilated or restrictive cardiomyopathy, congenital heart disease, or Brugada syndrome, in whom screening for ventricular ectopy or non-sustained ventricular arrhythmias might alter prognosis and therapy. (See "Incidence of and risk stratification for sudden cardiac death after myocardial infarction" and "Hypertrophic cardiomyopathy: Clinical manifestations, diagnosis, and evaluation", section on 'Ambulatory ECG monitoring' and "Congenital long QT syndrome: Diagnosis", section on 'Ambulatory ECG monitoring' and "Arrhythmogenic right ventricular cardiomyopathy: Diagnostic evaluation and diagnosis", section on 'Ambulatory monitoring'.) When to choose event monitoring An event monitor is the preferred ambulatory ECG monitoring test for patients with less frequent (ie, weekly to monthly) symptoms and for patients in whom a comprehensive assessment of all cardiac activity over a 24 to 48 hour interval is not required. Several types of event monitors are available with a gradation of features: Post Event Monitoring devices are generally small, lightweight devices that can be placed on the patient's chest upon the onset of symptoms. The patient's rhythm is stored for a specified amount of time after recording begins (eg, 30 to 150 seconds). Event/Loop Recorders are devices that constantly record for a pre-specified period, but do not save the data until they are triggered to do so by the patient pushing an event button. The device will record and save the patient's rhythm for a pre-specified amount of time before and after activation of the device (eg, 30 seconds prior to and 60 seconds after the event). Auto-triggered Event Recorders are more advanced devices. In addition to recording and saving symptomatic patient-triggered events, these devices have auto-detect features that will capture asymptomatic arrhythmias based on detection algorithms (eg, atrial fibrillation events or bradycardia events). Clinical scenarios in which an event monitor is a good choice include ( table 1): Patients with syncope, near syncope, or dizziness that occurs less frequently (ie, weekly to monthly). (See "Syncope in adults: Risk assessment and additional diagnostic evaluation", section on 'Ambulatory ECG monitoring'.) https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 4/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate Patients with palpitations that occur less frequently (ie, weekly to monthly). (See "Evaluation of palpitations in adults".) Patients with near syncope, dizziness, or palpitations that occur more frequently but in whom an initial Holter monitor was nondiagnostic. Patients undergoing evaluation for atrial fibrillation as a source of cryptogenic stroke. For this indication an event monitor with auto-triggered feature should be considered for the detection of asymptomatic atrial fibrillation events. (See "Overview of the evaluation of stroke", section on 'Monitoring for subclinical atrial fibrillation'.) When to choose mobile cardiac outpatient telemetry (MCOT) MCOT monitoring might be a preferred choice for patients with less frequent symptoms (ie, weekly to monthly) for whom comprehensive assessment of all cardiac activity is required. Clinical scenarios in which MCOT might be a good choice include ( table 1): Patients undergoing evaluation for atrial fibrillation as a source of cryptogenic stroke. (See "Overview of the evaluation of stroke", section on 'Monitoring for subclinical atrial fibrillation'.) Patients with syncope, near syncope, or dizziness that occurs less frequently (ie, weekly to monthly). (See "Syncope in adults: Risk assessment and additional diagnostic evaluation", section on 'Ambulatory ECG monitoring'.) Patients with palpitations that occur less frequently (ie, weekly to monthly). (See "Evaluation of palpitations in adults".) Patients in whom accurate information on arrhythmia burden is desired (eg, AF burden). Patients in whom information on nocturnal arrhythmias is desired, such as those associated with sleep apnea. When to choose insertable cardiac monitoring Intracardiac monitoring (ICM) can be a particularly useful diagnostic tool for patients with infrequent (ie, less than monthly) symptoms that are potentially injurious (eg, syncope). ICM can also be useful for patients with bothersome symptoms requiring a diagnosis, but whose prior assessments with Holter or event monitoring have been unrevealing of a diagnosis. ICM may also be helpful when searching for asymptomatic occult episodes of AF, such as following a cryptogenic stroke. (See "Overview of the evaluation of stroke", section on 'Monitoring for subclinical atrial fibrillation'.) https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 5/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate Other scenarios Occasionally, clinical circumstances will arise in which ambulatory ECG monitoring is not the optimal initial test ( table 1). Generally, if the patient has severe or potentially life-threatening symptoms (eg, syncope or presyncope with associated injury, sustained lightheadedness, etc), admission to the hospital for continuous in-hospital telemetry is warranted. TYPES OF AMBULATORY ECG MONITORING There are several different methods of performing ambulatory ECG monitoring ( table 2). Continuous ambulatory ECG (Holter) monitor The conventional continuous ambulatory ECG system, generally referred to as a Holter monitor, is most commonly performed for patients with frequent (ie, daily or near daily) symptoms of palpitations ( table 2). Continuous ambulatory ECG (Holter) monitoring includes a continuous recording of all ECG data for a period of 24 or 48 hours. This technology uses a small, lightweight, battery operated recorder ( figure 1) that typically records two or three channels of ECG data (although 12-lead monitors are also available) from electrodes placed on the patient's chest [6]. Holter monitoring devices have patient-activated event markers, and encoded time or time markers. The recorded data are analyzed with a playback instrument system that requires operator interaction with an arrhythmia analyzer, ST segment detector, RR interval analysis, signal-averaging computer, and a variety of audiovisual detection and review displays, as well as a printer for generating printouts of ECGs, trends, or a statistical summary. Holter monitors must be returned for analysis and, as such, do not provide real-time information on patient symptoms or arrhythmias. Holter monitor report A typical Holter monitor report includes the following information ( table 2): Total heart beats Average heart rate Maximum and minimum heart rates Number of premature beats (supraventricular and ventricular) Episodes of tachyarrhythmia and the etiology of the arrhythmias (eg, supraventricular or ventricular) https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 6/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate Longest R-R interval and any pauses greater than three seconds and etiology of the pauses (eg, sinus pauses versus AV block) ST segment changes Patient-reported symptoms and any associated ECG findings Representative samples of ECG tracings (eg, hourly samples) Event (loop) monitor Event monitors, historically referred to as loop monitors because the recording device continuously looped its recording tape, are most commonly used for patients with less frequent (ie, weekly to monthly) symptoms of palpitations, presyncope, or syncope ( table 2). Event monitors can be utilized for two to four weeks (although possibly up to three months). Similarly to Holter monitors, event monitors use a small, lightweight, battery operated recorder ( figure 1) that typically records two channels of ECG data from electrodes placed on the patient's chest. Typically, event monitors are worn continuously and are activated by patient trigger when symptoms arise. Alternatively, patients with longer lasting symptomatic episodes can be instructed to apply the event monitor during symptomatic events. Because these instruments store the ECG in continuous memory before activation, they are excellent for documenting transient symptomatic or incapacitating events, and often can display the antecedent onset and subsequent offset of a paroxysmal cardiac arrhythmia [7]. Several types of event monitors are available with a gradation of features: Post Event Monitoring devices are generally small, lightweight devices that can be placed on the patient s chest upon the onset of symptoms. The patient's rhythm is stored for a specified amount of time after recording begins (eg, 30 to 150 seconds). Event/Loop Recorders are devices that constantly record for a pre-specified period, but do not save the data until they are triggered to do so by the patient pushing an event button. The device will record and save the patient's rhythm for a pre-specified amount of time before and after activation of the device (eg, 30 seconds prior to and 60 seconds after the event). Auto-triggered Event Recorders are more advanced devices. In addition to recording and saving symptomatic patient-triggered events, these devices have auto-detect features that will capture asymptomatic arrhythmias based on detection algorithms (eg, atrial fibrillation events or bradycardia events). Real-time event monitors have been developed that transmit arrhythmic event data from an ambulatory patient to a continuously attended monitoring station [8,9]. Based on device specific algorithms and pre-specified alert criteria, these devices automatically record and transmit data https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 7/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate for review in real-time. Data can also be recorded through patient-triggered activation. (See 'Mobile cardiac outpatient telemetry (MCOT)' below.) The ECG data from event monitors are received at a base station equipped with a demodulator and an ECG strip chart recorder. Trained staff analyze live incoming patient data and can contact the patient's clinician if instructed to do so according to a prespecified set of alert criteria provided by the clinician. Event monitor report A typical event monitor report includes the following information ( table 2): ECG tracings for each patient-triggered (or auto-triggered) event Technician's interpretation of the tracing Reported symptoms and their duration Patch monitor Patch monitors ( picture 1) are all-in-one small adhesive devices that do not require separate leads, wires, or battery packs ( table 2). This makes them more convenient and less obtrusive than traditional Holter or event monitors. Patch monitors are capable of continuously recording an ECG for up to 14 days, although only a single lead is recorded [10,11]. Patch monitor report A typical patch monitor report includes the following information ( table 2): Average heart rate Maximum and minimum heart rates Number of premature beats (supraventricular and ventricular) Episodes of tachyarrhythmia and the etiology of the arrhythmias (eg, supraventricular or ventricular) Longest R-R interval and any pauses greater than three seconds and etiology of the pauses (eg, sinus pauses versus AV block) ECG tracings for each patient-triggered (or auto-triggered) event Mobile cardiac outpatient telemetry (MCOT) MCOT can be worn for up to 30 days ( table 2). Traditionally, a three-lead sensor transmits ECG telemetry data to a small portable monitor. The monitor transmits the ECG information to a monitoring center via built-in cell phone technology. The device can also be triggered to record symptomatic events by the patient or can be triggered automatically in response to programmed settings. Algorithms for https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 8/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate automatic arrhythmia detection analyze every heart beat and are based on rate, rhythm irregularity, P wave analysis, and QRS morphology. A single lead MCOT patch monitor is also available. As events occur, patient ECG data is automatically transmitted to the monitoring center for analysis. Certified technicians review the data and a daily report and service-summary report is made available for online review (or by fax) [12]. Immediate reports for potentially life- threatening arrhythmias can also be created and physicians are alerted to promptly review these episodes. MCOT has been shown to detect significantly more arrhythmias than standard patient triggered loop recorders and has been shown to be effective in diagnosing AF after cryptogenic stroke [13,14]. Daily reports on burden of arrhythmia (such as AF) are provided and MCOT can be useful to assess for possible nocturnal arrhythmias such as those associated with sleep apnea. Notably, prescribing physicians should be aware that MCOT can be more expensive than standard event monitoring. MCOT report A typical MCOT report includes the following information ( table 2): Daily and service-summary reports on heart rate and rhythm Daily and service-summary reports on classification, duration, and frequency of arrhythmic events ECG tracings for each patient-triggered (or auto-triggered) event Technician's interpretation of the tracing Reported symptoms and their duration Insertable cardiac monitor The insertable cardiac monitor (ICM) is a subcutaneous monitoring device ( picture 2) for the detection of cardiac arrhythmias ( table 2) [15]. ICMs are most commonly utilized in the evaluation of palpitations or syncope of undetermined etiology, particularly when symptoms are infrequent (eg, less than once per month) and/or other ambulatory monitoring has been unrevealing or inconclusive [16]. These devices are typically implanted in the left pectoral region and are MRI conditional. ICMs will store patient- triggered events and will also automatically store events according to programmed detection criteria (eg, a pause greater than four seconds). Depending on the manufacturer and the specific device, the battery longevity of ICMs can range between two to four years [17]. (See "Patient evaluation for metallic or electrical implants, devices, or foreign bodies before magnetic https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 9/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate resonance imaging", section on 'Safety labeling' and "Syncope in adults: Risk assessment and additional diagnostic evaluation", section on 'Ambulatory ECG monitoring'.) The ICM offers the ability to monitor for cardiac arrhythmias for prolonged periods of months to years. In one cohort of 312 patients who underwent ICM implantation at a single center between 2010 and 2015, ICM monitoring for a median of 12 months led to a diagnosis that changed management in 146 patients (47 percent) [18]. In one cohort of 157 patients with at least one episode of unexplained syncope who received an ICM, 70 (45 percent) were followed for greater than 18 months, some as long as four years [19]. Twenty-six percent of all diagnoses made in this cohort occurred more than 18 months following ICM implantation, indicating the utility of continued ICM monitoring in patients without a definitive diagnosis for their unexplained syncope. Among a registry cohort of 19,173 patients who underwent ICM implantation between 2007 and 2016 and were followed for an average of 2.1 years, syncope (54 percent) was the most common indication for ICM implantation; 25 percent of syncope patients ultimately had a diagnosis leading to implantation of a pacemaker or implantable cardioverter- defibrillator (ICD) [20]. Among patients with neurocardiogenic syncope, an ICM may more accurately establish a causative relationship between bradyarrhythmias and syncope than provocative tests (eg, upright tilt table testing or adenosine 5'-triphosphate [ATP] infusion). (See "Reflex syncope in adults and adolescents: Clinical presentation and diagnostic evaluation", section on 'Electrocardiographic monitoring'.) Implantation of the ICM is typically performed as an outpatient procedure using local anesthesia with few associated risks. Among 273 patients enrolled in a nonrandomized trial (151 patients) and an ICM registry (122 patients), device infections were noted in only four patients (1.5 percent), and serious adverse events (defined as procedure-related adverse events leading to death or serious deterioration in health) were noted in only three patients (1.1 percent) with no associated deaths [21]. ICM report A typical ICM report includes the following information ( table 2): Heart rate histograms, heart rate variability, and patient activity data Episode lists including classifications, frequency, and duration of arrhythmic events Single lead ECG tracings for each patient-triggered (or auto-triggered) event https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 10/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate Permanent pacemakers and implantable cardioverter-defibrillators Notably, permanent pacemakers and ICDs can also be used as a continuous monitoring devices, as these systems have built-in algorithms for the recognition of various arrhythmias (such as AF or VT) ( table 2). The devices can be interrogated to obtain data about arrhythmia frequency, duration, and atrial and ventricular rates. In addition, intracardiac ECGs meeting detection criteria can be recorded and stored for review. The data provided by such devices can be limited by the number and type of leads associated with the specific device (eg, a device with a single ventricular lead might not provide information on atrial arrhythmias) (see "Cardiac implantable electronic devices: Patient follow-up"). There are circumstances in which a patient with a PPM or ICD might be prescribed an external monitor to gain more clear and detailed information regarding a potential arrhythmia. For example, evaluation of a patient's single chamber defibrillator might reveal nonsustained ventricular tachycardia episodes that appear irregular. An external monitor can help differentiate if these episodes are atrial fibrillation or true nonsustained ventricular tachycardia events. Commercially available wearable heart rhythm monitors The use of commercially available, wearable heart rate and rhythm monitors is becoming more common ( table 2) [22]. These devices can be useful in the diagnosis of arrhythmia in certain patients. Many of these technologies come in the form of electronic wristbands and smartwatches. Optical sensors integrated into these devices use photoplethysmography to measure pulse rate [23]. Rapid rise in heart rate associated with symptoms can help detect arrhythmias such as supraventricular tachycardia or AF. In addition, some of these devices have automated algorithms to detect pulse irregularity and can notify the user regarding possible arrhythmia, such as AF. Photoplethysmographic algorithms for irregular heart rhythms in a number of smart watches and fitness trackers have been shown to have high positive predictive value for concurrent AF and to identify those who were likely to have AF demonstrated on subsequent traditional ECG monitoring [24-26]. Other commercially available devices use hand-held electrodes to record single or multiple-lead ECGs [23]. The ECGs are displayed and stored directly onto such devices or in another form, and are sent wirelessly to applications on the user's smartphone. Algorithms incorporated into these applications can also provide analysis for possible arrhythmia, including AF. Smartwatches are also now able to record single lead ECG by using electrodes built into the back of the watch and a finger from the opposite hand placed on the digital crown or onto an electrode built into the watch band to complete the circuit. Such devices can be useful in long term monitoring for arrhythmia recurrences, particularly in symptomatic patients with atrial fibrillation who have undergone rhythm control therapy (such as ablation). In addition to identifying cardiac rate and https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 11/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate rhythm, smartwatches have the potential to allow for QT interval measurement. In one study of 100 patients in which a single-lead ECG was recorded using a smartwatch in three different locations (left wrist, left ankle, left lateral chest wall), 94 percent of patients were able to obtain an accurate QT interval which correlated to the QT interval measured on 12-lead ECG [27]. As technology advances and arrhythmia detection algorithms further improve, the increased use of commercially available, wearable heart rhythm monitors is likely to prove valuable in the diagnosis and management of certain patients with arrhythmia. Data on how to best integrate such devices into optimal management of arrhythmia detection will be derived from ongoing clinical trials [28]. DIAGNOSTIC EFFICACY The ECG rhythm recorded using any ambulatory ECG monitoring modality must be correlated with the simultaneous occurrence of suggestive symptoms. Without this correlation in many cases, the detected rhythm abnormality is less likely to be clinically important. Bradycardia and sinus pauses are particularly common at night while the patient is asleep, as a result of enhanced vagal tone. Thus, the occurrence of such arrhythmias does not necessarily establish a causal mechanism for a transient disturbance in consciousness or for other symptoms. Among patients evaluated with 24-hour ambulatory monitoring, approximately 25 to 50 percent of patients will experience a symptom, between 2 and 15 percent will have an associated causal arrhythmia, and 35 percent will report symptoms without associated arrhythmias [29]. Extending the period of monitoring can increase the incidence of symptomatic events to approximately 50 percent at three days and 75 percent at 5 to 21 days (mean nine days) [30,31]. The diagnostic value of ambulatory monitoring seems to depend upon a number of variables: Hospitalized patients with symptoms are more likely to have a causal arrhythmia. In one study, which included 306 hospitalized patients and 278 outpatients, symptoms occurred far more frequently in the outpatients (55 versus 6 percent); however, symptomatic inpatients were significantly more likely to have symptoms that correlated with causal arrhythmias (95 versus 44 percent) [32]. The presenting symptom also may predict the findings of ambulatory ECG. In one report of 1010 hospitalized patients, those with syncope had a consistently higher rate of diary maintenance, of reporting symptoms during ambulatory ECG, and of a conclusive examination [33]. https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 12/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate When an insertable cardiac monitor (ICM) is implanted in patients with syncope of uncertain cause, transient bradycardia is frequently found to be responsible [34-38]. In the largest study of ICM use in the evaluation of patients with syncope of uncertain cause (The Place of Reveal In the Care pathway and Treatment of patients with Unexplained Recurrent Syncope, PICTURE registry) an ICM was implanted in 570 patients [39]. Patients were followed for at least one year or until their next syncopal episode (mean of 10 +/- 6 months). Recurrent syncope was noted in 218 patients (38 percent). Data acquired by the ICM during the syncopal event directly contributed to a diagnosis in 78 percent of patients. A cardiac etiology was found in 75 percent; however, a breakdown of specific cardiac arrhythmias noted on the ICM was not reported. (See "Syncope in adults: Risk assessment and additional diagnostic evaluation", section on 'Ambulatory ECG monitoring'.) Ambulatory ECG monitoring yields other potentially useful information regarding arrhythmias, including: Quantifying the number of ectopic morphologies (if multiple) and allowing qualitatively visual analysis of the different morphologies. Providing information regarding the onset and resolution of tachyarrhythmias and bradyarrhythmias. Providing information on a variety of arrhythmia characteristics such as coupling interval, rate dependence, and changes in QT interval. Ambulatory monitoring, typically with 24 to 48 hours of continuous ambulatory ECG, is useful for the evaluation of rate control of atrial fibrillation (AF) during daily activities or with exercise in patients who have permanent AF. Ambulatory monitoring may be used to assess AF burden (frequency or episodes and duration of episodes) in patients with paroxysmal or intermittent AF. (See "Control of ventricular rate in patients with atrial fibrillation who do not have heart failure: Pharmacologic therapy", section on 'Evaluation and goal ventricular rate'.) FOLLOW-UP AFTER AMBULATORY ECG MONITORING After the diagnostic monitoring period, the following scenarios of patient symptoms and ECG findings might be encountered: The patient had symptoms with corresponding ECG abnormalities (ie, arrhythmia or conduction abnormality) Such patients should be treated appropriately and, in most https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 13/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate instances, do not require additional ambulatory ECG monitoring for diagnostic purposes (although additional monitoring to assess the efficacy of therapy might be indicated). The patient had symptoms without corresponding ECG abnormalities (ie, arrhythmia or conduction abnormality) In such instances, assuming that the monitor was functioning properly and the available ECG data are interpretable, the diagnosis is likely non-cardiac and there is generally no need for additional ambulatory ECG monitoring. The patient had no symptoms and no ECG abnormalities during the monitoring period The decision to pursue additional testing following a period of ambulatory ECG monitoring depends on the potential severity of the clinical conditions. As examples: A patient who experiences only infrequent and mild palpitations who had an unremarkable four-week event monitor can usually be managed conservatively. Conversely, a patient with more severe symptoms might require more prolonged ambulatory ECG monitoring to make a diagnosis (such as with an insertable cardiac monitor [ICM]). For a patient with history of cryptogenic stroke who had an unremarkable four-week event monitor for whom a diagnosis of atrial fibrillation would prompt the need for stroke prevention therapy (ie, initiation of oral anticoagulation), prolonged ambulatory ECG monitoring should be instituted (such as with an ICM). A patient who has suffered syncope (particularly abrupt syncope without warning symptoms that resulted in injury) but who did not have recurrent symptoms or ECG abnormalities during the initial monitoring should usually proceed with prolonged ambulatory ECG monitoring (such as with an ICM). SOCIETY GUIDELINE LINKS Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Arrhythmias in adults" and "Society guideline links: Ventricular arrhythmias" and "Society guideline links: Atrial fibrillation" and "Society guideline links: Supraventricular arrhythmias".) SUMMARY AND RECOMMENDATIONS Background In contrast to the standard electrocardiogram (ECG), which provides a brief sample of cardiac electrical activity over 10 seconds, ambulatory ECG monitoring provides a https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 14/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate view of ECG data over an extended period of time, thereby permitting evaluation of dynamic and transient cardiac electrical phenomena. (See 'Introduction' above.) Indications Ambulatory ECG monitoring is the most widely employed technology to evaluate a patient with symptoms suggestive of a cardiac arrhythmia or a conduction abnormality (ie, unexplained syncope/near syncope or unexplained palpitations), but it is also used for a variety of other diagnostic and prognostic indications. (See 'Indications' above.) Types of monitors Ambulatory ECG monitoring is available in many forms ( table 2), including continuous (Holter) monitoring for 24 to 48 hours ( figure 1), event (loop) monitoring for several weeks, or insertable cardiac monitoring ( picture 2) for months to years. (See 'Types of ambulatory ECG monitoring' above.) The approach to choosing an ambulatory ECG monitoring strategy depends on the indication for the test along with the frequency and duration of symptoms ( table 1). If an ambulatory ECG monitor is being used for diagnostic purposes (ie, unexplained syncope or unexplained palpitations), extended duration monitoring might be required to increase the diagnostic yield. (See 'Diagnostic efficacy' above and 'Our approach to choosing an ambulatory ECG monitoring strategy' above.) Importance of symptom correlation The ECG rhythm recorded using any ambulatory ECG monitoring modality should be correlated with the simultaneous occurrence of suggestive symptoms. (See 'Diagnostic efficacy' above.) ACKNOWLEDGMENT The editorial staff at UpToDate acknowledge Philip Podrid, MD, who contributed to an earlier version of this topic review. Use of UpToDate is subject to the Terms of Use. REFERENCES 1. Boudoulas H, Schaal SF, Lewis RP, Robinson JL. Superiority of 24-hour outpatient monitoring over multi-stage exercise testing for the evaluation of syncope. J Electrocardiol 1979; 12:103. 2. Poblete PF, Kennedy HL, Caralis DG. Detection of ventricular ectopy in patients with coronary heart disease and normal subjects by exercise testing and ambulatory electrocardiography. Chest 1978; 74:402. https://www.uptodate.com/contents/ambulatory-ecg-monitoring/print 15/26 7/6/23, 10:40 AM Ambulatory ECG monitoring - UpToDate 3. Steinberg JS, Varma N, Cygankiewicz I, et al. 2017 ISHNE-HRS expert consensus statement on ambulatory ECG and external cardiac monitoring/telemetry. Heart Rhythm 2017; 14:e55. 4. Crawford MH, Bernstein SJ, Deedwania PC, et al. 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