A Holter monitor records cardiac activity while a patient is ambulatory for at least a period

Remote cardiac telemetry was developed to allow home ECG monitoring of patients with suspected cardiac arrhythmias. It was first introduced by the American biophysicist Norman J. Holter (1914–1983) in the 1940s. The original Holter monitor was a 75-lb backpack with a reel-to-reel FM tape recorder, analog patient interface electronics, and large batteries.1,2 It could record a single ECG lead for several hours and provided the first opportunity to record and analyze ambulatory ECG data outside a standard hospital or outpatient care setting. The clinical need to monitor outpatients has resulted in advances in technology that now allow us to monitor heart rhythms remotely through a wide variety of devices, including ambulatory external monitors, implantable event recorders, pacemakers, and cardioverter-defibrillators.

The rapid expansion of ambulatory monitoring technologies affords the clinician the obvious diagnostic advantage of more comprehensive and real-time data. It also presents the challenge of creating systems to handle and pay for this increased information and the potential liability associated with a continuous data stream. This review focuses on the available technology and factors that guide the choice of monitor.

Indications for Monitoring

Traditionally, ambulatory monitoring has been used to determine the cause of palpitations and syncope and, to a lesser degree, to identify ventricular ectopy or nonsustained ventricular tachycardia in patients at potential risk for sudden cardiac death.3 Atrial fibrillation (AF) has become an increasingly important indication for ambulatory monitoring, predominantly as a tool to monitor the efficacy and safety of pharmacological and nonpharmacological therapies.4,5 It is also used to identify asymptomatic AF as a potential source of cryptogenic stroke.6,7

Ambulatory Monitoring Technologies for the Assessment of Cardiac Rhythm Abnormalities

Several devices are currently available to remotely assess cardiac rhythm abnormalities in ambulatory patients (Table 1). Devices can record cardiac rhythm continuously or intermittently and can be worn externally or implanted subcutaneously. Aside from the Holter monitor described above, most of the new devices transmit recordings to a centralized monitoring station via telephone by converting an ECG signal into an audio signal. The signal is converted back to ECG data at the monitoring station.

Continuous Monitors (Holters)

The current state of Holter technology uses smaller recorders (size, 70×95×20 mm; weight, ≈190 g) with flashcard technology to record and store data from 2 to 3 ECG leads attached to the patient's chest and collected continuously over 24 to 48 hours. Once the monitor is returned, the data are analyzed in digital format. To increase the correlation between detected heart rhythm abnormalities and symptoms, patients are asked to keep a diary of their symptoms. The recorders use patient-activated event markers (annotations) specified for the time of day. The major advantages of Holter monitoring are the ability to continuously record ECG data and the lack of need for patient participation in the transmission of data. The short duration of monitoring can be inadequate if symptoms are infrequent. Newer Holter monitors are now available with up to 2 weeks of recording capability. Limitations of Holter monitoring include frequent noncompliance with keeping a log of symptoms and using event markers, which significantly limits the diagnostic value of these devices. The absence of real-time data analysis can also be an important clinical limitation of these devices.

Intermittent External Patient- or Event-Activated Recorders

Intermittent patient- or event-activated recorders make up the largest category of devices. They are also referred to as event monitors. Continuous looping monitors are attached to the patient through chest electrodes or a wrist band and record (save) data only when activated by the patient. Some of these devices have automatic triggers that recognize slow, fast, or irregular heart rates. Once activated, data are stored for a programmable fixed amount of time before the activation (looping memory) and a period of time after the activation. These devices are also referred to as external loop recorders (ELRs). Another less sophisticated form of event monitor is the postevent recorder. These devices are not worn continuously (nonlooping) but instead are applied directly to the chest area once a symptom develops. Therefore, they have no memory to allow recording of the rhythm before the device is activated. Event monitors are generally used for 14- to 30-day monitoring periods. The data are transmitted transtelephonically to a central monitoring station and then uploaded to a personal computer for analysis.

The major advantage of these devices compared with a traditional Holter monitor is that they are small, allow ECG monitoring for longer time periods, and can provide nearly real-time data analysis when the patient transmits a recording in proximity to the symptomatic event. The limitations of these devices include the following: The patient has to be awake and coherent enough to activate the device unless automatic activation/trigger for cardiac pauses, tachyarrhythmias, and bradyarrhythmias are built into the monitor; in the case of continuously worn devices, a significant percentage of patients are noncompliant with continuous application of the device (mostly because lead irritation/poor skin contact during exercise); and both continuous and postevent recorders require a degree of technological sophistication to transmit the stored data transtelephonically to the central monitoring station. The technical equivalent of this skill is the ability to use an automatic bank teller machine.8 Gula et al8 showed that 84.5% of patients were able to perform a test transmission, but a successful recording and diagnostic transmission was performed by only 58.9% of patients. Patients living alone were much less likely to use an ELR effectively, and factors such as worry about/fear of symptoms and their impact on quality of life were associated with successful use of the device.8 A new form of this device has recently become available that allows automatic transmission of triggered events over the cellular network (no requirement for the patient to transmit the data).

For devices that are not worn continuously (postevent recorders such as wristbands or handheld devices that need to be applied to the chest at the time of symptoms), the initiation of the arrhythmia that may provide a clue to the arrhythmic mechanism is missed, and short arrhythmias that terminate before the device is applied will not be recorded.

Real-Time Continuous Cardiac Monitoring Systems

Real-time continuous attended cardiac monitoring systems represent the newest form of external ambulatory monitors developed to combine the benefits and to overcome the limitations of Holter monitors and standard ELRs. They are worn continuously and are similar in size to the standard ELR. They automatically record and transmit arrhythmic event data from ambulatory patients to an attended monitoring station. Data can also be recorded through patient-triggered activation. This technology is referred to as mobile or real-time cardiac telemetry systems (MCOT). With these devices, cardiac activity is continuously monitored by 3 chest electrodes (some systems use a chest belt with built-in nonadhesive electrodes9,10) that are attached to a pager-sized sensor. The sensor transmits collected data to a portable monitor that has a built-in cell phone and needs to be in proximity to the patient to receive signals. The monitor is equipped with software that analyzes the rhythm data continuously and automatically. If an arrhythmia is detected by an arrhythmia algorithm, the monitor automatically transmits recorded data transtelephonically (by wireless network or land phone line) to a central monitoring station for subsequent analysis. Any patient-activated data also are transmitted. Trained staff members at a monitoring station analyze live incoming patient data and contact the referring physician and patient according to predetermined criteria.10 A built-in cellular phone allows transmission of data from the monitor to the central station when the patient is away from home.

The key features of these devices include continuous real-time ECG monitoring for an extended period of time (up to 30 days) without the requirement of patient activation and transmission of data. The data are transmitted and analyzed immediately by technicians who can contact the patient and/or the physician if an urgent intervention is needed. In 1 randomized controlled trial with 266 patients comparing real-time telemetry devices with ELRs in patients with a high clinical suspicion for a malignant arrhythmia, history of syncope, presyncope, or severe infrequent palpitations and a nondiagnostic 24-hour Holter, 41.4% of patients in the real-time telemetry group had detection of a clinically significant arrhythmia compared with 14.6% in ELR group.11

Implantable Loop Recorders

Implantable loop recorders (ILRs) are subcutaneously implanted arrhythmia-monitoring devices. These leadless devices record a single-lead ECG signal through 2 electrodes within the device. The device can be triggered automatically or by patient activation via placement of an activator over the device. The newest generations of these devices allow remote transmission of data and have a battery life in excess of 24 months.

Remote Monitoring of Pacemakers and Implantable Cardioverter-Defibrillators

Pacemakers and implantable cardioverter-defibrillators (ICDs) can also be used as continuous monitoring devices (Table 2). Most dual-chamber pacemakers and ICDs have built-in algorithms to detect supraventricular arrhythmias. Once a supraventricular tachycardia is detected, there is a switch in mode (“mode switch”) from DDD to VVI to avoid rapid tracking of the supraventricular tachycardia. The mode switch continues until the atrial arrhythmia terminates, at which time the device switches back to the dual-chamber tracking mode. Through continuous rhythm monitoring, modern devices provide information about arrhythmia burden with a detailed arrhythmia log that contains the number, duration, and dates of arrhythmia episodes, as well as the maximal atrial and ventricular rates associated with these episodes. Many devices provide histograms with arrhythmia trends, which have the number of hours a day spent in atrial arrhythmia for the prior 6 months. It should be noted that there is no clearly established standard for the minimum atrial rate or duration that constitutes an episode of AF. When programmed appropriately, implanted devices can identify AF with >95% sensitivity and specificity.12

Dual-chamber ICDs have built-in arrhythmia discriminator algorithms to avoid the delivery of inappropriate ICD therapies for supraventricular tachycardia that could otherwise be mistaken for ventricular tachycardia on the basis of the ventricular rate alone. In addition, ICDs store detailed information about ventricular tachyarrhythmia episodes, including time of occurrence, ventricular rate and duration, stored electrograms, and whether device therapy such as antitachycardia pacing or a shock was needed to abort an episode. The electrogram records a few seconds before and after an episode. This allows detailed evaluation of each device therapy to determine its appropriateness and success.

The conventional method of obtaining the stored record of arrhythmia occurrence and treatment requires device interrogation in the physician's office. Newer technology allows Internet-based remote monitoring of devices to evaluate symptomatic and asymptomatic arrhythmias and delivered therapies.

How Should These Technologies Be Used?

With all the available technology, it can be difficult to choose the right device for a particular indication. The newest continuous telemetry devices provide the benefit of real-time, comprehensive data without requiring the patient to participate in the process of data transmission. Compared with Holters, these devices allow immediate transmission of information; compared with looping event recorders, they gather more information and allow remote data transfer while overcoming the technical challenges of data transmission. This large amount of real-time data affords a higher diagnostic yield than standard devices but places a potential burden on the clinician who must be available to review large amounts of information (eg, daily) at any time of the day or night. In addition, standard monitoring devices (including long-term Holters) and loop recorders are inexpensive compared with the newer generation of real-time telemetry devices (Table 1). Insertable loop recorders, which are significantly more expensive than other monitoring devices, are generally reserved for patients with infrequent symptoms.

The choice of a monitoring modality depends on the presenting symptom, symptom frequency, and degree of suspicion of a life-threatening arrhythmia (the Figure). A number of considerations are useful to guide the selection of these devices.

Figure. Algorithm for the evaluation of palpitations, syncope, and atrial fibrillation. AAD indicates antiarrhythmic drug; PVI, pulmonary vein isolation.

Is 48 Hours a Sufficient Period of Monitoring?

The optimal duration of monitoring largely depends on symptom frequency. In the evaluation of palpitations, patients who experience daily symptoms can be evaluated with a Holter monitor. More often, palpitations are sporadic and require slightly longer monitoring. In a study in which patients with palpitations were prescribed an event monitor, the highest diagnostic yield was within the first week, when 80% of patients transmitted at least 1 rhythm strip corresponding to their symptoms. During the next 3 weeks of a standard 1-month monitoring period, only an additional 3.9% of patients received a diagnosis, and no patients received a diagnosis after week 2.13 In studies directly comparing a Holter with 48-hour monitoring and a longer evaluation with a loop recorder, the diagnostic yield of a loop recorder was up to 83% compared with a diagnostic yield of ≈39% for Holter monitoring for patients with palpitations3,13–15 (Table 3). ILRs have been compared with “standard conventional therapy” (24-hour Holter, a 4-week period of an evaluation with a loop recorder, and an electrophysiology study) in patients presenting with infrequent but recurrent unexplained palpitations. In 1 study of patients with infrequent palpitations (≤1 episode per month lasting >1 minute), in the absence of severe heart disease, ILRs provided a diagnostic yield of 73% compared with 21% in the group evaluated with standard conventional therapy.16

Syncope, in contrast, typically requires a significantly longer monitoring period, and the diagnostic yield of ambulatory monitors of any sort is extremely limited17 (the Figure and Table 3). The value of arrhythmia monitoring for syncope is both to identify an arrhythmia as a cause for syncope and to document a syncopal event without a corresponding arrhythmia, thus suggesting a nonarrhythmic cause. In 1 study of ambulatory Holter monitoring, symptoms correlated with a documented arrhythmia in 4% of patients and occurred without an arrhythmia on a monitor in 17%.18 Increasing the duration of monitoring from 24 to 72 hours does not appear to increase the diagnostic yield for syncope.18

ELRs have been directly compared with Holter monitors for the diagnosis of syncope or presyncope in several small trials (Table 3). In 1 trial, the overall probability of obtaining a symptom-rhythm correlation was 56% (44 of 78) for loop recorders worn for 1 month versus 22% (12 of 55) for 48-hour Holter monitors.19 In another study of ELRs, the median time for recording a symptom-rhythm correlation was 16 days (mean, 17+13 days) for patients assigned a loop recorder as their first diagnostic strategy. Symptom-rhythm correlation was obtained in 87% of patients by 1 month of monitoring.18 In our experience, identifying a compelling diagnosis for syncope in a 1-month monitoring period is rare. ILRs, which allow a prolonged monitoring period, have been demonstrated to improve the diagnostic yield for syncope, up to 85% in some studies.20,21 One study using ILRs for syncope demonstrated a 75% rate of meaningful change in management based on the findings of the device over a monitoring period of 40+10 months.20 In addition, the use of ILRs in patients with infrequent syncope has been associated with a reduction in recurrent syncopal events and has been shown to be more cost-effective than other conventional approaches.22–25

Circumstances in which a 48-hour monitoring period is preferred include the assessment of rate control in patients with AF and the identification of chronotropic insufficiency in patients with suspected sinus node dysfunction. Ventricular ectopy in patients in whom these arrhythmias may indicate an increased risk for sudden death or left ventricular dysfunction (eg, hypertrophic or dilated cardiomyopathy, post–myocardial infarction patients with left ventricular dysfunction, surgically repaired complex congenital heart diseases with known long-term arrhythmic risk, long-QT syndrome, congenital complete atrioventricular block26) also can be identified or quantified by a 48-hour monitoring period, but a prolonged Holter duration may be reasonable for this indication. At present, there are no clear recommendations for the preferred monitor type or duration for the identification of ventricular ectopy in high-risk cohorts.

Is It Necessary to Detect Asymptomatic Arrhythmias?

In the vast majority of circumstances, ambulatory monitors are used to identify a direct correlation between symptoms and the presence or absence of an arrhythmia. As noted above, there are some circumstances in which the identification of asymptomatic arrhythmias such as the frequency and morphology of symptomatic and asymptomatic ventricular premature depolarizations can be of interest. Prolonged asymptomatic pauses can be a clue to the cause of syncope, but caution must be used in the interpretation of the significance of these rhythms. For instance, pauses while sleeping are often related to heightened vagal tone and do not constitute an indication for a pacemaker. There are no available data to suggest whether a short (48 hour) monitoring period or a longer monitoring period afforded by a continuous telemetry device or a long-term Holter is advantageous in these cases.

The identification of AF is one of the commonest indications for monitoring in which the documentation of asymptomatic rhythms is desired (the Figure). Clinicians routinely monitor patients on antiarrhythmic drugs or after catheter or surgical AF procedures for arrhythmia recurrence.4,27,28 These recurrences are often asymptomatic, even in previously symptomatic patients, and may not be detected unless an aggressive monitoring strategy is undertaken.4,27,28 In addition, patients with cryptogenic stroke often undergo rhythm monitoring to identify asymptomatic AF as a potential cause.6 Most clinicians choose a device with the ability to pick up asymptomatic episodes of AF. These include event recorders with AF triggers, continuous telemetry devices, 2-week Holters, or ILRs with automatic triggers. In 1 study comparing MCOTs with ELRs, asymptomatic AF or atrial flutter with ventricular rate >150 bpm was detected in 17.3% of patients monitored with MCOTs versus 8.7% of patients monitored with ELR when monitored up to 30 days.11 The optimal tool for and duration of monitoring of the postintervention AF patient have not been determined, but in our practice, we monitor patients at least twice a year for a 2-week period each time.

Implanted devices (pacemakers, ICDs, and inserted loop recorders) afford the greatest opportunity for continuous and comprehensive monitoring of AF. The detection of atrial high-rate episodes has been shown to correlate with an increased risk of stroke and death.12,29,30 Recent analyses have demonstrated that a device-detected AF duration of >24 hours is associated with a 3-fold increase in the risk of stroke and an AF duration of <5 hours may not carry an excess stroke risk.29,30 New studies will attempt to determine whether anticoagulation can be tailored to the duration of the AF recurrence in a given individual.31 Specifically, with the availability of short-acting anticoagulants that do not require loading and monitoring, it is conceivable that some patients can use implanted devices to allow the initiation and discontinuation of anticoagulation, depending on the frequency and duration of AF recurrence.

Is It Critical to Have Real-Time Access to the Transmitted Rhythms?

There is an intrinsic appeal to real-time access to potentially serious arrhythmias. This is particularly true for patients who are being monitored for syncope or while starting an antiarrhythmic drug with a potential risk of proarrhythmia. In these instances, rapid access to data could result in clinically significant management decisions, and devices with real-time data access are preferred. Conversely, patients being monitored for other indications such as rate control in AF or arrhythmia recurrence after AF ablation may not necessarily require a real-time device and can be monitored with a long-term Holter. Along with continuous availability of data comes the physician's responsibility to be available to receive and act on the information. This can present a burden and theoretically a potential liability for the treating physician. In practice, the monitoring companies have standard rules for recommending that the patient seek emergency treatment and allow physicians to tailor the criteria for which they wish to be notified for non–life-threatening arrhythmias.

Obstacles to Compliance With Ambulatory Monitoring Devices

Common areas of noncompliance with ambulatory monitoring include the unwillingness to wear a device continuously, intolerance of the electrodes because of rash, failure to activate a monitor in association with symptoms, and inability to transtelephonically download the information. In 1 study, only 53% of patients wore the device and provided recordings 5 days a month during the entire 6-month monitoring period.4 Failure to activate a device in association with symptoms is a significant problem with monitoring with Holter and standard event recorders without automatic triggers.13 In a study mentioned earlier using loop recorders to diagnose syncope, despite patient education and test transmissions, 23% of patients who had recurrence of their syncopal symptoms failed to activate their loop recorder properly.18

Cost and Technical Considerations With Device Selection

A important consideration in our selection of devices is cost. At present, Holter monitors of any duration, postevent recorders, and continuous loop event recorders are relatively inexpensive (Table 1). Real-time continuous cardiac monitors require a greater degree of technical support than standard devices, and the current cost of these devices is approximately twice that of standard ambulatory monitoring devices. Finally, ILRs, which should be reserved for patients with very infrequent events, cost approximately $4000. When used in selected patient populations with infrequent syncope, monitoring with ILRs has been shown to be more cost-effective than conventional methods (ELR, electrophysiological study, and tilt) for the diagnosis of unexplained syncope.32 Remote monitoring of ICDs offers the potential advantage of reduced office visits with a possibility of cost savings approaching $3000 per patient over the lifetime of the device.33,34 Ambulatory monitoring of any type is subject to technical limitations. New devices that use cellular data transmission are subject to the limitation of nonuniform cellular coverage. Signal artifact or dropout is common and can be related to skin preparation at the time of electrode placement for externally worn devices. In the case of subcutaneous devices, the electrogram signal may be inadequate to identify P waves, and T waves may be oversensed and counted as QRS complexes. Signal dropout can also occur with subcutaneous devices, resulting in an artifactual pause. Oversensing of the QRS complex by the atrial lead is also common with pacemakers and ICDs and is an important cause of spurious mode switch episodes (AF detection). As a result, it is important to examine the primary data before clinical decisions are made.

Recommended Approach to Device Selection

In patients with a diagnosis of palpitations, we select a standard continuous loop or postevent recorder because of the low cost and the ability to provide a direct symptom-rhythm correlation. If the patient is unable to manage the technical requirements of a standard loop recorder, we choose a real-time continuous telemetry device. In patients with syncope, we do not routinely use externally applied ambulatory monitoring devices unless symptoms occur relatively frequently. In these cases, we choose a real-time continuous telemetry device as a first choice to allow documentation of asymptomatic rhythms that may provide a clue to the cause of syncope. We choose an ILR for syncopal events that occur infrequently and are suggestive of an arrhythmic cause. In patients with AF being monitored for rate control, we select a standard short-term Holter device. For the identification of AF recurrence or AF as a possible cause of stroke, we favor a device that provides at least 2 weeks of monitoring and captures asymptomatic arrhythmias. At present, this includes real-time continuous telemetry devices, ELRs with AF triggers, and 2-week Holter devices. In most cases, we do not require real-time access to this information, so a 2-week Holter, which is likely to be relatively inexpensive, is an acceptable choice. In patients being monitored for possible antiarrhythmic drug toxicity, immediate access to data is important and a real-time continuous telemetry device is preferred. A standard continuous loop monitor with daily asymptomatic transmissions is a less expensive second choice. It is routine practice to remotely assess tachyarrhythmias resulting in ICD therapy. In addition to ICDs, newer-generation pacemakers will have the capability of remote monitoring, which may allow the adjustment of therapies like antiarrhythmic drugs and possibly anticoagulants based on AF burden.

Disclosures

Dr Zimetbaum is a consultant for Medtronic. Dr Goldman reports no conflicts.

Footnotes

References

• 1. Buckles D, Aguel F, Brockman R, Cheng J, Demian C, Ho C, Jensen D, Mallis E. Advances in ambulatory monitoring: regulatory considerations. J Electrocardiol. 2004; 37(suppl):65–67.CrossrefMedlineGoogle Scholar
• 2. Holter NJ. New method for heart studies: continuous electrocardiography of active subjects over long periods is now practical. Science. 1961; 134:1214–1220.CrossrefMedlineGoogle Scholar
• 3. Zimetbaum PJ, Josephson ME. The evolving role of ambulatory arrhythmia monitoring in general clinical practice. Ann Intern Med. 1999; 130:848–856.CrossrefMedlineGoogle Scholar
• 4. Vasamreddy CR, Dalal D, Dong J, Cheng A, Spragg D, Lamiy SZ, Maininger G, Henrikson CA, Marine JE, Berger R, Calkins H. Symptomatic and asymptomatic atrial fibrillation in patients undergoing radiofrequency catheter ablation. J Cardiovasc Electrophysiol. 2006; 17:134–139.CrossrefMedlineGoogle Scholar
• 5. Hauser TH, Pinto DS, Josephson ME, Zimetbaum P. Safety and feasibility of a clinical pathway for the outpatient initiation of antiarrhythmic medications in patients with atrial fibrillation or atrial flutter. Am J Cardiol. 2003; 91:1437–1441.CrossrefMedlineGoogle Scholar
• 6. Douen A, Pageau N, Medic S. Usefulness of cardiovascular investigation in stroke management: clinical relevance and economic implications. Stroke. 2007; 38:1956–1958.LinkGoogle Scholar
• 7. Liao J, Khalid Z, Scallan C, Morillo C, O'Donnell M. Non-invasive cardiac monitoring for detecting of paroxysmal atrial fibrillation or flutter after acute ischemic stroke: a systematic review. Stroke. 2007; 38:2935–2940.LinkGoogle Scholar
• 8. Gula LJ, Krahn AD, Massel D, Skanes A, Yee R, Klein GJ. External loop recorders: determinants of diagnostic yield in patients with syncope. Am Heart J. 2004; 147:644–648.CrossrefMedlineGoogle Scholar
• 9. Gottipaty VK, Khoury L, Beard JT, Hamson V. A novel real-time ambulatory cardiac monitoring system is user-friendly and effective for identifying cardiac arrhythmias: clinical studies [abstract]. Columbia, SC: Biowatch Medical, Inc; 2008.Google Scholar
• 10. Joshi AK, Kowey PR, Prystowsky EN, Benditt DG, Cannom DS, Pratt CM, McNamara A, Sangrigoli RM. First experience with a Mobile Cardiac Outpatient Telemetry (MCOT) system for the diagnosis and management of cardiac arrhythmia. Am J Cardiol. 2005; 95:878–881.CrossrefMedlineGoogle Scholar
• 11. Rothman SA, Laughlin JC, Seltzer J, Walia JS, Baman RI, Siouffi SY, Sangrigoli RM, Kowey PR. The diagnosis of cardiac arrhythmias: a prospective multi-center randomized study comparing mobile cardiac outpatient telemetry versus standard loop event monitoring. J Cardiovasc Electrophysiol. 2007; 18:241–247.CrossrefMedlineGoogle Scholar
• 12. Glotzer T, Hellkamp A, Zimmerman J, Sweeney M, Yee R, Marinchak R, Cook J, Paraschos A, Love J, Radoslovich G, Lee KL, Lamas GA. Atrial high rate episodes detected by pacemaker diagnostics predict death and stroke. Circulation. 2003; 107:1614–1619.LinkGoogle Scholar
• 13. Zimetbaum PJ, Kim KY, Josephson ME, Goldberger AL, Cohen DJ. Diagnostic yield and optimal duration of continuous-loop event monitoring for the diagnosis of palpitations. Ann Intern Med. 1998; 128:890–895.CrossrefMedlineGoogle Scholar
• 14. Kinlay S, Leitch JW, Neil A, Chapman BL, Hardy DB, Fletcher PJ. Cardiac event recorders yield more diagnoses and are more cost-effective than 48-hour Holter monitoring in patients with palpitations. Ann Intern Med. 1996; 124:16–20.CrossrefMedlineGoogle Scholar
• 15. Fogel RI, Evans JJ, Prystowsky EN. Utility and cost of event recorders in the diagnosis of palpitations, presyncope and syncope. Am J Cardiol. 1997; 79:207–208.CrossrefMedlineGoogle Scholar
• 16. Giada F, Gulizia M, Francese M, Croci F, Santangelo L, Santomauro M, Occhetta E, Menozzi C, Raviele A. Recurrent unexplained palpitations (RUP) study: comparison of implantable loop recorder versus conventional diagnostic strategy. J Am Coll Cardiol. 2007; 49:1951–1956.CrossrefMedlineGoogle Scholar
• 17. DiMarco JP, Philbrick JT. Use of ambulatory electrocardiographic (Holter) monitoring. Ann Intern Med. 1990; 113:53–68.CrossrefMedlineGoogle Scholar
• 18. Bass EB, Curtiss EI, Arena VC, Hanusa BH, Cecchetti A, Karpf M, Kapoor WN. The duration of Holter monitoring in patients with syncope: is 24 hours enough?Arch Intern Med. 1990; 150:1073–1078.CrossrefMedlineGoogle Scholar
• 19. Sivakumaran S, Krahn AD, Klein GJ, Finan J, Yee R, Renner S, Skanes AC. A prospective randomized comparison of loop recorders versus Holter monitors in patients with syncope or presyncope. Am J Med. 2003; 115:1–5.CrossrefMedlineGoogle Scholar
• 20. Krahn AD, Klein GJ, Norris C, Yee R. The etiology of syncope in patients with negative tile table and electrophysiological testing. Circulation. 1995; 92:1819–1824.LinkGoogle Scholar
• 21. Krahn AD, Klein GJ, Skanes AC, Yee R. Use of implantable loop recorder in evaluation of patients with unexplained syncope. J Cardiovasc Electrophysiol. 2003; 14(suppl):S70–S73.CrossrefMedlineGoogle Scholar
• 22. Krahn AD, Klein GJ, Yee R, Norris C. Final results from a pilot study with an implantable loop recorder to determine the etiology of syncope in patients with negative noninvasive and invasive testing. Am J Cardiol. 1998; 82:117–119.CrossrefMedlineGoogle Scholar
• 23. Farwell DJ, Freemantle N, Sulke N. Clinical impact of implantable loop recorders in patients with syncope. Eur Heart J. 2006; 27:351–356.CrossrefMedlineGoogle Scholar
• 24. Krahn AD, Klein GJ, Yee R, Manda V. The high cost of syncope: cost implications of a new insertable loop recorder in the investigation of recurrent syncope. Am Heart J. 1999; 135:870–877.CrossrefGoogle Scholar
• 25. Nierop PR, van Mechelen R, van Elsäcker A, Luijten RH, Elhendy A. Heart rhythm during syncope and presyncope: results of implantable loop recorders. Pacing Clin Electrophysiol. 2000; 23:1532–1538.CrossrefMedlineGoogle Scholar
• 26. Crawford MH, Bernstein SJ, Deedwania PC, DiMarco JP, Ferrick KJ, Garson A, Green LA, Greene HL, Silka MJ, Stone PH, Tracy CM, Gibbons RJ, Alpert JS, Eagle KA, Gardner TJ, Gregoratos G, Russell RO, Ryan TH, Smith SC. ACC/AHA guidelines for ambulatory electrocardiography: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol. 1999; 34:912–948.CrossrefMedlineGoogle Scholar
• 27. Hanke T, Charitros E, Stierle U, Karluss A, Kraatz E, Graf B, Hagemann A, Misfeld M, Sievers H. Twenty-four hour Holter monitor follow-up does not provide accurate heart rhythm status after surgical atrial fibrillation therapy: up to 12 months experience with a novel permanently implantable heart rhythm monitor device. Circulation. 2009; 120(suppl):S177–S184.LinkGoogle Scholar
• 28. Pontoppidan J, Nielsen JC, Poulsen SH, Hansen PS. Symptomatic and asymptomatic atrial fibrillation after pulmonary vein ablation and the impact on quality of life. Pacing Clin Electrophysiol. 2009; 32:717–726.CrossrefMedlineGoogle Scholar
• 29. Capucci A, Santini M, Padeletti L, Gulizia M, Botto G, Boriani G, Ricci R, Favale S, Zolezzi F, Belardino N, Molon G, Drago F, Villani G, Mazzini E, Vimercati M, Grammatico A. Monitored atrial fibrillation duration predicts arterial embolic events in patients suffering from bradycardia and atrial fibrillation implanted with anti tachycardia pacemakers. J Am Coll Cardiol. 2005; 146:1913–1920.CrossrefGoogle Scholar
• 30. Glotzer T, Daoud E, Wyse G, Singer D, Ezekowitz M, Hilker C, Miller C, Qi D, Ziegler P. The relationship between daily atrial tachyarrhythmia burden from implantable device diagnostics and stroke risk. Circ Arrhythmia Electrophysiol. 2009; 2:474–480.LinkGoogle Scholar
• 31. Ip J, Waldo A, Lip G, Rothwell P, Martin D, Barsohn M, Choucair W, Akar J, Wather M, Rohani P, Halperin J. Multicenter randomized study of anticoagulation guided by remote rhythm monitoring in patients with implantable cardioverter-defibrillator and CRT-D devices: rationale, design, and clinical characteristics of the initially enrolled cohort: the IMPACT Study. Am Heart J. 2009; 158:364–370.MedlineGoogle Scholar
• 32. Krahn AD, Klein GJ, Yee R, Hoch JS, Skanes AC. Cost implications of testing strategy in patients with syncope: randomized assessment of syncope trial. J Am Coll Cardiol. 2003; 42:502–504.CrossrefMedlineGoogle Scholar
• 33. Raatikainen M, Uusimaa P, van Ginneken MM, Jannsen JP, Linnaluoto M. Remote monitoring of implantable cardioverter defibrillator patients: a safe, time saving, and cost effective means for follow-up. Europace. 2008; 10:1145–1151.CrossrefMedlineGoogle Scholar
• 34. Varma N. Rationale and design of a prospective study of the efficacy of a remote monitoring system used in implantable cardioverter defibrillator follow up: the Lumos-T Reduces Routine Office Device Follow-Up Study (TRUST). Am Heart J. 2007; 154:1029–1034.CrossrefMedlineGoogle Scholar