AMBULATORY ARRHYTHMIA MONITORING CHOOSING THE RIGHT DEVICE
by Admin | Category Article | Aug 16th, 2016
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. 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.
Ambulatory Monitoring Technologies for the Assessment of Cardiac Rhythm Abnormalities
Several devices are currently available to remotely assess cardiac rhythm abnormalities in ambulatory patients. 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. 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. 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).
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 electrodes) 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. A built-in cellular phone allows transmission of data from the monitor to the central station when the patient is away from home.
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.
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.
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 heighten 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. Clinicians routinely monitor patients on antiarrhythmic drugs or after catheter or surgical AF procedures for arrhythmia recurrence. These recurrences are often asymptomatic, even in previously symptomatic patients, and may not be detected unless an aggressive monitoring strategy is undertaken. In addition, patients with cryptogenic stroke often undergo rhythm monitoring to identify asymptomatic AF as a potential cause.6Most 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 more than 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. Recent analyses have demonstrated that a device-detected AF duration of more than 24 hours is associated with a 3-fold increase in the risk of stroke and an AF duration of less than 5 hours may not carry an excess stroke risk. New studies will attempt to determine whether anticoagulation can be tailored to the duration of the AF recurrence in a given individual. 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.
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. 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.
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. 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. 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. 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.
