14th International Congress
THE "NEW FRONTIERS"
OF ARRHYTHMIAS 2000

Jan. 29 - Feb. 5, 2000
Marilleva, Trento, Italy

S-108

Arrhythmia monitoring technologies: limitations and strengths

Peter Zimetbaum.
Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, USA

Holter’s contribution of the initial ambulatory arrhythmia monitoring device in 1957 has evolved to a highly sophisticated armementarium of tools for the outpatient monitoring of arrhythmia patients¹. In many instances our understanding of the optimal clinical utility for each device has lagged behind the development of technology. This review will detail the currently available technologies and provide guidelines for the application of these devices for common clinical problems.

The Holter monitor represents the prototype for the continuous ambulatory monitor. In its current form, this device records 24 to 48 hours of electrocardiographic data (Tab. I)². These devices are battery operated and record data onto a magnetic tape microcassette or compact disc. A series of 5 or 6 skin electrodes are placed on the patient to allow the acquisition of two bipolar leads of electrocardiographic data. The data is then converted to a digital format or in the case of newer devices is recorded directly into a digital form through the use of solid-state technology. The standard Holter monitor that records data onto a microcassette magnetic tape stores continuous electrical activity for the full duration of monitoring and allows complete analysis or “full disclosure” of the entire monitoring period. The analysis of this data is time consuming and sometimes marred by artifact related to difficulties with the tape and the acquisition of data in the analog form. Newer devices acquire data in a digital format and provide a higher fidelity of electrocardiographic signal. Devices with this solid state technology have a limited amount of storage capacity and require compression measures to select the data that will be stored. Newer digital devices have enhanced storage capacity that allows the advantages of full disclosure provided with analog devices coupled with the improved quality of signal provided by digital systems. New Holter devices have also been developed that allow greater than two and up to twelve reconstructed leads of electrocardiographic data. All of these devices are equipped with internal clocks and event markers that allow the patient to mark the time that a symptom is experienced.

The major limitation of continuous monitors is the short duration of monitoring that makes the diagnosis of episodic symptoms such as palpitations or syncope unlikely^2,3. Although these devices have event markers-patients frequently forget to press the button at the time they experience a symptom and therefore limit the specificity of these monitors. The relatively large size of these devices makes them cumbersome to wear and may hamper the ability of patients to perform activities that would normally trigger an arrhythmic event. Although newer digital monitors provide excellent electrocardiographic signals, the increased expense of these devices compared with the more conventional analog devices limits their popularity for general clinical purposes.

The major advantage of continuous monitors is the ability to document asymptomatic arrhythmias including those that occur while sleeping or arrhythmias in patients who are unable to reliably activate a monitor. These devices are also particularly helpful for assessing 24 hour rate control in patients with atrial fibrillation.

Continuous loop event recorders represent a more practical form of ambulatory electrocardiographic monitoring devices. These devices record continuously but do not save data unless the patient activates the monitor. These devices can be programmed to save upto 5 minutes of recorded data per event. The duration of saved data both prior to the time the device is activated as well as post activation can be programmed. The devices are the size of a standard beeper and may be clipped to a belt. Three electrodes are applied to the chest wall and one to two leads of data are acquired.

Event monitors are also available that are not worn continuously. These event monitors may be classified as post event or non looping devices. These devices range in size from a credit card to the size of standard continuous loop event recorders. These non looping event recorders are applied over the left chest at the time of a symptom. A third variety of non looping monitor is a wrist watch-like device that requires application of fingers from the opposite hand to complete the circuit. Compared with looping event recorders these non looping devices do not require the application of sticky electrodes but sacrifice the opportunity to record data immediately preceding (pre-event) the activation of the monitor. Event recorders, whether continuous looping devices or monitors applied at the time of an event have the important advantage of transtelephonic transmission of data. As with Holter monitors, newer event monitors are available that provide multiple (upto 12) reconstructed leads of data.

The major disadvantage of event recorders is the necessity of the patient to activate the monitor at the time of symptoms. Newer devices will likely be equipped with automatic triggers that will record data if the monitored rhythm falls outside of a prespecified heart rate. Ultimately, algorithms will also be available to help recognize more complex issues such as loss of RR regularity or P wave amplitude to aide in the diagnosis of atrial fibrillation. A second problem with these devices is the skin irritation that may be associated with wearing electrodes for an extended period of time.

A new form of event recorder has been developed that is implanted in the subcutaneous tissue⁴. This device functions like a standard continuous loop event recorder but allows upto fourteen months of monitoring. The newest form of this device contains automatic rate triggers but does not have transtelephonic monitoring capabilities. This device is particularly well suited for patients with infrequent symptoms and has proven more effective than other ambulatory monitoring devices for the diagnosis of syncope⁵.Another important indication for event recorders is the monitoring of recurrent arrhyhmias and drug toxicities. In the case of antiarrhythmic drug loading, patients transmit asymptomatic as well as symptomatic strips for a fixed period of time. The clinician is able to adjust medication to avoid conduction disturbances related to drugs.

Transtelephonic monitoring of pacemakers is currently standard practice⁶. Patients apply a transmitter over the pacemaker generator and transmit an electrocardiographic strip to a pacemaker clinic or monitoring service. Through a series of standardized maneuvers, the general function and remaining battery life can be determined. Current pacemakers and implantable cardioverter-defibrillators (ICDs) have very sophisticated monitoring capabilities called diagnostics. These diagnostics are provided through continuous rhythm monitoring and allow determination of important parameters such as percent of the day the patient is paced, trends and variance in heart rate and in the case of ICDs, logs as well as stored electrograms of tachyarrhythmias. Dual chamber pacemakers with mode switching capability are able to sense atrial tachyarrhythmias and switch to a single chamber pacing mode that will avoid tracking of the trial rhythm. Once the patient returns to an atrial rate in the normal range, the device switches back to a dual chamber tracking mode. Interrogation of the device will provide a record of the number and duration of mode switching episodes. In the case of atrial fibrillation, data provided from the mode switching log can help guide decisions regarding antiarrhythmic and anticoagulation medications.Many dual chamber defibrillators also have mode switching capability. For ICDs, the atrial lead provides the additional benefit of helping discriminate atrial from ventricular tachyarrhythmias. This added information has helped diminish the frequency of inappropriate shocks for supraventricular arrhythmias.

The most common reasons for clinicians to employ ambulatory arrhythmia monitoring devices are for the diagnosis of palpitations, presyncope or syncope⁷. These conditions are listed as Class 1 indications for ambulatory monitoring in the most recent ACC/AHA guidelines for ambulatory electrocardiography⁸.

Continuous loop event recorders provide an advantage over Holter monitors for the diagnosis of palpitations because they can be worn for longer periods of time and require the patient to activate them allowing a complete correlation between symptom and rhythm. There is now ample evidence that transtelephonic event monitors are more cost-effective than Holter monitors for palpitations and that two weeks of monitoring is sufficient to make a diagnosis in most patients^9,10.

Ambulatory monitoring devices remain very limited in their ability to diagnose syncope. Holter monitors provide the ability to record rhythms without patient activation but are limited by the short duration of available monitoring. Transtelephonic monitoring devices with pre and post event recording provide the advantage of long term monitoring but are limited by many patients’ inability to activate the device before or immediately following a syncopal episode. Implantable loop recorders allow for a more extended monitoring period and substantially increase the likelihood of diagnosis⁵. Ultimately, implantable monitors with preprogrammed upper and lower rate triggers should further increase the diagnostic yield of these devices for syncope.

The previously mentioned technologies are now utilized not only as diagnostic techniques but also as methods to identify patients at high risk for sudden death based on the presence of non-sustained ventricular tachycardia. Heart rate variability (HRV) and T wave alternans represent two new electrocardiographic technologies used to identify patients at risk for ventricular arrhythmias and sudden death. Heart rate variability refers to changes in RR intervals or heart rate that are modulated by changes in autonomic tone and reflect sympathovagal balance. Data for HRV analysis is generally obtained from 24 hour digitized Holter recordings and measured utilizing time or frequency domain methods. Efforts to standardize the conditions and methods employed for HRV analysis are being developed by the European Cardiology and North American Pacing and Electrophysiology Societies¹¹. Impaired or depressed HRV has been shown to be predictive of mortality in post myocardial infarction and congestive heart failure patients^12,13.

T wave alternans refers to an alternating shape of the T wave between successive complexes. The computerized analysis of microvolt changes in T wave amplitude in an alternating pattern is the basis for an important new electrocardiographic tool. The development of T wave alternans is favored by higher heart rates and the preferred method of analysis of T wave alternans involves the analysis of multiple beats (n=128) called the “spectral” method. An early study that evaluated T wave alternans in a patient population at risk for ventricular tachycardia and sudden death utilized a method of atrial pacing at a rate of 100 bpm¹⁴. In this study T wave alternans was equivalent to invasive electrophysiological testing at predicting arrhythmia-free survival. Newer methods of analysis allow for T wave alternans to be recorded in patients undergoing exercise treadmill testing¹⁵. Further studies will be necessary to determine the value of this technology in clinical practice (Fig. 1).

TABLE I – Ambulatory electrocardiographic monitoring devices

Fig. 1: Algorithm for the evaluation of syncope that is felt to be due to an arrhythmia.