Which of the following tests might be ordered for a patient with a dysrhythmia?

Curr Cardiol Rev. 2013 Nov; 9[4]: 299–307.

Abstract

Elucidating the cause of syncope is often a diagnostic challenge. At present, there is a myriad of ambulatory cardiac monitoring modalities available for recording cardiac rhythm during spontaneous symptoms. We provide a comprehensive review of these devices and discuss strategies on how to reach the elusive diagnosis based on current evidencebased recommendations.

Keywords: Syncope, loop recorder, holter monitor, cardiac monitor.

INTRODUCTION

Syncope is defined as a transient loss of consciousness secondary to transient global reduction in cerebral blood flow characterized by rapid onset, short duration and spontaneous complete recovery [1]. Syncope is a frequently encountered clinical conundrum with an estimated lifetime prevalence of up to 35% [2]. Syncope accounts for up to 3% of emergency department consultations and 6% of hospital admissions [3-5]. Although the diagnosis may be evident in a minor number of classic presentations, delineating the underlying cause of unexplained syncope can pose a clinical challenge, which is difficult yet worthwhile, as identification of underlying cardiac disease in patients with syncope is associated with higher rates of mortality and morbidity [6]. The most important aspect of the diagnostic challenge is to obtain a comprehensive history and physical examination [7, 8]. The ideal but often unattainable test for elucidating a cause is obtaining comprehensive physiologic data during spontaneous symptoms. Short of that goal, establishing an accurate symptom–rhythm correlation can often provide a diagnosis, making ambulatory outpatient monitoring a powerful diagnostic tool for the evaluation of cardiac arrhythmias. Evolving technologies have provided a wide array of monitoring options for patients suspected of having cardiac arrhythmias, with each modality differing in duration of monitoring, quality of recording, convenience, and invasiveness.

HOLTER MONITORING

A standard ECG should be ordered for all patients with syncope [3-9]. Short term ECG monitoring via 3 or, in some cases, 12 surface electrodes is the most common initial investigation in patients who present with syncope or palpitations. Typically this occurs in the emergency room or primary care setting with telemetry and continuous monitoring. More recently, however, wireless telemetry offers the possibility of reviewing continuous ECG recordings instantaneously at particular access points [10].

The findings on ECG monitoring must be correlated with symptoms, as heart rate, and even cardiac rhythm, is often uninformative in the absence of clinical correlation. Presyncope is a more common event during ambulatory monitoring but is less likely to be associated with an arrhythmia [11, 12]. Additionally, the ubiquitous nature of presyncope makes it a relatively poor surrogate for the assessment of syncope.

The Holter monitor is a portable battery-operated device that connects to the patient using bipolar electrodes, providing recordings from up to 12 ECG leads. Data are stored in the device using analog or digital storage media. The data are transformed into a digital format and analyzed using interpretive software. Additional markers for patient-activated events and time correlates are included, along with a patient event diary, to allow greater diagnostic accuracy. Continuous ECG monitoring is possible for a maximum of 72 hours [See Fig. ​1]. This allows the documentation of cardiac rhythm during symptomatic and/or asymptomatic events. Holter monitoring is useful if the clinical history is suggestive of an arrhythmic etiology and the symptoms are frequent enough to be detected within the period of monitoring.

Holter Monitor. The recording device is worn by the patient using a shoulder strap or belt loop, attached to 3-5 skin electrodes for continuous monitoring. An event button [not shown] at the top of the housing of the device is pressed in the event of symptoms to mark the recording. See text for discussion.

There are, however, a number of disadvantages to Holter monitoring. The major limitation is that patients may not experience symptoms or cardiac arrhythmias during the recording period. The physical size of the device may impair the ability of patients to sleep comfortably or engage in activities that precipitate or reproduce symptoms. Patients are further inconvenienced because the device has to be removed while bathing. There is also considerable variability in patient documentation and recollection of activated events, thereby compromising accurate symptom-rhythm correlation.

It is therefore not surprising that Holter monitoring has a low diagnostic yield. In several large series of patients undergoing 12 or more hours of ambulatory monitoring for investigation of syncope, only 4% had recurrence of symptoms during monitoring [9, 13, 14]. The overall diagnostic yield of Holter monitoring was 19%. Uncommon asymptomatic arrhythmias such as prolonged sinus pauses, atrio-ventricular block [such as Mobitz type II block], and non-sustained ventricular tachycardia can provide important clues to the diagnosis, often leading to further investigations to rule out structural heart disease and other precipitating factors. While these observations require prompt attention, it is important to interpret the results in the clinical context of the syncopal presentation so that common causes of syncope, such as neurocardiogenic syncope, are not unduly excluded. An example would be nocturnal pauses in a patient with sleep apnea, easily mistaken for intrinsic sinus node disease as a cause of syncope.

It is also important to recognize that normal ambulatory ECG monitoring does not exclude an arrhythmic cause for syncope. Risk stratification scores such as the EGSYS score can be used to estimate the pre-test probability for cardiac syncope [15]. If the pre-test probability is high for an arrhythmic cause, then further investigations such as prolonged monitoring or cardiac electrophysiological studies are required. In a study which evaluated extension of Holter monitoring duration to 72 h [13], there was an increase in the number of asymptomatic arrhythmias detected, but not the overall diagnostic yield.

EXTERNAL EVENT RECORDERS

External event recorders are external devices attached to patients via one to three electrodes with the ability to provide a longer period of monitoring than the standard Holter monitor. They may be patient activated or triggered automatically. The 3 main types of external event recorders are transtelephonic monitors, external cardiac loop recorders and mobile automated cardiac outpatient telemetry [MCOT] monitors.

Transtelephonic ECG monitors are recording devices that transmit data via an analog phone line to a base station [Fig. ​2]. The signal is then converted to an interpretable recording that is displayed or printed as a single lead rhythm strip. The ECG signals are collected on a real-time 1-2 minute loop.

Transtelephonic Monitors. The device is lightweight and portable. Four recording electrodes are present on the back of the device to permit single lead rhythm strip capture. A record button [top left] is pressed at the onset of symptoms, and the recorded event is transmitted to a base station over an analog phone line.

An external cardiac loop recorder continuously records and stores an external single modified limb lead electrogram with a 4-18 minute memory buffer [Fig. ​3, left]. After the onset of spontaneous symptoms the patient activates the device, which stores the previous 3-14 minutes, and the following 1-4 minutes, of recorded information. The captured rhythm strip can subsequently be uploaded and analyzed [Fig. ​4] and often provides critical information regarding the onset of the arrhythmia. This system can be used for weeks to months provided weekly battery changes are performed. The recording device is attached with two leads to the patient’s chest wall and needs to be removed for bathing, and can be uncomfortable during sleep. To allow detection of asymptomatic arrhythmias, external loop recorders with an automatic trigger algorithm have been introduced.

Loop Recorders. An external loop recorder [left] with cables that attach to the patient. The record button is pressed in the event of symptoms to store the previous 9 minutes, and the ensuing minute. The phone receiver is also placed over this button to transmit data over an analog phone line. An implantable cardiac monitor [center] and patient activator [right]. The patient activator is used to “freeze” symptomatic events that are retrieved with a pacemaker programmer. Automatic events can also be captured [see text for discussion].

External Loop Recorder Tracing. Sinus rhythm during presyncope is recorded in a 43-year-old female with recurrent unexplained syncope and presyncope. The fluctuation in heart rate is suggestive of neurocardiogenic syncope.

MCOT is the most recent advancement in external ambulatory arrhythmia monitoring [16]. Patients wear two to three chest leads attached to a portable sensor that continuously records rhythm strips and transmits the ECG data of pre-specified arrhythmias in real-time to a communication hub at the patient’s home. If the algorithms in the hub detect a significant arrhythmia in keeping with previously designated physician thresholds or if the patient activates the sensor to report symptoms, the monitor automatically transmits the patient’s ECG data to the central station using wireless communications. The data may be screened 24 hours a day by central monitoring station technicians, with potential immediate or deferred referral to the attending physician for evaluation of symptoms, rate and/or rhythm changes. The major drawback of this modality is patient compliance to wearing the device.

Linzer et al. reported the use of patient-activated loop recorders in 57 patients with syncope and non-diagnostic findings on history, physical examination and 24 hour Holter monitoring [17, 18]. A diagnosis was obtained in 14 of 32 patients who had recurrence of symptoms. In the remaining 18 patients, device malfunction, patient non-compliance or inability to activate the recorder was responsible for the lack of diagnosis. Other studies have also reported similar findings [18, 19] and demonstrated that loop recorders are complementary to 24 hour ambulatory electrocardiographic monitoring. The diagnostic yield for external loop recorders in these three studies [17-19] ranged from 24%-47%, with highest yield in patients with palpitations.

A prospective randomized clinical trial compared the utility of external loop recorders to conventional Holter monitoring in a community based referral population with syncope and presyncope [20]. Not surprisingly, the ability to obtain a symptom-rhythm correlation was 22% for Holter monitoring and 56% for the external loop recorder [p < 0.001], with duration of monitoring of 48 hours and 4 weeks, respectively. A higher diagnostic yield was also obtained among patients randomized to Holter monitoring who remained undiagnosed and crossed-over to use of a loop recorder. This trial suggests that loop recorders should be considered as first line monitoring when attempting to establish a symptom rhythm correlation in the initial workup of patients with syncope, unless symptoms are very frequent, or a rhythm sample of 24-48 hours is sought. Twenty-four percent of loop recorder patients failed to activate the device properly, suggesting limited usefulness in some patients [20]. Analysis of factors pertaining to use of external loop recorders has revealed a particularly low diagnostic yield among patients who are unfamiliar with technology, live alone, or have low motivation for achieving a diagnosis [21]. Reiffel et al. [22] retrospectively compared the results obtained by Holter monitoring, loop recording and auto-triggered loop recording in 600 patients from a database of approximately 100,000 patients. The auto-triggered loop recording approach provided a higher yield of diagnostic events [36%] compared to loop recording [17%] and Holter monitoring [6.2%].

External event recorders appear to have the greatest role in motivated patients with frequent spontaneous symptoms that are likely to recur within 4-6 weeks. Given that they are non-invasive and cost effective, they should be considered in all patients in whom an arrhythmic cause for syncope is suspected, keeping in mind that long-term compliance with these devices can be challenging because of electrode and skin-related problems and waning of patient motivation in the absence of recurrent symptoms.

IMPLANTABLE CARDIAC MONITORS

The implantable cardiac monitor [ICM] has become the investigative tool of choice in recurrent unexplained syncope following negative initial investigations. The ICM permits prolonged monitoring without external electrodes and is ideally suited to patients with infrequent recurrent syncope thought to be due to an arrhythmic cause. Similar to the external event recorders, it is designed to correlate physiology with recorded cardiac rhythms, but is implanted and therefore devoid of surface electrodes and accompanying compliance issues. The ICM also allows for monitoring over much longer time periods than an external event recorder. Commonly available ICMs include the Medtronic Reveal® and the St Jude Medical ConfirmTM series. A typical ICM [Medtronic Reveal DX Model 9528] has a pair of sensing electrodes with 4-cm spacing on a small elongated recording device 6.2 cm long, 1.9 cm wide, and 0.8 cm thick, weighing 15 g [Fig. ​3, center]. The projected battery longevity is 36 months. The device can be implanted subcutaneously in the left chest wall with local anesthesia and antibiotic prophylaxis.

Prior to implantation, cutaneous mapping should be performed to optimize the sensed signal and avoid T-wave over-sensing, which can be falsely interpreted as a high rate episode. An adequate signal can usually be obtained anywhere in the left hemithorax [23]. Grubb et al. [24] described an anatomic-based approach to ICM placement in 63 patients that did not require cutaneous mapping. Each underwent implantation of ILR in the left upper chest area midway between the supraclavicular notch and left breast area. In all patients, adequate electrocardiographic tracings were obtained at implant without need for preoperative cutaneous mapping. The mean P wave amplitude was 0.12 ± 0.20 mV at implant and at follow-up [6-14 months post-implant], the amplitude was 0.11 ± 0.19 mV. The peak-to-peak QRS amplitude was 0.48 ± 0.15 mV at implant and 0.44 ± 0.16 mV at a follow-up of 6-14 months. This strategy has not been validated.

The recorded bipolar signal is stored in the device as 42 minutes of compressed signal. A compressed signal maximizes memory capability with only marginal loss of quality. The patient, along with a spouse, family member or friend is instructed in the use of the activator at the time of implant. Once an episode is recorded [i.e. a presyncopal or syncopal event occurs] the memory is “frozen” by the patient or a relative using a non-magnetic hand held activator [Fig. ​3, right]. The episode is then uploaded for interrogation to a pacemaker programmer. Although heart rate is usually easily ascertained, p waves can occasionally be challenging to interpret. The most recent version of the ICM has programmable automatic detection of tachycardia-bradycardia arrhythmias, pauses and allows for comprehensive remote monitoring without an office visit. The Medtronic CareLink® Home Monitor allows patients to transmit data from their Medtronic Reveal® ICMs over a standard phone line for review by their physicians. The St Jude Medical ConfirmTM ICMs also has transtelephonic monitoring capability, enabling transmission of timely and accurate data. These features enhance the utility of ICMs, especially if patients have frequent saved events or live in remote areas where travel to a dedicated clinic is time consuming and costly.

A classification system for recorded events has been proposed by Brignole et al. [25] [Table ​1] that categorizes the probable mechanism of syncope according to the pattern of bradycardia recorded during spontaneous syncope. An example of the cardioinhibitory component of neurocardiogenic or vasovagal syncope is illustrated in [Fig. ​5]. This would be considered a 1A response. Figure 6 illustrates a primary bradycardia [1C response], highly suggestive of intrinsic AV node disease. This classification is useful for research purposes for event classification, and is useful in directing therapy once validated.

Automatic Event Detection from an ICM. This is a typical tracing of an event captured by an ICM during syncope in a patient. The arrow and letter A denotes automatic activation when the device detects a 3 second pause. Each line constitutes 10 seconds of a single lead rhythm strip. Note the slowing of the sinus rate prior to onset of a prolonged pause, which resulted in syncope. This is consistent with the diagnosis of neurocardiogenic syncope [ISSUE classification 1A].

Table 1.

ISSUE classification of detected rhythm from the ICM.