Holter Monitoring

The Holter monitor is a portable battery-operated device that connects to the patient with bipolar electrodes and provides recordings from up to 12 electrocardiographic leads. Data are stored in the device using analog or digital storage media. The data are transformed into a digital format and are analyzed with interpretive software, with technologist and physician editing and reporting. Additional markers for patient-activated events and time correlates are included to allow greater diagnostic accuracy. Continuous electrocardiographic monitoring is possible for 24 h to a maximum of 72 h with conventional Holter monitors. Novel extended-duration continuous recording technologies will be discussed subsequently. Complete rhythm capture allows documentation of rhythm for symptomatic and asymptomatic events. Holter monitoring is useful if the clinical history is suggestive of an arrhythmic origin and the symptoms are frequent enough to be detected within the period of monitoring. Holter monitoring may also yield a rhythm profile to provide evidence of sinus node dysfunction or ambient arrhythmias that are potentially significant and to assess rate control of atrial fibrillation.

Holter monitoring has several drawbacks. Patients may not experience symptoms or cardiac arrhythmias during the usual Holter recording periods. In patients with syncope, the likelihood of another syncopal episode occurring during the monitoring period is the major limiting factor. Presyncope is a more common event during ambulatory monitoring but is less likely to be associated with an arrhythmia.1,2 The ubiquity of presyncope as a symptom in the community makes its usefulness as a surrogate for syncope relatively uncertain. The physical size of the device may hinder the ability of patients to sleep comfortably or to engage in activities that precipitate symptoms. Patients are further inconvenienced because the devices have to be removed while showering. Observations on Holter monitoring must be correlated with clinical context in the absence of symptoms. Significant variability is often noted in patient documentation of activated events such that accurate symptom–rhythm correlation is undermined.

It is not surprising that Holter monitoring has a low diagnostic yield. In several large series using 12 h or more of ambulatory monitoring for investigation of syncope, only 4% had recurrence of symptoms during monitoring.3,4 The overall diagnostic yield of ambulatory or Holter monitoring was 19%. These studies reported symptoms that were not associated with arrhythmias in 15% of cases. The causal relationship between arrhythmia and syncope was frequently uncertain. Uncommon asymptomatic arrhythmias such as prolonged sinus pauses, atrioventricular block (such as Mobitz type II block), and nonsustained ventricular tachycardia can provide important contributions to the diagnosis, instigating further investigations to rule out structural heart disease and other precipitating factors. Although these observations necessitate prompt attention, it is important to interpret the results in the clinical context of the syncopal presentation and to not unduly exclude common causes of syncope such as neurocardiogenic syncope.

It is also important to understand that a normal Holter monitor does not exclude an arrhythmic cause for syncope. In fact, an arrhythmic cause is typically the case. If the pretest probability is high for an arrhythmic cause, further investigations such as more prolonged monitoring or cardiac electrophysiological studies are required. In a study that evaluated extension of ambulatory Holter monitoring duration to 72 h,⁵ an increased number of asymptomatic arrhythmias were detected, but the overall diagnostic yield was not increased.

In our institution, we typically use Holter monitoring for 24 h. It is a noninvasive test that provides information to establish a rhythm profile in patients and the diagnosis in those with frequent symptoms. The more frequent the symptoms, the higher the diagnostic yield. The apparent modest yield of Holter monitoring presumably reflects primary care use of the device in patients with frequent symptoms, facilitating a symptom–rhythm correlation. This leads to selection bias in the referral population, leading to an apparent futility in referred patients who, by definition, have failed short-term monitoring.

Nonelectrode Event Recorders

Transtelephonic monitors are a form of noncontinuous ambulatory recording that is convenient for patient use. During symptomatic episodes, the patient activates the device, which then records electrocardiographic signals. The recorded event must be directly transmitted by an analog telephone line to a receiving center (Figures 64-2 and 64-3). The received signal is then converted to an analog recording that is displayed or printed as a single lead rhythm strip. The device has solid-state memory capacity, allowing recording and storage of electrocardiographic signals during symptoms. Electrocardiographic signals are collected prospectively for 1 to 2 minutes upon patient activation. The major disadvantages of such devices include the need for patient activation; missing asymptomatic arrhythmias, which requiring that the symptoms persist long enough for the device to record the event; and the inability to record events that surround the onset of symptoms.

New Technology

Extended Holter

Continuous ambulatory monitoring with data transmission to a central monitoring station staffed by Health Professionals (Cardionet, San Diego California) has emerged in the United States as a useful resource to extend traditional Holter monitoring beyond 48 h. This technique is typically used for 7 to 14 d, and has shown incremental benefit over a standard external loop recorder in diagnosing or excluding arrhythmia.⁷ This approach also provides the added layer of monitoring center involvement, which enhances responsiveness to changes seen during monitoring. Unfortunately, this substantially increases cost.

Long-term compliance can be challenging even with these devices because of electrode and skin-related problems and waning of patient motivation in the absence of recurrent symptoms.

External Loop Recorders

An external loop recorder continuously records and stores a single external modified limb lead electrogram with a 4- to 18-min memory buffer (see Figure 64-2). After the onset of spontaneous symptoms, the patient activates the device storing the previous 3 to 14 min and the following 1 to 4 min of recorded information. The captured rhythm strip subsequently can be uploaded and analyzed, often providing critical information regarding onset and termination of the arrhythmia (see Figure 64-2