The two common recording systems are tape-based and solid-state systems. The tape-based system records at an extremely slow tape speed, with two or three ECG amplifiers and an analogue tape recorder. Irregularities in the tape drive can create artifacts that simulate tachycardia or bradycardia. Tape-based systems have been replaced by solid-state recording systems. Solid-state recording systems digitize the incoming ECG signal from two or three channels and then store this data in the solid-state memory. The digital signal is assessed by intricate algorithms to determine whether the QRS complex is normal or represents an ectopic beat or part of a run of tachycardia. Such systems combine the advantages of the tape-based system, in which all ECG complexes are recorded and analyzed, with the advantages of a smaller, quieter, and easier-to-maintain solid-state system. Solid-state devices depend on efficient and accurate algorithms (4,5).

Intermittent recorders are capable of storing only a limited number of short, patient-activated ECG recordings (12) (Fig. 60.3). These devices are lightweight, and some are as small as a thick credit card. Their advantage over continuous ambulatory systems is that the ECG is more likely to be obtained while the patient is experiencing clinical symptoms, thus allowing a direct correlation between the patient’s symptoms and the ECG recorded at that instant (6,7,8). Current devices use solid-state technology that allows recording of 30 to 60 seconds of ECG for three to six occasions when the patient is feeling symptoms. The ECG is stored in the solid-state memory of the device and
transmitted by telephone to the base station at a time that is convenient for the patient.

Development of this technology led to memory-loop circuitry, which is a hybrid between the ambulatory ECG monitor and the solid-state event recorder. With the memory-loop recorder, electrodes are applied to the patient and the device is worn on a continuous basis. When the patient activates the device, a permanent recording of stored ECG data is obtained that includes periods from 1 to 4 minutes before and 30 to 60 seconds after the recording button is pressed. The data can then be transmitted by telephone at the patient’s convenience. This device has the disadvantage that it must be worn on a continuous basis but the clear advantage that it obtains ECG data recorded before the patient presses the button and thus documents the rhythm at the onset of the patient’s palpitations and the initiating event (9). Approximately 25% of patients who have recurrent syncope obtain diagnostically useful information using this device. Memory-loop recorders are limited by malfunction and by the inability of 20% of patients to properly use the device (10).

The implantable memory-loop recorder is placed subcutaneously in the left upper chest, allowing continuous ECG recording using a loop-based memory system capable of recording for up to 15 months (11). The device can be activated by the patient or set to automatically record an arrhythmia that is below or above a predetermined heart rate. The data are then downloaded into a computer for analysis and review. This system has the unique advantage of not being dependent on patient interaction, avoiding problems caused by patients who are unable to activate the device or place the recording leads. The device can be used for extended periods of time, as opposed to the standard memory-loop recorder, which is less practical after 3 to 4 weeks because of skin irritation from the patch electrodes. The implantable memory-loop recorder also allows the ECG to be recorded while the patient is exposed to water, when other systems cannot be used. A report of 85 patients who had syncope and negative ambulatory monitoring, tilt-table and electrophysiologic testing, showed that 58 patients (68%) had recurrent syncope at a mean of 10.5 months after implantation (12). The diagnosis of the syncopal mechanism included bradycardia in 18 patients and tachycardia in 3 patients; normal rhythm was recorded at the time of syncope in the remaining patients, suggesting a vasodepressor mechanism.

The most recent development in this technology involves continuous ECG recording in an ambulatory setting with real-time analysis of the recorded data 24 hours/day, 7 days/week. Serious arrhythmias detected by the device are transmitted to a base station and then to the analysis center. The patient’s physician is notified that a serious rhythm disturbance has occurred. This device is useful for home monitoring while medications are being started to control an arrhythmia, adjusted to control heart rate during atrial fibrillation, or detection of arrhythmia after myocardial infarction (13).

Selection of 24-hour continuous ambulatory versus intermittent ECG recording depends on the patient and the symptoms (14). Both recording systems are most useful when a patient has typical symptoms while the ECG is being recorded. The absence of any significant rhythm abnormality while symptoms are occurring is helpful in patient management because it eliminates arrhythmia as a possible cause of the symptoms.

Patients with infrequent symptoms are more likely to benefit from the intermittent or memory-loop recorder. The memory-loop recorder is most useful for patients who are expected to have symptoms within 4 weeks after they begin using the device and in whom symptoms are brief and not expected to last long enough to find, apply, and record with the intermittent recorder. The implantable memory-loop recorder is helpful for patients who have syncope or near-syncope of unknown etiology despite standard evaluation but who have spells too infrequently to justify a standard memory-loop recorder (15,16).

Ambulatory ECG monitoring in persons without apparent heart disease has identified a wide range of arrhythmias (Table 60.1) (17,18,19,20,21). The frequency of arrhythmias increases with age (22,23,24,25). In athletes (26,27), 24-hour ambulatory ECG monitoring, in comparison to controls, identified a significantly lower average minimum heart rate (38 vs. 45 beats per minute), more sinus pauses that were longer than 2 seconds (37% vs. 6%), more episodes of Mobitz I block (23% vs. 6%), and more junctional rhythm (20% vs. 0%), but no significant difference in the number of premature ventricular complexes (PVCs) (33% vs. 43%). The results of ambulatory ECG monitoring should be placed in context with the patient’s clinical condition and associated symptoms.

Patients who experience sudden loss of consciousness are best evaluated with techniques other than ambulatory monitoring because of the severity of their symptoms. Such patients may be admitted to the hospital for long-term ECG assessment and consideration for electrophysiologic testing, tilt-table testing, or exercise assessment to identify the cause of their spells. Patients with syncope who undergo ambulatory ECG monitoring experience lightheadedness, presyncope, or syncope during the recording 25% of the time (28,29). Approximately 2% to 15% are identified as having an associated cardiac arrhythmia when the symptoms occurred, and 35% have no associated ECG abnormality (21,29,30). Extending the recording to 3 days
increases the number of patients who experience symptoms to 50% (31).

The criteria for determining a significant arrhythmia in the absence of associated symptoms are difficult to establish. For example, many authors indicate that a sinus pause of 2.0 to 2.5 seconds is abnormal, but other authors have shown that sinus pauses of 3 seconds or longer may not cause symptoms and do not predict an adverse prognosis (32,33). Therefore, a sinus pause should be correlated with symptoms before intervention is considered.

Patients who have cardiogenic syncope and associated heart disease, cardiac arrhythmia on ambulatory ECG monitoring, or abnormal 12-lead ECG results should be considered for electrophysiologic testing. Patients without these abnormalities are generally considered for tilt-table testing. Thirty percent of patients with negative electrophysiologic and tilt-table studies continue to have syncope and may benefit from use of a memory-loop recorder. The implantable memory-loop recorder is uniquely beneficial in this group of patients.

In patients with paroxysmal atrial fibrillation, 24-hour ambulatory ECG monitoring can help to identify the onset of the arrhythmia and potential approaches to treatment including vagal-dependent atrial fibrillation (34) and atrial fibrillation caused by an episode of atrial flutter or supraventricular reentrant arrhythmia. The 24-hour ambulatory recording system is also helpful in assessing a patient’s rate control during atrial fibrillation. Patients may demonstrate adequate rate control while they are sedentary but experience a rapid heart rate and symptoms when they are active. Finally, 24-hour ambulatory monitoring may be helpful in identifying patients in whom a pulmonary vein focus is the cause of atrial fibrillation and would benefit from catheter ablation to isolate the pulmonary veins.

The role of complex ventricular ectopy in predicting mortality after myocardial infarction has been confirmed in several multicenter trials (35,36,37,38,39) (Table 60.2). A 1996 study by Statters and associates (38) reported that PVCs are an independent predictor of mortality after myocardial infarction in patients who undergo thrombolytic therapy. The positive predictive accuracy of PVCs in predicting cardiac mortality and arrhythmic events in patients receiving thrombolytic therapy was 19.4% and 25.8%, respectively, compared to 16% and 16%, respectively, for patients who did not receive thrombolytic therapy.

Studies of this nature emphasize the importance of PVC triggers associated with structural heart disease in the development of life-threatening ventricular arrhythmias after myocardial infarction (40). Risk factors that can be assessed by 24-hour ambulatory monitoring, including PVC frequency, abnormal autonomic tone assessed by reduced heart-rate variability, and ischemia monitoring of ST-segment changes, further delineate the dynamic relationship among PVC trigger, substrate, and modulating factors that can converge to create a life-threatening arrhythmic event (Fig. 60.4). Nonsustained ventricular tachycardia (NSVT) with reduced ejection fraction has been used to identify postinfarction patients who are at potential risk for a cardiac event and would benefit from further evaluation by means of electrophysiologic testing or placement of the implantable cardioverter-defibrillator (2).

Ventricular arrhythmias recorded by 24-hour ambulatory ECG monitoring independently predict subsequent death in patients with dilated cardiomyopathy (41,42). The use of angiotensin-converting-enzyme inhibitors has significantly increased survival in patients with moderately symptomatic congestive heart failure and reduced ventricular function. Nonetheless, approximately 25% to 50% of deaths in patients with congestive heart failure are related to cardiac arrhythmias, despite the use of angiotensin-converting-enzyme inhibitors (43,44).

In 1981, NSVT recorded during 24-hour ambulatory monitoring was reported to be associated with sudden death (45,46), with an 8% annual mortality rate in patients with hypertrophic cardiomyopathy. A subsequent study by Spirito and associates (47) reviewed 151 asymptomatic patients with hypertrophic cardiomyopathy in whom the finding of NSVT on ambulatory monitoring was associated with a relative risk of sudden death of 2.4 compared with patients without NSVT. Although the risk of sudden death was slightly higher in the presence of NSVT, these authors did not recommend that a finding of NSVT should lead to the initiation of antiarrhythmic therapy but should alert the physician that further evaluation of the patient is needed.

Twenty-four-hour ambulatory ECG monitoring is useful in assessing the response to antiarrhythmic therapy and the performance of a permanent pacemaker (48,49,50,51). Ambulatory monitoring is useful in patients with atrial fibrillation, atrial flutter, and atrial tachycardia to determine whether adequate rate control is achieved with atrioventricular (AV) nodal–blocking drugs during activity and periods of emotional stress while avoiding prolonged pauses during periods of inactivity or sleep. Questions of pacemaker function are often assessed with 24-hour ambulatory monitoring. Failure to sense or pace that is not easily detected during a brief interrogation of the pacemaker may be apparent during 24-hour monitoring.

Ambulatory ECG monitoring has been found to be helpful for assessing autonomic tone, through measurement of RR intervals and assessment of heart-rate variability (see Chapter 77) and for ascertaining the frequency of silent and symptomatic myocardial ischemia through ST-segment analysis (see Chapter 18). QT-interval prolongation is a marker of repolarization changes associated with malignant ventricular arrhythmia, termed torsades de pointes (52,53,54).