1. What are the major indications for ambulatory electrocardiography monitoring?
Ambulatory electrocardiography monitoring (AECG) allows the noninvasive evaluation of a suspected arrhythmia during normal daily activities. It aids in the diagnosis, documentation of frequency, severity, and correlation of an arrhythmia with symptoms such as palpitations, lightheadedness, or overt syncope. AECG monitoring can be extremely helpful in excluding an arrhythmia as a cause for a patient’s symptoms if there is no associated event during monitoring. AECG can also be used to assess antiarrhythmic drug response in patients with defined arrhythmias. Occasionally AECG is also used in other situations. The current major indications for AECG monitoring, from the American College of Cardiology/American Heart Association (ACC/AHA), are given in Box 5-1.
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2. What are the different types of ambulatory ECG monitoring available?
The major types of ambulatory electrocardiography (ECG) monitoring include Holter monitors, event monitors, and implantable loop recorders (ILRs). The type and duration of monitoring is dependent on the frequency and severity of symptoms. Most modern devices have the capability for transtelephonic transmission of ECG data during or after a detected arrhythmia. Each system has advantages and disadvantages; selection must be tailored to the individual. With any system, however, patients must record in some fashion (e.g., diary) symptoms and activities during the monitored period. & A Holter monitor constantly monitors and records two to three channels of ECG data for
24 to 48 hours. It is ideal for patients with episodes that occur daily. & An event monitor constantly monitors two to three channels of ECG data for 30 to 60 days. However, it will only record events when the patient experiences a symptom and presses a button that triggers the event monitor to store ECG data 1 to 4 minutes before and 1 to 2 minutes after the event. Some event monitors will also store arrhythmias that are detected by the monitor itself, based on preprogrammed parameters. An event monitor is appropriate for patients with episodes that occur weekly or monthly. & An ILR is an invasive monitoring device allowing long-term monitoring and recording of a single ECG channel for over a year. It records events similar to an event monitor based on patient’s symptoms or automatically based on heart rate. It is best reserved for patients with more infrequent episodes occurring greater than 1 month apart from each other.
3. What is an implantable loop recorder?
An ILR is a surgically placed, long-term monitoring device used to record and identify potentially life-threatening arrhythmias. It is commonly placed subcutaneously below the left shoulder and continuously monitors bipolar electrocardiographic signals for up to 14 months. The patient may use a magnetic activator held over the device to trigger an event at the time of symptoms. In addition, the device automatically records episodes of bradycardia and tachycardia (Fig. 5-1). The device is then interrogated with an external programmer and recorded events reviewed in a similar manner to a permanent pacemaker. After a diagnosis is obtained, the device is surgically extracted. In patients with unexplained syncope, an ILR yields a diagnosis in more than 90% of patients after 1 year.
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4. When is Holter or event monitoring considered abnormal?
It is not uncommon to identify several arrhythmias that are not necessarily abnormal during ambulatory electrocardiographic monitoring. These include sinus bradycardia during rest or sleep, sinus arrhythmia with pauses less than 3 seconds, sinoatrial exit block, Wenckebach atrioventricular (AV) block (type I second-degree AV block), wandering atrial pacemaker, junctional escape complexes, and premature atrial or ventricular complexes.
Of concern are frequent and complex atrial and ventricular rhythm disturbances that are less commonly observed in normal subjects, including second-degree AV block type II, third-degree AV block, sinus pauses longer than 3 seconds, marked bradycardia during waking hours, and tachyarrhythmias. One of the most important factors for any documented arrhythmia is the correlation with symptoms. In some situations, even some ‘‘benign’’ rhythms may warrant treatment if there are associated symptoms.
5. How often are arrhythmias detected during ambulatory ECG monitoring? Approximately 25% to 50% of patients experience a complaint or symptom during a 24-hour recording. Of such symptoms, only 2% to 15% correlate with or are believed to be caused by arrhythmia. Approximately 35% of patients will log a symptom without a corresponding ECG abnormality. Extending the period of monitoring can increase the yield of identifying symptomatic events. In patients with presyncope or syncope, the incidence of symptomatic events can be increased to 50% at 3 days and to 75% at 5 to 21 days of ambulatory ECG monitoring.
6. How often are ventricular arrhythmias identified in apparently healthy subjects during ambulatory ECG monitoring?
Ventricular arrhythmias are found in 40% to 75% of normal persons as assessed by 24- to 48-hour Holter monitors. The incidence and frequency of ventricular ectopy increases with age, but this has no impact on long-term prognosis in apparently healthy subjects.
7. What is the role of ambulatory ECG monitoring in patients with known ischemic heart disease?
Although ejection fraction after myocardial infarction is one of the strongest predictors of survival, ambulatory ECG monitoring can be helpful in further risk stratification. Ventricular arrhythmias occur in 2% to 5% of patients after transmural infarction in long-term follow-up. In the post–myocardial infarction patient, the occurrence of frequent premature ventricular contractions (PVCs) (more than 10 per hour) and nonsustained ventricular tachycardia (VT) by 24-hour monitoring is associated with a 1.5- to 2.0-fold increase in death during the 2- to 5-year follow-up, independent of left ventricular (LV) function.
8. Can Holter monitors assist in the diagnosis of suspected ischemic heart disease?
Yes. Transient ST-segment depressions 0.1 mV or greater for more than 30 seconds are rare in normal subjects and correlate strongly with myocardial perfusion scans that show regional ischemia.
9. What have Holter monitors demonstrated about angina and its pattern of occurrence?
Holter monitoring has shown that the majority of ischemic episodes that occur during normal daily activities are silent (asymptomatic) and that symptomatic and silent episodes of ST-segment depression exhibit a circadian rhythm, with ischemic ST changes more common in the morning. Studies also have shown that nocturnal ST-segment changes are a strong indicator of significant coronary artery disease.
10. What is a signal-averaged ECG?
A signal-averaged ECG (SAECG) is a unique type of ECG initially developed to identify patients at risk for sudden cardiac death and complex ventricular arrhythmias. In patients susceptible to VT and ventricular fibrillation (VF), there can be slowing of electrical potentials through diseased myocardium, resulting in small, delayed electrical signals (late potentials) not easily visible on a normal ECG. Through the use of amplification and computerized signal averaging, these microvolt late potentials can be visualized on a SAECG and potentially assist in risk-stratifying patients susceptible to certain arrhythmias.
11. When should a signal-averaged ECG be considered?
The use of SAECG to identify late potentials and those post–myocardial infarction patients at greatest risk for sudden death has been extensively evaluated. Although an association exists between late potentials and increased risk of ventricular arrhythmias after myocardial infarction, the positive predictive value of SAECG is low. In the Coronary Artery Bypass Graft (CABG) Patch Trial, patients with an abnormal SAECG and depressed ejection fraction who were to undergo cardiac surgery were randomized to implantable cardioverter defibrillator (ICD) implantation or no implantation and then followed for an average of 32 months. The study could detect no benefit for ICD implantation in this patient population with abnormal SAECGs. A study of SAECG use in patients treated with reperfusion, mainly primary percutaneous coronary intervention (PCI), did not find SAECG to be a useful risk stratification tool in this patient population. In current practice, the test is rarely used for risk stratification.
12. What is microvolt T-wave alternans and does it predict outcomes in certain patients?
Microvolt T-wave alternans (MTWA) is a technique used to measure very small beat-to-beat variability in T-wave voltage not generally detectable on a standard ECG. Significant MTWA changes are associated with increased risk of sudden cardiac death and complex ventricular arrhythmias. The benefit of MTWA for risk stratification is greatest in patients with a history of coronary artery disease and reduced ejection fraction. A positive MTWA test predicts nearly a fourfold risk of ventricular arrhythmias compared with MTWA-negative patients. The ACC/AHA/ ESC guidelines on ventricular arrhythmias and prevention of sudden cardiac death state that it is reasonable to use T-wave alternans for improving the diagnosis and risk stratification of patients with ventricular arrhythmias or who are at risk for developing life-threatening ventricular arrhythmias (class IIa; level of evidence A).
13. Is heart-rate variability useful in certain patients?
Diminished heart-rate variability is an independent predictor of increased mortality after myocardial infarction and results from decreased beat-to-beat vagal modulation of heart rate. The predictive value of heart-rate variability is low after myocardial infarction. It is recommend by the ACC as a class IIb recommendation to assess risk for future events in asymptomatic patients who:
Are post–myocardial infarction with LV dysfunction
Have heart failure
Have idiopathic hypertrophic cardiomyopathy
