Exercise testing is particularly important when catecholaminergic polymorphic ventricular tachycardia is a possibility. However, exertion-related symptoms and findings may not be reliably reproducible with exercise testing, and long-term electrocardiographic monitoring with external or implantable recorders may be necessary. 3. A 12-lead ECG may indicate the presence of structural heart disease such as prior MI or chamber enlargement that would increase the likelihood that a patient s symptoms might be due to VA, or it may provide evidence of the underlying substrate for documented VA. An ECG may also reveal evidence of inherited arrhythmia disorders, such as long QT syn- drome, Brugada syndrome, and arrhythmogenic right ventricular cardiomyopathy. In patients with structural heart disease, QRS duration and the presence of conduction abnormalities pro- vide prognostic information.S4.2.1-7 S4.2.1-14 Data on the use of microvolt T wave alternans and the signal averaged ECG are inconclusive, as such these tests are not routinely used in clinical ; the one exception is the potential use of signal averaged ECG in patients with arrhythmogenic right ventricular cardiomy- opathy (see Section 7.3). CLINICAL STATEMENTS AND GUIDELINES 4.2.2. Ambulatory Electrocardiography Recommendation for Ambulatory Electrocardiography Referenced studies that support the recommendation are summarized in Online Data Supplements 3 and 4. COR LOE Recommendation I B-NR Ambulatory electrocardiographic monitoring is useful to evaluate whether symptoms, including palpitations, presyncope, or syncope, are caused by VA.S4.2.2-1 S4.2.2-4 Recommendation-Specific Supportive Text 1. Ambulatory electrocardiographic monitoring is often used to assess the effectiveness of treatments to sup- press arrhythmias, but more robust data are needed on the clinical use of this practice. Continuous or intermittent ambulatory electrocardiographic recording with a Holter monitor or an event recorder is helpful in diagnosing suspected arrhythmias, estab- lishing their frequency, relating them to symptoms, and assessing the response to therapy. 