Chapter 021. Syncope (Part 6) Diagnostic Tests The choice of diagnostic tests should be guided by the history and the physical examination. Measurements of serum electrolytes, glucose, and the hematocrit are usually indicated. Cardiac enzymes should be evaluated if myocardial ischemia is suspected. Blood and urine toxicology screens may reveal the presence of alcohol or other drugs. In patients with possible adrenocortical insufficiency, plasma aldosterone and mineralocorticoid levels should be obtained.Although the surface electrocardiogram is unlikely to provide a definitive diagnosis, it may provide clues to the cause of syncope and should be performed in almost all patients. The presence of conduction abnormalities (PR prolongation and bundle branch block) suggests a bradyarrhythmia, whereas pathologic Q waves or prolongation of the QT interval suggests a ventricular tachyarrhythmia. Inpatients should undergo continuous electrocardiographic monitoring; outpatients should wear a Holter monitor for 24–48 h. Whenever possible, symptoms should be correlated with the occurrence of arrhythmias. Continuous electrocardiographic monitoring may establish the cause of syncope in as many as 15% of patients. Cardiac event monitors may be useful in patients with infrequent symptoms, particularly in patients with presyncope. An implantable event monitor may be necessary for patients with extremely infrequent episodes. The presence of a late potential on a signal-averaged electrocardiogram is associated with increased risk for ventricular tachyarrhythmias in patients with a prior myocardial infarction. Low-voltage (visually inapparent) T wave alternans is also associated with development of sustained ventricular arrhythmias. Invasive cardiac electrophysiologic testing provides diagnostic and prognostic information regarding sinus node function, AV conduction, and supraventricular and ventricular arrhythmias (Chaps. 225 and 226). Prolongation of the sinus node recovery time (>1500 ms) is a specific finding (85–100%) for diagnosis of sinus node dysfunction but has a low sensitivity; continuous electrocardiographic monitoring is usually more effective for diagnosing this abnormality. Prolongation of the HV interval and conduction block below the His bundle indicate that His-Purkinje disease may be responsible for syncope. Programmed stimulation for ventricular arrhythmias is most useful in patients who have experienced a myocardial infarction; the sensitivity and specificity of this technique is lower in patients with normal hearts or those with heart disease other than coronary artery disease. Upright tilt table testing is indicated for recurrent syncope, a single syncopal episode that caused injury, or a single syncopal event in a "high-risk" setting (pilot, commercial vehicle driver, etc.), whether or not there is a history of preexisting heart disease or prior vasovagal episodes. In susceptible patients, upright tilt at an angle between 60° and 80° for 30–60 min induces a vasovagal episode. The protocol can be shortened if upright tilt is combined with administration of drugs that cause venous pooling or increase adrenergic stimulation (isoproterenol, nitroglycerin, edrophonium, or adenosine). The sensitivity and specificity of tilt-table testing is difficult to ascertain because of the lack of validated criteria. Moreover, the reflexes responsible for vasovagal syncope can be elicited in most, if not all, individuals given the appropriate stimulus. The specificity of tilt- table testing has been reported to be near 90%, but it is lower when pharmacologic provocation is employed. The reported sensitivity of the test ranges between 20 and 74%, the variability due to differences in populations studied, techniques used, and the absence of a true "gold standard" against which to compare test results. The reproducibility (in a time ranging from several hours to weeks) is 80–90% for an initially positive response, but may be less for an initially negative response (ranging from 30 to 90%). A variety of other tests may be useful to determine the presence of structural heart disease that may cause syncope. The echocardiogram with Doppler examination detects valvular, myocardial, and pericardial abnormalities. The echocardiogram is the "gold standard" for the diagnosis of hypertrophic cardiomyopathy and atrial myxoma. Cardiac cine MRI provides an alternative noninvasive modality that may be useful for patients in whom diagnostic-quality echocardiographic images cannot be obtained. This test is also indicated for patients suspected of having arrhythmogenic right ventricular dysplasia or right ventricular outflow tract ventricular tachycardia. Both are associated with right ventricular structural abnormalities that are better visualized on MR imaging than by echocardiogram. Exercise testing may detect ischemia or exercise-induced arrhythmias. In some patients, cardiac catheterization may be necessary to diagnose the presence or severity of coronary artery disease or valvular abnormalities. Ultrafast CT scan, ventilation-perfusion scan, or pulmonary angiography is indicated in patients in whom syncope may be due to pulmonary embolus.In cases of possible cerebrovascular syncope, neuroimaging tests may be indicated, including Doppler ultrasound studies of the carotid and vertebrobasilar systems, MRI, magnetic resonance angiography, and x-ray angiography of the cerebral vasculature (Chap. 364). Electroencephalography is indicated if seizures are suspected.[newpage] . Chapter 021. Syncope (Part 6) Diagnostic Tests The choice of diagnostic tests should be guided by the history. establish the cause of syncope in as many as 15% of patients. Cardiac event monitors may be useful in patients with infrequent symptoms, particularly in patients with presyncope. An implantable. pulmonary angiography is indicated in patients in whom syncope may be due to pulmonary embolus.In cases of possible cerebrovascular syncope, neuroimaging tests may be indicated, including