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 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. | 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 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
