Levy S. Cardiology Division, University of Marseille, France. J Cardiovasc Pharmacol 1991;17 Suppl 6:S24-31. The diagnostic approach to cardiac arrhythmias should be logical and starts with the clinical history, which provides two types of information: (a) the presence of symptoms, and (b) the clinical context, including the presence of an underlying heart disease. Clinical history and examination are helpful in the choice of pertinent invasive or noninvasive tests. The tolerance of the arrhythmia is not helpful in determining the type of arrhythmia because ventricular tachycardia, for example, may be well tolerated or even asymptomatic. The electrocardiogram (ECG) in sinus rhythm may be suggestive of the origin or etiology of arrhythmia as the presence, for example, of the Wolff-Parkinson-White pattern. An essential step in the diagnostic approach to arrhythmia is the ECG documentation. Ambulatory Holter monitoring, radiotelemetry, intermittent recorders, exercise testing, and electrophysiological testing will help in this endeavor. The latter is particularly useful in paroxysmal circus movement tachycardias. Once the tachycardia is recorded, a number of clues, including the regularity of the RR interval and the width of the QRS complex, may facilitate the diagnosis. In tachycardias with wide QRS complexes, preexcitation has to be excluded. The first step is then to look for atrioventricular dissociation, which is diagnostic of ventricular tachycardia. Other diagnostic clues (QRS duration, axis deviation, QRS morphology) may be useful. In case of difficulty because of preexisting bundle branch block or aberrancy, esophageal, right atrial, or His bundle recordings are indicated. If the tachycardia is not well tolerated, prompt termination with electrical DC shock should be performed.