The diagnostic value of ATP testing in patients with unexplained syncope.

A minority of patients with unexplained syncope has an increased susceptibility to adenosine triphosphate (ATP) injection. In these 'hypersensitive' patients, owing to its powerful cardiac and hypotensive effects, endogenous adenosine released under physiological and pathological conditions could trigger bradycardia and/or hypotension and cause syncope. This hypothesis still needs to be proven. However, there is some evidence that the ATP test identifies a group of patients with otherwise unexplained syncope with definite clinical features, absence of structural heart disease and benign prognosis. The mechanism of syncope is heterogeneous; indeed, in cases of electrocardiographic documentation of spontaneous syncope, either a long ventricular pause (mainly due to paroxysmal atrioventricular (AV) block) or no rhythm variations or even tachycardia were documented. ATP-positive patients have clinical features and mechanisms of syncope which are different from tilt-positive patients. Owing to its low positive predictive value, the ATP test is of little value in selecting treatment. A favourable outcome suggests a strategy of postponing treatment, in particular pacemaker therapy, until a definite diagnosis can be made by documenting a spontaneous syncopal relapse.