196 Role of endogenous adenosine during ATP-test in patients with syncope of unexplained origin

Abstract

For some authors, positive head-up tilt-test (HUT) and/or ATP-test could identify patients with vaso-vagal syncope. It has been shown that endogenous adenosine plays a role in the mechanism of syncope in patients with positive HUT. Indeed, patients with positive HUT have higher adenosine plasma levels (APL) than patients with negative HUT. Moreover, APL increase during HUT, only in patients with positive HUT. Adenosine triphosphate injection can induce in some patients with unexplained syncope a cardio-inhibitory but the role of endogenous adenosine in those patients is unknown. In this prospective study we studied APL before and during ATP-test. Twenty consecutive patients with syncope of unexplained origin were included (56.0 □ 23.1 y., 13 females). They underwent baseline APL measurement followed by ATP-test. Then APL was measured 1, 5 and 15 minutes after ATP injection. APL were determined by High Performance Liquid Chromatography method on venous blood samples obtained in resting patients. ATP-test consisted of the intravenous bolus injection of 20 mg ATP during continuous recording of the surface ECG. The test was considered positive when a ventricular pause of ≥ 6 sec. occurred, due either to complete atrioventricular block or sinoatrial block. Nine patients had positive ATP-test. There was no statistical difference between patients with positive and negative ATP-test at baseline, 1, 5 and 15 minutes after ATP injection: 0.32 □ 0.16 μmol/L vs 0.49 □ 0.61 μmol/L, 0.32 □ 0.16 μmol/L vs 0.37 □ 0.21 μmol/L, 0.29 □ 0.23 μmol/L vs 0.41 □ 0.26 μmol/L, 0.34 □ 0.20 μmol/L vs 0.28 □ 0.13 μmol/L, respectively. There was no variation of APL during ATP-test in patients with positive and negative ATP-test. Endogenous adenosine is not involved in the mechanism of ATP-test. These results suggest that ATP-test and HUT have different mechanism and could identify different groups of patients with syncope of unexplained origin.