Role of endogenous opioids in syncope induced by head-up tilt test and its relationship with isoproterenol-dependent and isoproterenol-independent neurally-mediated syncope.

This study was designed to evaluate the role of endogenous opioids in neurally-mediated syncope. Head-up tilt test was performed on 35 patients with syncope of unknown origin. Plasma beta-endorphin was measured (1) at baseline, (2) at the end of tilt test or at time of syncope, (3) 15 min before isoproterenol-test, (4) at the end of the isoproterenol-test or at time of syncope.