Significance of circulatory epinephrine levels in exercise-induced neurally mediated syncope

Previous research has failed to document temporal changes in epinephrine levels in patients with neurally mediated syncope associated with exercise. The purpose of this study was to investigate the role of circulatory catecholamines in exercise-induced neurally mediated syncope, specifically focusing on epinephrine levels.

The present study deals with temporal changes of circulatory catecholamine levels during head-up tilt tests (40 min, 80 degree tilt) in 62 patients with syncope of unknown origin, 7 of whom had syncope associated with exercise (exercise-induced group, 19+/-3 years). Data were compared with 10 control subjects (control group, 45+/-23 years). Of the 55 patients with syncope not associated with exercise, 32 tested positive for the head-up tilt tests (positive group, 31+/-16 years) and 23 patients tested negative (negative group, 46+/-19 years). Blood samples for circulatory catecholamine assay were obtained from the antecubital vein in the baseline supine position 2 min after the tilt started, every 10 min during tilt, and at the time of the onset of symptoms or the end of tilt. Levels of norepinephrine and epinephrine were determined using the high-pressure liquid chromatography (HPLC) method (pg/ml).

Plasma norepinephrine levels among the four groups were similar at the supine position and during tilt testing. In contrast, patients in the exercise-induced group had significantly higher maximum epinephrine levels during head-up tilt testing than the other three groups (288+/-191 vs. 148+/-117, 66+/-31, and 54+/-27 pg/ml, respectively, p < 0.05). Patients in the positive group had higher maximum epinephrine levels than those in the negative group (p <0.05). Also, patients in the exercise-induced group and those in the positive group had a significantly shorter tilt-testing time than patients in the negative and control groups.

A marked increase of epinephrine was observed during head-up tilt testing in patients with neurally mediated syncope associated with exercise. The present findings further accelerate the identification of the role of epinephrine in the mechanisms behind neurally mediated syncope associated with exercise.