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

Background and hypothesis: Previous research has failed to document tempora l 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 sy ncope, specifically focusing on epinephrine levels. Methods: The present study deals with temporal changes of circulatory catec holamine levels during head-up tilt tests (40 min, 80 degrees tilt) in 62 p atients with syncope of unknown origin, 7 of whom had syncope associated wi th exercise (exercise-induced group, 19 +/- 3 years). Data were compared wi th 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 neg ative (negative group, 46 +/- 19 years). Blood samples for circulatory cate cholamine assay were obtained from the antecubital vein in the baseline sup ine position 2 min after the tilt started, every 10 min during till, and at the time of the onset of symptoms or the end of tilt. Levels of norepine-p hrine and epinephrine were determined using the high-pressure liquid chroma tography (HPLC) method (pg/ml). Results: plasma norepinephrine levels among the four groups were similar at the supine position and during had testing. In contrast, patients in the e xorcise-induced group had significantly higher maximum epinephrine levels d uring head-up tilt resting 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 ti me than patients in the negative and control groups. Conclusions: A marked increase of epinephrine was observed during head-up t ilt testing in patients with neurally mediated syncope associated with exer cise. The present findings further accelerate the identification of the rol e of epinephrine in the mechanisms behind neurally mediated syncope associa ted with exercise.