title: Elevated parasympathetic nerve tone in isoproterenol-induced neurally mediated syncope during head-up tilt testing.
abstract: To clarify the autonomic nerve mechanisms whereby isoproterenol induces vasovagal reaction, 32 patients with recurrent syncopal episodes of unknown origin underwent head-up tilt testing. Syncope was induced in 11 patients by a control tilt (control/+ group). The remaining 21 patients (control/- group) underwent the procedure during isoproterenol infusion. Syncope was induced in 13 (isoproterenol/+ group), but not in the remaining 8 patients (isoproterenol/- group). High-frequency component (HF) and low-frequency component (LF)/HF, which are indices of heart rate variability, were assessed for 2-min intervals at baseline (T1) during the initial phase (T2) and at the end (T3) of tilting in each test. The ratios of HF and LF/HF at T2/T1 and T3/T2 were calculated. The HF value at T3 in the control/+ group was greater than that in the control/- group (1.47+/-0.91 vs 0.75+/-0.34; p<0.05). The HF value at T3 in the isoproterenol/+ group was significantly greater than those in the isoproterenol/- group (1.19+/-1.04 vs 0.43+/-0.23; p<0.05). The HF value at T2 in the isoproterenol/+ tended to be greater than in the isoproterenol/- group (1.30+/-0.85 vs 0.66+/-0.53; p=0.07). The ratio of HF for T2/T1 in isoproterenol/+ was greater than in the other groups (2.04+/-1.26 in isoproterenol/+ vs 0.58+/-0.39 in control/-, 0.47+/-0.18 in control/+ and 0.71+/-0.45 in isoproterenol/-; p=0.0001), whereas that of HF for T3/T2 in the control/+ group was greater than in the other groups (1.48+/-0.62 in control/+ vs 0.97+/-0.33 in control/-; p<0.05, 0.79+/-0.35 in isoproterenol/- and 0.88+/-0.37 in isoproterenol/+; p<0.01). No difference was observed in either LF/HF or the ratio of LF/HF among the groups. The early and persistent increase in parasympathetic nerve tone provoked by isoproterenol plays an important role in enhancing the vasovagal reaction during head-up tilt testing.
authors.affiliation: First Department of Internal Medicine, Kobe University School of Medicine, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Takei
authors.forename: Y
authors.initials: Y
authors.lastname: Ohnishi
authors.forename: M
authors.initials: M
authors.lastname: Yokoyama
mesh_terms: D000293:Adolescent; D000318:Adrenergic beta-Agonists; D000328:Adult; D000344:Afferent Pathways / Q000503:physiopathology; D001794:Blood Pressure / Q000187:drug effects; D004452:Echocardiography; D004562:Electrocardiography; D015716:Electrocardiography, Ambulatory; D005260:Female; D006339:Heart Rate / Q000187:drug effects; D006801:Humans; D007545:Isoproterenol; D008297:Male; D008465:Mechanoreceptors / Q000502:physiology; D008875:Middle Aged; D010275:Parasympathetic Nervous System / Q000503:physiopathology*; D012008:Recurrence; D013564:Sympathetic Nervous System / Q000187:drug effects / Q000503:physiopathology; D019462:Syncope, Vasovagal / Q000139:chemically induced / Q000175:diagnosis / Q000000981:diagnostic imaging / Q000209:etiology / Q000503:physiopathology*; D018667:Tilt-Table Test; D014630:Vagus Nerve / Q000503:physiopathology
chemical_list: D000318:Adrenergic beta-Agonists; D007545:Isoproterenol