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.