title: Vagal modulation of RR intervals during head-up tilt and the infusion of isoproterenol.
abstract: The objective of this study was to characterize the autonomic effects of 2 interventions, head-up tilt and isoproterenol infusion, which alter autonomic balance by different mechanisms but produce the same RR intervals. We compared the effect of head-up tilt with the effect of isoproterenol on autonomic balance as measured by power spectral analysis of RR variability. Fifteen normal subjects had baseline measurements and then underwent head-up tilt. After return to baseline supine values, isoproterenol was infused at a rate of 1 microgram/min (low-dose isoproterenol), which was then increased to 2.1 +/- 0.5 microgram/min (high-dose isoproterenol). All subjects underwent a second tilt during high-dose isoproterenol, and 9 subjects completed this second tilt study. During the experiment, normal RR intervals were recorded and 5-minute segments were used to calculate power spectra. High-frequency (HF) power (0.15 to 0.40 Hz) was used as a measure of vagal activity. The effects of head-up tilt were compared with the effects of low-dose isoproterenol. Despite nearly identical mean RR intervals (784 ms with tilt vs 792 ms with low-dose isoproterenol, p = NS), there was significantly (p < 0.05) less HF power during head-up tilt in the drug-free state (172 ms2) than during low-dose isoproterenol in the supine position (307 ms2). A second head-up tilt was performed during the infusion of high-dose isoproterenol. During high-dose isoproterenol, tilt caused a decrease in RR intervals (from 573 to 491 ms; p < 0.01) and a decrease in HF power (from 68 to 28 ms2; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Medicine, School of Public Health, Columbia University, New York, New York, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: Bloomfield
authors.forename: J T
authors.initials: JT
authors.lastname: Bigger
authors.forename: B B
authors.initials: BB
authors.lastname: Pavri
authors.forename: J
authors.initials: J
authors.lastname: Han
authors.forename: J L
authors.initials: JL
authors.lastname: Fleiss
authors.forename: L M
authors.initials: LM
authors.lastname: Rolnitzky
authors.forename: R C
authors.initials: RC
authors.lastname: Steinman
mesh_terms: D000328:Adult; D004562:Electrocardiography; D005260:Female; D005583:Fourier Analysis; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D007262:Infusions, Intravenous; D007545:Isoproterenol / Q000008:administration & dosage / Q000494:pharmacology*; D008297:Male; D011187:Posture; D016683:Supine Position; D018667:Tilt-Table Test; D014630:Vagus Nerve / Q000502:physiology*
chemical_list: D007545:Isoproterenol