title: Sympathetic restraint of respiratory sinus arrhythmia: implications for vagal-cardiac tone assessment in humans.
abstract: Clinicians and experimentalists routinely estimate vagal-cardiac nerve traffic from respiratory sinus arrhythmia. However, evidence suggests that sympathetic mechanisms may also modulate respiratory sinus arrhythmia. Our study examined modulation of respiratory sinus arrhythmia by sympathetic outflow. We measured R-R interval spectral power in 10 volunteers that breathed sequentially at 13 frequencies, from 15 to 3 breaths/min, before and after beta-adrenergic blockade. We fitted changes of respiratory frequency R-R interval spectral power with a damped oscillator model: frequency-dependent oscillations with a resonant frequency, generated by driving forces and modified by damping influences. beta-Adrenergic blockade enhanced respiratory sinus arrhythmia at all frequencies (at some, fourfold). The damped oscillator model fit experimental data well (39 of 40 ramps; r = 0.86 +/- 0.02). beta-Adrenergic blockade increased respiratory sinus arrhythmia by amplifying respiration-related driving forces (P < 0.05), without altering resonant frequency or damping influences. Both spectral power data and the damped oscillator model indicate that cardiac sympathetic outflow markedly reduces heart period oscillations at all frequencies. This challenges the notion that respiratory sinus arrhythmia is mediated simply by vagal-cardiac nerve activity. These results have important implications for clinical and experimental estimation of human vagal cardiac tone.
authors.affiliation: Department of Internal Medicine, Hunter Holmes McGuire Department of Veterans Affairs Medical Center, Richmond 23249, USA. ataylor@mail.hrca.harvard.edu
authors.forename: J A
authors.initials: JA
authors.lastname: Taylor
authors.forename: C W
authors.initials: CW
authors.lastname: Myers
authors.forename: J R
authors.initials: JR
authors.lastname: Halliwill
authors.forename: H
authors.initials: H
authors.lastname: Seidel
authors.forename: D L
authors.initials: DL
authors.lastname: Eckberg
mesh_terms: D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000328:Adult; D001146:Arrhythmia, Sinus / Q000503:physiopathology / Q000517:prevention & control; D001683:Biological Clocks / Q000187:drug effects / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D002245:Carbon Dioxide / Q000378:metabolism; D006328:Cardiac Catheterization; D005260:Female; D005583:Fourier Analysis; D006339:Heart Rate / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D008297:Male; D008955:Models, Cardiovascular; D018727:Muscarinic Antagonists / Q000494:pharmacology; D012016:Reference Values; D015203:Reproducibility of Results; D012119:Respiration; D012849:Sinoatrial Node / Q000187:drug effects; D013564:Sympathetic Nervous System / Q000187:drug effects / Q000502:physiology*; D013990:Tidal Volume / Q000502:physiology; D018667:Tilt-Table Test; D014630:Vagus Nerve / Q000502:physiology*
chemical_list: D000319:Adrenergic beta-Antagonists; D018727:Muscarinic Antagonists; D002245:Carbon Dioxide