Vagal modulation of RR intervals during head-up tilt and the infusion of isoproterenol

• Published in: The American journal of cardiology Vol. 75; no. 16; pp. 1145 - 1150
• Main Authors: Bloomfield, Daniel M., Bigger, J. Thomas, Pavri, Behzad B., Han, Jennifer, Fleiss, Joseph L., Rolnitzky, Linda M., Steinman, Richard C.
• Format: Journal Article
• Language: English
• Published: New York, NY Elsevier Inc 01-06-1995; Elsevier; Elsevier Limited
• Subjects: Adult; Biological and medical sciences; Cranial nerves. Spinal roots. Peripheral nerves. Autonomic nervous system. Gustation. Olfaction; Electrocardiography; Female; Fourier Analysis; Heart; Heart - drug effects; Heart - physiology; Humans; Infusions, Intravenous; Isoproterenol - administration & dosage; Isoproterenol - pharmacology; Male; Medical research; Medical sciences; Medical screening; Nervous system (semeiology, syndromes); Neurology; Posture; Space life sciences; Supine Position; Tilt-Table Test; Vagus Nerve - physiology; Human; RR interval; Electrodiagnosis; Nervous system diseases; Intravenous administration; Syncope; Consciousness impairment; Electrocardiography; Vagovagal reflex; Neurological disorder; Tilt table
• ISSN: 0002-9149; 1879-1913
• DOI: 10.1016/S0002-9149(99)80747-4

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 μg/min (low-dose isoproterenol), which was then increased to 2.1 ± 0.5 μg/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 drugfree state (172 ms
2) than during low-dose isoproterenol in the supine position (307 ms
2). 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 ms
2; p < 0.05). The autonomic response to tilt, as measured by relative changes in RR interval and HF power, was similar whether tilt was performed in the drugfree state or during the infusion of isoproterenol, In conclusion, there was less vagal modulation of RR intervals during tilt than during low-dose isoproterenol infusion despite similar RR intervals. Despite the short RR intervals achieved with high-dose isoproterenol, tilt caused a striking decrease in vagal activity. Power spectral analysis of RR variability can distinguish between different autonomic states that produce similar RR intervals.