VAGAL MODULATION OF RR INTERVALS DURING HEAD-UP TILT AND THE INFUSION OF ISOPROTERENOL

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 mu g/min (low-dose isoproterenol), which was then increased to 2.1 +/- 0.5 mu g/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-vp tilt in the drug-free 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 drug-free 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.