Characterization of the factors that determine the effect of sympathetic stimulation on heart rate variability

Heart rate variability analysis has been used to derive indices of sympathetic tone. As different sympathetic stimuli may give rise to divergent changes in heart rate variability, this study was designed to characterize the factors responsible for these divergent responses. Twelve healthy subjects (7 males, age 24.8 ± 3.1 years) were evaluated. Five-minute electrocardiographic recordings were obtained at baseline, following upright tilt, and during isoproterenol infusion (25 ng/kg per min) under control conditions and following parasympathetic blockade. Data were acquired during spontaneous respiration and when breathing was timed with a metronome (15 breaths/min). Under control conditions, both upright tilt and isoproterenol infusion resulted in significant decreases in the SD and MSSD from baseline values of 69 ± 3 ms and 64 ± 5 ms to 48 ± 4 ms and 21 ± 5 ms during tilt and 44 ± 4 ms and 20 ± 5 ms during isoproterenol infusion, respectively. LF power also significantly increased from 0.47 ± 0.17 ln (beats/min)² at baseline to 1.90 ± 0.20. In (beats/min)² and 1.34 ± 0.18. In (beats/min)² during tilt and isoproterenol infusion, respectively. No change in HF power was noted. Following parasympathetic blockade, all heart rate variability parameters were significantly decreased. No significant change from baseline in the SD, MSSD, or HF power was noted with either tilt or isoproterenol infusion. The LF power increased only with tilt from a baseline value of -3.17 ± 0.17 in (beats/min)² to -0.41 ± 0.19 in (beats/min)². Similar changes were noted during spontaneous respiration and metronome breathing. These findings demonstrate that the response of the sinus node to β-adrenergic stimulation depends on the mode of stimulation. In addition, the associated level of parasympathetic tone affects the observed changes in heart rate variability that are associated with sympathetic stimulation.