Dissociation of heart rate variability from parasympathetic tone

Time and frequency domain measurements of heart rate variability have been used as indexes of parasympathetic tone. This study evaluates the effects of parasympathetic stimulation on these indexes. Ten normal subjects (5 females, 5 males; age 27.4 +/- 5.1 yr) were evaluated in the Clinical Research Center. Five-minute electrocardiographic recordings were obtained at baseline and during 1) combined alpha-adrenergic stimulation with baroreflex-increased cardiac parasympathetic activity produced by phenylephrine infusion; 2) parasympathetic blockade (atropine 0.04 mg/kg); and 3) isolated alpha-adrenergic stimulation produced by phenylephrine infusion after parasympathetic blockade. The infusion rate of phenylephrine was titrated to increase the systolic blood pressure by 20-30 mmHg. Heart rate variability analysis was performed using standard time and frequency domain parameters. Phenylephrine infusion resulted in a decrease in the time domain measures and in the high-frequency power. After parasympathetic blockade, alpha-adrenergic stimulation had no significant effect on the heart rate variability parameters. These findings suggest that the heart rate variability parameters traditionally associated with parasympathetic tone do not always reliably measure parasympathetic tone, since they decrease with baroreflex parasympathetic stimulation.