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 s ubjects (5 females, 5 males; age 27.4 +/- 5.1 yr) were evaluated in th e Clinical Research Center. Five-minute electrocardiographic recording s were obtained at baseline and during 1) combined alpha-adrenergic st imulation with baroreflex-increased cardiac parasympathetic activity p roduced by phenylephrine infusion; 2) parasympathetic blockade (atropi ne 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 pre ssure by 20-30 mmHg. Heart rate variability analysis was performed usi ng standard time and frequency domain parameters. Phenylephrine infusi on resulted in a decrease in the time domain measures and in the high- frequency power. After parasympathetic blockade, alpha-adrenergic stim ulation had no significant effect on the heart rate variability parame ters. These findings suggest that the heart rate variability parameter s traditionally associated with parasympathetic tone do not always rel iably measure parasympathetic tone, since they decrease with barorefle x parasympathetic stimulation.