EFFECT OF PHYSIOLOGICAL AND PHARMACOLOGICAL ADRENERGIC-STIMULATION ONHEART-RATE-VARIABILITY

Objectives. The aim of this study was to evaluate and compare the effe cts of physiologic and pharmacologic sympathetic stimulation on time a nd frequency domain indexes of heart rate variability. Background. Mea surements of heart rate variability have bees used as indexes of sympa thetic tone. To date, the effects of circulating catecholamines on hea rt rate variability have not been evaluated. Methods. Fourteen normal subjects (eight men, six women, mean [+/-SD] age 28.5 +/- 4.8 years) w ere evaluated. Five-minute electrocardiographic recordings were obtain ed in triplicate after physiologic and pharmacologic sympathetic stimu lation: during upright tilt, after maximal exercise, during epinephrin e and isoproterenol infusions at 50 ng/kg body weight per min, during beta-adrenergic blockade and during combined beta-adrenergic and paras ympathetic blockade. Results. Beta-adrenergic stimulation resulted in a significant decrease in time domain measures of heart rate variabili ty. The frequency domain indexes showed variable responses, depending on the individual stimulus. Tilt caused an increase in low frequency p ower and in the ratio of low to high frequency power. These changes we re not necessarily observed with other conditions of beta-adrenergic s timulation. Double blockade suppressed baseline heart rate variability , but beta-adrenergic blockade had no significant effect. Time domain measures of heart rate variability demonstrated excellent reproducibil ity over the three recordings, but the frequency domain variables demo nstrated fair to excellent reproducibility. Conclusions. These finding s suggest that different modes of beta-adrenergic stimulation may resu lt in divergent heart rate variability responses. Thus, current heart rate variability techniques cannot be used as general indexes of ''sym pathetic'' tone. Studies utilizing heart rate variability to quantify sympathetic tone need to consider this.