Background: Although heart rate variability (HRV) at 0.1 Hz has been proposed as a noninvasive clinical measure of cardiac sympathetic nerve firing, this premise has not been sufficiently validated by comparison with techniques such as microneurography and the measurement of norepinephrine spillover from the heart that more directly reflect presynaptic sympathetic activity.
Methods and results: We compared the three techniques under conditions of effective cardiac sympathetic denervation, pure autonomic failure (n = 4), dopamine beta-hydroxylase deficiency (n = 1), and after cardiac transplantation (n = 9) as well as in the context of sympathetic nervous activation in cardiac failure (n = 15) and with aging (n = 10). Age-matched comparisons were made in each case with healthy individuals drawn from a pool of 52 volunteers. In pure autonomic failure and early after transplantation, cardiac norepinephrine spillover was negligible, and HRV was low. Late after transplantation, however, cardiac norepinephrine spillover returned to normal levels, and HRV remained low. In comparison to younger subjects (18 to 35 years old), older individuals (60 to 75 years old) had higher muscle sympathetic nerve activity (young, 22.9 +/- 1.9; old, 31.3 +/- 5.8 bursts per minute; P < .05) and cardiac norepinephrine spillover (young, 14.3 +/- 2.5; old, 20.1 +/- 3.0 ng/min; P < .05). In contrast, total HRV was reduced by 89%, and at 0.1 Hz it was reduced by 93% (P < .05). Cardiac failure was also characterized by elevated cardiac norepinephrine spillover (cardiac failure patients, 59 +/- 4; healthy volunteers, 18 +/- 3 ng/min; P < .01) but reduced 0.1 Hz HRV (cardiac failure patients, 49 +/- 17; healthy volunteers, 243 +/- 4 ms2; P < .05).
Conclusions: HRV at 0.1 Hz depends on factors in addition to cardiac sympathetic nerve firing rates, including multiple neural reflexes, cardiac adrenergic receptor sensitivity, postsynaptic signal transduction, and electrochemical coupling, and is not directly related to cardiac norepinephrine spillover, which is a more direct measure of the sympathetic nerve firing rate.