Heart rate as marker of sympathetic activity

To determine the value of the supine heart rate as a marker of sympathetic tone by assessing, in a large group of subjects, the relationships between this parameter and two other indices of sympathetic activity, plasma norepinephrine and sympathetic nerve traffic.

We studied 243 subjects aged 50.0+/-12.1 years (mean +/- SD). Of these, 38 were normotensive healthy controls, 113 subjects had untreated essential hypertension, 27 were obese normotensives and 65 had congestive heart failure. In each subject, over a 10 min supine period, we measured mean arterial pressure (Finapres), heart rate (electrocardiogram), venous plasma norepinephrine (high-performance liquid chromatography) and efferent postganglionic muscle sympathetic nerve activity (microneurography at a peroneal nerve).

In the whole study group, supine heart rate was correlated with both plasma norepinephrine (r = 0.32, P < 0.0001) and muscle sympathetic nerve activity (r = 0.38, P < 0.0001). This was also the case in the normotensive obese subjects and the heart failure subjects considered separately. Heart rate values were greater in the obese and the heart failure patients than in controls (75.1+/-13.0 and 78.2+/-13.0 versus 69.2+/-11.6 beats/min; P < 0.05 and P < 0.001, respectively), as were plasma norepinephrine (362.7+/-202 and 400.3+/-217 versus 230.4+/-126 pg/ml; P < 0.01 and P < 0.001, respectively) and muscle sympathetic nerve activity (44.1+/-14.7 and 55.3+/-14.3 versus 27.8+/-11.0 bursts/min; P < 0.001 for both). In contrast, in the essential hypertensive subjects, no significant relationship was found between these three indices of sympathetic activity. Furthermore, in the hypertensives, the heart rate was not increased, at variance with the sympathetic nerve traffic, which was greater than in controls (36.2+/-10.0 versus 27.8+/-11.0 bursts/min, P < 0.001).

These data suggest that the supine heart rate can be regarded as a marker of intersubject differences in sympathetic tone, and that this is the case both in the general population and in those with cardiovascular diseases. Its value for this purpose is limited, however, and the limitations may be more evident in essential hypertension than in conditions such as obesity and heart failure.