Effects of clonidine on power spectral analysis of heart rate variability in mild essential hypertension

Patients with essential hypertension often show alterations of the autonomi c nervous system. We evaluated the sympathetic and parasympathetic drive to the heart in 12 mildly hypertensive patients and 9 healthy subjects by pow er spectral analysis of heart rate variability. All subjects underwent meas urements of RR interval, low (LF) and high frequency (HF) components of hea rt rate variability, LF/HF ratio and blood pressure in the resting and sitt ing positions, both before and after oral clonidine (300 mu g), a central s ympatholytic agent. In the supine position before clonidine, hypertensive p atients had higher blood pressure and lower HF values than healthy subjects . Clonidine induced increases in RR interval and HF in both groups, while L F and LF/HF ratio decreased in healthy subjects, but not in hypertensive pa tients. On assuming the sitting position, both groups showed reductions in RR and HF and increments in LF and LF/HF. In healthy subjects, the response to the postural challenge was unaffected by clonidine. In contrast, hypert ensive patients showed no changes in LF and LF/HF ratio, and a significantl y lower decrease in HF. These differences were probably due to the existenc e of two subsets of patients, one exhibiting similar responses to clonidine as healthy subjects, and the other showing no appreciable response to the drug. These results suggest that hypertensive patients have an altered symp atho-vagal balance to the heart, which can be unmasked by clonidine. This p henomenon should be taken into account to achieve a better control of the o verall cardiovascular risk of hypertensive patients. (C) 1998 Elsevier Scie nce B.V. All rights reserved.