EFFECTS OF ACUTE CLONIDINE ADMINISTRATION ON POWER SPECTRAL-ANALYSIS OF HEART-RATE-VARIABILITY IN HEALTHY HUMANS

1 To investigate the relationship between the autonomic drive to the h eart and heart rate variability, as evaluated by power spectral analys is, we studied the effect of clonidine (300 mu g), a central sympathol ytic agent, on heart rate variability. 2 Six healthy subjects (mean ag e 31 +/- 3 years) were studied in the supine and the sitting position (15 min each) on two different occasions, respectively, before and aft er clonidine administration. Using an autoregressive approach, the low frequency (LF) and the high frequency (HF) components of power spectr al analysis were measured, and their ratio was calculated. Blood press ure was monitored throughout the study and plasma catecholamines were measured at the end of each period. 3 Before clonidine, assumption of the sitting position induced increases in LF, LF/HF ratio, blood press ure and plasma noradrenaline. Clonidine induced remarkable reductions in the normalized values of the LF component and the LF/HF ratio in bo th the resting and the sitting position (supine: LF = -68%, LF/HF rati o = -80%; sitting: LF = -23%, LF/HF ratio = -55%) without affecting th e central frequencies of LF and HF components. Blood pressure and plas ma catecholamines also significantly decreased after clonidine. 4 Thes e results support the hypothesis that the LF component of HRV, express ed in normalized units, is an indicator of the sympathetic control of the heart. In addition, this component seems to be largely of central origin, because it is markedly reduced by the central sympatholytic ac tion of clonidine.