The influence of the alpha(2)-adrenoceptor agonist, clonidine, on the EEG and on the MAC of isoflurane

We investigated the influence of intravenous clonidine 2, 4 and 6 mu g kg(- 1) on the electroencephalogram and on the minimal alveolar concentration of isoflurane in 40 patients aged 20-60 years undergoing elective surgery. Mi nimal alveolar concentration was determined using the Dixon 'up-and-down' m ethod. Thirty min after the clonidine infusion anaesthesia was induced with etomidate, 0.25 mg kg(-1). Skin incision was made after stable end-tidal i soflurane concentrations had been maintained for at least 15 min. Clonidine caused significant decreases in the spectral edge (95%) and median power f requency of the electroencephalogram and in the bispectral index. The minim al alveolar concentration of isoflurane decreased in a dose-dependent manne r from 1.32% (95% CI, 1.28%-1.36%) in the control group to 1.03% (0.9%-1.18 %) in patients given clonidine 6 mu g kg(-1). Clonidine 4 and 6 mu g kg(-1) was associated with a moderate reduction in heart rate and arterial systol ic blood pressure. We recommend the use of clonidine intravenously as an ad junct to general anaesthesia in a dose of 4 mu g kg(-1) given 15 min before induction of anaesthesia.