The influence of the alpha2-adrenoceptor agonist, clonidine, on the EEG and on the MAC of isoflurane

Abstract

We investigated the influence of intravenous clonidine 2, 4 and 6 microg kg-1 on the electroencephalogram and on the minimal alveolar concentration of isoflurane in 40 patients aged 20-60 years undergoing elective surgery. Minimal alveolar concentration was determined using the Dixon 'up-and-down' method. Thirty min after the clonidine infusion anaesthesia was induced with etomidate, 0.25 mg kg-1. Skin incision was made after stable end-tidal isoflurane concentrations had been maintained for at least 15 min. Clonidine caused significant decreases in the spectral edge (95%) and median power frequency of the electroencephalogram and in the bispectral index. The minimal alveolar concentration of isoflurane decreased in a dose-dependent manner 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 microg kg-1. Clonidine 4 and 6 microg kg-1 was associated with a moderate reduction in heart rate and arterial systolic blood pressure. We recommend the use of clonidine intravenously as an adjunct to general anaesthesia in a dose of 4 microg kg-1 given 15 min before induction of anaesthesia.