NO EFFECT OF AGE ON THE DOSE REQUIREMENT OF THIOPENTAL IN THE RAT

With increasing human age (20-80 years), the electroencephalogram (EEG ) dose requirement for the intravenous anesthetic thiopental decreases approximately 10% per decade of life. The goal of this study was to c ompare the dose required to attain isoelectric EEG in young (4-5-month ) vs. aged (24-25-month) Fischer 344 rats. One second isoelectricity w as found to be an endpoint where minimal cardiorespiratory depression occurred. The effects of age, infusion rate, and repeated administrati on were examined in nine young and nine old rodents. Thiopental dose r equirement increased with increasing infusion rates. Repeated administ ration at two-day intervals did not demonstrate tolerance to thiopenta l. No difference in thiopental dose requirement was detected in the yo ung vs. elderly rats. In a separate group of five young and five old r ats, thiopental plasma, brain, heart, and CSF concentrations were meas ured when five seconds of EEG isoelectricity was achieved; no consiste nt differences were noted. The rat may not be an appropriate model to investigate acute age-related anesthetic effects in humans, because ca rdiovascular changes with age are dissimilar between species. (C) 1997 Elsevier Science Inc.