Influence of volatile and intravenous anesthetics on the threshold of the acoustically evoked stapedius reflex

The influence of volatile and intravenous anesthetics on the threshold of t he acoustically evoked stapedius reflex (SR) was studied prospectively in 4 5 patients undergoing elective ENT surgical procedures. After premedication with flunitrazepam the patients were randomly assigned to one of nine grou ps. Group I: 70% nitrous oxide (N2O) in oxygen (O-2); Groups II-VII: induct ion of anesthesia with intravenous thiopental, followed by mask inhalation with 100% O-2 and 1.13% halothane (Group II), 2.52% enflurane (Group III) o r 1.73% isoflurane (Group IV); or 70% N2O in oxygen, and 0.44% halothane (G roup V), 0.86% enflurane (Group VI) or 0.75% isoflurane (Group VII); Group VIII: intravenous midazolam and ketamine; and Group IX: intravenous midazol am and alfentanil. Tympanometry and ipsilateral and contralateral SR measur ements were performed when the effects of the anesthetics had achieved a st eady state. Flunitrazepam raised the SR threshold only slightly. Substances applied during inhalation anesthesia either markedly increased the thresho ld contralaterally more than ipsilaterally (thiopental, N2O), or suppressed the reflex completely (thiopental, all volatile anesthetics with or withou t N2O). Under intravenous anesthesia the reflex was always present. The mid azolam-ketamine combination influenced the threshold bilaterally only sligh tly, while the midazolam-alfentanil combination led to a pronounced, contra laterally significant elevation of the threshold. Based on its minimal infl uence on the SR threshold, flunitrazepam is especially suitable for sedatio n and the midazolam-ketamine combination for anesthesia in audiological dia gnostic procedures.