Effect of Subanesthetic Concentrations of Halothane and Methoxyflurane on Pain Threshold in Conscious Volunteers

Abstract

Algesimetric studies were carried out in volunteers during the inhalation of subanesthetic concentrations of methoxyflurane and halothane. Blood levels of agents were measured in venous samples obtained immediately after each determination of pain threshold. Significant increases in threshold were seen with both agents but a linear relationship between pain threshold and blood levels was not seen consistently with either agent. Significant reductions in threshold were seen occasionally with both methoxyflurane and halothane and were most probably related to the onset of drowsiness. The occurrence of analgesia with low blood levels in the majority of subjects lends support to the current practice of reinforcing nitrous oxide oxygen anesthesia with low concentrations of halothane or methoxyflurane.