MINIMUM ALVEOLAR CONCENTRATIONS OF NOBLE-GASES, NITROGEN, AND SULFUR-HEXAFLUORIDE IN RATS - HELIUM AND NEON AS NONIMMOBILIZERS (NONANESTHETICS)

We assessed the anesthetic properties of helium and neon at hyperbaric pressures by testing their capacity to decrease anesthetic requiremen t for desflurane using electrical stimulation of the tail as the anest hetic endpoint (i.e., the minimum alveolar anesthetic concentration [M AC]) in rats. Partial pressures of helium or neon near those predicted to produce anesthesia by the Meyer-Overton hypothesis (approximately 80-90 atm), tended to increase desflurane MAC, and these partial press ures of helium and neon produced convulsions when administered alone. Ln contrast, the noble gases argon, krypton, and xenon were anesthetic with mean MAC Values of (+/- SD) of 27.0 +/- 2.6, 7.31 +/- 0.54, and 1.61 +/- 0.17 atm, respectively. Because the lethal partial pressures of nitrogen and sulfur hexafluoride overlapped their anesthetic partia l pressures, MAC values were determined for these gases by additivity studies with desflurane. Nitrogen and sulfur hexafluoride MAC values w ere estimated to be 110 and 14.6 atm, respectively. Of the gases with anesthetic properties, nitrogen deviated the most from the Meyer-Overt on hypothesis. Implications: It has been thought that the high pressur es of helium and neon that might be needed to produce anesthesia antag onize their anesthetic properties (pressure reversal of anesthesia). W e propose an alternative explanation: like other compounds with a low affinity to water, helium and neon are intrinsically without anestheti c effect.