Inhaled anesthetics have hyperalgesic effects
at 0.1 minimum alveolar anesthetic concentration

Zhang Y, Eger EI 2nd, Dutton RC, Sonner JM.
Department of Anesthesia and Perioperative Care,
University of California,
San Francisco 94143-0464, USA.
Anesth Analg. 2000 Aug;91(2):462-6


We investigated the hyperalgesic (antianalgesic) effect of the inhaled anesthetics isoflurane, halothane, nitrous oxide, and diethyl ether, or the nonimmobilizer 1, 2-dichlorohexafluorocyclobutane at subanesthetic partial pressures (or, for the nonimmobilizer, subanesthetic partial pressures predicted from lipid solubility) in rats. Hyperalgesia was assessed as a decrease in the time to withdrawal of a rat hind paw exposed to heat. All four anesthetics, including nitrous oxide and diethyl ether, produced hyperalgesia at low partial pressures, with a maximal effect at 0.1 minimum alveolar anesthetic concentration (MAC) required to prevent response to movement in 50% of animals, and analgesia (an increased time to withdrawal of the hind paw) at 0. 4 to 0.8 MAC. The nonimmobilizer had neither analgesic nor hyperalgesia effects. We propose that inhaled anesthetics with a higher MAC-Awake (the MAC-fraction that suppresses appropriate responsiveness to command), such as nitrous oxide and diethyl ether, can be used as analgesics because patients are conscious at higher anesthetic partial pressures, including those which have analgesic effects, whereas anesthetics with a lower MAC-Awake do not produce analgesic effects at concentrations that permit consciousness. Implications: The inhaled anesthetics isoflurane, halothane, nitrous oxide, and diethyl ether produce antianalgesia at subanesthetic concentrations, with a maximal effect at approximately one-tenth the concentration required for anesthesia. This effect may enhance perception of pain when such small concentrations are reached during recovery from anesthesia.
Adverse effects
Obstetric anaesthesia
Molecular mechanisms
Inhalational techniques
Contemporary anaesthesia
Anaesthesia and anaesthetics
Nitrous oxide: adverse effects
Anaesthesia: rivalries and discoveries
Consciousness, anaesthesia and anaesthetics
First use of anaesthetics in different countries
Early religious/military opposition to anaesthetics

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