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ANESTHETIC PHARMACOLOGY

Halothane and Isoflurane Have Additive Minimum Alveolar Concentration (MAC) Effects in Rats

Edmond I Eger, II, MD^*, Yilei Xing, MD^*, Michael Laster, DVM^*, James Sonner, MD^*, Joseph F. Antognini, MD, and Earl Carstens, PhD

*Department of Anesthesia and Perioperative Care, University of California, San Francisco, and Department of Anesthesiology and the Section of Neurobiology, Physiology, and Behavior, University of California, Davis

Address correspondence and reprint requests to Edmond I Eger II, MD, Department of Anesthesia, S-455, University of California, San Francisco, CA 94143-0464. Address e-mail to egere{at}anesthesia ucsf.edu.

Studies suggest that at concentrations surrounding MAC (the minimum alveolar concentration suppressing movement in 50% of subjects in response to noxious stimulation), halothane depresses dorsal horn neurons more than does isoflurane. Similarly, these anesthetics may differ in their effects on various receptors and ion channels that might be anesthetic targets. Both findings suggest that these anesthetics may have effects on movement in response to noxious stimulation that would differ from additivity, possibly producing synergism or even antagonism. We tested this possibility in 20 rats. MAC values for halothane and (separately) for isoflurane were determined in duplicate before and after testing the combination (also in duplicate; six determinations of MAC for each rat). The sum of the isoflurane and halothane MAC fractions for individual rats that produced immobility equaled 1.037 ± 0.082 and did not differ significantly from a value of 1.00. That is, the combination of halothane and isoflurane produced immobility in response to tail clamp at concentrations consistent with simple additivity of the effects of the anesthetics. These results suggest that the immobility produced by inhaled anesthetics need not result from their capacity to suppress transmission through dorsal horn neurons.

IMPLICATIONS: Despite differences in their capacities to inhibit spinal dorsal horn cells, isoflurane and halothane are additive in their ability to suppress movement in response to a noxious stimulus.

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