Minimum alveolar concentration of I-653 and isoflurane in pigs: definition of a supramaximal stimulus.

We determined the anesthetic potencies of a new fluorinated anesthetic, I-653, and isoflurane in pigs as a preliminary to a study of the relative cardiovascular and electroencephalographic effects of these agents. Clamps were sequentially applied to the dew claw and/or tail of each animal to determine the minimum alveolar concentration (MAC) that suppressed movement in response to each of these stimuli. MAC obtained by clamping the tail (8.28 +/- 1.34% [mean +/- standard deviation] for I-653 and 1.65 +/- 0.36% for isoflurane) was more variable and lower than MAC obtained by clamping the dew claw (10.00 +/- 0.94% for I-653 and 2.04 +/- 0.19% for isoflurane). We conclude that the type of stimulus applied affects the MAC value obtained for I-653 and isoflurane. Clamping the tail is not a supramaximal stimulus in pigs; a greater stimulus is provided by clamping the dew claw.