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Estimates of anesthetic requirement based on inspired anesthetic gas concentrations assume equilibration with alveolar, and consequently brain, anesthetic gas tensions. This assumption may be rendered invalid by the solubility of the anesthetic agent and its effects on ventilation. To test this assumption, ED50 values of anesthesia using halothane and a less soluble anesthetic, isoflurane, were determined in rats with and without tracheostomies. Alveolar ED50 or MAC for halothane and isoflurane were 1.11 ± 0.03 per cent and 1.38 ± 0.02 per cent, respectively. Inspired ED50 values 2–3 hours after induction in non-tracheostomized rats were 14 and 17 per cent greater, respectively, than alveolar ED50 values 2–3 hours following tracheostomy during halothane and isoflurane anesthesia in the same animals. These data demonstrate that previous studies in which the anesthetic requirement for halothane was determined using inspired gas samples are inaccurate. Inspired ED50 values for halothane and isoflurane in rats without tracheostomies were 2 and 11 per cent greater, respectively, than inspired ED50 values in the same animals with tracheostomies. Even after 3 hours of anesthesia in tracheostomized rats, measurements of inspired-to-alveolar differences in anesthetic gas tension revealed 9 and 3 per cent differences for halothane and isoflurane, respectively. Thus, the use of inspired concentration for this purpose in rats should be limited to inhalation anesthetics with low blood–gas partition coefficients and with minimal effects on ventilation. A moderately or highly soluble anesthetic agent or one which results in significant respiratory depression necessitates the direct measurement of “alveolar” anesthetic gas tension to determine anesthetic requirement.
White et al. (Tue,) studied this question.