Recent determinations of the minimum alveolar anesthetic concentration (MAC) for nitrous oxide in rats yield values of 1.51-1.55 atm. When combined with results from other reports, these results suggest a deviation from linear additivity, and call into question the unitary theory of narcosis. The present report provides evidence that nitrous oxide does act in an additive manner. We directly determined the MAC for nitrous oxide in groups of 10 Sprague-Dawley and Long-Evans rats, using electrical stimulation of the tail or abdomen. MAC equaled 2.35 +/- 0.20 atm (mean +/- SD) in Long-Evans rats with tail stimulation; 2.21 +/- 0.19 in Sprague-Dawley rats using tail stimulation; and 1.99 +/- 0.21 atm in Sprague-Dawley rats using abdominal stimulation (tail versus abdominal stimulation was significantly different). Our MAC values are higher than those recently reported by others. Differences from the previous reports may be explained by differences in experimental design or interpretation. Our findings are consistent with an additive effect of nitrous oxide with other inhaled anesthetics (data from other reports) and thereby support the unitary theory of narcosis.
Whether anesthetized patients register emotionally charged information remains controversial. We tested this possibility using subanesthetic concentrations of propofol or desflurane. Twenty-two volunteers (selected for hypnosis susceptibility) received propofol and desflurane (on separate occasions, and in a random order) at a concentration 1.5-2 times each individual's minimum alveolar anesthetic concentration (MAC)-awake (or equivalent for propofol). We gave vecuronium, intubated the trachea of each volunteer, controlled ventilation, and then presented a neutral (control) drama or a "crisis" drama stating that the oxygen delivery system had failed, assigning crisis and control dramas in a blinded, randomized, and balanced manner. One day later, interviewers blinded to the assigned drama conducted a 2-h structured interview (including hypnosis) to determine whether the contents of the interviews after crisis and control dramas differed. In addition, messages permitting subsequent assessment of learning of matter-of-fact information (Trivial Pursuit-type question task and a behavior task) were presented at the anesthetic concentration just sufficient to prevent response to command in each volunteer. No analyses of the tasks involving matter-of-fact information revealed learning except one which correlated hypnosis susceptibility with behavior task performance. Both propofol and desflurane suppressed memory of the crisis. Consistent with previous findings for isoflurane and nitrous oxide, propofol and desflurane suppressed learning of matter-of-fact information at concentrations just above MAC-awake, except that volunteers' susceptibility to hypnosis correlated with performance of a behavior suggested during anesthesia. Propofol and desflurane suppressed learning of emotionally charged information at anesthetic concentrations 1.5-2 times MAC-awake (less than MAC), a different result from that previously reported for ether.
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