Feasibility Study of an Indicator of Equivalent Potency of Multiple Anesthetics Normalized by Minimum Alveolar Concentration Derived From Response Surface Models

BACKGROUND: Minimum alveolar concentration (MAC) is used as the standard measure of potency for volatile anesthetic agents. However, there is a lack of effective and quantitative indicator of the combined potency of multiple coadministered inhalation and intravenous anesthetics. We hypothesized that an indicator of equivalent potency of multiple anesthetics, normalized by MAC and derived from response surface models as a fraction (abbreviated as eMAC fraction), can reflect the total potency of multiple anesthetics. METHOD: Fifty-three patients receiving general anesthesia were enrolled. A random dose combination of propofol and remifentanil was administrated before a tetanic electric stimulus which was used to simulate incision. The vital signals and responses of patients were recorded to tetanic stimulus and in turn used to calculate the prediction probability (P k ) of the response, using the eMAC fraction and the bispectral index (BIS). After induction, the doses administered during anesthesia maintenance were entirely determined by anesthesiologists. During emergence, the anesthesiologists facilitated the awakening of patients through a combination of auditory and tactile stimuli at eMAC fraction levels of 0.8, 0.6, 0.4, and 0.2, or every 2 minutes after the certain level was reached, whichever arrived first. RESULTS: The eMAC fraction for predicting the loss of movement response to tetanic electric stimulus yielded a mean ± standard deviation (SD) P k of 0. 80 ± 0.06, which was higher than the Pk of the BIS value for predicting the loss of movement response to tetanic electric stimulus (0.71 ± 0.07, P < .001). During maintenance of anesthesia, the eMAC fraction showed changes related to anesthetic dose and surgical phase. In all patients, approximately 71.9% of eMAC fraction values were within the range of 1.3 to 2.6. During emergence, the mean eMAC fraction values at awakening were 0. 30 ± 0.15. CONCLUSIONS: The eMAC fraction showed a superior performance in indicating the loss of response to electric stimulus compared to BIS. Anesthesiologists are familiar with the clinical use range of MAC fraction, and the distribution of eMAC fraction values during maintenance is similar to this range. This similarity allows anesthesiologists to easily use eMAC fraction in practice. These results indicate that the eMAC fraction has the potential to assist anesthesiologists in titrating multiple anesthetics to estimate the depth of anesthesia during general anesthesia, and should further be evaluated in clinical studies.