Mathematical method to build an empirical model for inhaled anesthetic agent wash-in

BACKGROUND: The wide range of fresh gas flow - vaporizer setting (FGF - F(D)) combinations used by different anesthesiologists during the wash-in period of inhaled anesthetics indicates that the selection of FGF and F(D )is based on habit and personal experience. An empirical model could rationalize FGF - F(D )selection during wash-in. METHODS: During model derivation, 50 ASA PS I-II patients received desflurane in O(2 )with an ADU(® )anesthesia machine with a random combination of a fixed FGF - F(D )setting. The resulting course of the end-expired desflurane concentration (F(A)) was modeled with Excel Solver, with patient age, height, and weight as covariates; NONMEM was used to check for parsimony. The resulting equation was solved for F(D), and prospectively tested by having the formula calculate F(D )to be used by the anesthesiologist after randomly selecting a FGF, a target F(A )(F(At)), and a specified time interval (1 - 5 min) after turning on the vaporizer after which F(At )had to be reached. The following targets were tested: desflurane F(At )3.5% after 3.5 min (n = 40), 5% after 5 min (n = 37), and 6% after 4.5 min (n = 37). RESULTS: Solving the equation derived during model development for F(D )yields F(D)=-(e((-FGF*-0.23+FGF*0.24))*(e((FGF*-0.23))*F(At)*Ht*0.1-e((FGF*-0.23))*FGF*2.55+40.46-e((FGF*-0.23))*40.46+e((FGF*-0.23+Time/-4.08))*40.46-e((Time/-4.08))*40.46))/((-1+e((FGF*0.24)))*(-1+e((Time/-4.08)))*39.29). Only height (Ht) could be withheld as a significant covariate. Median performance error and median absolute performance error were -2.9 and 7.0% in the 3.5% after 3.5 min group, -3.4 and 11.4% in the 5% after 5 min group, and -16.2 and 16.2% in the 6% after 4.5 min groups, respectively. CONCLUSIONS: An empirical model can be used to predict the FGF - F(D )combinations that attain a target end-expired anesthetic agent concentration with clinically acceptable accuracy within the first 5 min of the start of administration. The sequences are easily calculated in an Excel file and simple to use (one fixed FGF - F(D )setting), and will minimize agent consumption and reduce pollution by allowing to determine the lowest possible FGF that can be used. Different anesthesia machines will likely have different equations for different agents.