An Improved Method Of Calculating The Venous Blood Concentration Of Isoflurane By Fick's Equation

Background: In 1994, Lin proposed a method that was purported to be able to calculate the concentration of inhaled anesthetic in mixed venous blood. Then Wai from Taiwan upgraded his method, and got that the correlation coefficient between the measured concentration and the calculated was 0. 7. In order to use the calculated concentration as an input variable in target controlled infusion (TCI) of inhaled anesthetic, we upgraded their method. The relationship between the measured concentration and that calculated from the Wai's or our equation was evaluated.Methods: Disposed standard gas samples of isoflurane, and measured their peak area with gas chromatograph, we got the curve between concentration and peak area. Seventeen ASA grade I II patients undergoing abdominal surgeries received balanced anesthesia. 20, 40, 60, 80min after the initiation of isoflurane, we drew the central venous bloods, and measured the actual concentration of isoflurane(C_v) with gas chromatography. When drawing the bloods, the C_I and C_ET were measured and recorded, then we calculated the venous blood concentrations with wai's equation and our equation(C_v1,C_v2), and compared them with C_v. Linear regression, correlation coefficient(r), the median performance error (MDPE), the median absolute performance error (MDAPE) and the wobble(MDADPE) were used for statistical analysis. Results: The calculations from our equation and the actual concentrations were correlated well (r=0.9), and bias (MDPE') was 5. 37%, accuracy (MDAPE') was 16.02%, the wobble(MDADPE') was 15.86%, they showed much better than the calculations from wai's equation (r=0.8,MDPE=16.76%, MDAPE=29.09%, MDADPE=2l.61%).Conclusion: the calculating method of the fraction of uptake(F) in our experiment is better than that of wai's. C_v2 is more accurate than C_v1 for representing the actual concentration of isoflurane, showed that the functional residual capacity(FRC) be a part of holding the inhaled anesthetic. The bias, the accuracy and the wobble between C_v2 and C_v is lower than that between C_v1 and C_v, showed that C_v2 represents C_v better.