A real-time method for estimating the concentrations of isoflurane in mixed venous blood by a derived Fick's equation

We propose an equation derived from Pick's laws and Lin's concept of effective blood concentration to calculate the blood concentration of inhaled anesthetics in mixed venous blood (MVBC) without direct blood sampling. We investigated the relationship between the calculated concentrations and the actual blood sample concentrations in mixed venous blood of patients undergoing cardiac surgery during isoflurane anesthesia in this study. Sixteen patients were recruited for Experiment 1. At different time points, pulmonary arterial blood samples were collected for gas chromatography/mass spectrometric determination via the pulmonary artery catheter: these samples represented the actual concentrations. The inspired and expired concentrations of isoflurane measured by a gas monitor were used for the MVBC calculations. Lin's effective blood concentration method was also used, and the obtained results were then compared with MVBC. Studies were conducted on 11 additional patients (Experiment 2) to confirm the results obtained from Experiment 1. The MVBC and the actual blood concentrations showed a similar kinetic pattern and level during anesthesia and had high correlation coefficients within subjects. We have demonstrated that MVBC could represent the actual pulmonary blood concentrations of isoflurane during cardiac surgery. The results suggest that MVBC could be a useful method of estimating the real-time pulmonary blood concentration of isoflurane. The clinical significance and importance of the method merit further investigation.