A simple method for evaluation of the uptake of isoflurane and its comparison with the square root of time model

BACKGROUND: The square root of time (SqRT) model had been used to predict the uptake of volatile agents. METHODS: We studied the rate of uptake of isoflurane in 10 patients using liquid isoflurane infusion through syringe pump into the closed circuit. The infusion rates were titrated to maintain a constant end tidal concentration of isoflurane of 1.5%. The predicted uptake values were also calculated from the square root principle and compared with the derived uptake. RESULTS: The observed rate of uptake was higher than predicted from the Lowe and Ernst equation (P<0.001). There exists considerable inter-individual variability in uptake pharmacokinetics and it showed statistically significant correlation with ideal body weight, body weight (P<0.01), body surface area, and body weight¾ from 30 min of start of isoflurane infusion (P<0.05). CONCLUSION: SqRT model is inaccurate in predicting isoflurane uptake and underestimates it during closed circuit anaesthesia.