[Initial washout time of inhaled anesthetic agents]

Recovery of inhalation anesthesia is generally considered the inverse of induction. While other factors remain unchanged, the rate of elimination of an inhaled anesthetic agent with a lower solubility such as nitrous oxide (N2O) is faster than one with a high solubility. Therefore, at the end of surgery it is a common practice to discontinue the volatile agent prior to discontinuing N2O. However, the results of our routine monitoring of end-tidal concentrations of anesthetics and physiological gases by employing a mass spectrometer lead us to question the practice. We studied 24 non-premedicated ASA I/II patients requiring endotracheal intubation with an anesthesia time over 2 h. All patients were mechanically ventilated at an appropriate tidal volume and respiratory rate to maintain an ETCO2 of 35-38 mmHg with an anesthetic mixture of 40% O2 and 60% N2O and isoflurane (Forane). At the end of surgery, both N2O and isoflurane were discontinued simultaneously and total oxygen flow was increased to 6 or 10 l/min by random. The exhaled partial pressure of both inhaled anesthetic agents were assessed at the time of anesthetic termination, after 1, 2 min, and then every 2 min up to 20 min or at the time of extubation. The results obtained were plotted against real time using two different scales. Despite the marked difference in their plasma solubilities, these two inhaled anesthetic agents reached to low exhaled partial pressure that had no clinical significance about ten minutes after discontinuation.(ABSTRACT TRUNCATED AT 250 WORDS)