Effects of Fresh Gas Flow Rate on the Ratio of Expired to Inspired Anesthetic Concentration / 대한마취과학회지

BACKGROUND: One way to make rapid increase in alveolar anesthetic concentration includes using high fresh gas flow rates. Fresh gas flow rates should be increased to compensate the amount of uptake either. This study was performed to elucidate optimal fresh gas flow rates for rapid induction by comparison of changes of ratio of expired to inspired concentration. METHODS: The study population was composed of 107 patients undergoing thyroidectomy. Patients were randomly allocated to one of three groups who received desflurane or sevoflurane or isoflurane. Each group was randomly subdivided into three groups who received one of the fresh gas flow rate: 2, 5 or 10 L/min. Inspired anesthetic concentration (Fi) and expiratory anesthetic concentration (Fe), delivered concentration (FD) were recorded. RESULTS: With same fresh gas flow rates, there were significant differences between Fe/Fi of desflurane, sevoflurane, isoflurane. With same anesthetics, Fe/Fi of desflurane and sevoflurane were not influenced by fresh gas flow rates. But Fe/Fi of isoflurane at 2 L/min was significantly lower than 5 L/min and 10 L/min. Fi/FD of desflurane at 10 L/min did not differ from sevoflurane. At 2 L/min and 5 L/min, Fi/FD of desflurane was highest and then sevofluane, isoflurane in that order. CONCLUSIONS: Because rates of Fe/Fi of desflurane and sevoflurane were not influenced by fresh gas flow rates, 2 L/min of fresh gas flow rates could be selected. However, considering the wash-in time in circuit, optimal choice of fresh gas flow rate for desflurane and sevoflurane could be 5 L/min, that of isoflurane be 10 L/min.

Uptake of Inhalation Anesthetics at the Different Inflow Rate in Semiclosed Circuit / 대한마취과학회지

Any factor which increases rebreathing (such as a decrease in inflow rate or an increase in ventilation) or increase uptake (such as an increase in solubility or in cardiac output) will lower the inspired anesthetic concentration. This in turn, will be reflected in an slower rate of anesthesia induction. In a circle absorption system, the inspired anesthetic concentration may be reduced from the inflowing concentration by rebreathing of anesthetic depleted gas. Semiclosed system, most widely used modern anesthetic system was tested to determine what effect the inflow rate had on the rate of rise of anesthetizing alveolar concentration of halothane and enflurane. The results were as follows: 1) High fresh gas inflow rate put the inspired anesthetic concentration close to that of inflow more rapidly. 2) In halothane anesthesia with 4 and 6 L/min of fresh gas flow, the anesthesia induction time was about 20 and 15 minutes respectively. But there was marked delay of the development of anesthesia by 2 L/min of fresh gas fiow. 3) In enflurane anesthesia, the anesthesia induction time was within about 10 minutes without respect to inflow rate. With these results, in general inhalation anesthesia with halothane or enflurane when a semiclosed technique and 100% oxygen are employed, a total gas flow at 4 to 6L per minute is recommended to establish a satisfactory induction and maintenance level of anesthesia.