Introduction to the quantitative technique of closed circuit anesthesia in dogs.
In humans, anesthetic uptake in a closed system with constant arterial concentration has been shown to be inversely proportional to the square root of time. A practical method for quantitative dosage of volatile anesthetic was derived from this. The method was evaluated in nine dogs anesthetized with a closed circle system using halothane and isoflurane. A unit dose (UD) of anesthetic was calculated in milliliters of vapor which was converted to milliliters of liquid and repeatedly administered into the expiratory limb between the squares of integer units of time (0-1, 1-4, 4-9 minutes, etc). The UD was derived as follows: UD = 2 f MAC X lambda B/G X 2 (kg)3/4, where f MAC was the desired alveolar concentration, lambda B/G the blood-gas partition coefficient, and 2 (kg)3/4 was an approximation of cardiac output. The method resulted in a stable plane of anesthesia and permitted continuous monitoring of O2 consumption. There was no significant difference between predicted and measured values of O2 consumption, cumulative doses, or alveolar concentrations at 9 and 16 minutes of anesthesia.