General anesthesia is a necessary condition for most complex surgical operations. General anesthesia is intended to induce unconsciousness and reduce pain of the patient. The main principle of anesthesia is to main� tain the concentration of anesthetic in the brain at a nec� essary level during all stages of surgical intervention. Inhalation anesthesia (InA) is based on inhalation of gas or vapor anesthetics and their further diffusion into the blood. Intravenous anesthesia (IvA) is based on intra� venous infusion of liquid anesthetics into the blood. Blood flow provides transport of anesthetics to the brain and their removal. Inhalation anesthetics are partial� ly removed by the lungs, whereas intravenous anesthetics are metabolized in the liver, kidney, etc. The rate of anes� thetic diffusion and elimination depends on solubility of the anesthetic in blood and tissues, concentration of inhalation anesthetic in inhaled gas, and concentration of intravenous anesthetic in infusion solution. These effects also depend on the patient’s respiration and circulation. If the concentration of inhalation anesthetic in inhaled gas is higher or lower than in the blood, the anes� thetic is transported to the brain or eliminated, respec� tively. Therefore, at the initial stages of anesthesia blood intensely takes up anesthetics. As the concentrations of anesthetic in inhaled gas and blood are leveled, the rate of the uptake is reduced and becomes equal to zero in the equilibrium state. However, the anesthetic supply should not be switched off, because this would cause its elimina� tion from the patient’s body. High (impact) concentration of anesthetic is used to accelerate the initial stages of anesthesia. Then, the con� centration of anesthetic is gradually decreased to the required level. The accuracy of concentration of anes� thetic setting at this moment should be maximized. Because an impact dose of anesthetic cannot be applied through lungs at high rate, the rate of InA is slower than the rate of IvA. On the other hand, the time of elimina� tion of InA does not exceed tens of minutes. In recent years IvA has gradually become more pop� ular because of adverse effects of anesthetic vapors used in InA on personnel and the environment. Overdose of IvA is dangerous because the concentra� tion of infused drug cannot be reduced without compli� cated blood transfusion. The time of IvA clearance may reach tens of hours. A combination of IvA and InA is used in 45% of cases. This combination is based on the notion of mini� mal alveolar concentration of anesthetic (MAC). At MAC = 1 and MAC = 1.3, the surgical stage of anesthe� sia is observed in 50 and 100% of patients, respectively. Although nitrous oxide is mutagenic, it is widely used in InA. However, MAC = 1 would be attained only at an impossible concentration of nitrous oxide (101%). Because the concentration of O2 in the respiratory mix� ture should be at least 21%, the use of only nitrous oxide does not allow the surgical stage to be reached. A method based on combination of gas and liquid vapor anesthetics shifts the threshold concentration to a lower level. Intravenous infusion of anesthetic (dose, 1/2�1/3 MAC) makes the situation even more difficult. Therefore, the working range of InA is shifted toward the area of high concentration errors.