Commonly used clinical methods of general anesthesia include inhalation anesthesia, intravenous anesthesia and compound anesthesia. The implementation of general anesthesia can be divided into several steps, such as preanesthesia treatment, anesthesia induction, anesthesia maintenance and anesthesia recovery. 1. Inhalation anesthesia (1) Inhalation anesthesia refers to the method of general anesthesia produced by the absorption of volatile anesthetics or anesthetic gases into the blood through the respiratory system by the anesthesia machine and the suppression of the central nervous system. Inhalation anesthesia is the earliest anesthesia method used in the history of anesthesia, ether is a widely known inhalation anesthetic, but because of its instability and flammable and explosive properties, modern operating rooms require more electric knives and other equipment, which may lead to the explosion of ether, now the clinic has abandoned. Inhalation anesthesia has developed into the main method of administering general anesthesia. Inhalation anesthesia has a high degree of controllability, safety and effectiveness because most of the inhalation anesthetics are metabolized and decomposed in the body, and most of them are excreted from the lungs in their original form. Inhalation anesthesia can be divided into four types: open, semi-open, semi-compact and compact, according to the contact method between breathing gas and air, the degree of repeated inhalation and the presence or absence of carbon dioxide absorption devices. According to the size of the fresh air flow is divided into low-flow anesthesia, minimum flow anesthesia and tightly closed circuit anesthesia. (2) The implementation of inhalation general anesthesia ① Pre-anesthesia treatment mainly includes the physical and psychological preparation of the patient, pre-anesthesia assessment, the choice of anesthesia method, and the preparation and inspection of the corresponding equipment, as well as the reasonable pre-anesthesia medication. In addition, we should explain to the patient the characteristics of inhalation anesthesia induction itself and prepare the respiratory tract. The induction is divided into slow induction method with increasing concentration and fast induction method with high concentration. Simple inhalation anesthesia induction is suitable for pediatric patients who are not suitable for intravenous anesthesia and not easy to keep the vein open, difficult airway and laryngeal mask intubation, etc. It is not suitable for alcoholics and physically strong people. The slow induction method is to fix the mask on the patient’s mouth and nose with the left hand, gently hold the airbag with the right hand, open the volatile canister after oxygen inhalation and start to give low concentration of inhaled anesthetics. Halothane is the best choice of anesthetic, but other inhaled anesthetics may also be used. If needed, an oropharyngeal or nasopharyngeal ventilation catheter can be inserted to maintain the usual airway while detecting the patient’s response to stimulation, and if the response disappears, the surgeon can be notified to prepare for surgery. Venous access should be established as early as possible after the start of anesthesia with venous dilatation. This slow induction method with increasing concentration allows for a smoother induction of anesthesia, but the prolonged induction time increases the possibility of accidents during the excitation period and the patient is prone to uncooperative conditions. The high concentration fast induction method is to first inhale pure oxygen 6L/min denitrogenated by mask for 3 minutes, then inhale high concentration anesthetics and let the patient breathe deeply for several times to lose consciousness and then change to medium concentration anesthetics until the surgical anesthesia period. Tracheal intubation may be performed to implement assisted or controlled breathing. In clinical practice, many patients will ask whether inhalation induction is like gauze covering the mouth and nose in film and television works leading to loss of consciousness. In fact, clinically applied inhalation anesthetics will not work that fast and require special airtight apparatus to be stored and are prone to volatilization in an open environment. ③After the induction of maintenance anesthesia is completed, it enters the maintenance phase of anesthesia. This period should meet the requirements of surgery, maintain the patient pain-free, unconscious, normal muscle relaxation and organ function, stress response is suppressed, water, electrolytes and acid-base balance is maintained, and blood loss is timely replenished. At present, low-flow inhalation anesthesia is the main method to maintain anesthesia. Intraoperatively, the depth of anesthesia should be adjusted according to the characteristics of the surgery, preoperative medication and the patient’s response to anesthesia and surgical stimulation. Changing the depth of anesthesia without changing the patient’s minute ventilation is achieved primarily by adjusting the volatile tank opening concentration and increasing the fresh air flow. Inhaled anesthetics by themselves produce weak inotropic effects, and in order to obtain perfect inotropy that satisfies major surgery, intravenous administration of inotropic agents is often required to avoid circulatory depression caused by simply increasing the inhalation concentration in order to enhance inotropic effects. Volatile anesthetics can significantly enhance the nerve blocking effect of non-depolarizing myorelaxants, and the dosage of myorelaxants can be reduced when the two are used together. ④Waking up and recovery The process of waking up in patients with inhalation anesthesia is the opposite of the induction process and can be seen as a wash-out process of inhaled anesthetics. Due to the low flow rate of the gas in the circuit, the anesthetic cannot be washed out quickly, so the volatile tank should be closed earlier at the end of the operation than with high-flow anesthesia. At the end of the entire surgical operation, high-flow pure oxygen is used to quickly wash out the patient and the residual anesthetics in the circuit. When the intra-alveolar inhaled anesthetic concentration drops to 0.4 MAC (minimum effective alveolar gas concentration), approximately 95% of patients are able to open their eyes as ordered by the surgeon. The cleaner the inhalation anesthetic is washed out the smoother the awakening process and the patient’s recovery; excessive residuals may not only cause irritability, vomiting, or even inhibit wakefulness and breathing. When washing out the inhaled anesthetics, certain painkillers can be given intravenously to increase the patient’s tolerance to the tracheal tube, in order to facilitate the early discharge of the inhaled drugs, and also to reduce the stress reaction during extubation. Intravenous anesthesia (1) intravenous general anesthesia refers to the method of injecting one or more drugs intravenously to produce general anesthesia by acting on the central nervous system through blood circulation. According to the different ways of drug delivery, intravenous anesthesia can be divided into single dosing method, fractionated dosing method and continuous dosing method. The use of intravenous general anesthesia was once limited due to some limitations of its own. But since the 1980s, with the continuous improvement of clinical pharmacology research methods, the development of new strong and short-acting intravenous anesthetics and the introduction of computerized intravenous automatic drug delivery systems, intravenous anesthesia has been greatly improved and developed. According to the different ways of drug delivery, intravenous anesthesia can be divided into single injection, fractionated injection, continuous injection and target-controlled infusion (TCI). (2) Implementation of intravenous general anesthesia ① Pre-anesthesia treatment is the same as other general anesthesia, mainly including physical and psychological preparation of patients, pre-anesthesia assessment, selection of anesthesia methods, and preparation and inspection of corresponding equipment, as well as reasonable pre-anesthesia medication. The induction of anesthesia is more comfortable and suitable for most conventional anesthesia situations (including inhalational general anesthesia), which is especially suitable for patients who need rapid induction. This can be achieved using a single intravenous injection of anesthetic drugs, or the induction of intravenous anesthesia can be accomplished using the TCI technique. Among the various stimuli produced by surgical anesthesia, tracheal intubation is higher than in general surgery, and thus the blood concentration required for anesthesia induction may be greater than that required for intraoperative anesthesia maintenance. The first dose of intravenous infusion can be calculated based on the loading dose formula CTVd peak effect and should also take into account the actual situation of the patient. The anesthesiologist should also be familiar with the peak effect time of the drugs used, which is important for the induction of anesthesia. When using the TCI technique for intravenous induction, care should be taken to select the appropriate target concentration based on the individual patient’s condition. The time required for the patient to lose consciousness during induction decreases as the target concentration selected increases. Some characteristics of intravenous anesthesia itself should also be noted when using intravenous anesthesia to induce anesthesia. First of all, the principle of individualization should be emphasized. Drug selection and dosage should be adjusted according to the patient’s specific situation, such as weight, age, circulatory status, and preoperative medication. Secondly, for elderly patients or patients with slow circulatory time (such as shock, hypovolemia and cardiovascular disease) the amount of medication should be reduced, and the injection should be slow speed, while closely monitoring the changes of cardiovascular system. Finally, the injection of some anesthetics during induction may cause local pain. Preoperative or pre-induction administration of opioids or lidocaine mixed with the injected intravenous general anesthetics can reduce the occurrence of pain. (iii) Anesthesia maintenance uses continuous intravenous drip or pumping of anesthetic drugs to maintain patient anesthesia and requires a dose that includes both, a dose of drug eliminated from the central compartment plus a dose of drug transferred to the peripheral compartment. Adjusting the infusion rate of intravenous anesthetic drugs according to the intensity of the surgical stimulus and the specific circumstances of each patient also provides relatively reasonable anesthetic maintenance blood concentrations. Using the TCI technique, these objectives can be achieved more precisely and easily by target concentration setting. It should be noted, however, that since the injury stimulus is not invariable intraoperatively, the appropriate target concentration should be selected according to the specific situation (size of the procedure, degree of stimulation and patient response, etc.). It should also be emphasized that pre-active adjustment of target concentration to adapt to an imminent strong stimulus is much more effective than passive adjustment after an injurious stimulus has occurred. Combination of drugs should be emphasized for anesthesia maintenance. A perfect anesthesia should at least achieve loss of consciousness, complete analgesia, muscle relaxation, and suppression of autonomic reflexes, provided that the patient’s vital signs are stable. In order to achieve these four objectives, it is obvious that it is not feasible to rely on a particular type of anesthetic, which requires the combined use of anesthetic drugs. The perfect intravenous general anesthesia mainly involves three major categories of drugs: first, intravenous general anesthetics, such as isoproterenol, imipramine, etc., which can make the patient fall asleep, lose consciousness, and have no memory of the surgical procedure; second, narcotic analgesics, such as fentanyl, dulcolax and other opioids, which can reduce pain and inhibit the stress response; third, skeletal muscle relaxants, such as depolarizing myorelaxants succinylcholine and non-depolarizing myorelaxants drugs, such as vecuronium bromide and pancuronium bromide, can relax the muscles and provide a good surgical view, but a ventilator is needed to control breathing. ④ After resumption of intravenous anesthesia, the patient’s awakening time is closely related to the concentration of central chamber (plasma) anesthetics. For a single injection, the decrease in blood concentration depends mainly on the distribution half-life and clearance half-life of the drug. The order of rapid recovery for single-infusion administration at equivalent doses is: isoproterenol, etomidate, sodium thiopental, imipramine, and ketamine. For longer continuous infusions of anesthetic drugs, the speed of decline in blood concentration depends not only on the distribution half-life and clearance half-life, but also on the sluggishness of their peripheral compartments. In addition to rapid recovery, good recovery should be free of side effects and still have adequate analgesia. Isoproterenol has the fewest side effects during recovery. After ketamine and etomidate anesthesia, agitation often occurs during the wake period. Imipramine can better reduce these side effects but makes recovery delayed. Haloperidol may increase the incidence of nightmares. Patients with agitation during the recovery period should first exclude hypoxia, carbon dioxide accumulation, wound pain, and residual inotropic drugs; if inhaled anesthetics are used, the thoroughness of their washout should also be considered. 3. Complex anesthesia Currently, several different anesthetic drugs or techniques are used simultaneously or successively in clinical anesthesia to obtain a state of general anesthesia. This kind of anesthesia method that applies more than two general anesthetic drugs or techniques simultaneously or successively to achieve analgesia, amnesia, muscle relaxation, suppression of voluntary reflexes and maintenance of stable vital signs is called balanced anesthesia. Balanced anesthesia emphasizes the combination of drugs, which can not only maximize the pharmacological effects of each type of drug, but also reduce the dosage and side effects of each drug. This method has played a very important role in improving the quality of anesthesia, ensuring patient safety and reducing medical costs, and is an anesthesia concept in line with China’s national conditions. Intravenous-inhalation compound anesthesia is a typical representative of balanced anesthesia. There are various methods, such as induction of intravenous anesthesia and maintenance of inhalation anesthesia; or inhalation anesthesia induction and maintenance of intravenous anesthesia; or induction of static inhalation complex and maintenance of static inhalation complex. Because of the rapid onset of intravenous anesthesia and smooth induction, while inhalation anesthesia is easy to manage and the depth of anesthesia is easy to control, so inhalation anesthesia or static inhalation compound anesthesia maintenance after induction of intravenous anesthesia occupies a major position in clinical anesthesia work. The induction of intravenous anesthesia and the maintenance of inhalation anesthesia fully demonstrate the respective advantages of intravenous anesthesia and inhalation anesthesia, which is the sublimation of anesthesia technology to the art of anesthesia. In addition to the above three kinds of general anesthesia, there are basic anesthesia, supervised anesthesia and other general anesthesia techniques, which have different degrees of anesthesia, but there is no obvious difference in essence. Nowadays, more and more painless examination/treatment techniques are carried out clinically, such as painless gastroscopy, painless abortion, etc. This is actually also a general anesthesia technique, giving intravenous anesthetics (propofol is commonly used) and analgesic drugs to achieve a state of sleep and painlessness for the patient, but mostly short operations, mostly without intubation to control breathing, but with the risk of respiratory depression, aspiration pneumonia, etc.