Induction of General Anesthesia. General anesthesia can be induced by giving drugs intravenously, by inhalation, or by a combination of both methods.

1. Rapid-sequence induction: Anesthesia is most commonly induced by the method of rapid-sequence induction, in which

rapid administration of an ultra-short acting barbiturate (e.g. thiopental) is followed by a depolarizing muscle relaxant (e.g.

succinylcholine). This allows anesthesia to be induced within 30 seconds and the trachea to be intubated within 60~90 seconds. Oxygen is usually given by mask beforehand to allow maximum time for intubation while the patient is apneic. A nondepolarizing neuromuscular blocking drug (e.g. vecuronium, atracurium or pancuronium) can be substituted for succinylcholine, but the onset of paralysis is delayed by about 60 seconds.

Rapid-sequence induction minimizes the time during which the trachea is unprotected. Consequently, this method is often used in emergency surgery in patients who have eaten recently. The disadvantage of giving depressant drugs rapidly is that by potension may occur in patients with questionable cardiovascular status or marginal circulatory volume.

2. Inhalation induction: Inhalation of nitrous oxide plus a potent volatile anesthetic (e.g. halothane, enflurane, or isoflurane) can produce anesthesia within 3~5 minutes. After induction, a depolarizing or nondepolarizing neuromuscular blocking drug can be given intravenously to facilitate tracheal intubation. If there is some question about the difficulty of intubation, it can be attempted while the patient is breathing spontaneously, without giving a muscle relaxant. Although conditions for intubation may not be as good with this method, the patient will still be breathing if difficulties with intubation prolong the time before complete airway control is achieved.

The advantage of inhalation induction is that anesthetic drugs can be titrated according to the patient's needs. This allows for administration of more precise doses and minimizes the risk of an accidental overdose with resultant cardiovascular depression. The disadvantages are a slower induction time and the lack of protection for the airway for a longer period of time.

3. Combined intravenous-inhalation ind'uction: Short-acting anesthetic drugs such as thiopental or diazepam are often administered intravenously before inhalation of a volatile anesthetic. This is done to minimize the discomfort of wearing the anesthetic mask and to facilitate inhalation of the anesthetic agent, which many people consider to have an offensive odor. This technique combines the advantages of both the intravenous and inhalation approaches. Anesthesia is induced rapidly and anesthetic drug dosages can be titrated according to the patient's requirements.

Maintaining General Anesthesia. The main objectives of general anesthesia are analgesia, unconsciousness, skeletal muscle relaxation, and control of sympathetic nervous system responses to noxious stimulation. Inhaled and intravenous anesthetics, narcotics, and muscle relaxants should be selected with specific pharmacologic goals in mind.

Although, paralysis by muscle relaxants simplifies exposure of the operative site and decreases the need for volatile anesthetics, many signs of anesthesia are absent in the paralyzed patient. It is essential thạt the anesthesiologist continuousty assess the depth of anesthesia. Failure to do so may result in the patient being awake but paralyzed during the procedure.

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