Introduction to Maintenance of General Anesthesia

Introduction to Maintenance of General Anesthesia

Maintenance of general anesthesia is continuation anesthesia following induction to ensure the patient is in state of hypnosis (unaware of the procedure), pain free and relaxed while the patient vital sign and other physiologic parameters are within normal range or within an acceptable alteration which will not endanger the patient's health/ life.

As it has been explained in module one of session II, general anesthesia can be induced by the administration of intravenous and intramuscular drugs or inhalation of volatile anesthetics. For example, inhalation induction using halothane is produced by applying the anesthetic mask over the patient face at the concentration of 0.5% to 4%. By allowing the patient to breathe a mixture of halothane and oxygen hypnosis can developed gradually. Induction using intravenous drugs such as ketamine 1to 2 mg/kg intravenously (5- 10 mg/kg intramuscularly) or thiopentone 3 to 5 mg/kg intravenously produce fast onset of anesthesia. Maintenance of clear airway is the most important care during induction of anesthesia. Administering short acting muscle relaxant such as suxamethonium 1 to 2 mg/kg intravenously for endotracheal tube insertion to maintain clear air way, continuous delivery of inhalation anesthetic and other indications may be required.

The duration of action of intravenous induction agents is in general 5 to 10 minutes, after which time spontaneous recovery of consciousness will occur. In order to prolong anesthesia for the required duration of surgery, anesthesia must be maintained. Usually this is achieved by allowing the patient to breathe a carefully controlled mixture of oxygen, and a volatile anesthetic agent with muscle relaxant, opioids and other adjuncts (additional substance, treatment used for increasing the efficacy or safety of the primary treatment) or by having a carefully controlled infusion of medication, usually propofol but other anesthetics such as ketamine and thiopentone can be used, through an intravenous catheter. Recovery of consciousness occurs when the concentration of anesthetic in the brain drops below a certain level (usually within 1 to 30 minutes, depending upon the duration of surgery and the total dose administered). Combined inhalational and intravenous anesthetic can be titrated to utilize the good properties of each anesthetic agent.

The maintenance of anesthesia can be applied to patients who are self ventilating mostly short and superficial procedure and those techniques applied to patients who are receiving artificial ventilation of their lungs (e.g., procedures that need relaxation, long procedure, intrabodominal procedure...).