Several options are available to maintain anaesthesia. Inhalational agents, intravenous anaesthetic agents or intravenous opioids can be used, either alone or in combination. Muscle relaxants are commonly used to facilitate tracheal intubation and subsequent ventilation of the lungs. Regional anaesthesia can be used to supplement any of these techniques.
Inhalational anaesthesia with spontaneous ventilation is appropriate for superficial operations where profound muscle relaxation is not required. The patient is allowed to breathe spontaneously and a volatile anaesthetic agent such as halothane, enflurane, isoflurane or sevoflurane is used with carrier gases nitrous oxide and oxygen. Most anaesthetic machines will not deliver less than 30% oxygen and have an interlocking device to ensure this. These volatile agents are delivered from vaporisers, which are specially calibrated containers that will deliver a constant percentage of agent irrespective of the gas flow through them, temperature or pressure. Analgesia as determined by the patient's response might also be required.
Minimal alveolar concentration (MAC) is the minimal alveolar concentration of an inhaled anaesthetic agent that prevents reflex movement in response to surgical incision in 50% of subjects. This value is commonly used as an index of relative potency and may be affected by the patient's age and concomitant use of other drugs.
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