All commonly used volatile anesthetics, including sevoflurane, desflurane, and isoflurane, can be used to maintain general anesthesia. The strength of these agents is measured by their minimum alveolar concentration (MAC), which indicates the concentration needed to prevent movement in response to surgical stimuli in 50% of patients. MAC values are additive, so 0.5 MAC of nitrous oxide plus 0.5 MAC of sevoflurane gives 1 MAC. During maintenance, the dosing of inhaled agents is adjusted based on the MAC value and is often combined with nitrous oxide or other intravenous agents like opioids or sedative-hypnotics to create a balanced anesthesia technique. This approach allows for a combination of hypnosis, amnesia, and analgesia while minimizing the doses of individual agents, reducing the risk of side effects.
Approximate MAC values in a 40-year-old adult are isoflurane 1.2%, sevoflurane 2.0%, desflurane 6.0%, and nitrous oxide 104%, which is why nitrous oxide cannot be a sole anesthetic at sea level. MAC peaks at about 6 months of age and then falls roughly 6% per decade after age 40, so an 80-year-old needs about three quarters of the dose of a 40-year-old. MAC is lowered by opioids, benzodiazepines, hypothermia, pregnancy, acute alcohol intoxication, and hyponatremia, and raised by chronic alcohol use, hyperthermia, and acute amphetamine or cocaine use. MAC-awake, the concentration at which half of patients respond to a verbal command, is about 0.3 to 0.4 MAC, and roughly 1.3 MAC prevents movement in nearly all patients.
Anesthetics must be carefully titrated during the maintenance phase to avoid overdosing, which could lead to prolonged emergence or hemodynamic instability. As the surgery nears completion, the anesthesia providers must plan when to discontinue the inhaled anesthetic to ensure a smooth and timely emergence from anesthesia. The rate of emergence depends on how long the anesthesia has been maintained, which agents were used, and at what doses. Anesthetics with low blood-gas solubility, like desflurane and sevoflurane, allow for faster emergence, which is beneficial for procedures where quick recovery is desired.
