Effect of Dexmedetomidine on the Minimum Alveolar Concentration of Sevoflurane

The anesthetic potency of an inhalational anesthetic is measured by the minimum alveolar concentration (MAC), or the dose required to suppress movement to a surgical stimulus in 50% of patients. This measure of potency may also be used to assess the effects of other agents on the MAC of the inhalational agent. Sedative agents reduce MAC of inhalational anesthetic agents. Two human adult studies have shown that Dexmetomidine decreased the minimum alveolar concentration of Isoflurane and Sevoflurane. Age has an important effect on the MAC of inhalational anesthetics in children. Sevoflurane is a commonly used polyflourinated methyl ether with a low blood:gas partition coefficient, which facilitates a rapid increase in alveolar and tissue anesthetic partial pressures and subsequent rapid emergence from anesthesia. Additionally, Sevoflurane is non-stimulating to airway reflexes facilitating smooth inhalational induction of anesthesia. The MAC of Sevoflurane is also dependent on age with the MAC in neonates (\<28 days) 3.3%, infants between 6-12 months of age 3.2%, and infants 6-12 months of age and children 1-12 years of age 2.5%. MAC is affected by several factors. Hypothermia, hyponatremia, hypo-osmolality, metabolic acidosis, hypoxia, hypercarbia, anemia, pregnancy, nitrous oxide, opioids, propofol, benzodiazepines, and acute alcohol use decrease MAC whereas hyperthermia, cocaine, amphetamines, hypernatremia, chronic alcohol use increase MAC. Dexmetomidine is an alpha-2 agonist that has been used as a premedication, an adjunct to analgesia, and an adjunct to general inhalational anesthesia. Alpha-2 adrenergic agonist additionally decease the stress response to surgery. Alpha-2 adrenergic agonists have sedative, hypnotic, and analgesic properties and have been reported to decrease the amount of other sedative, analgesic, and general inhalational anesthetics in not only animal studies but also human studies. A concentration-response relationship has been reported in adults receiving Dexmedetomidine using the visual analog scale (VAS) sedation scale (0-100) with a 0 as very alert and 100 as very sedated. A score of 40 correlated with a Dexmedetomidine concentration of 0.7mcg/mL and a score of 60 with a plasma concentration of 1.2mcg/mL. The sedated target concentration from a pooled population of children and neonates has been reported to be between 0.4 and 0.8 mcg/mL. Additionally, a plasma concentration of 0.6mcg/mL has been estimated as satisfactory for sedation in the adult ICU. Inhalational anesthetics have been shown to cause neuroapoptosis and neurodegenerative changes in various animal models; however, the human data from retrospective and epidemiologic studies in children exposed to inhalational anesthetics is inconclusive. There are ongoing trials to determine the effect of exposure from inhalational anesthetics on neurodevelopment outcomes. There is emerging evidence that Dexmedetomidine is not associated with neuroapoptosis or other neurodegenerative changes. Dexmedetomidine has actually been shown to attenuate Isoflurane-induced neurocognitive impairment in neonatal rats. There is no information regarding the reduction of MAC of Sevoflurane in children with clinically applicable Dexmetomidine dosing regimens. Additionally, determining the degree of reduction of Sevoflurane MAC by Dexmedetomidine may be helpful in providing an alternative anesthetic regimen such as the routine use of Dexmedetomidine and Sevoflurane in order to decrease the potential neuroapoptotic effects of inhalational anesthetic agents.