Inhalation Anesthetics - Minimum Alveolar Concentration MAC -

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Minimum Alveolar Concentration MAC

Anesthetic potency of volatile anesthetics is measured by the minimum alveolar concentration (MAC). This value represents the alveolar concentration of an anesthetic (at one atmosphere) that prevents movement in 50 percent of the subjects in response to pain. A variety of noxious stimuli have been used to provoke response. For determination of MAC in humans, the usual stimulus used is surgical skin incision. In daily practice, MAC must be exceeded by a factor of 1.3 in order to assure sufficient surgical anesthesia for most of our patients. 1.3 times MAC will prevent movement in about 95 percent of the patients. The idea of measuring MAC is that after a short period of equilibration the alveolar concentration of the gas equals the blood concentration and a little later equals the brain concentration. It represents after a short time the partial pressure of the anesthetic in the central nervous system (CNS) and it is therefore the most useful index of anesthetic potency. MAC is age-dependent, being lowest in newborns, reaching a peak in infants, and then decreasing progressively with increasing age. MAC values for inhaled anesthetics are additive, which means that the addition of nitrous oxide will decrease the MAC of another volatile anesthetic. The MAC can also be altered following administration of opioids. Inhalation anesthetics alone are not able to suppress hemodynamic responses to painful stimuli nor does MAC for skin incision predict the concentrations of inhalation anesthetics necessary to avoid the motor responses to other painful stimuli such as endotracheal intubation. As a rule of thumb, the addition of every one percent of alveolar nitrous oxide to another inhalation anesthetic will decrease in the MAC of that gas about one percent. Increases in MAC result from hyperthermia and hypernatremia. Decreases in MAC can result from hypothermia, hyponatremia, pregnancy, hypotension, and drugs such as lithium, lidocaine, opioids, and a2 agonists.

Minimum alveolar concentration of inhaled anesthetics in 100% oxygen:

* Halothane 0.74 percent
* Enflurane 1.68 percent
* Isoflurane 1.15 percent
* Desflurane 6.3 percent
* Sevoflurane 2.0 percent
* Nitrous oxide 104 percent