The depth of anesthesia is dependent on the anesthetic agent’s partial pressure (or tension) in the brain. The goal in giving an inhalation anesthetic is to achieve an adequate partial pressure of anesthetic (Panes) in the brain to cause a desired level of CNS depression to achieve general anesthesia. Inhalation anesthetics move down a series of partial pressure gradients (from higher to lower tension) until equilibrium (i.e. equal pressure throughout the anesthetic delivery equipment and body tissues) is established. The alveolar partial pressure (PA) of an anesthetic is a balance between anesthetic input (i.e. anesthetic delivery to the alveoli) and loss (uptake of the anesthetic by blood and body tissues) from the lungs.
A rapid rise in the PA of an anesthetic is associated with rapid anesthetic induction or change in anesthetic depth. Factors that contribute to a rapid change in the PA of an anesthetic are those related to anything that increases inspired anesthetic concentration, anything that increases alveolar ventilation, and/or anything that decreases removal of the anesthetic from the alveoli.
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 to surgical skin incision. The standard deviation of MAC is ~10%, thus 95% of patients will not respond to 1.2 MAC, and 99% will not respond to 1.3 MAC.
The concept of measuring MAC is that after equilibration, the alveolar concentration of the gas equals the blood concentration and a little later equals the brain concentration, and is therefore a useful indicator of anesthetic potency. MAC is age-dependent, being lowest in newborns, reaching a peak in infants, and then decreasing progressively with increasing age. It is independent of sex, height, weight, and anesthetic duration. MAC values for inhaled anesthetics are additive, which means that the addition of nitrous oxide will decrease the MAC of another volatile anesthetic.
|Factors that INCREASE anesthetic requirements||Factors that DECREASE anesthetic requirements
|Infancy (peaks at 6 mo)||Elderly patients|
|Chronic alcohol use||Acute alcohol intoxication|
|Acute cocaine or amphetamine use||Pregnancy|
|CNS depressant drugs (opioids, benzos, TCAs)|