In order to increase the rate at which FA reaches FI, several components can be manipulated.

Elimination of inhalational anesthetics is primarily accomplished by exhalation, though minimal amount of elimination occurs percutaneously. Least soluble agents, that is, those with the lower blood gas partition coefficients, will have the fastest decrease of FA. These agents readily diffuse from the pulmonary venous blood into the alveoli and exhaled. Once the concentration of the agent in the venous blood drops below that of peripheral tissues, the agent stored in these tissues diffuses back into the venous blood, slowing the decline in FA (Table 10.2). Inhalational anesthetics with higher blood tissue coefficients take longer to wash out. It is generally accepted that 80–90 % of the inhalational anesthetic concentration must be eliminated for emergence to occur.

Metabolism plays a smaller role in elimination, as inhalational agents are exhaled primarily via the lungs. Halothane, isoflurane, sevoflurane, and desflurane are metabolized in the liver to fluoride compounds, which can accumulate after prolonged exposure. But these fluoride levels have not been shown to cause postoperative renal dysfunction.

The exact mechanism of action of inhalational anesthetics is unknown. The most common understanding is that there are multiple sites of action, which may not be uniform for all inhalational agents. Inhalational anesthetics are presumed to act on voltage-gated ion channels in cell membranes, thereby altering permeability and impairment of neurotransmitter function (see Chap. 9).

The minimum alveolar concentration (MAC) of inhalational anesthetics is defined as the concentration of gas necessary to prevent movement to surgical stimulus in 50 % of patients. The MACs of common gases are shown in Table 10.3. The MAC of nitrous oxide is 104 %. This above impossible percentage means that the MAC of nitrous oxide cannot be achieved (except in a hyperbaric chamber) and it is a weak anesthetic.

In order to achieve a level of anesthesia in which 95 % of patients do not move, it takes MAC + 25 % (1.25 MAC). In order for a patient to not recall but lose self-awareness (MAC Aware), it generally takes 0.4 to 0.5 MAC. This means for induction of anesthesia at least 0.4–0.5 MAC is needed. Emergence takes place at approximately 10–20 % MAC, which equates to 0.15–0.2 MAC, meaning that it takes more to induce anesthesia in a patient but lower levels to emerge. Yet another term is the minimum alveolar concentration which blunts the adrenergic response (MAC-BAR), which is usually 1.5 MAC. Therefore, a 1.25 MAC may stop patient movement in 95 % of patients, but it will not suppress the physical signs of pain (tachycardia, tachypnea, and hypertension).

The MAC of anesthetics is additive. Since only one volatile anesthetic can be used at a time, adding nitrous oxide as an adjunct allows the practitioner to avoid the negative side effects associated with higher concentrations of the volatile anesthetic. For example, using nitrous oxide at 0.5 MAC (52 %) combined with desflurane at 0.5 MAC (3.3 %) would result in 1 MAC and achieve a state where 50 % of patients do not react to stimulation. MAC is affected by many physiological states and pharmacologic interactions (Table 10.4). Of important note is that MAC is variable with age. From birth to one year, it is generally accepted that MAC increases to a peak. From this point on there is a decline. MAC declines at a rate of 5 % for every decade of life over 40 years of age.