Editing General anaesthetic (section)

==Pharmacokinetics==

=== Intravenous general anesthetics ===

====Induction====

Intravenously delivered general anesthetics are typically small and highly [[Lipophilicity|lipophilic]] molecules. These characteristics facilitate their rapid preferential distribution into the brain and spinal cord, which are both highly vascularized and lipophilic. It is here where the actions of these drugs lead to general anesthesia induction.<ref name=":16" />

====Elimination====
Following distribution into the [[central nervous system]] (CNS), the anesthetic drug then diffuses out of the CNS into the muscles and viscera, followed by [[adipose tissue]]s. In patients given a single injection of drug, this redistribution results in termination of general anesthesia. Therefore, following administration of a single anesthetic [[Bolus (medicine)|bolus]], duration of drug effect is dependent solely upon the redistribution kinetics.<ref name=":16" />

The [[half-life]] of an anesthetic drug following a prolonged infusion, however, depends upon both drug redistribution kinetics, [[drug metabolism]] in the liver, and existing drug concentration in fat. When large quantities of an anesthetic drug have already been dissolved in the body's fat stores, this can slow its redistribution out of the brain and spinal cord, prolonging its CNS effects. For this reason, the half-lives of these infused drugs are said to be [[Context-sensitive half-life|context-dependent]]. Generally, prolonged anesthetic drug infusions result in longer drug half-lives, slowed elimination from the brain and spinal cord, and delayed termination of general anesthesia.<ref name=":16" />

=== Inhalational general anesthetics ===
[[Minimum alveolar concentration|Minimal alveolar concentration]] (MAC) is the concentration of an inhalational anesthetic in the lungs that prevents 50% of patients from responding to surgical incision. This value is used to compare the [[Potency (pharmacology)|potencies]] of various inhalational general anesthetics and impacts the [[Partial pressure|partial-pressure]] of the drug utilized by healthcare providers during general anesthesia induction and/or maintenance.<ref name=":16" /><ref name=":25" />

==== Induction ====
[[File:General Anesthesia WIKI - Figure.tif|thumb|437x437px]]
Induction of anesthesia is facilitated by diffusion of an inhaled anesthetic drug into the brain and spinal cord. Diffusion throughout the body proceeds until the drug's [[partial pressure]] within the various tissues is equivalent to the partial pressure of the drug within the lungs.<ref name=":16" /> Healthcare providers can control the rate of anesthesia induction and final tissue concentrations of the anesthetic by varying the partial pressure of the inspired anesthetic. A higher drug partial pressure in the lungs will drive diffusion more rapidly throughout the body and yield a higher maximum tissue concentration. Respiratory rate and inspiratory volume will also affect the promptness of anesthesia onset, as will the extent of pulmonary blood flow.<ref name=":25" />

The [[partition coefficient]] of a gaseous drug is indicative of its relative solubility in various tissues. This metric is the relative drug concentration between two tissues, when their partial pressures are equal (gas:blood, fat:blood, etc.). Inhalational anesthetics vary widely with respect to their tissue solubilities and partition coefficients.<ref name=":16" /> Anesthetics that are highly soluble require many molecules of drug to raise the partial pressure within a given tissue, as opposed to minimally soluble anesthetics which require relatively few.<ref name=":25" /> Generally, inhalational anesthetics that are minimally soluble reach equilibrium more quickly. Inhalational anesthetics that have a high fat:blood partition coefficient, however, reach equilibrium more slowly, due to the minimal vascularization of fat tissue, which serves as a large, slowly-filling reservoir for the drug.<ref name=":16" />

==== Elimination ====
Inhaled anesthetics are eliminated via expiration, following diffusion into the lungs. This process is dependent largely upon the anesthetic [[Blood–gas partition coefficient|blood:gas partition coefficient]], tissue solubility, blood flow to the lungs, and patient respiratory rate and inspiratory volume.<ref name=":25" /> For gases that have minimal tissue solubility, termination of anesthesia generally occurs as rapidly as the onset of anesthesia. For gases that have high tissue solubility, however, termination of anesthesia is generally [[Context-sensitive half-life|context-dependent]]. As with intravenous anesthetic infusions, prolonged delivery of highly soluble anesthetic gases generally results in longer drug half-lives, slowed elimination from the brain and spinal cord, and delayed termination of anesthesia.<ref name=":16" />

Metabolism of inhaled anesthetics is generally not a major route of drug elimination.<ref name=":25" />