Intravenous and Inhalation Anesthetics - Pharmacology of Intravenous Anesthetics
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Intravenous (IV) anesthetics are commonly used for the induction and maintenance of general anesthesia and for sedation. The rapid onset and offset of these drugs are due to their physical translocation into and out of the brain. After a bolus IV injection, lipid-soluble drugs such as propofol, thiopental (not available in the United States), and etomidate rapidly distribute into the vessel-rich group of highly perfused tissues (e.g., brain, heart, liver, and kidneys), causing an extremely rapid onset of effect. Plasma concentrations decrease as the drug is taken up by the less well-perfused tissues (e.g., muscle and fat), and the drug rapidly leaves the brain. This redistribution from the brain is responsible for the termination of effects, but the clearance of the active drug must still occur, typically by hepatic metabolism and renal elimination. Elimination half-life is defined as the time required for the plasma concentration of a drug to decrease by 50% during the elimination phase of clearance. Context-sensitive half-time (CSHT) is defined as the time required for a 50% decrease in the central compartment drug concentration after a steady-state infusion of specified duration (duration is the “context”).