Pharmacokinetics

Intravenous anaesthetics allow an extremely rapid induction because the blood concentration can be raised rapidly, establishing a steep concentration gradient and expediting diffusion into the brain.

The rate of transfer depends on the lipid solubility and arterial concentration of the unbound, non-ionised fraction of the drug. After a single, induction dose of an intravenous anaesthetic recovery occurs quite rapidly as the drug is redistributed around the body and the plasma concentration reduces. Recovery from a single dose of intravenous anaesthetic is not related to its rate of metabolic breakdown. With the exception of propofol, repeated doses or infusions of intravenous anaesthetics will result in considerable accumulation and prolonged recovery. Attempts to use thiopental as the sole anaesthetic in war casualties led to its being described as an ideal form of euthanasia.3 It is common practice to induce anaesthesia intravenously and then to use a volatile anaesthetic for maintenance. When administration of a volatile anaesthetic is stopped, it is eliminated quickly through the lungs and the patient regains consciousness. The recovery from propofol is rapid, even after repeat doses or an infusion. This advantage, and others, has resulted in propofol displacing thiopental as the most popular intravenous anaesthetic.

Propofol

Propofol (2,6-diisopropylphenol) is available as a 1% or 2% emulsion, which contains soya bean oil and purified egg phosphatide. Induction of anaesthesia with 1.5-2.5 mg/kg occurs within 30 seconds and is smooth and pleasant with a low incidence of excitatory movements. It causes pain on injection but adding lidocaine 20 mg to an ampoule of propofol eliminates this. The recovery from propofol is rapid and the incidence of nausea and vomiting is extremely low, particularly when propofol is used as the sole anaesthetic. Recovery from a continuous infusion of propofol is relatively rapid. On stopping the infusion the plasma concentration decreases rapidly as a result of both redistribution and clearance of the drug. Special syringe pumps incorporating pharmacokinetic algorithms allow the anaesthetist to select a target plasma propofol concentration (e.g. 6 micrograms/ml for induction of anaesthesia) once details of the patient's age and weight have been entered. This technique of target-

3 Halford J J 1943 A critique of intravenous anaesthesia in war surgery. Anesthesiology 4: 67.

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