(1989; 60 pages) [French]
Group: intravenous anaesthetic agent
Powder for injection: 0.5 g, 1 g and 2.5 g (sodium salt) in ampoule
Thiopental is a very short-acting barbiturate which, administered parenterally, rapidly induces hypnosis and anaesthesia without analgesia. It is extensively bound to plasma albumin and is initially distributed most extensively in the highly vascular tissues of the brain and other organs. It subsequently diffuses selectively into fatty tissues where it is pharmacologically inactive. It is slowly but almost entirely metabolized in the liver. Traces are excreted unchanged in the urine.
Thiopental usually exerts its cerebral depressant effect within 30 seconds and it persists for about 4-7 minutes. Anaesthesia is induced rapidly, pleasantly and without excitement.
Thiopental has little analgesic action. Any muscular relaxation that occurs is too short to be of practical value. In contrast to ketamine, it cannot be used alone as an anaesthetic agent because the large and repeated doses required accumulate in fatty tissues and are subsequently only slowly released; this results in prolonged anaesthesia and delayed recovery characterized by somnolence and respiratory and circulatory depression.
Induction of anaesthesia prior to administration of inhalational and other anaesthetics.
Dosage and administration
Adults and children: 3-5 mg/kg given by slow intravenous injection over 10-15 seconds and repeated, if necessary, after 20-30 seconds.
Dosage requirements vary; they are reduced in the elderly, in hypovolaemic patients, and in patients heavily premedicated with narcotics or other cerebral depressants.
The injection should be administered slowly until the patient becomes unconscious. Recovery after a single dose is rapid because of redistribution of the drug from the central nervous system into other tissues.
Solutions of 25 mg/ml should be freshly prepared by mixing 20 ml of water for injection with the contents of the 0.5-g ampoule, 40 ml with the 1-g ampoule or 100 ml with the 2.5-g ampoule. Any solution made up over 24 hours previously or in which cloudiness, precipitation or crystallization is evident should be discarded.
• Thiopental should not be used if there is doubt that a clear airway can be maintained.
• Hypersensitivity to barbiturates.
• Severe cardiovascular disease or hypo-tension.
• Dyspnoea or obstructive respiratory disease.
• Status asthmaticus.
• Addison’s disease.
• Hepatic dysfunction.
• A history of acute intermittent or variegate porphyria.
Thiopental should, whenever possible, be administered under the supervision of an experienced specialist anaesthetist.
Equipment for resuscitation and endotracheal intubation should be immediately available and ready for use.
The patient must always lie supine as even a small overdose can cause hypotension.
Concentrations greater than 25 mg/ml are liable to cause thrombophlebitis. Local extravasation can result in extensive necrosis and sloughing. Intra-arterial injection causes intense pain and may result in arteriospasm necessitating local use of vasodilators, supplemented, if necessary, by brachial plexus block and anticoagulants.
Outpatients must be discharged in the care of a responsible adult and advised against driving, operating machinery and taking alcohol for 24 hours.
Use in pregnancy and lactation
Thiopental should be used during pregnancy only when the need outweighs any potential risk to the fetus; repeated doses should not be administered during labour. It readily crosses the placental barrier and appears in breast milk.
A short period of apnoea may follow intravenous injection.
Rapid injection may result in severe hypo-tension and hiccoughs. Coughing, sneezing or laryngeal spasm may occur during induction.
Allergic reactions and hypersensitivity have been documented.
Acute attacks of variegate or acute intermittent porphyria may be triggered in susceptible individuals.
Other cerebral depressants may augment the action of thiopental. Antihypertensives or diuretics may augment the hypotensive effect.
Serious overdosage results in respiratory depression necessitating assisted ventilation with oxygen, and hypotension progressing to circulatory collapse. In the latter event the head of the table must immediately be tilted down. Plasma expanders and pressor agents may be of value in patients who are unresponsive to this measure.
Thiopental should be stored in ampoules.