Thiopental sodium

Pharmacology

Thiopental sodium (also known as thiopentone and sodium pentothal) is a barbiturate which is used for induction of anaesthesia. It has a rapid onset. It has no analgesic properties.

Pharmacokinetics and metabolism

Thiopental rapidly and easily crosses the blood–brain barrier as it is a lipophilic molecule. As with all lipid-soluble anaesthetic drugs the short duration of action of sodium thiopental is due almost entirely to its redistribution away from central circulation into muscle and fat. Loss of consciousness occurs in around 30 seconds with return of consciousness typically within 10 minutes.

Rapid redistribution means thiopental sodium is short acting and recovery is fast. It doesn however have a relatively long half-life of around 9 hours Once redistributed, the free fraction in the blood is metabolised in the liver by zero-order kinetics hence sedative effects may persist for several hours.

Comparison of commonly used anaesthetic agents

Drug	Key Points	Dose	Side effects
Propofol 1%	Produces significant venodilation Myocardial depression Can reduce cerebral perfusion pressure	1.5-2.5 mg/kg	Hypotension Respiratory depression Pain at site of injection
Ketamine	Produces dissociative state Does not display a dose-response continuum Causes bronchodilation (useful in asthma)	Initially 0.5-2 mg/kg Typical IV dose is 1.5mg/kg	Tachycardia Hypertension Laryngospasm Unpleasant hallucinations Nausea and vomiting Hypersalivation Increased intracranial and intraocular pressure Nystagmus and diplopia Abnormal movements Skin reactions
Thiopentone (thiopental sodium, sodium pentothal)	Ultra-short acting barbiturate Acts on the GABA receptor complex Decreases cerebral metabolic oxygen Reduces cerebral blood flow & ICP	Adult: 3-5 mg/kg Child: 5-8 mg/kg Doses halved in hypovolaemia	Venodilation Myocardial depression Hypotension Contraindicated in: Acute porphyrias Myotonic dystrophy
Etomidate	Most haemodynamically stable induction agent Useful in hypovolemia, anaphylaxis & asthma Similar cerebral effects to thiopentone	0.15-0.3 mg/kg	Injection site pain Movement disorders Adrenal insufficiency Apnoea (rare) Contraindicated in: Sepsis due to adrenal suppression

Thiopental sodium can reduce intracranial pressure but can cause cardiorespiratory depression