Describe Thiopentone in detail.

Pharmacology

Thiopentone (thiopental sodium) is an ultra-short-acting barbiturate used for induction (dose 3–5 mg/kg). It acts via GABA_A potentiation, has a very rapid onset (one arm–brain circulation time), and its brief action after a single dose is due to redistribution. It is prepared as an alkaline solution and was for decades the standard induction agent.

Effects & Uses

Thiopentone causes dose-dependent cardiovascular depression (hypotension) and respiratory depression/apnoea. It reduces cerebral metabolic rate and intracranial pressure, and has anticonvulsant properties — hence uses in neuroanaesthesia and in the control of status epilepticus. It is a useful agent for rapid-sequence induction. It has no analgesic action (indeed it may be anti-analgesic at low dose).

Comparison with Propofol

Compared with propofol, thiopentone has a similarly rapid onset but a less clear recovery (more hangover, accumulation), is not suitable for maintenance infusion, and is not anti-emetic — which is why propofol has largely replaced it for routine induction, though thiopentone retains niche roles.

Pharmacokinetics & Cautions

Thiopentone illustrates the principle of redistribution more clearly than any other agent: a single dose wakes the patient within minutes as the drug moves from brain to muscle and fat, yet the drug is only slowly metabolised and eliminated, so repeated doses or an infusion saturate the tissues and produce a prolonged ‘hangover’ and delayed recovery. It is highly protein-bound and its free (active) fraction rises in hypoalbuminaemia, and its cardiovascular depression is exaggerated in the hypovolaemic or shocked patient, so the dose is reduced in the elderly, the frail and the haemodynamically compromised, in whom an over-large dose can cause dangerous hypotension.

Uses Beyond Routine Induction

Although largely displaced from routine induction, thiopentone retains value where its particular properties matter: its ability to lower cerebral metabolic rate and intracranial pressure makes it useful in neuroanaesthesia and in the management of raised intracranial pressure, and its anticonvulsant action underlies its use in the control of refractory status epilepticus. It also remains a familiar, inexpensive and rapidly-acting agent for rapid-sequence induction in settings where propofol is unavailable, so a sound understanding of its pharmacology and its dangers — porphyria, intra-arterial injection and extravasation — remains clinically relevant.

Contraindicated in porphyria.

• Thiopentone: ultra-short-acting barbiturate; GABA_A; induction 3–5 mg/kg; offset by redistribution.
• Reduces cerebral metabolic rate/ICP & is anticonvulsant → neuroanaesthesia, status epilepticus.
• Causes hypotension & apnoea; no analgesia; accumulates with repeat dosing.
• Absolute contraindication: porphyria; danger: intra-arterial injection (gangrene), extravasation (necrosis).
• Largely replaced by propofol for routine induction (less clear recovery, not infusible/anti-emetic).