Thiopental: Mechanism of Action as an Anesthetic

Thiopental (brand name Trapanal and generics) is an ultrashort-acting barbiturate that has been established in anesthesia since the 1930s. In Germany, thiopental is nowadays primarily used for induction of anesthesia, treatment of refractory status epilepticus, and reduction of intracranial pressure in traumatic brain injury in intensive care medicine. The substance is one of the historically most significant anesthetics and ranks alongside modern intravenous anesthetics such as propofol and etomidate.

With the advent of propofol in the early 1990s, thiopental has lost importance in routine anesthesia because propofol provides better awakening quality and causes less nausea. However, thiopental remains indispensable in specific neurological and intensive care indications because of its effect on intracranial pressure and its anticonvulsant properties. Application is performed exclusively by experienced anesthesiologists or intensive care specialists with the capability to secure the airway.

Mechanism of Action

Thiopental is a thiobarbiturate that enhances the action of GABA at the GABA-A receptor. It binds to the barbiturate binding site of the receptor, prolongs the opening time of the chloride channel, and leads to hyperpolarization of the neuronal membrane. This results in strong central depression with hypnosis, sedation, and anticonvulsant effects. Unlike benzodiazepines, barbiturates work even in the absence of GABA, which enhances the hypnotic effect but also promotes respiratory depression.

Thiopental has a pronounced cerebral effect with reduction of cerebral metabolism (CMRO2), cerebral blood flow, and intracranial pressure. These properties make it an established substance in neurosurgical anesthesia and in the treatment of severe traumatic brain injuries. The anticonvulsant effect is very strong, making thiopental reserve therapy for refractory status epilepticus.

Pharmacokinetically, thiopental shows very rapid penetration to the brain after intravenous administration (loss of consciousness within 30 seconds). The duration of action after a single dose is 5 to 10 minutes because thiopental rapidly redistributes from the brain to peripheral tissues. The elimination half-life is 8 to 12 hours, so accumulation occurs after multiple doses or continuous infusion and the awakening time is significantly prolonged. Metabolism occurs predominantly in the liver.

Indications

• Induction of anesthesia in anesthesia, especially in patients with elevated intracranial pressure or during neurosurgical procedures
• Refractory status epilepticus, when benzodiazepines, phenytoin, and levetiracetam show insufficient efficacy
• Reduction of intracranial pressure in severe traumatic brain injury, ischemic stroke with cerebral edema, or other conditions with increased intracranial pressure
• Brain protection in neurosurgery, in specific indications such as carotid endarterectomy
• Sedation in intensive care medicine, rather as a reserve solution when modern hypnotics fail or are not tolerated

Thiopental is not indicated for routine anesthesia in ambulatory anesthesia because propofol provides better awakening quality. Also not in obstetrics because thiopental crosses the placenta and can depress the newborn.

Dosage and Administration

Induction of anesthesia in adults: 3 to 5 mg per kg body weight intravenously, injected slowly over approximately 20 to 30 seconds. In elderly or hemodynamically unstable patients, reduced dose (1 to 3 mg per kg).

Refractory status epilepticus: Loading dose 3 to 5 mg per kg, followed by continuous infusion 3 to 5 mg per kg per hour. Adjustment based on EEG (burst suppression) and clinical response.

Intracranial pressure management: Loading dose 5 to 10 mg per kg, continuous infusion 1 to 5 mg per kg per hour according to intracranial pressure and EEG.

Pediatric: 4 to 6 mg per kg for induction. In children, dosing is weight-adapted in specialized pediatric anesthesia and intensive care settings.

Administration: exclusively intravenous via secure access, injected slowly. During bolus injection, it should not be administered too rapidly to avoid pronounced hypotension. Continuous EEG monitoring is advisable during continuous infusion.

Renal impairment: generally no dose adjustment is required because hepatic elimination predominates. Hepatic impairment: caution with severe impairment because of prolonged duration of action.

Important: accidental intraarterial injection can cause severe vasospasm with tissue necrosis. Immediate therapy with intraarterial heparin administration and sympatholytics is required.

Side Effects

Very common: respiratory depression up to apnea, hypotension, bradycardia, pain at injection site, cough, and hiccup.

Common: postoperative fatigue, drowsiness, nausea, vomiting, paradoxical agitation during awakening.

Occasional to rare: allergic reactions including anaphylaxis, bronchospasm, histamine release with skin redness, severe hypotension in patients with hypovolemia or cardiac disease, laryngospasm with shallow anesthesia.

With intraarterial injection: severe vasospasm with ischemic tissue damage, possibly loss of the affected limb. Acute therapy is mandatory.

Acute intermittent porphyria: thiopental is absolutely contraindicated because it can trigger a life-threatening porphyria crisis.

With continuous infusion: accumulation with prolonged awakening time, possibly days. With prolonged use, reduced immune function is possible.

Drug Interactions

• Other centrally depressing agents (opioids, benzodiazepines, inhalational anesthetics): additive central depression and respiratory depression. Dose adjustment and airway management are mandatory.
• Antihypertensives, diuretics, vasodilators: additive hypotension, caution with hypovolemia.
• Beta blockers and calcium antagonists: additive bradycardia and negative inotropic effects.
• CYP450 inducers (rifampicin, carbamazepine, phenytoin, phenobarbital): increased thiopental requirements with chronic use.
• Theophylline, caffeine: reduce central depression.
• Sulfonamides: reduce plasma protein binding of thiopental, resulting in higher free drug levels.
• Acidic solutions: chemical incompatibility, thiopental solution is stable only at alkaline pH. No direct mixing with other infusions without verification.

Special Precautions

Pregnancy: thiopental crosses the placenta. Use in obstetrics and before cesarean section with caution because respiratory depression in the newborn is possible. Modern alternatives such as propofol are often preferred in obstetrics. Breast-feeding: passage into breast milk, a brief breast-feeding pause after single dose is usually sufficient.

Children: established in pediatric anesthesia and intensive care medicine.

Acute intermittent porphyria: absolute contraindication because thiopental can trigger crises.

Before use: obtain history of allergies, porphyria, cardiac disease, volume status, ensure airway management capability, have emergency equipment with defibrillator and vasopressors available.

Monitoring: continuous ECG, blood pressure, pulse oximetry, EtCO2 during anesthesia. With continuous infusion, EEG to control burst suppression.

Postoperative care: because of the long elimination half-life, extended postoperative monitoring is required, especially with repeated doses.

Lifestyle: not relevant due to acute use.

Ability to drive: after anesthesia with thiopental, do not drive independently or operate heavy machinery for at least 24 hours.

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Frequently Asked Questions

Sources

• Gelbe Liste, Thiopental Drug Profile
• BfArM, Federal Institute for Drugs and Medical Devices
• AWMF, Guidelines for Anesthesia, Status Epilepticus, and Traumatic Brain Injury
• German Society for Anesthesiology and Intensive Care Medicine