Thiopental Sodium
Thiopental sodium is an ultra-short-acting thiobarbiturate used primarily for the induction of general anesthesia or as a sole anesthetic for very brief procedures. **Clinical Pearls:** * **Redistribution Kinetics:** Its short duration of action (10-30 minutes) is due to rapid redistribution from the highly perfused CNS to muscle and adipose tissue, rather than rapid metabolism. * **Sight Hound Sensitivity:** Greyhounds and other sight hounds exhibit prolonged recoveries due to differences in metabolic handling and lower body fat; alternative induction agents (e.g., propofol, alfaxalone, or methohexital) are generally preferred. * **Tissue Toxicity:** Thiopental solutions are highly alkaline (pH > 10). **Perivascular extravasation causes severe tissue irritation and necrosis.** It must be administered via a secure IV catheter. * **Arrhythmogenic:** It can sensitize the myocardium to catecholamines, commonly causing transient ventricular bigeminy in dogs.
Mechanism: Thiopental is a highly lipid-soluble thiobarbiturate. * **Mechanism:** It binds to the **GABA_A receptor** complex in the central nervous system. * **Pathway:** Binding โ increases the duration of chloride ion channel opening โ enhances chloride influx โ hyperpolarization of the postsynaptic neuronal membrane โ profound CNS depression, hypnosis, and anesthesia. * Due to its high lipid solubility, it rapidly crosses the blood-brain barrier, resulting in anesthesia within 15-30 seconds of IV injection.
Dosing by species
- Anesthesia ยท 9.9-15 mg/kg ยท IV ยท once ยท depending on depth required
- Anesthesia ยท 20-22 mg/kg ยท IV ยท once ยท Given after atropine (0.7 mg/kg) IM
- Anesthesia ยท 13.2-26.4 mg/kg ยท IV ยท once ยท depending on duration of anesthesia required
- Anesthesia (unpremedicated) ยท 22 mg/kg ยท IV ยท once
- Anesthesia (after tranquilization) ยท 15.4 mg/kg ยท IV ยท once
- Anesthesia (after narcotic premedication) ยท 11 mg/kg ยท IV ยท once
- Anesthesia induction ยท 12-15 mg/kg ยท IV ยท once ยท One-third administered rapidly, additional amount to effect
- Anesthesia ยท 8.14-15.4 mg/kg ยท IV ยท once
- Deep surgical anesthesia (unweaned calves fasted 6-12h) ยท no more than 6.6 mg/kg ยท IV ยท once
- Anesthesia (calves under 2 weeks of age) ยท 15-22 mg/kg ยท IV ยท once ยท 10-12 minutes ยท Administer slowly until complete muscular relaxation
Routes of administration
Contraindications
- Absence of suitable veins for IV administration (Absolute)
- History of hypersensitivity to barbiturates (Absolute)
- Status asthmaticus (Absolute)
- Preexisting leukopenia in horses
- Use with extreme caution or avoid in greyhounds and other sight hounds
- Relative: Severe cardiovascular disease, ventricular arrhythmias, shock, increased intracranial pressure, myasthenia gravis, severe hepatic disease
Adverse effects
- Ventricular bigeminy (dogs)
- Apnea (especially in cats)
- Arterial hypotension
- Excitement and severe ataxia during recovery (horses, if used alone)
- Transient leukopenia and hyperglycemia (horses)
- Vascular dilatation and hypoglycemia (with rapid IV administration)
- Severe tissue necrosis (if administered perivascularly)
- Prolonged recovery with repeated doses
Drug interactions
- Clonidine ยท IV clonidine prior to induction may reduce thiopental dosage requirements by up to 37%
- CNS Depressants ยท May enhance respiratory and CNS depressant effects
- Diazoxide ยท Potential for hypotension
- Epinephrine / Norepinephrine ยท Ventricular fibrillatory effects may be potentiated when used with thiobarbiturates and halothane
- Metoclopramide ยท Given prior to induction may reduce thiopental dosage requirements
- Midazolam ยท May potentiate hypnotic effects
- Opiates ยท Given prior to induction may reduce thiopental dosage requirements
- Phenothiazines ยท May potentiate thiopental effects; hypotension possible
- Probenecid ยท May displace thiopental from plasma proteins
- Sulfonamides ยท Thiopental and sulfas may displace one another from plasma proteins
Monitoring
- Level of hypnosis/anesthesia
- Respiratory status (rate, depth, apnea)
- Cardiac status (heart rate, rhythm, blood pressure)
Overdose
Treatment of thiobarbiturate overdosage consists of: * **Respiratory Support:** Provide oxygen (O2) and mechanical ventilation. * **Cardiovascular Support:** Provide IV fluids and supportive care. > **Note:** Do NOT use catecholamines (e.g., epinephrine) as they may induce fatal ventricular arrhythmias in the presence of thiobarbiturates.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.