Pentobarbital
**Pentobarbital** is a short-acting oxybarbiturate primarily used in veterinary medicine for general anesthesia, chemical restraint, and the management of refractory status epilepticus. While historically popular as a general anesthetic, its use has largely been supplanted by safer, more titratable agents (like propofol or alfaxalone) due to its narrow therapeutic index and profound cardiopulmonary depressant effects. **Clinical Pearls:** * **Status Epilepticus:** Pentobarbital is often used to induce a "barbiturate coma" in patients with refractory seizures. However, it is a general anesthetic with negligible true anticonvulsant properties at standard doses; it stops the physical manifestations of seizures but may not halt the underlying electrical seizure activity in the brain. * **Ventilatory Support:** Patients treated with pentobarbital for severe seizures typically require mechanical ventilation and intensive care monitoring due to profound respiratory depression. * **Euthanasia:** At significantly higher doses, pentobarbital is the primary active ingredient in most veterinary euthanasia solutions (note: euthanasia dosing is typically covered under separate combination monographs). * **Enzyme Induction:** Chronic use strongly induces hepatic cytochrome P450 enzymes, accelerating the metabolism of many concurrent medications.
Mechanism: Pentobarbital exerts its profound central nervous system (CNS) depressant effects by interacting with the inhibitory neurotransmitter system. * **GABA_A Receptor Modulation:** Binds to the barbiturate site on the **GABA_A receptor** โ increases the *duration* of chloride channel opening โ enhances chloride influx โ hyperpolarization of the postsynaptic neuronal membrane. * **Glutamate Inhibition:** At higher anesthetic doses, it also depresses the actions of the excitatory neurotransmitter glutamate via **AMPA receptors**. * **Result:** Dose-dependent CNS depression ranging from mild sedation to general anesthesia, coma, and fatal respiratory depression.
Dosing by species
- For chemical restraint for ventilatory support ยท 1-3 mg/kg/hour ยท IV ยท CRI ยท Switch to shorter acting drug like propofol ~12 hours prior to weaning. Adjunctive drugs: Midazolam 0.3-0.5 mg/kg/hr; Medetomidine 0.3-1 mcg/kg/hr (caution with medetomidine).
- As a sedative ยท 2-4 mg/kg ยท IV ยท single dose
- As a sedative ยท 2-4 mg/kg ยท PO ยท q6h
- For status epilepticus ยท 5-15 mg/kg to effect ยท IV ยท single dose
- For status epilepticus ยท 3-15 mg/kg SLOWLY to effect ยท IV ยท single dose ยท Goal is heavy sedation, not surgical planes of anesthesia. May need to repeat in 4-8 hours.
- For anesthesia ยท 25 mg/kg ยท IV ยท single dose ยท An additional 10 mg/kg IV may be given if initial dose is inadequate.
- Euthanasia ยท 80 mg/kg in debilitated animals, up to 120-160 mg/kg in younger and fitter animals ยท IV ยท once ยท single dose ยท Rapid IV injection. Premedication recommended to prevent narcotic excitement.
- General ยท 30 mg/kg to effect ยท IV ยท single dose
- As an anesthetic ยท 15-30 mg/kg ยท IV ยท single dose
Routes of administration
Contraindications
- Severe hepatic impairment
- Severe respiratory depression or airway obstruction
- Porphyria
- Hypovolemia or severe cardiovascular instability (relative contraindication)
- Intramuscular (IM) administration (painful and slow to act)
- Use of euthanasia-specific solutions for seizure control or anesthesia
Adverse effects
- Profound respiratory depression (apnea)
- Depression of myocardial metabolism
- Vasodilation and decreased venous return
- Hypotension and decreased cardiac perfusion
- Poikilothermia (hypothermia)
- Decreased urinary output
- Seizure-like movements or excitement during recovery from anesthesia
- Excitement and injury during induction/recovery in large animals (especially horses)
- Narcotic excitement (if not premedicated)
- Agonal gasping (reflexive, not a sign of pain)
- Muscle twitching
- Vocalization (rare, usually associated with excitement phase)
Drug interactions
- Oral Anticoagulants (Warfarin) ยท Decreased effect by lowering serum concentration (due to hepatic enzyme induction)
- Beta-blockers ยท Decreased effect by lowering serum concentration
- Chloramphenicol ยท Decreased effect by lowering serum concentration
- Clonazepam ยท Decreased effect by lowering serum concentration
- Corticosteroids ยท Decreased effect by lowering serum concentration
- Cyclosporine ยท Decreased effect by lowering serum concentration
- Doxorubicin ยท Decreased effect by lowering serum concentration
- Doxycycline ยท Decreased effect (may persist for weeks after barbiturate is discontinued)
- Estrogens ยท Decreased effect by lowering serum concentration
- Griseofulvin ยท Decreased effect by lowering serum concentration
- Methadone ยท Decreased effect by lowering serum concentration
- Metronidazole ยท Decreased effect by lowering serum concentration
- Quinidine ยท Decreased effect by lowering serum concentration
Monitoring
- Levels of consciousness and/or seizure control
- Respiratory rate, rhythm, and depth (capnography and pulse oximetry highly recommended)
- Cardiac signs (heart rate, blood pressure, ECG)
- Body temperature (monitor for poikilothermia)
- Routine blood counts and liver function tests (if used chronically)
- Absence of heartbeat (auscultation)
- Absence of respiration
- Loss of corneal reflex
- Pupillary dilation
Overdose
**Overdose** of pentobarbital leads to profound central nervous system depression, progressing rapidly to coma, severe respiratory depression (apnea), cardiovascular collapse, and death. * **Treatment:** There is no specific reversal agent for barbiturates. Treatment is entirely supportive. * **Interventions:** Immediate intubation and mechanical ventilation are required. Intravenous fluids and vasopressors (e.g., dopamine) should be administered to support blood pressure and cardiac output. Maintain body temperature to prevent severe hypothermia.
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.