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.
Mecanismo: 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** → increases the *duration* of chloride channel opening → enhances chloride influx → 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.
Dosificación por especie
- For chemical restraint for ventilatory support · 1-3 mg/kg/hour · IV · CRI · 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 · 2-4 mg/kg · IV · single dose
- As a sedative · 2-4 mg/kg · PO · q6h
- For status epilepticus · 5-15 mg/kg to effect · IV · single dose
- For status epilepticus · 3-15 mg/kg SLOWLY to effect · IV · single dose · Goal is heavy sedation, not surgical planes of anesthesia. May need to repeat in 4-8 hours.
- For anesthesia · 25 mg/kg · IV · single dose · An additional 10 mg/kg IV may be given if initial dose is inadequate.
- Euthanasia · 80 mg/kg in debilitated animals, up to 120-160 mg/kg in younger and fitter animals · IV · once · single dose · Rapid IV injection. Premedication recommended to prevent narcotic excitement.
- General · 30 mg/kg to effect · IV · single dose
- As an anesthetic · 15-30 mg/kg · IV · single dose
Vías de administración
Contraindicaciones
- 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
Efectos adversos
- 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)
Interacciones farmacológicas
- Oral Anticoagulants (Warfarin) · Decreased effect by lowering serum concentration (due to hepatic enzyme induction)
- Beta-blockers · Decreased effect by lowering serum concentration
- Chloramphenicol · Decreased effect by lowering serum concentration
- Clonazepam · Decreased effect by lowering serum concentration
- Corticosteroids · Decreased effect by lowering serum concentration
- Cyclosporine · Decreased effect by lowering serum concentration
- Doxorubicin · Decreased effect by lowering serum concentration
- Doxycycline · Decreased effect (may persist for weeks after barbiturate is discontinued)
- Estrogens · Decreased effect by lowering serum concentration
- Griseofulvin · Decreased effect by lowering serum concentration
- Methadone · Decreased effect by lowering serum concentration
- Metronidazole · Decreased effect by lowering serum concentration
- Quinidine · Decreased effect by lowering serum concentration
Monitoreo
- 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
Sobredosis
**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.
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