Amobarbital

Pronunciation

(am oh BAR bi tal)

U.S. Brand Names

Amytal®

Synonyms

Amylobarbitone

Generic Available

Canadian Brand Names

Amytal®

Use

Hypnotic in short-term treatment of insomnia; reduce anxiety and provide sedation preoperatively

Use - Unlabeled/Investigational

Therapeutic or diagnostic "Amytal® Interviewing"; Wada test

Restrictions

C-II

Pregnancy Risk Factor

Pregnancy Implications

Barbiturates cross the placenta and distribute in fetal tissue. Teratogenic effects have been reported with 1st trimester exposure. Exposure during the 3rd trimester may lead to symptoms of acute withdrawal following delivery; symptoms may be delayed up to 14 days.

Lactation

Excretion in breast milk unknown/use caution

Contraindications

Hypersensitivity to barbiturates or any component of the formulation; marked hepatic impairment; dyspnea or airway obstruction; porphyria; pregnancy

Warnings/Precautions

Tolerance and/or psychological and physical dependence may occur with prolonged use; withdraw gradually if used over extended periods of time. Use caution in patients with depression or suicidal tendencies, or in patients with a history of drug abuse. When used as a hypnotic for the treatment of insomnia, effectiveness is limited to

2 weeks. Use caution in acute or chronic pain; paradoxical excitement may occur. Use caution with hepatic dysfunction, decreased dosage may be needed. Use caution in patients with hypoadrenalism; effects of exogenous or endogenous corticosteroids may be decreased. Use caution in CHF, renal impairment, hypovolemic shock, the elderly, or in children. Rapid I.V. administration may cause respiratory depression, apnea, and hypotension. Solution for injection is highly alkaline and extravasation may cause local tissue damage. May cause CNS depression, which may impair physical or mental abilities. Patients must be cautioned about performing tasks which require mental alertness (eg, operating machinery or driving). Effects with other sedative drugs or ethanol may be potentiated. Safety and efficacy have not been established in children <6 years of age.

Adverse Reactions

Frequency not defined.

Cardiovascular: Bradycardia, hypotension, syncope

Central nervous system: Agitation, anxiety, ataxia, confusion, CNS depression, dizziness, fever, hallucinations, headache, insomnia, nightmares, nervousness, psychiatric disturbances, somnolence, thinking abnormal

Gastrointestinal: Constipation, nausea, vomiting

Hematologic: Megaloblastic anemia (following chronic phenobarbital use)

Hepatic: Liver damage

Local: Injection site reaction

Neuromuscular & skeletal: Hyperkinesia

Respiratory: Apnea, atelectasis (postoperative), hypoventilation

Miscellaneous: Hypersensitivity reaction

Overdosage/Toxicology

Symptoms of overdose include unsteady gait, slurred speech, confusion, jaundice, hypothermia, fever, and hypotension. If hypotension occurs, administer I.V. fluids and place the patient in the Trendelenburg position. If unresponsive, an I.V. vasopressor (eg, dopamine, epinephrine) may be required. Forced alkaline diuresis is of no value in the treatment of intoxications with short-acting barbiturates. Charcoal hemoperfusion or hemodialysis may be useful in severe intoxications.

Drug Interactions

Induces CYP2A6 (strong)

Acetaminophen: Barbiturates may enhance the hepatotoxic potential of acetaminophen overdoses

Antiarrhythmics: Barbiturates may increase the metabolism of antiarrhythmics, decreasing their clinical effect; includes disopyramide, propafenone, and quinidine

Anticonvulsants: Barbiturates may increase the metabolism of anticonvulsants; includes ethosuximide, felbamate (possibly), lamotrigine, phenytoin, tiagabine, topiramate, and zonisamide; does not appear to affect gabapentin or levetiracetam

Antineoplastics: Limited evidence suggests that enzyme-inducing anticonvulsant therapy may reduce the effectiveness of some chemotherapy regimens (specifically in ALL); teniposide and methotrexate may be cleared more rapidly in these patients

Antipsychotics: Barbiturates may enhance the metabolism (decrease the efficacy) of antipsychotics; monitor for altered response; dose adjustment may be needed

Beta-blockers: Metabolism of beta-blockers may be increased and clinical effect decreased; atenolol and nadolol are unlikely to interact given their renal elimination

Calcium channel blockers: Barbiturates may enhance the metabolism of calcium channel blockers, decreasing their clinical effect

Chloramphenicol: Barbiturates may increase the metabolism of chloramphenicol and chloramphenicol may inhibit barbiturate metabolism; monitor for altered response

Cimetidine: Barbiturates may enhance the metabolism of cimetidine, decreasing its clinical effect

CNS depressants: Sedative effects and/or respiratory depression with barbiturates may be additive with other CNS depressants; monitor for increased effect; includes ethanol, sedatives, antidepressants, narcotic analgesics, and benzodiazepines

Corticosteroids: Barbiturates may enhance the metabolism of corticosteroids, decreasing their clinical effect

Cyclosporine: Levels may be decreased by barbiturates; monitor

CYP2A6 substrates: Amobarbital may decrease the levels/effects of CYP2A6 substrates. Example substrates include ifosfamide and rifampin.

Doxycycline: Barbiturates may enhance the metabolism of doxycycline, decreasing its clinical effect; higher dosages may be required

Estrogens: Barbiturates may increase the metabolism of estrogens and reduce their efficacy

Felbamate may inhibit the metabolism of barbiturates and barbiturates may increase the metabolism of felbamate

Griseofulvin: Barbiturates may impair the absorption of griseofulvin, and griseofulvin metabolism may be increased by barbiturates, decreasing clinical effect

Guanfacine: Effect may be decreased by barbiturates

Immunosuppressants: Barbiturates may enhance the metabolism of immunosuppressants, decreasing its clinical effect; includes both cyclosporine and tacrolimus

Loop diuretics: Metabolism may be increased and clinical effects decreased; established for furosemide, effect with other loop diuretics not established

MAO inhibitors: Metabolism of barbiturates may be inhibited, increasing clinical effect or toxicity of the barbiturates

Methadone: Barbiturates may enhance the metabolism of methadone resulting in methadone withdrawal

Methoxyflurane: Barbiturates may enhance the nephrotoxic effects of methoxyflurane

Oral contraceptives: Barbiturates may enhance the metabolism of oral contraceptives, decreasing their clinical effect; an alternative method of contraception should be considered

Theophylline: Barbiturates may increase metabolism of theophylline derivatives and decrease their clinical effect

Tricyclic antidepressants: Barbiturates may increase metabolism of tricyclic antidepressants and decrease their clinical effect; sedative effects may be additive

Valproic acid: Metabolism of barbiturates may be inhibited by valproic acid; monitor for excessive sedation; a dose reduction may be needed

Warfarin: Barbiturates inhibit the hypoprothrombinemic effects of oral anticoagulants via increased metabolism; this combination should generally be avoided

Ethanol/Nutrition/Herb Interactions

Ethanol: Avoid ethanol (may increase CNS depression).

Stability

Powder should be stored at 15°C to 30°C (59°F to 86°F). Reconstitute with SWFI to make a 10% I.V. solution; a 20% solution may be made for I.M. use. Rotate vial to dissolve, do not shake. Do not use unless a clear solution forms within 5 minutes. Following reconstitution, solution should be used within 30 minutes.

Compatibility

Stable in D5LR, D5NS, D5W, D10W, D20W, LR, NS

Compatibility when admixed: Compatible: Amikacin, aminophylline, sodium bicarbonate. Incompatible: Cefazolin, cimetidine, clindamycin, diphenhydramine, droperidol, hydroxyzine, insulin (regular), levorphanol, meperidine, morphine, norepinephrine, pancuronium, pentazocine, penicillin G potassium, procaine, streptomycin, vancomycin. Variable (consult detailed reference): Atracurium, dimenhydrinate, hydrocortisone sodium succinate, isoproterenol, metaraminol, methyldopate, norepinephrine, succinylcholine

Mechanism of Action

Interferes with transmission of impulses from the thalamus to the cortex of the brain resulting in an imbalance in central inhibitory and facilitatory mechanisms

Pharmacodynamics/Kinetics

Onset of action: I.V.: Within 5 minutes

Distribution: Readily crosses placenta; small amounts enter breast milk

Metabolism: Primarily hepatic via microsomal enzymes

Half-life elimination: 15-40 hours (mean: 25 hours)

Excretion: Urine, feces

Dosage

Children:

Sedative: I.M., I.V. : 6-12 years: Manufacturer's dosing range: 65- 500 mg

Hypnotic: I.M.: 2-3 mg/kg (maximum: 500 mg)

Adults:

Hypnotic: I.M., I.V.: 65-200 mg at bedtime (maximum I.M. dose: 500 mg)

Sedative: I.M., I.V.: 30-50 mg 2-3 times/day

"Amytal® interview" (unlabeled use): I.V.: 50-100 mg/minute for total dose of 200-1000 mg or until patient experiences drowsiness, impaired attention, slurred speech, or nystagmus

Wada test (unlabeled use): Intra-arterial: 100 mg over 4-5 seconds via percutaneous transfemoral catheter

Dosing adjustment in renal/hepatic impairment : Dosing should be reduced; specific recommendations not available.

Administration

I.M.: Administer deeply into a large muscle; do not use more than 5 mL at any single site (may cause tissue damage). I.M. dosages should not exceed 500 mg.

I.V.: Use only when I.M. administration is not feasible; administer by slow I.V. injection (maximum: 50 mg/minute in adults)

Monitoring Parameters

Vital signs should be monitored during injection and for several hours after administration

Reference Range

Therapeutic: 1-5 mcg/mL (SI: 4-22 mol/L)

Toxic: >10 mcg/mL (SI: >44 mol/L)

Lethal: >50 mcg/mL

Patient Education

I.M./I.V.: Patient instructions and information are determined by patient condition and therapeutic purpose. Drug may cause physical and/or psychological dependence. While using this medication, do not use alcohol or other prescription or OTC medications (especially pain medications, sedatives, antihistamines, or hypnotics) without consulting prescriber. Maintain adequate hydration (2-3 L/day of fluids) unless instructed to restrict fluid intake. You may experience drowsiness, dizziness, or blurred vision (use caution when driving or engaging in tasks requiring alertness until response to drug is known); nausea, vomiting, or loss of appetite (small, frequent meals, frequent mouth care, chewing gum, or sucking lozenges may help); constipation (increased exercise, fluids, fruit, or fiber may help). If medication is used to control seizures, wear identification of epileptic status and medication. Report skin rash or irritation; CNS changes (eg, confusion, depression, increased sedation, excitation, headache, insomnia, or nightmares); respiratory difficulty or shortness of breath; changes in urinary pattern or menstrual pattern; muscle weakness or tremors; or difficulty swallowing or feeling of tightness in throat. Pregnancy/breast-feeding precautions: Do not get pregnant while taking this medication; use appropriate barrier contraceptive measures. Consult prescriber if breast-feeding.

Nursing Implications

Raise bed rails at night

Dental Health: Effects on Dental Treatment

No significant effects or complications reported

Dental Health: Vasoconstrictor/Local Anesthetic Precautions

No information available to require special precautions

Dosage Forms

Injection, powder for reconstitution, as sodium: 500 mg

References

Kavirajan H, "The Amobarbital Interview Revisited: A Review of the Literature Since 1966," Harv Rev Psychiatry , 1999, 7(3):153-65.

International Brand Names

Amital® (RO); Amobarbital® (RO); Amytal® (CA, GB); Amytal Sodium® (AU); Barbamyl® (IL); Dorlotyn® (HU); Isomytal® (JP); Neur-Amyl® (AU); Sodium Amytal® (GB); Tuinal® (GB)

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