Kaplan + Sadock's Synopsis of Psychiatry, 11e

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29.8 Barbiturates and Similarly Acting Drugs

Table 29.8-1 Barbiturate Dosages (Adult)

Drug

Trade Name Available Preparations

Hypnotic Dose Range Anticonvulsant Dose Range

Amobarbital Aprobarbital Butabarbital

Amytal Alurate Butisol

200 mg

50–300 mg 40–120 mg 45–120 mg

65–500 mg IV Not established Not established

40 mg/5 mL elixir

15, 30, and 50 mg tablets 30 mg/5 mL elixir 32, 50, and 100 mg tablets

Mephobarbital Methohexital

Mebaral Brevital

100–200 mg

200–600 mg

500 mg/50 mL

1 mg/kg for electro­ convulsive therapy

Not established

Pentobarbital

Nembutal

50 and 100 mg capsules 50 mg/mL injection or elixir 30, 60, 120, and 200 mg suppository 20 mg/5 mL elixir 30 to 130 mg/mL injection 100 mg capsule, 50 mg/mL injection

100–200 mg

100 mg IV, each minute up to 500 mg

Phenobarbital

Luminal

Tablets range from 15–100 mg 30–150 mg

100–300 mg IV, up to 600 mg/day

Secobarbital

Seconal

100 mg

5.5 mg/kg IV

IV, intravenous.

should be administered slowly at 10 to 20 mg/kg for status epilepticus.

have an important role in the treatment of certain mental and convulsive disorders.

Pharmacological Actions The barbiturates are well absorbed after oral administration. The binding of barbiturates to plasma proteins is high, but lipid solubility varies. The individual barbiturates are metabolized by the liver and excreted by the kidneys. The half-lives of specific barbiturates range from 1 to 120 hours. The barbiturates may also induce hepatic enzymes (cytochrome P450, CYP), thereby reducing the levels of both the barbiturate and any other concur- rently administered drugs metabolized by the liver. The mecha- nism of action of barbiturates involves the g -aminobutyric acid (GABA) receptor–benzodiazepine receptor–chloride ion chan- nel complex. Methohexital (Brevital) is commonly used as an anesthetic agent for electroconvulsive therapy (ECT). It has lower cardiac risks than other barbiturate anesthetics. Used intravenously (IV), methohexital produces rapid unconsciousness, and because of its rapid redistribution, it has a brief duration of action (5 to 7 minutes). Typical dosing for ECT is 0.7 to 1.2 mg/kg. Metho- hexital can also be used to abort prolonged seizures in ECT or to limit postictal agitation. Seizures Phenobarbital (Solfoton, Luminal), the most commonly used barbiturate for treatment of seizures, has indications for the treat- ment of generalized tonic–clonic and simple partial seizures. Parenteral barbiturates are used in the emergency management of seizures independent of cause. Intravenous phenobarbital Therapeutic Indications Electroconvulsive Therapy

Narcoanalysis Amobarbital (Amytal) has been used historically as a diagnos- tic aid in a number of clinical conditions, including conversion reactions, catatonia, hysterical stupor, and unexplained mute- ness, and to differentiate stupor of depression, schizophrenia, and structural brain lesions. The Amytal interview is performed by placing the patient in a reclining position and administering amobarbital IV at 50 mg a minute. Infusion is continued until lateral nystagmus is sus- tained or drowsiness is noted, usually at 75 to 150 mg. After this, 25 to 50 mg can be administered every 5 minutes to maintain narcosis. The patient should be allowed to rest for 15 to 30 min- utes after the interview before attempting to walk. Because of the risk of laryngospasm with IV amobarbital, diazepam has become the drug of choice for narcoanalysis. Sleep The barbiturates reduce sleep latency and the number of awak- enings during sleep, although tolerance to these effects gener- ally develops within 2 weeks. Discontinuation of barbiturates often leads to rebound increases on electroencephalographic measures of sleep and a worsening of the insomnia. Withdrawal from Sedative-Hypnotics Barbiturates are sometimes used to determine the extent of tolerance to barbiturates or other hypnotics to guide detoxifica- tion. After intoxication has resolved, a test dose of pentobarbital (200 mg) is given orally. One hour later, the patient is examined. Tolerance and dose requirements are determined by the degree to which the patient is affected. If the patient is not sedated, another 100 mg of pentobarbital can be administered every