29.8 Barbiturates and Similarly Acting Drugs
945
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.
Therapeutic Indications
Electroconvulsive Therapy
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
should be administered slowly at 10 to 20 mg/kg for status
epilepticus.
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
Table 29.8-1
Barbiturate Dosages (Adult)
Drug
Trade Name Available Preparations
Hypnotic Dose Range Anticonvulsant Dose Range
Amobarbital
Amytal
200 mg
50–300 mg
65–500 mg IV
Aprobarbital
Alurate
40 mg/5 mL elixir
40–120 mg
Not established
Butabarbital
Butisol
15, 30, and 50 mg tablets
30 mg/5 mL elixir
45–120 mg
Not established
Mephobarbital
Mebaral
32, 50, and 100 mg tablets
100–200 mg
200–600 mg
Methohexital
Brevital
500 mg/50 mL
1 mg/kg for electro
convulsive therapy
Not established
Pentobarbital
Nembutal
50 and 100 mg capsules
100–200 mg
100 mg IV, each minute up to 500 mg
50 mg/mL injection or elixir
30, 60, 120, and 200 mg
suppository
Phenobarbital
Luminal
Tablets range from 15–100 mg 30–150 mg
100–300 mg IV, up to 600 mg/day
20 mg/5 mL elixir
30 to 130 mg/mL injection
Secobarbital
Seconal
100 mg capsule, 50 mg/mL
injection
100 mg
5.5 mg/kg IV
IV, intravenous.
