Phenobarbital Overdose

OVERDOSAGE Signs and Symptoms The onset of symptoms following a toxic oral exposure to phenobarbital may not occur until several hours following ingestion.

The toxic dose of barbiturates varies considerably.

In general, an oral dose of 1 g of most barbiturates produces serious poisoning in an adult.

Death commonly occurs after 2 to 10 g of ingested barbiturate.

The sedated, therapeutic blood levels of phenobarbital range between 5 to 40 µg/mL; the usual lethal blood level ranges from 100 to 200 µg/mL.

Barbiturate intoxication may be confused with alcoholism, bromide intoxication, and various neurologic disorders.

Potential tolerance must be considered when evaluating significance of dose and plasma concentration.

The manifestations of long-acting barbiturate in overdose include nystagmus, ataxia, CNS depression, respiratory depression, hypothermia, and hypotension.

Other findings may include absent or depressed reflexes and erythematous or hemorrhagic blisters (primarily at pressure points).

Following massive exposure to phenobarbital, pulmonary edema, circulatory collapse with loss of peripheral vascular tone, cardiac arrest, and death may occur.

In extreme overdose, all electrical activity in the brain may cease, in which case a "flat" EEG normally equated with clinical death should not be accepted.

This effect is fully reversible unless hypoxic damage occurs.

Consideration should be given to the possibility of barbiturate intoxication even in situations that appear to involve trauma.

Complications such as pneumonia, pulmonary edema, cardiac arrhythmias, congestive heart failure, and renal failure may occur.

Uremia may increase CNS sensitivity to barbiturates if renal function is impaired.

Differential diagnosis should include hypoglycemia, head trauma, cerebrovascular accidents, convulsive states, and diabetic coma.

Treatment To obtain up-to-date information about the treatment of overdose, a good resource is your certified Regional Poison Control Center.

Telephone numbers of certified poison control centers are listed in the Physicians' Desk Reference (PDR) .

In managing overdosage, consider the possibility of multiple drug overdoses, interaction among drugs, and the unusual drug kinetics in your patient.

Protect the patient's airway and support ventilation and perfusion.

Meticulously monitor and maintain, within acceptable limits, the patient's vital signs, blood gases, serum electrolytes, etc.

Absorption of drugs from the gastrointestinal tract may be decreased by giving activated charcoal, which, in many cases, is more effective than emesis or lavage; consider charcoal instead of or in addition to gastric emptying.

Repeated doses of charcoal over time may hasten elimination of some drugs that have been absorbed.

Safeguard the patient's airway when employing gastric emptying or charcoal.

Alkalinization of urine hastens phenobarbital excretion, but dialysis and hemoperfusion are more effective and cause less troublesome alterations in electrolyte equilibrium.