TOXBASEŽ - Updated 03/2004

 

Type of product:

Long-acting barbiturate, used in the management of epilepsy and status epilepticus.

 

Other names:

Phenobarbitone.

 

Ingredients:

Phenobarbital
Tablets - 15 mg, 30 mg, 60 mg
Elixir - 15 mg/5 mL
Injection - 200 mg/mL

 

Toxicity:

Toxicity varies between patients; tolerance will develop with chronic use. Features of poisoning are to be expected after ingestion of 1 g in an adult.

1.2 g has caused deep coma, hypotension and respiratory arrest in a 21 year old patient, but a 32 year old ingested 1.2 g and remained asymptomatic (NPIS case data). The fatal dose is reported to be 6-10 g for an adult (Lindberg et al, 1992), although deaths from ingestion of phenobarbital alone are rare. The half-life in adults is 80-120 hours (Lindberg et al, 1992).

Phenobarbital is slowly absorbed after an oral dose, with peak levels in 6-18 hours. The half-life is longer in neonates, and shorter (approximately 40-75 hours) and more variable in children. Following an overdose the half-life may be extended to 4-7 days (Product SPC).

 

Features:

Drowsiness, dysarthria, ataxia, nystagmus and disinhibition. There may also be coma, hypotonia, hyporeflexia, hypothermia, hypotension and respiratory depression.

In serious poisoning cardiac features include hypotension, cardiovascular collapse and cardiac arrest. The duration of coma may be prolonged in phenobarbital poisoning, due to its slow elimination.

Acute renal failure may occur secondary to hypotension or rhabdomyolysis if the patient has lain unconscious for several hours before being found. Blisters (erythematous or haemorrhagic) may occur, particularly at pressure points. Paradoxical excitement and irritability can occur, particularly in elderly patients, children and patients in acute pain. Agitation may also occur during recovery.

Barbiturates decrease gut motility, which may lead to a slow onset and worsening of symptoms, or cyclical improvement and worsening of symptoms.

 

Management:

1. Maintain a clear airway and adequate ventilation. Endotracheal intubation may be necessary in unconscious patients.

2. Consider activated charcoal (50 g for an adult, 10-15 g for a child under 5 years) if more than 10 mg/kg body weight of phenobarbital has been ingested within 1 hour, provided the airway can be protected.

3. Repeat dose activated charcoal is the best method of enhancing elimination of phenobarbital in symptomatic patients. For information follows. Care should be taken to protect the airway in obtunded patients.

4. Observe for at least 6 hours after ingestion. Monitor pulse, BP, temperature, respiratory rate, oxygen saturation and assess conscious level. Perform a 12-lead ECG.

5. Check blood gases in obtunded patients. Measure U & Es and CK. Ensure a good urine output.

6. Correct hypotension by raising the foot of the bed and/or by expanding the intravascular volume. If severe hypotension persists despite the above measures consider use of inotropes such as dopamine (2-10 micrograms/kg body weight/minute) or dobutamine (2.5-10 micrograms/kg body weight/minute).

7. Treat rhabdomyolysis with urinary alkalinisation e.g. 1 litre 1.26% sodium bicarbonate over 2 hours, repeated as necessary aiming for a urinary pH >7.5. Mannitol diuresis may be of benefit. Give 50 mL 20% mannitol by slow bolus followed by 2 g/hour IV infusion if there is a good response. Haemodialysis or haemofiltration may be required for cases of acute renal failure or severe hyperkalaemia.

8. Rewarm slowly using conventional methods. Treat skin blisters as burns.

9. Haloperidol or diazepam may occasionally be required for behaviour disturbance during recovery.

10. In cases of prolonged coma, or in patients with irregular or absent bowel sounds, charcoal haemoperfusion should be considered to increase elimination of phenobarbital. Discuss with a doctor at your local poisons service: in the UK NPIS 0870 600 6266, in Ireland NPIC (01) 809 2566.

11. Any other measures according to the patient's clinical condition.