Safe Intramuscular Phenobarbital Dosing
The safe intramuscular (IM) dose of phenobarbital is 4-6 mg/kg for anticonvulsant purposes in pediatric patients, with a maximum of 20 mg/kg for status epilepticus, while adult dosing ranges from 100-200 mg for preoperative sedation and up to 320 mg for acute convulsions. 1
Pediatric Dosing Guidelines
Status Epilepticus
- 15-20 mg/kg IM over 10-15 minutes 2
- Can be repeated once after 15 minutes if necessary (maximum total dose: 40 mg/kg) 2
- Be prepared to provide respiratory support as there is increased risk of apnea, especially when combined with other sedative agents 2
Anticonvulsant Therapy
- 4-6 mg/kg/day IM for 7-10 days to achieve blood levels of 10-15 mcg/mL 1
- Alternative dosing: 10-15 mg/kg/day IM 1
Preoperative Sedation
Administration Technique
Critical Safety Points
- Inject deeply into a large muscle 1
- Do not exceed 5 mL at any one injection site to minimize tissue irritation 1
- Avoid injection into or near peripheral nerves to prevent permanent neurological deficit 1
- Monitor vital signs after administration of hypnotic doses 1
- Subcutaneous administration is not recommended 1
Pharmacokinetics and Bioavailability
- IM phenobarbital has approximately 80% bioavailability compared to oral administration 3
- Peak levels occur 1-3 hours after IM administration 3
- Half-life is approximately 90 hours in adults 3, 103 hours in term infants, and 141 hours in preterm infants 4
Potential Adverse Effects
Respiratory
- Transient respiratory depression (oxygen saturation less than 10% below baseline) has been reported in 1.2% to 7.5% of pediatric patients 2
- Most cases of oxygen desaturation respond to interventions such as head positioning or supplemental oxygen 2
Emergence Reactions
- Paradoxical reactions (sustained inconsolability, severe irritability, combativeness) occur in approximately 1.5% of children 2
- Hyperactivity may occur in 5-7% of children, with higher rates (8.4%) in children older than 8 years 2
Other Adverse Effects
- Emesis (0.53-1% of patients) 2
- Coughing (3% incidence) 2
- Prolonged sedation (2% rate of sedation >120 minutes when maximum suggested dose is exceeded) 2
Special Considerations
- Fatal respiratory depression has been reported in children younger than 2 years 5
- Monitoring should continue until discharge criteria are met, as re-sedation can occur after apparent recovery 5
- Extravasation into subcutaneous tissues causes tissue irritation ranging from slight tenderness and redness to necrosis 1
- For treatment of extravasation, apply moist heat and inject 0.5% procaine solution into the affected area 1
Alternative Routes When IM Is Not Ideal
- For status epilepticus, IV administration is preferred when possible due to more predictable absorption 6
- In cases where IV access is not available and IM is not feasible, rectal administration has been used successfully in refractory status epilepticus 7
Remember that phenobarbital administration requires careful monitoring of respiratory status and level of consciousness, particularly in young children and when combined with other sedative medications.