What is the maximum recommended dosage of fosphenytoin (antiepileptic medication)?

Maximum Dosage of Fosphenytoin

The maximum infusion rate for fosphenytoin is 150 mg PE/min in adults and 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients. 1

Dosing Guidelines for Fosphenytoin

Status Epilepticus (Emergency Dosing)

• Adults: 15-20 mg PE/kg at a rate of 100-150 mg PE/min, not exceeding 150 mg PE/min 2, 1
• Pediatric patients: 15-20 mg PE/kg at a rate of 1-3 mg PE/kg/min (maximum rate: 150 mg PE/min or 2 mg PE/kg/min, whichever is slower) 2, 1

Non-Emergent Loading and Maintenance Dosing

• Adults:

  • Loading dose: 10-20 mg PE/kg at a rate not exceeding 150 mg PE/min 3, 1
  • Maintenance dose: 4-6 mg PE/kg/day in divided doses 1

• Pediatric patients:

  • Loading dose: 10-15 mg PE/kg at a rate of 1-2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) 1
  • Initial maintenance dose: 2-4 mg PE/kg given 12 hours after loading dose 1
  • Subsequent maintenance doses: 4-8 mg PE/kg/day in divided doses 1

Important Administration Considerations

Route of Administration

• Intravenous (IV) administration is preferred for status epilepticus 1
• Intramuscular (IM) administration should ordinarily not be used in status epilepticus or in pediatric patients 1

Preparation and Dilution

• Dilute in 5% Dextrose Injection or 0.9% Sodium Chloride Injection 1
• Concentration should range from 1.5 to 25 mg PE/mL 1
• Maximum concentration should not exceed 25 mg PE/mL 1

Safety Precautions

• Continuous monitoring of ECG, blood pressure, and respiratory function is essential during administration 1
• Observe patients throughout the period where maximal serum phenytoin concentrations occur (approximately 10-20 minutes after the end of infusion) 1
• Monitor heart rate via ECG and reduce infusion rate if heart rate decreases by 10 beats per minute 2
• When given IV, itching is common and can be controlled by reducing the flow rate 2

Common Pitfalls and Caveats

1. Medication errors: Fatal overdoses have occurred when the concentration of the vial (50 mg PE/mL) was misinterpreted to mean that the total content of the vial was 50 mg PE. Always ensure the appropriate volume is withdrawn from the vial 1

2. Cardiovascular risks: Exceeding the maximum infusion rate can cause cardiac arrhythmias and hypotension 3

3. Compatibility issues: Fosphenytoin should be diluted in normal saline only, as it is incompatible with glucose-containing solutions 3

4. Therapeutic monitoring: After fosphenytoin administration, phenytoin concentrations should not be monitored until conversion to phenytoin is essentially complete (approximately 2 hours after IV infusion) 1

5. Target therapeutic concentrations: Aim for serum total phenytoin concentrations of 10-20 mcg/mL (unbound phenytoin concentrations of 1-2 mcg/mL) 1

Fosphenytoin offers advantages over phenytoin due to its greater aqueous solubility, resulting in fewer adverse effects and less soft-tissue injury 4. It can be administered more rapidly than phenytoin, allowing for quicker achievement of therapeutic levels in emergency situations 5.