IV Fosphenytoin Dosing
For status epilepticus, administer IV fosphenytoin at 15-20 mg PE/kg (maximum 1500 mg PE) at an infusion rate not exceeding 150 mg PE/min in adults or 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients. 1
Loading Dose by Clinical Indication
Status Epilepticus (Emergency)
- Adults: 15-20 mg PE/kg IV at 100-150 mg PE/min 1
- Pediatric patients: 15-20 mg PE/kg IV at 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) 2, 1
- The conversion half-life from fosphenytoin to phenytoin is 7-15 minutes, allowing rapid achievement of therapeutic levels 3
Non-Emergent Loading and Maintenance
- Adults: 10-20 mg PE/kg IV at a slower rate, with initial maintenance of 4-6 mg PE/kg/day in divided doses 1
- Pediatric patients: 10-15 mg PE/kg at 1-2 mg PE/kg/min (or 150 mg PE/min, whichever is slower), with maintenance of 2-4 mg PE/kg every 12 hours 1
Critical Administration Parameters
Infusion Rate Limits (Boxed Warning)
- Never exceed 150 mg PE/min in adults - severe hypotension and cardiac arrhythmias can occur 1
- Never exceed 2 mg PE/kg/min in pediatric patients (or 150 mg PE/min, whichever is slower) 1
- Fosphenytoin can be infused 3 times faster than phenytoin (150 mg PE/min vs 50 mg/min) due to its improved tolerability 3, 4
Dilution and Compatibility
- Dilute in normal saline only to a final concentration ≥5 mg PE/mL 5
- Never mix with dextrose-containing solutions - causes precipitation 2, 5
- The near-neutral pH of 8.6 (vs pH 11 for phenytoin) reduces local tissue reactions 6
Mandatory Monitoring During Administration
Cardiac Monitoring
- Continuous ECG monitoring is required during and after infusion 1
- Reduce infusion rate if heart rate decreases by 10 beats/min 2, 5
- Monitor for bradycardia, arrhythmias, heart block, and hypotension 5, 1
Respiratory Monitoring
- Be prepared to provide respiratory support, especially when combined with benzodiazepines 2, 7
- Increased risk of apnea when fosphenytoin follows benzodiazepine administration 7
Special Population Considerations
Neonates and Young Infants
- Phenobarbital is preferred over fosphenytoin/phenytoin in neonates due to increased toxicity risk from decreased protein binding 2, 7
- If fosphenytoin must be used in infants, careful individual dosing is required - maintaining therapeutic levels on standard doses (5-8 mg PE/kg/day) is difficult 6
- Neonates may require higher or more frequent dosing to maintain therapeutic phenytoin levels 6
Renal or Hepatic Disease
- Reduce infusion rate by 25-50% in patients with decreased protein binding capacity (renal disease, hepatic disease, hypoalbuminemia, elderly) 3
- Caution in end-stage renal disease - fosphenytoin releases phosphate as a metabolite and can cause acute hyperphosphatemia 8
Common Pitfalls to Avoid
Dosing Errors
- Always express dose in mg PE (phenytoin equivalents), not mg - 1 mg PE fosphenytoin = 1 mg phenytoin 1, 4
- Verify the appropriate volume is withdrawn from the 50 mg PE/mL vial 1
- Ensure prescription specifies "mg PE" to prevent 10-fold dosing errors 7
Administration Errors
- Infusing too rapidly increases risk of hypotension and cardiac arrhythmias 5, 1
- Using glucose-containing solutions causes drug precipitation 2, 5
- Failing to prepare for respiratory support when combining with benzodiazepines 7
Intramuscular Alternative
- IM administration is possible but not preferred for emergencies - therapeutic phenytoin levels achieved within 30 minutes, with peak at 2-4 hours 1, 3
- IM dose: 10-20 mg PE/kg with 100% bioavailability 1, 3
- Well tolerated with minimal injection site irritation (99% of sites had no irritation) 9