Fosphenytoin Dosing in Milliliters
The dose of fosphenytoin cannot be expressed in milliliters without knowing the concentration of your specific formulation, but the standard loading dose is 15-20 mg PE/kg (phenytoin equivalents per kilogram), which you must calculate based on your vial's concentration (typically 50 mg PE/mL or 75 mg PE/mL). 1, 2, 3
Calculating the Volume in Milliliters
To determine the volume in mL, you must:
- Identify your vial concentration (standard fosphenytoin comes as 50 mg PE/mL or 75 mg PE/mL) 3
- Calculate total dose needed: Patient weight (kg) × 15-20 mg PE/kg 1, 2, 3
- Divide total dose by concentration to get mL volume 3
Example Calculation:
For a 70 kg adult using 20 mg PE/kg with 50 mg PE/mL concentration:
- Total dose = 70 kg × 20 mg PE/kg = 1,400 mg PE
- Volume = 1,400 mg PE ÷ 50 mg PE/mL = 28 mL 3
Critical Dosing Parameters
Loading Dose
- Adults: 15-20 mg PE/kg IV 1, 2, 3
- Children: 15-20 mg PE/kg IV 1, 4
- Neonates: 10 mg PE/kg IV (lower dose due to decreased protein binding) 1
- Maximum initial dose: 1,000-1,500 mg PE 1, 2
Infusion Rate
- Standard rate: 100-150 mg PE/min 3, 5
- Pediatric rate: Maximum 1-3 mg PE/kg/min or 50 mg PE/min, whichever is slower 1, 2
- Never exceed 150 mg PE/min to avoid cardiovascular complications 2, 3, 5
Administration Protocol
Preparation
- Dilute in normal saline only - fosphenytoin is incompatible with dextrose-containing solutions 1, 2, 6
- Final concentration should be ≥5 mg PE/mL 2
- Unlike phenytoin, fosphenytoin's water solubility eliminates the need for propylene glycol and extreme alkalinity 3, 7
Monitoring Requirements
- Continuous ECG monitoring for bradycardia, arrhythmias, and heart block 1, 2
- Blood pressure monitoring for hypotension 2, 5
- Reduce infusion rate if heart rate decreases by 10 beats/min 1, 2
- Continuous respiratory monitoring with oxygen saturation 1
Key Advantages Over Phenytoin
Fosphenytoin can be administered faster (150 mg PE/min vs 50 mg/min for phenytoin) and has fewer infusion site reactions, making it the preferred agent when available 1, 6, 3, 7
Safety Profile
- Lower incidence of purple glove syndrome compared to phenytoin 7
- Can be given intramuscularly with complete absorption, unlike phenytoin 3, 8
- Less soft-tissue injury due to improved water solubility 3, 7
Common Pitfalls to Avoid
- Do not confuse mg PE with mg - fosphenytoin is dosed in phenytoin equivalents (PE), not actual fosphenytoin weight 3, 4
- Do not mix with dextrose solutions - causes precipitation 1, 2, 6
- Do not exceed infusion rates - rapid administration causes hypotension, bradyarrhythmias, and cardiac arrest 1, 2
- Expect transient overshoot of free phenytoin levels at end of infusion, though this is generally well-tolerated 5
Special Populations
Status Epilepticus
- Initial dose: 20 mg PE/kg over 10 minutes 1
- May repeat once after 15 minutes if necessary (maximum total: 40 mg PE/kg) 1
- For refractory cases, up to 30 mg PE/kg may be administered before switching to another antiepileptic 6
Intramuscular Administration
- 10-20 mg PE/kg loading dose can be given IM for nonemergency situations 3, 8
- Complete absorption achieved with more predictable serum concentrations than IM phenytoin 3, 8
- Time to peak plasma concentration: approximately 0.38 hours (range 0.17-2.0 hours) 8